The effects of water deficit on carbon and nitrogen metabolism were investigated in flag leaves of wild-type and transgenic rice ( Oryza sativa japonica ‘Kitaake’) plants expressing ISOPENTENYLTRANSFERASE ( IPT ; encoding the enzyme that mediates the rate-limiting step in cytokinin synthesis) under the control of P SARK , a maturation- and stress-induced promoter. While the wild-type plants displayed inhibition of photosynthesis and nitrogen assimilation during water stress, neither carbon nor nitrogen assimilation was affected by stress in the transgenic P SARK :: IPT plants. In the transgenic plants, photosynthesis was maintained at control levels during stress and the flag leaf showed increased sucrose (Suc) phosphate synthase activity and reduced Suc synthase and invertase activities, leading to increased Suc contents. The sustained carbon assimilation in the transgenic P SARK :: IPT plants was well correlated with enhanced nitrate content, higher nitrate reductase activity, and sustained ammonium contents, indicating that the stress-induced cytokinin synthesis in the transgenic plants played a role in maintaining nitrate acquisition. Protein contents decreased and free amino acids increased in wild-type plants during stress, while protein content was preserved in the transgenic plants. Our results indicate that the stress-induced cytokinin synthesis in the transgenic plants promoted sink strengthening through a cytokinin-dependent coordinated regulation of carbon and nitrogen metabolism that facilitates an enhanced tolerance of the transgenic plants to water deficit. Glossary CK cytokinin C carbon N nitrogen ABA abscisic acid RWC relative water content Vcmax maximum rate of carboxylation Jmax maximum rate of electron transport TPU triose phosphate utilization F v / F m ratio of variable to maximal chlorophyll fluorescence SPS sucrose phosphate synthase SuSy sucrose synthase GDH glutamate dehydrogenase NR nitrate reductase NiR nitrite reductase GOGAT glutamate synthase GS glutamine synthetase TCA tricarboxylic acid DNP-hydrazone 2,4-dinitrophenylhydrazone Ci calculated substomatal CO 2 concentration G6PDH Glc-6-P dehydrogenase A assimilation rate HK hexokinase
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