Abstract
Salt stress, considered as one of the major factors, decreases crop productivity world-wide and hence, investigations are being made to understand the cellular basis of salt tolerance in plants. In our earlier studies, maintenance of redox homeostasis and energetics were found as key determinants of salt tolerance in a halophyte Sesuvium portulacastrum (high salt accumulator). The redox homeostasis is defined as integrated ratio of different redox couples present inside the cell. In recent years, it has also been proposed as general stress response regulator in plants, bacteria as well as animals. In view of this, present study was performed to compare responses of redox state and energetics of S. portulacastrum with a glycophyte Brassica juncea (low salt accumulator). The data revealed activation of antioxidant defense in S. portulacastrum which either avoided or delayed the accumulation of different reactive oxygen species (ROS). In contrast, due to the lack of co-ordination, although the non-enzymatic antioxidants were increased, significant oxidative damage was seen in B. juncea. Further, the decreased NADPH oxidase activity suggested that basal redox signaling was also affected in B. juncea. In order to correlate these changes with chloroplastic and mitochondrial electron transport chain, NADP/NADPH and NAD/NADH ratios were measured. The NADP/NADPH ratio suggested that the process of photosynthesis was minimally affected in S. portulacastrum which might have contributed to its lower level of ROS under salt stress. The comparatively lower NAD/NADH and ATP/ADP ratios in S. portulacastrum as compared to B. juncea indicated the active and better utilization of energy generated to support different processes associated with salt tolerance. Thus, the findings suggest that co-ordinated regulation of antioxidant defense to avoid oxidative damage and proper utilization of energy are the key determinants of salt-tolerance in plants.
Highlights
A major challenge toward world agriculture involves production of 70% more food crop for an additional 2.3 billion people by 2050 (FAO, 2009)
Physiological responses of a halophyte, S. portulacastrum have been reported and redox homeostasis and energetics were found to be the key determinants of salt tolerance (Lokhande et al, 2011)
In the presence of salt stress, series of events leading to perturbation of cellular metabolism are: less water availability, stomata closure, altered gaseous exchange, inhibition of photosynthesis, effect on electron flow in electron transport chain (ETC) in chloroplast and mitochondria, increase in the production of reactive oxygen species (ROS) and disturbed status of adenine (ATP) and pyridine nucleotides (NADH, NADPH)
Summary
A major challenge toward world agriculture involves production of 70% more food crop for an additional 2.3 billion people by 2050 (FAO, 2009). Salinity is a major stress limiting the increase in the demand for food crops. Availability of atmospheric CO2 is reduced because of an increased stomatal closure in order to avoid water loss via transpiration and consumption of NADPH by the Calvin cycle is decreased. Plants have complex antioxidant defense mechanisms including superoxide dismutase (SOD) and the ascorbate (ASC)-glutathione (GSH) cycle (Mittler, 2002). SOD constitutes the first line of defense converting O2− to hydrogen peroxide (H2O2), which is further reduced to water and oxygen by ascorbate peroxidase (APX) and catalase (CAT). Regeneration of ASC from MDHA occurs in sequential steps and utilizes GSH. This results in generation of oxidized glutathione (GSSG), which is in turn, re-reduced to GSH by NADPH, a reaction catalyzed by glutathione reductase (GR). Ascorbate and GSH both can accumulate in millimolar concentrations in cells and function as molecular antioxidants, in addition to serving various other roles, reacting directly with various ROS (Noctor and Foyer, 1998)
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