An environmental factor that limits crop productivity or destroys biomass is referred to as stress or disturbance.
By Ashwani Kumar
| November 26th 2009 04:54 AM | Print
An environmental factor that limits crop productivity or destroys biomass is referred to as stress or disturbance. Salinity in soil or water is one of the major stresses and especailly in arid and semi-arid regions, can severely limit crop production.
Sugars play a critical role in regulating overall cellular metabolism and owing to their general compatibility with various cellular events plants invariably showenhanced levels of sugars for maintaining desired osmoticum under osmotic stress. Sugars (sucrose and trehalose) and sugar-alcohols (glycerol, mannitol, inositol, and sorbitol) with the exception of sorbitol lowered oxygenase activity of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase, EC 188.8.131.52) without altering carboxylase activity under unstressed conditions. Most interestingly, these solutes including sorbitol fully curtailed NaCl-induced enhancement in oxygenase activity, even at concentrations as lowas 50mM. However, none of these solutes could alleviate NaCl-suppressed carboxylase activity. In summary, our .findings demonstrate that one of the most important roles of sugars and sugar-alcohols in plants exposed to salt stress is to curtail oxygenase activity of Rubisco. Most remarkably, NaCl-induced enhancement in oxygenase activity of Rubisco was totally nulli.ed in the presence of any of the compatible sugars or sugar-alcohols tested including sorbitol, even at concentrations as lowas 50mM . Unlike proline, whose presence brought about an additive inhibitory e.ect on the NaCl-suppressed carboxylase activity of Rubisco .
no change/recovery in the NaCl-induced suppression in the carboxylase activity was observed in the presence of any of the compatible sugars/sugar-alcohols.the oxygenase activity and hence in curtailing yield
losses due to abiotic stresses ( Sivakumar, 2002).
Despite some reservations (Biehler & Foch 1996), it is generally accepted that a reduction in internal CO2 levels
resulting from stress-induced stomatal closure results in elevated photorespiratory activity (Bohnert & Jensen
1996; Foyer 1997). Heat stress may also decrease the intracellular CO2 content, owing to the increased solubility of
O2 and decreased solubility of CO2 at elevated temperatures (Leegood et al. 1995). Thus, irrespective of whether
or not photorespiration decreases in absolute terms, in stressed plants, activity of the PCO cycle is likely to
increase as a proportion of total photosynthesis (Biehler & Fock 1996; Hare & Cress 1997). Long-standing suggestions
as to the importance of photorespiration during stressare that ribulose 1,5-bisphosphate (RuBP) oxygenase
activity may provide a sink for photosynthetic electron transport and that the glycolate pathway may assist in
mobilization of carbon reserves into the reductive pentose phosphate pathway. By dissipating excess photochemically
generated energy, photosynthetic membranes are protected against light-induced damage when CO2 assimilation is limited. Despite an ability to down-regulate choline synthesis, glycine betaine deficiency in maize
(Zea mays) was associated with accumulation of serine and a significant expansion of the free choline pool (Yang et al. 1995). Environmental stress is the major factor limiting plant productivity (Hare et al. 1996). Abiotic stresses which cause depletion of cellular water (drought, high soil salinity and temperature extremes) are responsible for the greatest agricultural losses. Upon exposure to these prevalent stresses, many plants accumulate organic osmolytes, most commonly polyhydroxylic compounds (saccharides and polyhydric alcohols) and zwitterionic alkylamines (amino acids and quaternary ammonium compounds).
Several recent reviews discuss osmolyte accumulation in plants (Ingram & Bartels 1996; Bohnert & Jensen 1996;
Serrano 1996). It is generally accepted that the increase in cellular osmolarity which results from the accumulation of
non-toxic (thus ‘compatible’) osmotically active solutes is accompanied by the influx of water into, or at least a
reduced efflux from, cells, thus providing the turgor necessary for cell expansion. None the less, a conclusive demonstration that osmotic adjustment contributes to fitness in stressful environments has yet to be achieved (Munns
1993). Since all subcellular structures must exist in an aqueous environment, tolerance to dehydration also
depends on the ability of cells to maintain membrane integrity and prevent protein denaturation. Hypotheses that
attribute special protective properties of osmolytes to protein structure, dry membranes and liposomes under
adverse environmental conditions (dehydration, temperature extremes or denaturants) have been discussed extensively
(Csonka 1989; Crowe, Hoekstra & Crowe 1992), but remain largely a matter of conjecture. Likewise, high
concentrations of many but not all compatible solutes (Rhodes & Hanson 1993) have been proposed to confer
protection against oxidative damage by scavenging free radicals in addition to their roles in maintenance of osmotic
equilibrium without perturbing macromolecule-solvent interactions. It has proved difficult to identify the relative
importance of these postulated overlapping effects mediated by compatible solutes in vivo. Consistent with the
high, diffusion-rate-limited reactivity of hydroxyl radicals towards most metabolic intermediates, chloroplastic accumulation of mannitol was recently shown to increase resistance to oxidative stress in tobacco (Shen, Jensen &
Bohnert 1997). None the less, despite its impressive implications for agriculture, this study contributes little insight
into whether compatible solutes normally play a significant role in terminating free radical chain reactions.
Incharoensakdi et al.  had reported that sugars like sucrose and sugar-alcohols like glycerol protect
carboxylase activity of cyanobacterial Rubisco under salt stress. Sugars and sugar-alcohols have been well
demonstrated to safeguard other enzymes such as phosphoenolpyruvate carboxylase, nitrate reductase,
malate dehydrogenase, glucose-6-phosphate dehydrogenase, isocitrate dehydrogenase, and lyceraldehyde phosphate dehydrogenase against the adverse e.ects of various abiotic stresses.
The regulation of carbon partitioning between source and sink tissues in higher plants is not only important for plant growth and development, but insight into the underlying regulatory mechanisms is also a prerequisite to modulating assimilate partitioning in transgenic plants. Hexoses as well sucrose have been recognised as important signal molecules in source-sink regulation. Components of the underlying signal transduction pathways have become apparent. There is accumulating evidence for cross talk, modulation and integration between signalling pathways responding to phytohormomes, phosphate, light, sugars, and biotic and abiotic stress related stimuli. These complex interactions at the signal trasduction levels and co-ordinated regulation of gene expression seem to play a central role in source sink regulation ( Thomas, 1999).