Plants encounter many abiotic stresses which severely affect their growth and ultimately the yield
(Mei et al., 2018). The most detrimental one is certainly water-deficit, particularly given that environmental stresses as high temperature, freezing and salinity are also accompanied with or result in water deficit. Plants that receive inadequate water, experience water deficit stress. There is hardly a physiological process in plants which is not affected when the amount of water transpired exceeds the amount of water uptake which is caused by insufficient rainfall or decreased ground water level
(Kapoor et al., 2020). The initial response to water-deficit is stomata closure to prevent tissue dehydration which results in reduced transpiration and a limited carbon dioxide uptake, consequently, the photosynthetic rate of plants decreases
(Kumawat and Sharma, 2018). The declining photosynthetic activity negatively affects further vegetative growth. In the worst case, flowers and fruits may also be shed, negatively affecting the yield. Drought stress also causes decline in leaf water relations and membrane stability that results in membrane injury
(Abid et al., 2018). In the present review, attempts have been made to comprehend the available literature on plant physiology and carbohydrate metabolism with special emphasis on plant glycolysis under water deficit stress.
Physiological responses elicited by water deficit stress
Plant growth
Since water is an essential element for plant growth, it is obvious that water deficit stress, depending on its severity and duration affects the growth and yield of the plant. Water loss causes lowering of the water potential in the cell and a corresponding decrease of cell turgor that has a direct impact on the rate of cell expansion and cell size, ultimately resulting in a reduction of plant growth
(Lang et al., 2014; Abid et al., 2018; Prakash and Singh 2020). Water deficit reduces the number of leaves per plant and individual leaf size and leaf longevity
(Bhargavi et al., 2017). Reduction in shoot growth, especially leaves is beneficial for the plant under drought stress as it reduces the surface area exposed for transpiration, hence minimizing water loss. Growth arrest can also be considered as a medium by which plants can preserve carbohydrates for sustained metabolism thus prolonging energy supply and faster recovery after stress relief. Decrease in leaf area index, leaf dry weight, shoot height and fresh and dry weight of root and shoot has been reported by many workers in different crop plants
(Mohammadian et al., 2005; Yucel et al., 2010; Khanna et al., 2014a; Gheidary et al., 2017).
Chlorophyll content
Chlorophyll is one of the major components of photosynthesis and even a short-term water deficiency changes the total contents of chlorophylls
(Daoqian et al., 2016). As water deficit stress accelerates chlorophyll decomposition, chlorophyll content is one of the most commonly used metrics for the severity of drought stress
(Efeoglu et al., 2009; Ying et al., 2015). Decreased chlorophyll level during drought stress has been reported in many plant species
(Manivannan et al., 2007; Mafakheri et al., 2010; Khanna et al., 2014a). Maintaining lower chlorophyll content in drought stress conditions can help plants reduce photo-oxidative damage, which occurs when photosynthesis is inhibited and light excitation energy is in excess
(Aranjuelo et al., 2011).
Water relations
Drought stress alters leaf water relations by decreasing leaf water potential and relative water content (RWC) but increasing osmotic adjustment (OA)
(Abid et al., 2018). RWC is the appropriate measure of the plant water status in terms of the physiological consequences of cellular water deficit
(Khanna et al., 2014a and b). It estimates the current water content of the sampled leaf tissue relative to the maximal water content it can hold at full turgidity. Decrease in RWC in response to drought stress has been noted in wide variety of plants
(Yucel et al., 2010; Rahbarian et al., 2011).
Osmolyte accumulation
Under drought, plants accumulate different types of organic and inorganic solutes like proline, sucrose, soluble carbohydrates, glycine betaine in the cytosol to lower osmotic potential
(Suprasanna et al., 2016; Ozturk et al., 2021). These solutes protect cellular structures and functions as well as maintain water balance and delay dehydrative damage by maintaining cell turgor and other physiological mechanisms under water-deficit conditions
(Taiz and Zeiger, 2006). In addition, compatible solutes have some other functions in plants such as, protecting of enzymes, maintaining membrane structure and its integrity, stabilizing of protein conformation and scavenging of free oxygen radicals at low water potentials
(Slama et al., 2015). In plants, sucrose is the usual osmoprotectant sugar
(Pinheiro et al., 2001). Koster, (1991) suggested glass formation being a possible way for sugars shielding cellular structures. Liquids become supersaturated in the presence of sugars and enter the state of plastic solids rather than solutes crystallizing and disrupting membranes. Sugars have also been shown to directly protect membranes and proteins
in vitro, possibly by replacing water molecules and altering physical properties through the formation of hydrogen bonds
(Crowe et al., 1992).
Importance of sugar accumulation in plants during water deficit stress
In response to drought, plants accumulate a large amount of water-soluble carbohydrates such as glucose, fructose, sucrose, stachyose, mannitol and pinitol. Sugars protect cells during drought by two mechanisms
(Leopold et al., 1994). First, the hydroxyl groups of sugars may substitute for water to maintain hydrophilic interactions in membranes and proteins during dehydration, thereby preventing protein denaturation. Secondly, sugars are a major contributing factor to vitrification, which is the formation of a biological glass in the cytoplasm of dehydrated cells. These intracellular glasses by virtue of their high viscosity, drastically reduce molecular movement, impede the diffusion of reactive compounds in the cell and may maintain the structural and functional integrity of macromolecules. The available reports (Table 1) stated that the content of soluble sugars and other carbohydrates in the leaves of various water stressed plants is altered and may act as metabolic signal in response to drought.
During stress, sugars not only function as osmoprotectants, but they are also considered important regulators of gene expression. Sugars act as key players in stress perception, signaling and are a regulatory hub for stress-mediated gene expression. The expression of stress responsive genes corresponding to enzymes of carbohydrate metabolism are down or upregulated by the sugar status of the cell, indicating the role of sugars during abiotic stresses
(Seki et al., 2002; Price et al., 2004). Changes in expression of genes involved in carbohydrate metabolism during drought stress have been reported in many different studies
(Bray, 2002;
Chaves et al., 2002; Yamaguchi-Shinozaki and Shinozaki, 2006;
Pan et al., 2016). Fluctuations in carbohydrate composition, genes and enzymes involved in carbohydrate metabolism are of particular importance because of their direct relationship with physiological processes such as photosynthesis, translocation and respiration.
Effect of water deficit stress on respiration
Respiratory pathway has a critical function in providing energy to the cell for the various metabolic activities. Environmental variables profoundly affect respiration. Despite its well-recognized importance, the regulation of respiration by drought at the plant physiological level is largely unknown, because of the apparent contradictions among different studies with either increased
(Gratani et al., 2007; Slot et al., 2008) or decreased
(Ribas-Carbo et al., 2005) or unaffected
(Lawlor and Fock, 1977) rates of respiration. Reduced availability of the substrate to the mitochondria under conditions of low photosynthesis as well as inhibition of leaf growth may explain reduced respiration
(Gimeno et al., 2010). However, an increased demand for respiratory ATP under severe water stress (to compensate for the lowered ATP production in the chloroplasts) may be required to support photosynthesis repair mechanisms, as suggested by
Atkin and Macherel, (2009).
De Vries et al. (1979) conducted studies in maize and wheat and observed that while respiration rate remained unaffected at low or moderate water stress, it decreased at severe water deficit stress.
Flexas et al., (2005) attributed this controversy to three possible causes: (i) the use of different species, organs and techniques for respiration studies; (ii) the presence of complex interactions of respiration rates with other environmental factors and (iii) the presence of a threshold of water stress intensity in which a change in the response of respiration to water stress occurs.
Effect of water deficit stress on glycolysis
Glycolysis is a central metabolic pathway whose main function is to provide ATP, NADH and precursor metabolites for biomass production. In plants, glycolysis is organized as a network of reactions that provide elasticity when production of energy for survival is challenged. Evidence for the participation of glycolytic genes
(Roche et al., 2007; Watkinson et al., 2008; Kim et al., 2009; Pan et al., 2016), proteins and enzymes
(Xu and Huang, 2010a;
Ford et al., 2011; Oliver et al., 2011, Khanna et al., 2014b; Khanna et al., 2016) in response to stress is established by the fact that their expression is deeply affected by exposure of plant tissues to environmental stresses such as drought. However, different workers quoted different and contrasting results. For instance, genes and proteins related to glycolytic pathway were either down regulated
(Kim et al., 2009; Xu and Huang, 2010a), up regulated
(Roche et al., 2007; Ford et al., 2011; Oliver et al., 2011) or remained unchanged
(Watkinson et al., 2008, Khanna et al., 2016) under water deficit stress.
Minhas and Grover, (1999) examined the changes in transcript levels of genes encoding various glycolytic enzymes in response to different abiotic stresses in rice. Selective alterations were noted with respect to triose phosphate isomerase induction in response to desiccation, salt and high temperature stresses, aldolase transcript in response to desiccation and salt stresses, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) transcript in response to salt stress, enolase transcript in response to desiccation stress, glucose phosphate isomerase transcript in response to high temperature stress and pyruvate kinase transcript in response to salt stress. The gene probe corresponding to phosphoglycerate kinase showed no inducibility with respect to all the stresses tested in this work.
Velasco et al., (1994) also found that dehydration and abscisic acid (ABA) increased mRNA levels and enzyme activity of cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the resurrection plant
Craterostigma plantagineum. Cytosolic GAPDH was also found to accumulate under dehydration in chickpea
(Boominathan et al., 2004). However,
Garg et al., (2010) and
Khanna et al., (2014b) reported stability in gene expression of GAPDH for different stress series in chickpea.
Gao et al., (2008) found downregulation of phosphoglycerate kinase (PGK) and NADP-GAPDH genes in chickpea in response to drought stress.
Several recent studies have dealt with protein expression changes in response to drought with the relatively limited number of studies reporting changes in glycolytic enzymes (Table 2). Phosphoglucomutase involved in glycolysis was decreased in leaves of
S.
stapfianus (Oliver et al., 2011) and
O.
sativa (Raorane et al., 2015) but increased in leaves of
C.
lanatus (Akashi et al., 2011), O.
sativa (Pandey et al., 2010) and
M.
paradisiaca (Vanhove et al., 2012) in response to drought. Fructose-bisphosphate aldolase (FBPA), another glycolysis-related protein, was found to increase in response to drought in plants such as
O.
sativa (Pandey et al., 2010; Raorane et al., 2015), S.
stapfianus (Oliver et al., 2011) and
M.
paradisiaca (Vanhove et al., 2012). Besides, FBPA abundance was decreased in drought-sensitive cultivars of
M.
domestica (Zhou et al., 2015) and
P.
pratensis (Xu and Huang, 2010b) but was increased in tolerant cultivars. Glycolytic protein, triose-phosphate isomerase (TPI) was also found to be induced in maize
(Riccardi et al., 1998), rice
(Salekdeh et al., 2002) and wheat
(Ford et al., 2011) in response to water stress, whereas a decrease in protein content of TPI was reported in water-stressed
Quercus ilex plants
(Echevarria-Zomeno et al., 2009).
From the above ongoing discussion, it could be inferred that genes, proteins and enzymes related to glycolysis are affected differently in different crops under water-deficit stress. The literature contains the contrasting information and the direction of change is also not uniform. The induction of glycolytic proteins/enzymes is thought to be essential for the activation of the entire energy-producing pathway to maintain homeostasis and activating stress defenses in stressed cells. It may be a mechanism by which plant cells prepare for a demand of ATP and NADH during recovery. On the other hand, the decline in the glycolytic proteins/enzymes is argued as a consequence of overall reduction in biochemical activities of the plant cells under water stress due to slow growth in shoots. Inhibition of glycolysis has also been suggested to be a mechanism for accumulating sugars as an energy source for recovery and fast growth once water is available
(Echevarria-Zomeno et al., 2009). Such discrepancies can be attributed to specific features of different species and cultivars of crops on which stress is applied; different stages of plant growth and development; different degrees, length and method of imposing water stress; and other environmental conditions of the plants.
Molecular cloning of genes related to glycolytic pathway increases stress tolerance in transgenic plants
Several recent studies have demonstrated that overexpression of glycolytic genes/enzymes confers tolerance to several abiotic stresses. Transgenic yeast cells that overexpress the P.sajor-caju GAPC gene increased their tolerance to cold, salt, heat and drought stresses
(Jeong et al., 2000). Moreover, the overexpression of GAPDH gene improved salt tolerance in transgenic potato plants
(Jeong et al., 2001) and transgenic rice
(Zhang et al., 2011; Lim et al., 2021). Zhang et al., (2011) also reported that the elevated stress tolerance of
Oryza sativa-GAPC3-overexpressing plants coincided with the upregulation of several stress-responsive genes, including dehydration responsive element-binding protein (DREB2A), transcriptional inhibitory protein (Lip9) and catalase (catA). Several reports have described that the mRNA accumulation of fructose-1,6-bisphosphate aldolase (FBA) in plants increased in response to high salinity and the chloroplast FBA-overproduced tobacco could improve its salt-tolerance
(Yamada et al., 2000; Zhang et al., 2003). Cloning and characterization of the cytosol FBA gene from
S.
portulacastrum roots showed that the SpFBA was involved in responding to abiotic stimuli such as seawater, NaCl, ABA and polyethylene glycol (PEG) and the SpFBA overproduction promoted the survival ability of the transgenic
Escherichia coli (Fan et al., 2009). The transgenic potatoes with antisense of plastid aldolase gene displayed decreased enzymatic activities of photosynthetic pathway, including, phosphoribulokinase (PRK), sedoheptulose-1,7-biphosphatase (SBPase) and plastid fructose-1,6-bisphosphatase (FBPase) and very low starch synthesis, whereas sucrose synthesis was less strongly inhibited
(Haake et al., 1998; 1999). Co-overexpression of the rice fructose-1,6-bisphosphate aldolase (FBA), spinach triosephosphate isomerase (TPI) and wheat FBPase genes could significantly improve the photosynthetic yield in transgenic cells via stimulating sedoheptulose-1,7-bisphosphatase (SBPase) activity and consequently accelerating the ribulose-1,5-bisphosphate (RuBP) regeneration rate
(Ma et al., 2007). OsPGK is differentially regulated in contrasting genotypes of rice under salinity stress and its overexpression provides salinity tolerance to transgenic tobacco
(Joshi et al., 2016).
