Rice spikelet formation inhibition caused by decreased sugar utilization under high temperature is associated with brassinolide decomposition

Rice (Oryza sativa) spikelet formation is dependent on carbohydrate metabolism. High temperature inhibits young panicle growth and spikelet formation, and brassinolides (BRs) are involved in the regulation of spikelet formation. Elucidation of the mechanism by which endogenous BRs mediate carbohydrate metabolism to influence spikelet formation under high temperature may provide a theoretical and technical basis to alleviate heat injury. In this study, rice cultivar 'IR36' was subjected to high temperature (HT; 40 degrees C) or normal temperature (NT; 33 degrees C) for 7 days at the panicle initiation stage in conjunction with application of exogenous 2, 4-epibrassinolide (EBR) and brassinazole (BRZ) at the beginning of the treatments. HT inhibited spikelet differentiation and aggravated spikelet degeneration to reduce the number of spikelets per panicle. Exogenous EBR reduced the decrease in spikelet number induced by HT, whereas exogenous BRZ enhanced the reduction in spikelet number. Analyses of the transcript levels of genes associated with BR biosynthesis and decomposition showed that HT promoted BR decomposition to reduce the endogenous BR content. HT blocked sucrose transport from leaves to young panicles, and inhibited the activities of crucial enzymes involved in sucrose hydrolysis, glycolysis, and the tricarboxylic acid cycle to reduce sugar utilization. In addition, HT reduced the activities of antioxidant enzymes and increased the contents of hydrogen peroxide and malondialdehyde. Exogenous EBR application promoted sucrose transport to young panicles and improved sucrose utilization under NT and HT, whereas application of BRZ resulted in the opposite physiological response. These results show that inhibition of spikelet formation owing to decreased sugar utilization under HT is associated with BR decomposition.