Germination of non-dormant seeds generally requires hydration under several favorable conditions such as light, suitable temperature and presence of oxygen. Following imbibition of seeds, metabolic activities resume resulting in radicle emergence
(Bradford 2017). Thus, germination of seeds and emergence of seedlings are the crucial stages for obtaining optimum crop stand in fields. The quality seeds of newly released varieties of various field and horticultural crops by plant breeders along with proper agronomic management practices can assure augment crop yield with supplemented nutritional values
(Singh et al., 2015; Dutta 2018;
Dutta et al., 2020 a and b, 2021 a and b). But inadequate crop stand because of poor seedling emergence still remains a constraint that farmers have to face
(Singh et al., 2015). Unsynchronised seedling emergence due to variable environmental factors
viz., low soil moisture content is a major area of concern directly affecting the crop stand and consequently yield of the crop. Under such a scenario, there must be some innovative and cost-effective approaches to overcome these hurdles. Seed priming is one such approach. It allows seeds to proceed through the first reversible stage of germination but does not permit radical protrusion through the seed coat. The primed seeds retain desiccation tolerance and are dried and preserved until final planting
(Lutts et al., 2016). The different approaches to priming have found applications in various economically important crops. These methods have not only been instrumental in tackling abiotic stress (osmopriming, hydropriming) but have also been able to handle biotic stress (biopriming). The final result of seed priming is improved seed vigour defined as the entire set of properties influencing a seed lot’s performance in diverse environmental conditions
(Zhang et al., 2015). So, the current context mainly focuses on seed biopriming, its methods and applications as a way of ensuring better crop stands and enhancing agricultural productivity.
Seed priming: A comprehensive approach
Seed Priming is primarily considered as a seed quality enhancement method, wherein seeds are hydrated to initialize their metabolic activities followed by dehydration prior to radicle emergence. This results in rapid and uniform germination to improve crop establishment under field conditions
(Singh et al., 2020). During seed priming, a series of physiological events take place inside the seeds leading to metabolic activation. These reversible changes upon priming treatments include activation of cell cycles, DNA repair mechanism, precise regulation of oxidative status, change in phytohormones levels, synthesis of enzymes and consequently mobilization of storage reserves
(Chatterjee et al., 2018, Lutts et al., 2016). The hydrated seeds are then dehydrated to a safe moisture content causing a temporary stoppage of metabolic activities. It is assumed that rehydration of the seeds will allow uniform emergence and crop stand as the primed seeds retain desiccation tolerance
(Lutts et al., 2016). Thus, this process can also be described as a method for seed invigoration. Furthermore, seed priming is an approach concerned with the whole plant rather than the seed only. It has been observed that plants that have been exposed to stress early on in their lives undergo a series of transient metabolic adaptations that result in a stress memory, helping them to better adapt to future exposures to stresses
(Tanou et al., 2012).
Types of priming
Different methods of priming are used for different economically important crop plants such as rice
(Reddy et al., 2013), maize
(Reddy et al., 2013), pearl millet
(Raj et al., 2004) and chickpea
(Singh et al., 2012). In this context, hydropriming is the most simple and cost-effective approach for enhancing of planting value of the seeds. It involves soaking of seeds in pure water followed by drying to original moisture content without the use of any other chemicals. On the other side, nutri-priming is an improvement over hydropriming, wherein solution containing nutrients instead of pure water is used for treatment of seeds. This method allows for better seedling establishment due to improved seed quality
(Farooq et al., 2012). The next approach termed as osmo-priming, where seeds are soaked in salt solution with low water potential and thereby permitting the seed to imbibe water slowly followed by initiation of enzymes activity without radicle protrusion
(Di Girolamo and Barbanti 2012). In hormo-priming, plant growth regulators (PGR) are used during the soaking process which has a direct influence on germination and metabolism of the seed
(Galhaut et al., 2014). The mostly used growth regulators include salicylic acid, gibberellic acid, auxin, ethylene, kinetin, polyamides and polyethylene glycol. Chemo-priming is another variant of the seed priming where an array of natural and synthetic compounds is used as a priming agent. Studies indicated that this method could be used to counter salinity stress
(Fercha et al., 2014). Solid matrix priming involves exposing seeds to wet solids for a length of time, then separating, rinsing and drying the seeds. This method was adopted to counter high cost involved in procurement of the osmotic agents used in osmo-priming
(Paparella et al., 2015). In biopriming, bacterial inoculum is used during the initial imbibition process without the involvement of the agrochemicals
(Callan et al., 1990). This is the only approach that provides protection against several seed-borne and soil-borne pathogens without sacrificing ecological harmony. Studies also suggest that biopriming is more effective for management of disease as compared to seed pelleting or seed coating methods
(Muller and Berg 2008). Even though the other seed priming methods help to enhance the abiotic stress and uniform crop establishment, biopriming plays an important role in crop establishment as well as amelioration of biotic stress.
Biopriming as an approach to enhance nutrient use efficiency
Biopriming is a potential approach involving plant growth-promoting bacteria (PGPB). It is not only effective in dealing with pathogens but also substantially improves the nutrient uptake efficiency after treatment (Table 1). The process improves plant growth and establishment by releasing compounds involved in mineral solubilization
(Sukanya et al., 2018). Most of the fertilizers added to the soil are not available to the plant as there are either run-off or leaching losses. This results in poor nutrient uptake and low nutrient use efficiency. In this context, various techniques have been followed to improve nutrient use efficiency. To achieve this, microorganisms have a very important role to enhance nutrient mobilization and their uptake. For instance, phosphate solubilizing microorganisms such as
Enterobacter, Bacillus and Pseudomonas release crystal dissolving compounds and phosphate-solubilizing enzymes that make phosphorus available to the plants
(Glick 2012). Use of plant growth promoting rhizobacteria (PGPR) during biopriming also revealed higher production of plant growth hormones
viz., auxins and gibberellins to enhance crop establishment and uniformity
(Kavino et al., 2010). PGPR has also been reported to mobilize potassium ions from potassium-bearing minerals by breakdown of rock potassium or chelating silicon ions
(Sheng and He 2006). In addition, bioprimed seeds recorded higher amount of soluble protein as compared to untreated seeds
(Dhanya 2014). Moreover, bio-primed seeds supplemented with PGPR have shown higher free amino acids and protein content pool in the cellular context
(Warwate 2017;
Ahmed et al., 2014).
Biopriming to counter biotic stress
Biopriming is also used to improve crop stand by incorporating systemic resistance against several disease-causing organisms (Table 2).
Jain et al., (2012) revealed that a combination of selected microorganisms showed synergistic effect with around 1.4 to 2.3 folds increase in defence parameters of the plants. They also added that interaction between plants and microbial populations results in production of antioxidant enzymes in plant cells to protect them from oxidative stress caused by pathogen infection. Beneficial microbes also help plants in regulation of reactive oxygen species
(Singh et al., 2016). For instance, inoculation of
Pseudomonas fluorescens in the carnation stem results deposition of phytoalexins in stem leading to low
Fusarium wilt infection
(Van Peer et al., 1991). A similar finding was also evident from cucumber while seeds were treated with PGPR strains resulting in lower anthracnose disease suggesting induced systemic resistance following application of PGPR strains to protect the leaves of the plants against
Colletotrichum orbiculare (Wei et al., 1991). In another scenario,
Trichoderma harzianum is the most often employed as bio-priming fungi for its wide range of antagonism against plant diseases caused by fungi and nematodes
(Singh et al., 2004). Bacteria being the most abundant microorganism in soil play an indispensable role in maintaining soil fertility and nutrient upcycling. Seed bio-priming with PGPR improves plant performance in stressful situations, thereby increasing plant yield both directly and indirectly
(Dimkpa et al., 2009). PGPR mainly exerts its effect by inducing direct stimulation of plant growth and improvement through the supply of common nutrients and phytohormones released from bacterial siderophores
(Hayat et al., 2010).
Applications of biopriming
Experimental evidence from various crop models made biopriming instrumental in support of biotic stress resistance management and augmenting nutrient use efficiency
(Sukanya et al., 2018; Glick 2012). The use of biocontrol agents (BCAs) such as
Pseudomonas and
Trichodema have been proven to ameliorate plant diseases. As root rhizosphere is the home for many microbial colonies, seed biopriming allows better interaction of plants with the microbial population leading to better plant growth. The capacity of a group of suitable soil microorganisms to collaborate among themselves and with the plant favors the adoption of microbial consortiums rather than inoculation of seeds with a single microbe species. Plant development and abiotic stress tolerance may benefit from the synergistic action of microbial consortia. In addition, microbial consortiums may potentially improve the plants’ resistance responses to a wide range of soil and seed-borne diseases
(Jain et al., 2012; Wei et al., 1991). In rice, use of
Trichoderma and
Pseudomonas as a biopriming agent improves the antagonistic effects against infection by
Rhizoctonia (Reddy et al., 2013). Other beneficial microorganisms such as
Pseudomonas and
Bacillus have been implemented in pulses crops (pea and chickpea) to reduce the pathogenic effect exerted by
Fusarium spp.
(El-Mohamedy and Abd El-Baky 2008). Improvement of several growth parameters (root length, dry weight and shoot length) and reduced occurrence of root rot were also evident in the field trial of mung beans when seeds were treated with
Trichoderma harzianum (Sarkar and Bhattacharya 2008). Similarly, treatment of
Pseudomonas aeruginosa on soybean was successful to control damping-off disease caused by
Colletotrichum truncatum (Begum
et al., 2009)
. Mung bean seeds treated with
Pseudomonas fluorescens were shown to enhance growth and better resistance against
Macrophomina phaseolina causing charcoal rot disease. The number of seeds per pod and the number of pods per plant had increased significantly when the inoculum was added as compared to the control
(Saxena 2010).
