Polyamines and ethylene in rice young panicles in response to soil drought during panicle differentiation

This study tested the hypothesis that polyamines (PA) and ethylene (ETH) mediate the effects of soil drought on spikelet development in rice (Oryza sativa L.). Two rice cultivars, Yong You-2640 and Yang Dao-6, with vastly different panicle sizes were grown in pots under three soil moisture treatments: well-watered (WW), moderate soil drought (MD) and severe soil drought (SD), from the onset of panicle initiation to the pollen completion stage. MD treatment significantly increased spikelet differentiation, spikelet number per panicle, fully filled grain percentage and grain yield, decreasing the percentage of degenerated spikelets, sterile spikelets and partially filled grains compared to WW treatment. In contrast, SD treatment showed opposite effects. MD also increased the contents of free spermidine (Spd), free spermine (Spm) and the ratios of free putrescine, free-Spd and free-Spm to 1-aminocylopropane-1-carboxylic acid (ACC), decreasing the ETH evolution rate and ACC content in young panicles. In contrast, SD treatment showed opposite effects. Furthermore, free-Spd and free-Spm contents increased significantly, while ETH and ACC levels, and the percentage of degenerated and sterile spikelets decreased significantly under application of Spd or an inhibitor of ETH synthesis. The results were reversed when ACC or an inhibitor of Spd and Spm synthesis was applied. These findings suggest antagonistic interactions between free-PA (Spd and Spm) and ETH in response to soil drought, mediating spikelet development in rice.