Drought, the most prominent threat to agricultural production worldwide, accelerates leaf senescence, leading to a decrease in canopy size, loss in photosynthesis and reduced yields. On the basis of the assumption that senescence is a type of cell death program that could be inappropriately activated during drought, we hypothesized that it may be possible to enhance drought tolerance by delaying drought-induced leaf senescence through the stress-induced synthesis of cytokinins. We generated monocot and dicot transgenic plants expressing an isopentenyltransferase gene driven by pSARK, a stress- and maturation-induced promoter. Remarkably, the suppression of drought-induced leaf senescence resulted in outstanding drought tolerance as shown by, among other responses, vigorous growth following a long drought period that killed the control plants. The transgenic plants expressing pSARK-IPT maintained high water contents and retained photosynthetic activity (albeit at a reduced level) during the drought period. A comparison of the CO2-dependent rate of photosynthesis (A/Ci curve) from wild-type and transgenic plants showed that drought severely affected the RuBP regeneration and triose phosphate use of wild type plants but not of transgenic plants. Ultrastructural analysis of leaf parenchyma cells and metabolite analysis of leaf contents indicated the occurrence of enhanced photorespiration in the pSARK-IPT trangenic plants leading to CO2 fixation under drought conditions. The transgenic plants displayed minimal yield loss when watered under reduced watering regimes of only 30% of the amount of water used under control conditions. Transgenic plants grown under reduced watering also displayed the induction of photorespiration observed under severe drought conditions. Array-based analysis indicated major expression level difference between the tolerant and wild-type plants for genes involved in photosynthesis processes localized to the chloroplasts. Some of the key genes of PSII, PSI and, ATPase complexes were strongly repressed in the wild-type stressed plants but were unaffected or marginally repressed in stressed PSARK::IPT plants. Transcript abundance of genes associated with oxidative stress tolerance and photo-protection were significantly higher in pSARK::IPT compared to wild-type plants. At the protein level, photosynthesis genes such as D1, Cyt b6, and, α-ATPase were strongly repressed only in the stressed wild-type plants. The production of drought-tolerant crops able to grow under restricted water regimes without diminution of yield would minimize drought-related losses and ensure food production in water-limited lands.
Rivero RM, Shulaev V, Blumwald, E. (2009) Cytokinin-dependent photorespiration and the protection of photosynthesis during water deficit. Plant Physiol. 150:1380-1393.
Rivero, RM., Kojima, M., Gepstein A., Sakakibara, H., Mittler, R., Gepstein, S., and Blumwald, E. (2007). Delayed leaf senescence induces extreme drought tolerance in a flowering plant. Proc. Natl. Acad. Sci. USA 104:19631-19636.