Auxin protects spikelet fertility and grain yield under drought and heat stresses in rice

Rice is highly sensitive to drought and high temperature (HT) stresses at reproductive phase. Since auxin is a major regulator of reproductive development, we analysed the role of indole-3-acetic acid (IAA) in spikelet fertility of rice under drought and HT stresses in 4 different experiments with 23 diverse rice genotypes. The results showed that both of these stresses reduced the pollen viability, spikelet fertility, and yield components. Exogenous IAA (10(-5) M) sprayed on plants during drought or heat stresses mitigated the adverse stress effects on pollen viability, spikelet fertility and yield components across experiments in most genotypes. Exogenous IAA application reduced the membrane lipid peroxidation and ROS accumulation in spikelets under both stresses. Drought and HT stresses significantly reduced the mean endogenous IAA content of rice panicles. Endogenous IAA levels significantly and positively correlated with pollen viability and yield component traits, and negatively correlated with lipid peroxidation of spikelets. Expression of most of the YUCCA genes and their transcriptional regulators SPL, NGA and TFL were downregulated in the spikelets of rice genotypes under stress. Further, auxin co-receptor genes TIR1/AFB2-AUX/I4A, ARF target gene HAF TF, 3 out of 4 PIN genes, and 5 ARFs analysed in this study were significantly downregulated by drought/heat stresses in rice spikelets. These results showed that drought and HT stress-mediated decline in endogenous IAA level and impairment of IAA signalling are the major causes of spikelet sterility in rice. Exogenous auxin and genetic manipulation of auxin synthesis and signalling will be useful to mitigate spikelet sterility and stabilize the grain yield of rice under drought and heat stresses.