Delayed senescence and drought tolerance in rice

Leaf senescence is one of rice mechanisms to cope and survive drought stress, though often leading to dramatic reduction in yield. Delaying drought-induced senescence has been shown to increase drought tolerance in tobacco and japonica rice by manipulating the timing of cytokinin (CK) synthesis by over expressing isopentenyl-transferase (IPT) gene under a stress and maturation-induced promoter (PSARK). Transgenic rice plants were generated at IRRI genetic Transfermation Laboratory by inserting PSARK : : IPT in IR64 and MTU1010 mega varieties. Single copy lines were molecularly charterized and further screened under drought in paddy like conditions in screen house. Days to flowering, biomass, grain yield and yield components and chlorophyll content were measured. Promising lines were then selected based on yield under drought for subsequent screening. Around 300 and 74 transgenic independent events were regenerated for IR64 and MTU1010 out of which ten and twelve were subjected to reproductive stage stress respectively. 80% of the T1 IR64 transgenic plants showed significant grain yield advantage under both well water and drought condition with greater biomass under drought. Chlorophyll retention was tested and significantly higher in two events compared to the wild type. Only four T1 MTU1010 lines show greater grain yield than wild type under drought stress. While in IR64 background, trangenics reached 50% heading under drought a week before the wild type, it ranged from 1 day earlier to a four day delay in MTU1010. Four promising IR64 T2 lines were again subjected to stress. Three have shown significantly higher yield and early heading compared to IR64 in the drought treatment. Those results suggest that delaying rice leaf senescence can improve drought tolerance and additional work on stress timing and cytokinin content are needed to further elucidate the underlying mechanisms. The authors also observed a high genotype x transgene interaction and further studies in MTU1010 are underway.