Physiological performance and differential expression profiling of genes associated with drought tolerance in contrasting varieties of two Gossypium species

Cotton is mostly cultivated under rain-fed conditions in India, thus faces frequent drought conditions during its life cycle. Drought being a major stress factor responsible for yield penalty, there has always been a high priority to generate knowledge on adaptation and tolerance of cotton. In the present study, four cotton varieties, JKC-770 and KC-2 (Gossypium hirsutum), and JKC-717 and RAHS-187(Gossypium herbaceum), were imposed to drought. Under drought condition, differential changes in physiological characters like net photosynthesis, transpiration, stomatal conductance, chlorophyll fluorescence, relative water content (RWC), and predawn water potential (psi (0)) showed a change. While proline, malondialdehyde (MDA), and glutathione-S-transferase (GST) content increased along with a concomitant change in the expression of their associated genes. Under moderate stress, tolerant varieties maintain lower psi (0) probably due to higher proline content as compared to sensitive varieties. Cyclic electron flow (CEF) also plays an important role in tolerance under mild water stress in G. hirsutum varieties. CEF not only activates at high light but also initiates at a very low light intensity. Expression analysis of genes reveals that drought-tolerant varieties showed enhanced detoxifying mechanism by up-regulation of asparagine synthase (AS), glutathione-S-transferase (GST), and methyl glyoxalase (GlyI) genes under drought stress. Up-regulation of Delta(1)-pyrroline-5-carboxylase synthase (Delta(1)P5CS) enhanced accumulation of proline, an osmolyte, under drought in tolerant varieties. While the drought-sensitive varieties showed up-regulation of ethylene responsive factor (ERF) and down-regulation of WRKY70 responsible for senescence of the leaf which correlated well with the high rate of leaf fall in sensitive varieties under water stress.