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Current IssueSpecial IssueEditorial BoardOnline First ArticlesArchive

Review Article

Role of Pathogenesis-related (PR) Proteins in Enhancing Cotton’s Disease Resistance: A Review

V
Valarmathi Pandian1,*
T
Tamizharasi Alagesan1
1ICAR-Central Institute for Cotton Research (CICR), Regional station, Coimbatore-641 003, Tamil Nadu, India.

ABSTRACT

Plants respond to pathogen attack by activating Pathogenesis-related (PR) proteins, a group of defense-associated proteins first identified in the 1970s following their strong induction during Tobacco mosaic virus infection. PR proteins are classified into 17 families (PR1-PR17), most of which are secreted into the apoplast and accumulate in various plant tissues, particularly leaves, where they may constitute up to 5-10% of total soluble proteins. Several PR proteins possess specific enzymatic activities, including β-1,3-glucanases (PR2), chitinases (PR3, PR4, PR8 and PR11), proteinases (PR7) and ribonucleases (PR10). Subsequent studies demonstrated that PR proteins are induced in response to a broad spectrum of pathogens, including fungi, oomycetes, bacteria, viruses, viroids, nematodes and insect pests. Based on their isoelectric properties, PR proteins are categorized into acidic and basic forms, with acidic proteins typically secreted into the apoplast during defense responses. They are characterized by low molecular weight (6-43 kDa), stability under acidic conditions, thermostability and resistance to proteolytic degradation. This review discusses the classification and biological functions of PR proteins, emphasizing their antifungal, antibacterial and antiviral activities, as well as their role in enhancing resistance against Verticillium and Fusarium wilts in cotton. Notably, the cotton germin-like protein GhGLP2 contributes to defense against Verticillium dahliae, Fusarium oxysporum and oxidative stress. Recombinant GhGLP2 exhibits superoxide dismutase (SOD) activity and effectively inhibits pathogen spore germination, highlighting its importance in cotton disease resistance.

INTRODUCTION

Plants are continuously exposed to a wide range of pathogenic microorganisms, including fungi, oomycetes, bacteria, viruses and nematodes, which pose significant threats to their growth, development and productivity. These pathogens infect plants through diverse mechanisms, disrupting physiological and metabolic processes, impairing nutrient uptake, reducing photosynthetic efficiency and causing tissue damage. Severe infections can lead to stunted growth, reduced yield and quality and, in extreme cases, plant death. These pathogens cause substantial yield losses in agricultural crops and represent a major challenge to global food security. To combat pathogen invasion, plants have evolved sophisticated defense mechanisms that enable them to survive and maintain fitness (Roux et al., 2014). Plant immunity is governed by two interconnected defense systems: pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI is activated when pattern-recognition receptors (PRRs) on the plant cell surface detect conserved pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin, lipopolysaccharides and fungal cell wall components including chitin and β-glucans. This recognition initiates a cascade of signaling events that trigger basal defense responses, including reactive oxygen species production, cell wall reinforcement, defense-related gene expression and the synthesis of antimicrobial compounds, thereby restricting pathogen invasion and colonization. These are detected by plant receptors known as pattern recognition receptors (PRRs), which trigger the activation of PTI (Zipfel and Felix, 2005). Microbial pathogens secrete effector proteins that are recognized by a specific group of resistance (R) proteins in plants, which in turn activate an induced defense response known as effector-triggered immunity (ETI) (Dangl and Jones, 2001). Effector proteins are crucial virulence factors produced by fungal pathogens, especially during the biotrophic phase of infection (Sonah et al., 2016). However, PR proteins have also been widely recognized for their importance in plant-fungal pathogen interactions, with a growing number of pathogen effectors identified that directly interact with PR proteins during infection (Breen et al., 2017). The complexity and effectiveness of plant defense systems against pathogens vary across different plant species (Jones and Dangl 2006). Plants utilize both preformed (structural and biochemical) and inducible defense responses to combat various biotic stresses (Slusarenko et al., 2000). Preformed defenses, such as cutin, waxes, lignin deposition in cell walls and antimicrobial molecules like phytoanticipins, serve as the first line of defense to prevent pathogen invasion (VanEtten et al., 1994; Osbourn, 1996). However, many pathogens overcome this barrier, requiring plants to activate alternative defenses. One such mechanism is inducible defense, which includes the hypersensitive response, production of reactive oxygen species (ROS), cell wall cross- linking, synthesis of antimicrobial compounds like phytoalexins and the production of PR proteins (Van Loon et al., 1994; Van Loon and Van Strien, 1999; Van Baarlen et al., 2007). Pathogenesis-related (PR) proteins are essential mediators of systemic acquired resistance (SAR), an inducible and systemic defense response that enhances protection in uninfected plant tissues following localized pathogen infection.
 
History and classification of PR proteins
 
“PR proteins” refer to a diverse group of proteins activated by phytopathogens and defense signalling molecules. Upon pathogen attack, defense pathways like salicylic acid (SA) and jasmonic acid (JA) are triggered, leading to PR protein accumulation, which helps reduce pathogen load and prevent disease spread in uninfected plant organs. Pathogens are classified into two types: biotrophic and necrotrophic. The SA pathway was being activated by biotrophic pathogens, stimulating NPR1 and leading to the accumulation of SA-related genes (PR1, PR2, PR5) for systemic acquired resistance (SAR). Necrotrophic pathogens activate the JA pathway, inducing JA-related genes (PR3, PR4, PR12) for local acquired resistance (LAR) (Fig 1) (Ali et al., 2017). Pathogenesis-related proteins were first discovered in tobacco plants infected by Tobacco mosaic virus (TMV) (Van Loon and Van Kammen, 1970; Bol et al., 1990). Initially, only five major classes of PR proteins viz., PR1, PR2, PR3, PR4 and PR5 were reported in tobacco plants based on the biochemical and molecular approaches (Bol et al., 1990). Several new PR proteins were identified in various plants by the subsequent researchers. To standardize their classification, PR proteins were grouped into distinct families in 1994 based on molecular structure, biochemical and serological features and enzymatic activity. Further, PR proteins were classified into 11 families in tobacco and tomato plants which serve as a platform for isolating the homologs of PR proteins in other plant species including both monocots and dicots (Van Baarlen et al., 2007). To classify a newly isolated protein as a PR protein, it should satisfy two criteria: it should have low basal expression that significantly increases upon pathogen exposure and this increase must be confirmed across various plant pathology laboratories or in different plant-pathogen interactions. Currently, about 19 families of PRs have been reported (Table 1) including β- 1,3glucanases, chitinases, thaumatin-like proteins, peroxidases, ribosome-inactivating proteins, defensins, nonspecific lipid transfer proteins, oxalate oxidase and oxalate-oxidase-like proteins, amongst others (dos Santos and Franco, 2023).

Table 1: Pathogenesis-related proteins (PRs) properties and roles in plant defense.



Fig 1: An overview of activation of signaling cascades in plants after biotrophic and necrotrophic pathogenic infection.


 
Molecular mechanisms of PR proteins
 
A series of signals are being triggered to induce plant defense responses during the process of plant-pathogen interactions (Fig 2). Plant defense against pathogens is regulated through an interconnected network of signaling pathways. Upon recognition of pathogen elicitors, host plants activate signal transduction cascades involving Ca2+ influx, MAPK activation and resistance (R) genes. Calcium signaling through calmodulins (CaMs) and calmodulin-like proteins (CMLs) promotes nitric oxide (NO) production, triggering hypersensitive response (HR) and programmed cell death (PCD). Reactive oxygen species (ROS) and NO act synergistically to strengthen defense responses, while ROS also contribute directly to antimicrobial activity and systemic acquired resistance (SAR). R gene-Avr gene interactions further induce HR, PCD and salicylic acid (SA)-mediated SAR against biotrophic pathogens. SA biosynthesis is regulated through the EDS1-PAD4-GDG1 signaling module and maintained by a positive feedback loop. In contrast, jasmonic acid (JA) and ethylene (ET) signaling pathways primarily mediate resistance against necrotrophic pathogens, wounding and induced systemic resistance (ISR). Crosstalk between SA and JA/ET pathways occurs through NPR1-mediated suppression of JA signaling. The JA pathway activates defense responses through SCFCOI1-dependent degradation of JAZ repressors, releasing transcription factors such as MYC2. These signaling networks ultimately enhance plant immunity through reinforcement of physical barriers, production of antimicrobial secondary metabolites and defense proteins, modulation of ROS homeostasis and activation of multiple disease-resistance mechanisms (Ding et al., 2022).

Fig 2: Schematic representation of the plant defense signal transduction network (Ding et al., 2022).


       
Fungal pathogens secrete hydrolytic enzymes such as cutinases, pectinases, cellulases and proteases to break down plant cell walls. In defense, plants recognize pathogens, activate defense signaling pathways and produce antifungal compounds like PR proteins, which inhibit pathogen invasion and replication (Bowles 1990; Sels et al., 2008). The potent antifungal proteins in plants were identified as PR2, PR3, PR4, PR5 and PR12. In addition, over-expression of PR genes alone or in combination of various crops leads to enhanced disease resistance against biotrophic and necrotrophic fungal phytopathogens (Wang et al., 2021). β-1,3-glucanases, part of the PR-2 family and classified as endonucleases (E.C. 3.2.1.39), are multifunctional enzymes found in bacteria, fungi, some invertebrates and plants. Four subfamilies (A, B, C and D) have been identified, with ten β-1,3-glucanases classified based on amino acid sequence similarities. In plants, β-1,3-glucanases are one of three types of β-glucans, alongside β-1,4-glucanases and β-1,3 and β-1,4- glucanases. (Linton et al., 2020). In a fungal invasion, specifically in cell wall degradation through the action of β-1,3 glucanases, oligomers are released, namely β1,3/1,6-D-glucan. These released oligomers can be considered as elicitor oligosaccharides. The release of these elicitors induces a plant defense response, representing direct antimicrobial activity. Chitinases are enzymes (E.C. 3.2.1.14) belonging to groups 3, 4, 8 and 11 of the PRs. Chitinases, like chitosanases, are induced in plants during pathogenic interactions and can degrade chitosan, a component of the cell walls of certain fungi, such as those in the order Mucorales. These enzymes play an important defensive role by degrading chitin, a structurally important polysaccharide present in fungal cell walls and insect exoskeletons, thereby limiting pathogen growth and insect attack. Chitinases are also produced by some plants in response to phytopathogenic viruses (Pusztahelyi et al., 2018). Some identified chitinases have shown lysozyme activity, which enables them to degrade bacterial cell walls. This antibacterial action is due to their ability to hydrolyze the β-1,4 bonds between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan-like heterosaccharides, key components of prokaryotic cell walls (Wanke et al., 2021). In Morus notabilis plants, 26 chitinase genes were identified, with one, MnChi18, showing increased defense against Botrytis cinerea. Overexpression of MnChi18 protected the plants from damage and contributed to resistance against B. cinerea (Xin et al., 2021). Additionally, another study found that the chitinase gene positively regulates the hypersensitive and defense responses of Capsicum annuum L. against Colletotrichum acutatum infection (Ali et al., 2020). Bacterial pathogens enter the host through various routes, including stomata, lenticels, mechanical wounds, insect feeding, or chemoattraction. Plants defend against bacterial pathogens through immune responses, with the initial defense being pathogen recognition by host pattern recognition receptors (PRRs). This plant-pathogen interaction activates two key immune responses: pattern-triggered immunity (PTI) and effector-triggered immunity (ETI).
       
Pathogenesis-related (PR) proteins are effective tools in developing bacterial-resistant plants. In vitro studies have demonstrated that the antibacterial properties of various PR proteins, such as PR10 (ribonuclease-like proteins), PR12 (defensins), PR13 (thionins) and PR14 (lipid- transfer proteins) (Park et al., 2004; Patkar and Chattoo 2006; Jiang et al., 2015). PR10 exhibits broad-spectrum antibacterial activity against Pseudomonas syringae, Agrobacterium tumefaciens, A. radiobacter, P. aureofaciens and Serratia marcescens (Xie et al., 2010; Jiang et al., 2015). Overexpression of PR14 in rice increased resistance to both bacterial and fungal pathogens (Patkar and Chattoo, 2006).
       
During viral infection, PR proteins and antimicrobial peptides (AMPs) accumulate in non- infected plant organs, preventing further viral propagation. PR2a and PR3, known for antifungal activity in Nicotiana tabacum, also exhibit strong antiviral activity against TMV (Sindelarova and Sindelar, 2005). Additionally, PR9 (peroxidase), a novel PR protein, displays antiviral activity (Nawrot et al., 2014). Capsicum annuum PR10 protein (CaPR10) shows ribonucleolytic activity against TMV, with phosphorylation significantly enhancing its antiviral potential (Park et al., 2004). AMPs such as PR12, PR13, PR14, knottin and hevein-type peptides also possess antiviral activity, inhibiting viral fusion and targeting virus envelopes, causing lysis (Yount and Yeaman, 2013; Nawrot et al., 2014). Overexpression of PR1b in tobacco plants has been linked to increased resistance to TMV (Cutt et al., 2005). These findings suggest that PR proteins and AMPs are promising candidates for developing viral-resistant transgenic crops, alongside their antifungal and antibacterial roles. Mahmoud et al. (2020) demonstrated, with exogenous products to induce the plant’s systemic resistance, after treatment the plants showed increased abundance of PR10 as well as another protein, which is also used as a marker of systemic acquired resistance (SAR), namely, phenylalanine ammonia lyase (PAL). In vivo experiments confirmed the antiviral efficacy of the protein against Tobacco Mosaic Virus (TMV), a major viral pathogen of tomato.
 
Role of PR proteins in plant defense
 
Pathogenesis-related (PR) proteins serve as important molecular markers of the salicylic acid (SA) and jasmonic acid (JA) signaling pathways in both model and crop plants. Enhanced expression of PR1, PR2 and PR5 is commonly associated with activation of the SA pathway (Ali et al., 2017). This relationship is supported by studies showing that SA-deficient or signaling-impaired mutants, including nim1, npr1, sai1 and nahG transgenic Arabidopsis, fail to induce these PR genes (Shah et al., 1997). Conversely, elevated expression of PR3, PR4 and PR12 is indicative of JA-mediated defense responses. JA-signaling mutants such as fad3/7/8, coi1 and jar1 exhibit impaired induction of these genes and increased susceptibility to diverse pathogens, highlighting the critical role of JA signaling in plant defense (Ali et al., 2017).
       
Treatment with salicylic acid (SA) or jasmonic acid (JA) markedly induces the expression of PR proteins and other defense-related genes, reflecting their central role in plant immunity. The activation of SA- and JA-responsive signaling pathways enhances the accumulation of antimicrobial compounds and strengthens defense responses. Consequently, plants exhibiting elevated expression of SA- and JA-marker genes, including PR genes, display enhanced resistance against a broad spectrum of pathogens (Kaur et al., 2016). Besides SA and JA, other phytohormones such as abscisic acid (ABA), auxins, cytokinins, gibberellins and brassinosteroids are known to modulate plant immunity, though the molecular mechanisms remain largely unclear (Pieterse et al., 2012).
 
PR proteins in cotton
 
Verticillium wilt is a destructive disease characterized by leaf necrosis, wilting and vascular discoloration, while Fusarium wilt begins at the seedling stage and causes interveinal necrosis and leaf shedding. The effectiveness of current management strategies remains limited due to climatic variability, pathogen diversity and differences in cultivar susceptibility (López-Escudero et al., 2011). Thus, identifying new disease-resistant genes in cotton is vital for developing resistant cultivars. Germins and germin-like proteins (GLPs) are extracellular proteins that are induced in response to various biotic and abiotic stresses, including pathogen attack, herbivory, drought and salinity. Classified under the PR-16 family, they play important roles in plant defense and stress tolerance. First identified in wheat (Triticum aestivum), germins and GLPs belong to the cupin superfamily and are water-soluble glycoproteins that typically assemble into highly stable hexameric structures, exhibiting remarkable resistance to proteolytic degradation, heat and detergents (Zhang et al., 2017). Recent evidence highlights the role of germin- like proteins (GLPs) in modulating basal penetration resistance during plant-pathogen interactions. This is characterized by the deposition of callose-rich papillae and lignin at attempted penetration sites (Banerjee et al., 2010). Callose, a glucose polymer, accumulates between the plasma membrane and the inner cell wall as part of the plant’s defense response against pathogens. Lignin, an amorphous heteropolymer, is deposited in cell walls and is a key component of plant defense, particularly in the immune response against (hemi) biotrophic pathogens (Miedes et al., 2014). Both callose and lignin deposition are considered biochemical markers of activated defense mechanisms. Previous research by Wei et al. (1998) identified HvOxOLP as a structural protein involved in cell wall reinforcement, while OsGLP1 has been shown to contribute to disease resistance as a cell wall- associated protein.
       
The study assessed the effectiveness of the resistance-inducing chemicals salicylic acid (SA) and β-aminobutyric acid (BABA) in enhancing pathogenesis-related (PR) proteins and suppressing Cotton leaf curl disease (CLCuD). Activities of PR 2, PR 3, PR 9 and total soluble proteins were significantly increased following SA and BABA treatments, indicating enhanced defense responses. These findings suggest that both elicitors can induce resistance against CLCuD; however, multi-location field studies are needed to validate their efficacy and stability under diverse environmental conditions and disease pressures (Archana Kumari et al., 2025).
 
Case studies (GhGLP2, GhPR5-PevD1)
 
GLPs (Germin-like proteins) play a vital role in plant defense by functioning both as signaling molecules and as components of structural defense mechanisms. They contribute to disease resistance through the generation of reactive oxygen species, particularly H2O2, which activates salicylic acid (SA)- and jasmonic acid (JA)-mediated defense pathways and induces the expression of pathogenesis-related (PR) proteins (Rietz et al., 2012). A proposed model illustrating the role of GhPLP2 in mediating plant resistance against Verticillium dahliae is presented in Fig 3. as proposed by Zhu et al., (2021). For instance, BvGLP1 confers resistance against Verticillium longisporum and Rhizoctonia solani by enhancing H‚ O‚  accumulation and activating defense-related genes such as PR-1, PR-2, PR-3, PR-4 and PDF1.2 (Knecht et al., 2010), while OsRGLP1 protects plants against Fusarium oxysporum through H2O2 -mediated activation of SA and JA signaling pathways (Sultana et al., 2016. In cotton, GhGLP2 contributes to resistance against Fusarium oxysporum, Verticillium dahliae and oxidative stress. The recombinant GhGLP2 protein displays superoxide dismutase (SOD) activity and contributes to disease resistance by restricting pathogen spore germination. Silencing of GhGLP2 increases susceptibility, leading to severe wilting, vascular browning and reduced callose deposition, whereas its overexpression in Arabidopsis thaliana enhances resistance by restricting pathogen growth, promoting callose deposition and lignification and inducing defense- and oxidative stress-related genes such as PDF1.2, LOX2, VSP1, RbohD and RbohF (Pei et al., 2020). Furthermore, many GLPs are localized in the extracellular matrix, where they contribute to cell wall reinforcement following pathogen attack. Enhanced deposition of callose and lignin forms physical barriers that restrict pathogen ingress and spread, with lignification serving as a critical defense mechanism against fungal penetration (Nishimura et al., 2003). Overall, GLPs play a pivotal role in stress tolerance and defense, making them promising targets for developing resilient, high-yielding cotton cultivars.

Fig 3: A proposed model for the involvement of GhPLP2 in plant resistance against Verticillium dahliae. LOX, lipoxygenase; AOS, allene oxide synthase; OPR3, cis-(+)-12-oxo-phytodienoic acid reductase 3 (Zhu et al., 2021).


       
PR-5 proteins, commonly known as thaumatin-like proteins (TLPs), are an important class of pathogenesis-related proteins that contribute to plant defense through their antimicrobial and antifungal activities. These proteins are induced in response to pathogen attack and environmental stresses and have been identified in numerous plant species (Van Loon et al., 2006). Several PR-5 proteins exhibit direct antifungal activity against important vascular wilt pathogens, including Verticillium dahliae and Fusarium oxysporum. Similarly, the Arabidopsis thaumatin-like protein ATLP3 displays antifungal activity against V. dahliae, V. albo-atrum and F. oxysporum (Hu and Reddy, 1997) and overexpression of a rice thaumatin-like protein confers increased resistance to Alternaria alternata in tobacco (Velazhahan and Muthukrishnan, 2004). Despite the protective role of PR-5 proteins, successful pathogens have evolved mechanisms to overcome their antifungal activity. One such example is PevD1, an elicitor-like protein secreted by V. dahliae, which can induce plant defense responses as well as programmed cell death (Wang et al., 2012). Along with other V. dahliae elicitors such as VdCP1 and VdNEP, PevD1 activates host defense pathways but also contributes to pathogen virulence. Zhang et al. (2019) demonstrated that pevD1 is highly expressed throughout the infection process in cotton, Arabidopsis, tomato and tobacco and that pevD1 knockout mutants exhibit significantly reduced pathogenicity. Further investigations identified a cotton PR-5-like protein, GhPR5, as an interacting partner of PevD1 (Fig 4). The interaction occurs through the C-terminal region of PevD1 (PevD1b), which suppresses the antifungal activity of GhPR5. Reintroduction of pevD1b into knockout mutants restored virulence to wild-type levels, confirming its role in pathogenicity. Moreover, PevD1 accumulates during the later stages of infection and induces dose-dependent plant cell death, thereby facilitating nutrient release and disease development. These findings reveal a sophisticated infection strategy in which V. dahliae secretes PevD1 to neutralize the antifungal function of GhPR5, suppress host defense responses and promote successful colonization. Thus, while PR-5 proteins constitute a critical component of plant immunity, their effectiveness can be compromised by pathogen-derived effectors that specifically target and inhibit their defensive functions.

Fig 4: Proposed model of PevD1 function during Verticillium dahliae-host interaction (Zhang et al., 2019).


 
Limitations of PR protein-mediated resistance

Although pathogenesis-related (PR) proteins play a key role in plant defense, their capacity to confer durable resistance is often constrained by several biological and environmental factors. Individual PR proteins generally provide only partial protection and are frequently effective against a limited spectrum of pathogens, making them insufficient as standalone resistance factors. Their effectiveness can vary considerably under field conditions due to the influence of environmental factors such as temperature, humidity, nutrient status and abiotic stresses, which can affect both PR protein expression and activity. In addition, pathogens can evolve mechanisms to evade, suppress, or degrade PR proteins, thereby reducing their long-term efficacy. The induction and function of PR proteins are closely regulated by complex signaling networks involving salicylic acid, jasmonic acid and ethylene pathways and disruptions in these pathways can compromise resistance. Furthermore, constitutive overexpression of PR proteins may impose metabolic costs that negatively affect plant growth and productivity. Consequently, PR proteins are now regarded as important components of a multilayered defense system rather than as a sole source of durable resistance and their successful deployment is often dependent on integration with other resistance genes and disease-management strategies (Van Loon et al., 2006; Ali et al., 2018; Sels et al., 2008).
 
Evaluating the effectiveness of PR proteins under field conditions
       
The effectiveness of pathogenesis-related (PR) proteins has been demonstrated extensively under laboratory, greenhouse and controlled-environment conditions, where overexpression or induction of specific PR proteins often results in reduced disease severity and enhanced resistance against a range of fungal, bacterial and viral pathogens. However, evidence supporting their consistent effectiveness under natural field conditions remains comparatively limited. Field performance is influenced by numerous factors, including environmental variability, pathogen diversity, inoculum pressure, plant developmental stage and interactions with other biotic and abiotic stresses, which may affect PR protein expression and functionality. Consequently, resistance levels observed in controlled studies are not always replicated in field environments. Several studies have reported improved disease tolerance in transgenic or induced plants expressing PR proteins, but long-term, multi-location field evaluations demonstrating stable and durable resistance are relatively scarce. Therefore, conclusions regarding the effectiveness of PR proteins are still derived predominantly from laboratory and greenhouse experiments, highlighting the need for more comprehensive field-based validation before their widespread deployment in crop protection programs (Van Loon et al., 2006; Ali et al., 2018).
 
Applications and future perspectives
 
Biotic stresses remain a major constraint in modern agriculture, driving efforts to develop disease-resistant crop varieties. Pathogenesis-related (PR) proteins have gained considerable attention due to their broad-spectrum activity against fungi, bacteria, insects and other pathogens. Genetic engineering of PR proteins offers a promising strategy for enhancing crop resistance and several transgenic plants expressing PR genes have already demonstrated improved disease tolerance. However, research has largely focused on a limited number of well-characterized PR proteins. Future studies should explore the functional potential of diverse PR genes and alleles across crop and model plant species. With climate change accelerating the emergence of more virulent and adaptable phytopathogens, the identification and characterization of novel PR genes are increasingly important. Advances in genomics, transcriptomics, proteomics and metabolomics will facilitate a deeper understanding of PR gene networks and plant-pathogen interactions, enabling the development of crop varieties with durable resistance to multiple biotic stresses.

CONCLUSION

Pathogenesis-related (PR) proteins are important components of the cotton defense system and play diverse roles in resistance against fungal, bacterial and viral pathogens. Their activities, including β-1,3-glucanase, chitinase, antimicrobial and oxidative stress-related functions, contribute to restricting pathogen growth and disease development. In cotton, proteins such as GhGLP2 and GhPR5 have demonstrated significant potential in enhancing resistance against Fusarium oxysporum and Verticillium dahliae. PR protein induction through SA- and JA-mediated signaling further strengthens systemic and localized defense responses. However, individual PR proteins may provide only partial resistance, and their effectiveness can be influenced by environmental conditions and pathogen adaptation. Therefore, integration of PR proteins with resistance genes, elicitors and other integrated disease-management strategies is essential for achieving durable resistance. Future research combining genomics, transcriptomics, proteomics and field validation will facilitate the development of resilient and disease-resistant cotton cultivars.

ACKNOWLEDGEMENT

I like to acknowledge the Indian Council of Agricultural Research, Department of Agricultural Research and Education, Government of India.
 
Funding
 
No funding was received.
 
Data availability
 
Data sharing does not apply to this article as no new data were created or analysed in this study.
 
Author information
 
ICAR-Central Institute for Cotton Research (CICR), Regional Station, Coimbatore, Valarmathi Pandian.

Ethics declarations
 
Ethics approval and consent to participate
 
Not applicable.

CONFLICT OF INTEREST

The authors declared that they have no competing interests concerning the work reported in this review article.

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    Review Article

    Role of Pathogenesis-related (PR) Proteins in Enhancing Cotton’s Disease Resistance: A Review

    V
    Valarmathi Pandian1,*
    T
    Tamizharasi Alagesan1
    1ICAR-Central Institute for Cotton Research (CICR), Regional station, Coimbatore-641 003, Tamil Nadu, India.

    ABSTRACT

    Plants respond to pathogen attack by activating Pathogenesis-related (PR) proteins, a group of defense-associated proteins first identified in the 1970s following their strong induction during Tobacco mosaic virus infection. PR proteins are classified into 17 families (PR1-PR17), most of which are secreted into the apoplast and accumulate in various plant tissues, particularly leaves, where they may constitute up to 5-10% of total soluble proteins. Several PR proteins possess specific enzymatic activities, including β-1,3-glucanases (PR2), chitinases (PR3, PR4, PR8 and PR11), proteinases (PR7) and ribonucleases (PR10). Subsequent studies demonstrated that PR proteins are induced in response to a broad spectrum of pathogens, including fungi, oomycetes, bacteria, viruses, viroids, nematodes and insect pests. Based on their isoelectric properties, PR proteins are categorized into acidic and basic forms, with acidic proteins typically secreted into the apoplast during defense responses. They are characterized by low molecular weight (6-43 kDa), stability under acidic conditions, thermostability and resistance to proteolytic degradation. This review discusses the classification and biological functions of PR proteins, emphasizing their antifungal, antibacterial and antiviral activities, as well as their role in enhancing resistance against Verticillium and Fusarium wilts in cotton. Notably, the cotton germin-like protein GhGLP2 contributes to defense against Verticillium dahliae, Fusarium oxysporum and oxidative stress. Recombinant GhGLP2 exhibits superoxide dismutase (SOD) activity and effectively inhibits pathogen spore germination, highlighting its importance in cotton disease resistance.

    INTRODUCTION

    Plants are continuously exposed to a wide range of pathogenic microorganisms, including fungi, oomycetes, bacteria, viruses and nematodes, which pose significant threats to their growth, development and productivity. These pathogens infect plants through diverse mechanisms, disrupting physiological and metabolic processes, impairing nutrient uptake, reducing photosynthetic efficiency and causing tissue damage. Severe infections can lead to stunted growth, reduced yield and quality and, in extreme cases, plant death. These pathogens cause substantial yield losses in agricultural crops and represent a major challenge to global food security. To combat pathogen invasion, plants have evolved sophisticated defense mechanisms that enable them to survive and maintain fitness (Roux et al., 2014). Plant immunity is governed by two interconnected defense systems: pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI is activated when pattern-recognition receptors (PRRs) on the plant cell surface detect conserved pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin, lipopolysaccharides and fungal cell wall components including chitin and β-glucans. This recognition initiates a cascade of signaling events that trigger basal defense responses, including reactive oxygen species production, cell wall reinforcement, defense-related gene expression and the synthesis of antimicrobial compounds, thereby restricting pathogen invasion and colonization. These are detected by plant receptors known as pattern recognition receptors (PRRs), which trigger the activation of PTI (Zipfel and Felix, 2005). Microbial pathogens secrete effector proteins that are recognized by a specific group of resistance (R) proteins in plants, which in turn activate an induced defense response known as effector-triggered immunity (ETI) (Dangl and Jones, 2001). Effector proteins are crucial virulence factors produced by fungal pathogens, especially during the biotrophic phase of infection (Sonah et al., 2016). However, PR proteins have also been widely recognized for their importance in plant-fungal pathogen interactions, with a growing number of pathogen effectors identified that directly interact with PR proteins during infection (Breen et al., 2017). The complexity and effectiveness of plant defense systems against pathogens vary across different plant species (Jones and Dangl 2006). Plants utilize both preformed (structural and biochemical) and inducible defense responses to combat various biotic stresses (Slusarenko et al., 2000). Preformed defenses, such as cutin, waxes, lignin deposition in cell walls and antimicrobial molecules like phytoanticipins, serve as the first line of defense to prevent pathogen invasion (VanEtten et al., 1994; Osbourn, 1996). However, many pathogens overcome this barrier, requiring plants to activate alternative defenses. One such mechanism is inducible defense, which includes the hypersensitive response, production of reactive oxygen species (ROS), cell wall cross- linking, synthesis of antimicrobial compounds like phytoalexins and the production of PR proteins (Van Loon et al., 1994; Van Loon and Van Strien, 1999; Van Baarlen et al., 2007). Pathogenesis-related (PR) proteins are essential mediators of systemic acquired resistance (SAR), an inducible and systemic defense response that enhances protection in uninfected plant tissues following localized pathogen infection.
     
    History and classification of PR proteins
     
    “PR proteins” refer to a diverse group of proteins activated by phytopathogens and defense signalling molecules. Upon pathogen attack, defense pathways like salicylic acid (SA) and jasmonic acid (JA) are triggered, leading to PR protein accumulation, which helps reduce pathogen load and prevent disease spread in uninfected plant organs. Pathogens are classified into two types: biotrophic and necrotrophic. The SA pathway was being activated by biotrophic pathogens, stimulating NPR1 and leading to the accumulation of SA-related genes (PR1, PR2, PR5) for systemic acquired resistance (SAR). Necrotrophic pathogens activate the JA pathway, inducing JA-related genes (PR3, PR4, PR12) for local acquired resistance (LAR) (Fig 1) (Ali et al., 2017). Pathogenesis-related proteins were first discovered in tobacco plants infected by Tobacco mosaic virus (TMV) (Van Loon and Van Kammen, 1970; Bol et al., 1990). Initially, only five major classes of PR proteins viz., PR1, PR2, PR3, PR4 and PR5 were reported in tobacco plants based on the biochemical and molecular approaches (Bol et al., 1990). Several new PR proteins were identified in various plants by the subsequent researchers. To standardize their classification, PR proteins were grouped into distinct families in 1994 based on molecular structure, biochemical and serological features and enzymatic activity. Further, PR proteins were classified into 11 families in tobacco and tomato plants which serve as a platform for isolating the homologs of PR proteins in other plant species including both monocots and dicots (Van Baarlen et al., 2007). To classify a newly isolated protein as a PR protein, it should satisfy two criteria: it should have low basal expression that significantly increases upon pathogen exposure and this increase must be confirmed across various plant pathology laboratories or in different plant-pathogen interactions. Currently, about 19 families of PRs have been reported (Table 1) including β- 1,3glucanases, chitinases, thaumatin-like proteins, peroxidases, ribosome-inactivating proteins, defensins, nonspecific lipid transfer proteins, oxalate oxidase and oxalate-oxidase-like proteins, amongst others (dos Santos and Franco, 2023).

    Table 1: Pathogenesis-related proteins (PRs) properties and roles in plant defense.



    Fig 1: An overview of activation of signaling cascades in plants after biotrophic and necrotrophic pathogenic infection.


     
    Molecular mechanisms of PR proteins
     
    A series of signals are being triggered to induce plant defense responses during the process of plant-pathogen interactions (Fig 2). Plant defense against pathogens is regulated through an interconnected network of signaling pathways. Upon recognition of pathogen elicitors, host plants activate signal transduction cascades involving Ca2+ influx, MAPK activation and resistance (R) genes. Calcium signaling through calmodulins (CaMs) and calmodulin-like proteins (CMLs) promotes nitric oxide (NO) production, triggering hypersensitive response (HR) and programmed cell death (PCD). Reactive oxygen species (ROS) and NO act synergistically to strengthen defense responses, while ROS also contribute directly to antimicrobial activity and systemic acquired resistance (SAR). R gene-Avr gene interactions further induce HR, PCD and salicylic acid (SA)-mediated SAR against biotrophic pathogens. SA biosynthesis is regulated through the EDS1-PAD4-GDG1 signaling module and maintained by a positive feedback loop. In contrast, jasmonic acid (JA) and ethylene (ET) signaling pathways primarily mediate resistance against necrotrophic pathogens, wounding and induced systemic resistance (ISR). Crosstalk between SA and JA/ET pathways occurs through NPR1-mediated suppression of JA signaling. The JA pathway activates defense responses through SCFCOI1-dependent degradation of JAZ repressors, releasing transcription factors such as MYC2. These signaling networks ultimately enhance plant immunity through reinforcement of physical barriers, production of antimicrobial secondary metabolites and defense proteins, modulation of ROS homeostasis and activation of multiple disease-resistance mechanisms (Ding et al., 2022).

    Fig 2: Schematic representation of the plant defense signal transduction network (Ding et al., 2022).


           
    Fungal pathogens secrete hydrolytic enzymes such as cutinases, pectinases, cellulases and proteases to break down plant cell walls. In defense, plants recognize pathogens, activate defense signaling pathways and produce antifungal compounds like PR proteins, which inhibit pathogen invasion and replication (Bowles 1990; Sels et al., 2008). The potent antifungal proteins in plants were identified as PR2, PR3, PR4, PR5 and PR12. In addition, over-expression of PR genes alone or in combination of various crops leads to enhanced disease resistance against biotrophic and necrotrophic fungal phytopathogens (Wang et al., 2021). β-1,3-glucanases, part of the PR-2 family and classified as endonucleases (E.C. 3.2.1.39), are multifunctional enzymes found in bacteria, fungi, some invertebrates and plants. Four subfamilies (A, B, C and D) have been identified, with ten β-1,3-glucanases classified based on amino acid sequence similarities. In plants, β-1,3-glucanases are one of three types of β-glucans, alongside β-1,4-glucanases and β-1,3 and β-1,4- glucanases. (Linton et al., 2020). In a fungal invasion, specifically in cell wall degradation through the action of β-1,3 glucanases, oligomers are released, namely β1,3/1,6-D-glucan. These released oligomers can be considered as elicitor oligosaccharides. The release of these elicitors induces a plant defense response, representing direct antimicrobial activity. Chitinases are enzymes (E.C. 3.2.1.14) belonging to groups 3, 4, 8 and 11 of the PRs. Chitinases, like chitosanases, are induced in plants during pathogenic interactions and can degrade chitosan, a component of the cell walls of certain fungi, such as those in the order Mucorales. These enzymes play an important defensive role by degrading chitin, a structurally important polysaccharide present in fungal cell walls and insect exoskeletons, thereby limiting pathogen growth and insect attack. Chitinases are also produced by some plants in response to phytopathogenic viruses (Pusztahelyi et al., 2018). Some identified chitinases have shown lysozyme activity, which enables them to degrade bacterial cell walls. This antibacterial action is due to their ability to hydrolyze the β-1,4 bonds between N-acetylmuramic acid and N-acetylglucosamine in peptidoglycan-like heterosaccharides, key components of prokaryotic cell walls (Wanke et al., 2021). In Morus notabilis plants, 26 chitinase genes were identified, with one, MnChi18, showing increased defense against Botrytis cinerea. Overexpression of MnChi18 protected the plants from damage and contributed to resistance against B. cinerea (Xin et al., 2021). Additionally, another study found that the chitinase gene positively regulates the hypersensitive and defense responses of Capsicum annuum L. against Colletotrichum acutatum infection (Ali et al., 2020). Bacterial pathogens enter the host through various routes, including stomata, lenticels, mechanical wounds, insect feeding, or chemoattraction. Plants defend against bacterial pathogens through immune responses, with the initial defense being pathogen recognition by host pattern recognition receptors (PRRs). This plant-pathogen interaction activates two key immune responses: pattern-triggered immunity (PTI) and effector-triggered immunity (ETI).
           
    Pathogenesis-related (PR) proteins are effective tools in developing bacterial-resistant plants. In vitro studies have demonstrated that the antibacterial properties of various PR proteins, such as PR10 (ribonuclease-like proteins), PR12 (defensins), PR13 (thionins) and PR14 (lipid- transfer proteins) (Park et al., 2004; Patkar and Chattoo 2006; Jiang et al., 2015). PR10 exhibits broad-spectrum antibacterial activity against Pseudomonas syringae, Agrobacterium tumefaciens, A. radiobacter, P. aureofaciens and Serratia marcescens (Xie et al., 2010; Jiang et al., 2015). Overexpression of PR14 in rice increased resistance to both bacterial and fungal pathogens (Patkar and Chattoo, 2006).
           
    During viral infection, PR proteins and antimicrobial peptides (AMPs) accumulate in non- infected plant organs, preventing further viral propagation. PR2a and PR3, known for antifungal activity in Nicotiana tabacum, also exhibit strong antiviral activity against TMV (Sindelarova and Sindelar, 2005). Additionally, PR9 (peroxidase), a novel PR protein, displays antiviral activity (Nawrot et al., 2014). Capsicum annuum PR10 protein (CaPR10) shows ribonucleolytic activity against TMV, with phosphorylation significantly enhancing its antiviral potential (Park et al., 2004). AMPs such as PR12, PR13, PR14, knottin and hevein-type peptides also possess antiviral activity, inhibiting viral fusion and targeting virus envelopes, causing lysis (Yount and Yeaman, 2013; Nawrot et al., 2014). Overexpression of PR1b in tobacco plants has been linked to increased resistance to TMV (Cutt et al., 2005). These findings suggest that PR proteins and AMPs are promising candidates for developing viral-resistant transgenic crops, alongside their antifungal and antibacterial roles. Mahmoud et al. (2020) demonstrated, with exogenous products to induce the plant’s systemic resistance, after treatment the plants showed increased abundance of PR10 as well as another protein, which is also used as a marker of systemic acquired resistance (SAR), namely, phenylalanine ammonia lyase (PAL). In vivo experiments confirmed the antiviral efficacy of the protein against Tobacco Mosaic Virus (TMV), a major viral pathogen of tomato.
     
    Role of PR proteins in plant defense
     
    Pathogenesis-related (PR) proteins serve as important molecular markers of the salicylic acid (SA) and jasmonic acid (JA) signaling pathways in both model and crop plants. Enhanced expression of PR1, PR2 and PR5 is commonly associated with activation of the SA pathway (Ali et al., 2017). This relationship is supported by studies showing that SA-deficient or signaling-impaired mutants, including nim1, npr1, sai1 and nahG transgenic Arabidopsis, fail to induce these PR genes (Shah et al., 1997). Conversely, elevated expression of PR3, PR4 and PR12 is indicative of JA-mediated defense responses. JA-signaling mutants such as fad3/7/8, coi1 and jar1 exhibit impaired induction of these genes and increased susceptibility to diverse pathogens, highlighting the critical role of JA signaling in plant defense (Ali et al., 2017).
           
    Treatment with salicylic acid (SA) or jasmonic acid (JA) markedly induces the expression of PR proteins and other defense-related genes, reflecting their central role in plant immunity. The activation of SA- and JA-responsive signaling pathways enhances the accumulation of antimicrobial compounds and strengthens defense responses. Consequently, plants exhibiting elevated expression of SA- and JA-marker genes, including PR genes, display enhanced resistance against a broad spectrum of pathogens (Kaur et al., 2016). Besides SA and JA, other phytohormones such as abscisic acid (ABA), auxins, cytokinins, gibberellins and brassinosteroids are known to modulate plant immunity, though the molecular mechanisms remain largely unclear (Pieterse et al., 2012).
     
    PR proteins in cotton
     
    Verticillium wilt is a destructive disease characterized by leaf necrosis, wilting and vascular discoloration, while Fusarium wilt begins at the seedling stage and causes interveinal necrosis and leaf shedding. The effectiveness of current management strategies remains limited due to climatic variability, pathogen diversity and differences in cultivar susceptibility (López-Escudero et al., 2011). Thus, identifying new disease-resistant genes in cotton is vital for developing resistant cultivars. Germins and germin-like proteins (GLPs) are extracellular proteins that are induced in response to various biotic and abiotic stresses, including pathogen attack, herbivory, drought and salinity. Classified under the PR-16 family, they play important roles in plant defense and stress tolerance. First identified in wheat (Triticum aestivum), germins and GLPs belong to the cupin superfamily and are water-soluble glycoproteins that typically assemble into highly stable hexameric structures, exhibiting remarkable resistance to proteolytic degradation, heat and detergents (Zhang et al., 2017). Recent evidence highlights the role of germin- like proteins (GLPs) in modulating basal penetration resistance during plant-pathogen interactions. This is characterized by the deposition of callose-rich papillae and lignin at attempted penetration sites (Banerjee et al., 2010). Callose, a glucose polymer, accumulates between the plasma membrane and the inner cell wall as part of the plant’s defense response against pathogens. Lignin, an amorphous heteropolymer, is deposited in cell walls and is a key component of plant defense, particularly in the immune response against (hemi) biotrophic pathogens (Miedes et al., 2014). Both callose and lignin deposition are considered biochemical markers of activated defense mechanisms. Previous research by Wei et al. (1998) identified HvOxOLP as a structural protein involved in cell wall reinforcement, while OsGLP1 has been shown to contribute to disease resistance as a cell wall- associated protein.
           
    The study assessed the effectiveness of the resistance-inducing chemicals salicylic acid (SA) and β-aminobutyric acid (BABA) in enhancing pathogenesis-related (PR) proteins and suppressing Cotton leaf curl disease (CLCuD). Activities of PR 2, PR 3, PR 9 and total soluble proteins were significantly increased following SA and BABA treatments, indicating enhanced defense responses. These findings suggest that both elicitors can induce resistance against CLCuD; however, multi-location field studies are needed to validate their efficacy and stability under diverse environmental conditions and disease pressures (Archana Kumari et al., 2025).
     
    Case studies (GhGLP2, GhPR5-PevD1)
     
    GLPs (Germin-like proteins) play a vital role in plant defense by functioning both as signaling molecules and as components of structural defense mechanisms. They contribute to disease resistance through the generation of reactive oxygen species, particularly H2O2, which activates salicylic acid (SA)- and jasmonic acid (JA)-mediated defense pathways and induces the expression of pathogenesis-related (PR) proteins (Rietz et al., 2012). A proposed model illustrating the role of GhPLP2 in mediating plant resistance against Verticillium dahliae is presented in Fig 3. as proposed by Zhu et al., (2021). For instance, BvGLP1 confers resistance against Verticillium longisporum and Rhizoctonia solani by enhancing H‚ O‚  accumulation and activating defense-related genes such as PR-1, PR-2, PR-3, PR-4 and PDF1.2 (Knecht et al., 2010), while OsRGLP1 protects plants against Fusarium oxysporum through H2O2 -mediated activation of SA and JA signaling pathways (Sultana et al., 2016. In cotton, GhGLP2 contributes to resistance against Fusarium oxysporum, Verticillium dahliae and oxidative stress. The recombinant GhGLP2 protein displays superoxide dismutase (SOD) activity and contributes to disease resistance by restricting pathogen spore germination. Silencing of GhGLP2 increases susceptibility, leading to severe wilting, vascular browning and reduced callose deposition, whereas its overexpression in Arabidopsis thaliana enhances resistance by restricting pathogen growth, promoting callose deposition and lignification and inducing defense- and oxidative stress-related genes such as PDF1.2, LOX2, VSP1, RbohD and RbohF (Pei et al., 2020). Furthermore, many GLPs are localized in the extracellular matrix, where they contribute to cell wall reinforcement following pathogen attack. Enhanced deposition of callose and lignin forms physical barriers that restrict pathogen ingress and spread, with lignification serving as a critical defense mechanism against fungal penetration (Nishimura et al., 2003). Overall, GLPs play a pivotal role in stress tolerance and defense, making them promising targets for developing resilient, high-yielding cotton cultivars.

    Fig 3: A proposed model for the involvement of GhPLP2 in plant resistance against Verticillium dahliae. LOX, lipoxygenase; AOS, allene oxide synthase; OPR3, cis-(+)-12-oxo-phytodienoic acid reductase 3 (Zhu et al., 2021).


           
    PR-5 proteins, commonly known as thaumatin-like proteins (TLPs), are an important class of pathogenesis-related proteins that contribute to plant defense through their antimicrobial and antifungal activities. These proteins are induced in response to pathogen attack and environmental stresses and have been identified in numerous plant species (Van Loon et al., 2006). Several PR-5 proteins exhibit direct antifungal activity against important vascular wilt pathogens, including Verticillium dahliae and Fusarium oxysporum. Similarly, the Arabidopsis thaumatin-like protein ATLP3 displays antifungal activity against V. dahliae, V. albo-atrum and F. oxysporum (Hu and Reddy, 1997) and overexpression of a rice thaumatin-like protein confers increased resistance to Alternaria alternata in tobacco (Velazhahan and Muthukrishnan, 2004). Despite the protective role of PR-5 proteins, successful pathogens have evolved mechanisms to overcome their antifungal activity. One such example is PevD1, an elicitor-like protein secreted by V. dahliae, which can induce plant defense responses as well as programmed cell death (Wang et al., 2012). Along with other V. dahliae elicitors such as VdCP1 and VdNEP, PevD1 activates host defense pathways but also contributes to pathogen virulence. Zhang et al. (2019) demonstrated that pevD1 is highly expressed throughout the infection process in cotton, Arabidopsis, tomato and tobacco and that pevD1 knockout mutants exhibit significantly reduced pathogenicity. Further investigations identified a cotton PR-5-like protein, GhPR5, as an interacting partner of PevD1 (Fig 4). The interaction occurs through the C-terminal region of PevD1 (PevD1b), which suppresses the antifungal activity of GhPR5. Reintroduction of pevD1b into knockout mutants restored virulence to wild-type levels, confirming its role in pathogenicity. Moreover, PevD1 accumulates during the later stages of infection and induces dose-dependent plant cell death, thereby facilitating nutrient release and disease development. These findings reveal a sophisticated infection strategy in which V. dahliae secretes PevD1 to neutralize the antifungal function of GhPR5, suppress host defense responses and promote successful colonization. Thus, while PR-5 proteins constitute a critical component of plant immunity, their effectiveness can be compromised by pathogen-derived effectors that specifically target and inhibit their defensive functions.

    Fig 4: Proposed model of PevD1 function during Verticillium dahliae-host interaction (Zhang et al., 2019).


     
    Limitations of PR protein-mediated resistance

    Although pathogenesis-related (PR) proteins play a key role in plant defense, their capacity to confer durable resistance is often constrained by several biological and environmental factors. Individual PR proteins generally provide only partial protection and are frequently effective against a limited spectrum of pathogens, making them insufficient as standalone resistance factors. Their effectiveness can vary considerably under field conditions due to the influence of environmental factors such as temperature, humidity, nutrient status and abiotic stresses, which can affect both PR protein expression and activity. In addition, pathogens can evolve mechanisms to evade, suppress, or degrade PR proteins, thereby reducing their long-term efficacy. The induction and function of PR proteins are closely regulated by complex signaling networks involving salicylic acid, jasmonic acid and ethylene pathways and disruptions in these pathways can compromise resistance. Furthermore, constitutive overexpression of PR proteins may impose metabolic costs that negatively affect plant growth and productivity. Consequently, PR proteins are now regarded as important components of a multilayered defense system rather than as a sole source of durable resistance and their successful deployment is often dependent on integration with other resistance genes and disease-management strategies (Van Loon et al., 2006; Ali et al., 2018; Sels et al., 2008).
     
    Evaluating the effectiveness of PR proteins under field conditions
           
    The effectiveness of pathogenesis-related (PR) proteins has been demonstrated extensively under laboratory, greenhouse and controlled-environment conditions, where overexpression or induction of specific PR proteins often results in reduced disease severity and enhanced resistance against a range of fungal, bacterial and viral pathogens. However, evidence supporting their consistent effectiveness under natural field conditions remains comparatively limited. Field performance is influenced by numerous factors, including environmental variability, pathogen diversity, inoculum pressure, plant developmental stage and interactions with other biotic and abiotic stresses, which may affect PR protein expression and functionality. Consequently, resistance levels observed in controlled studies are not always replicated in field environments. Several studies have reported improved disease tolerance in transgenic or induced plants expressing PR proteins, but long-term, multi-location field evaluations demonstrating stable and durable resistance are relatively scarce. Therefore, conclusions regarding the effectiveness of PR proteins are still derived predominantly from laboratory and greenhouse experiments, highlighting the need for more comprehensive field-based validation before their widespread deployment in crop protection programs (Van Loon et al., 2006; Ali et al., 2018).
     
    Applications and future perspectives
     
    Biotic stresses remain a major constraint in modern agriculture, driving efforts to develop disease-resistant crop varieties. Pathogenesis-related (PR) proteins have gained considerable attention due to their broad-spectrum activity against fungi, bacteria, insects and other pathogens. Genetic engineering of PR proteins offers a promising strategy for enhancing crop resistance and several transgenic plants expressing PR genes have already demonstrated improved disease tolerance. However, research has largely focused on a limited number of well-characterized PR proteins. Future studies should explore the functional potential of diverse PR genes and alleles across crop and model plant species. With climate change accelerating the emergence of more virulent and adaptable phytopathogens, the identification and characterization of novel PR genes are increasingly important. Advances in genomics, transcriptomics, proteomics and metabolomics will facilitate a deeper understanding of PR gene networks and plant-pathogen interactions, enabling the development of crop varieties with durable resistance to multiple biotic stresses.

    CONCLUSION

    Pathogenesis-related (PR) proteins are important components of the cotton defense system and play diverse roles in resistance against fungal, bacterial and viral pathogens. Their activities, including β-1,3-glucanase, chitinase, antimicrobial and oxidative stress-related functions, contribute to restricting pathogen growth and disease development. In cotton, proteins such as GhGLP2 and GhPR5 have demonstrated significant potential in enhancing resistance against Fusarium oxysporum and Verticillium dahliae. PR protein induction through SA- and JA-mediated signaling further strengthens systemic and localized defense responses. However, individual PR proteins may provide only partial resistance, and their effectiveness can be influenced by environmental conditions and pathogen adaptation. Therefore, integration of PR proteins with resistance genes, elicitors and other integrated disease-management strategies is essential for achieving durable resistance. Future research combining genomics, transcriptomics, proteomics and field validation will facilitate the development of resilient and disease-resistant cotton cultivars.

    ACKNOWLEDGEMENT

    I like to acknowledge the Indian Council of Agricultural Research, Department of Agricultural Research and Education, Government of India.
     
    Funding
     
    No funding was received.
     
    Data availability
     
    Data sharing does not apply to this article as no new data were created or analysed in this study.
     
    Author information
     
    ICAR-Central Institute for Cotton Research (CICR), Regional Station, Coimbatore, Valarmathi Pandian.

    Ethics declarations
     
    Ethics approval and consent to participate
     
    Not applicable.

    CONFLICT OF INTEREST

    The authors declared that they have no competing interests concerning the work reported in this review article.

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