Ethylene sensitivity underscores the yield advantage of high-grain numbers in cylinder-shaped rice panicles

Abstract

Panicle-wise grain-filling of individual spikelets is location-specific in rice; apical spikelets dominate over their basal counterparts. This pattern of grain filling changes with panicle architecture among cultivars, but the underlying mechanisms are not understood. The newly introduced rice cultivars with cylinder-shaped panicles, although suited for high yield, exhibit several instances of poor filling. In this study shoot apical meristem gene expressions in the ontogeny stage and their influence on spikelet survival at the post-anthesis stage were elucidated by screening two types of rice Recombinant Inbreed Lines (RIL) having similar spikelet density in the cylinder-shaped panicles, such as SR-157 (low-sterile) and SR-159 (high-sterile) and their parents NPT-V12 (low-sterile) and SPS-1–6–1 (high-sterile). Between cultivars, SR-157 versus SR-159, and spikelets located apically versus basally, grain filling was more vulnerable in the latter than the former owing to a more remarkable evolution of ethylene at anthesis. Combined genome analyses revealed that branching genes like DEP2, SPL-14, PTR, and FZP over-expressed in high-sterile compared to low-sterile cultivars during the ontogeny stage. But this over-expression had no matching improvement in grain-setting in the post-anthesis stage. Grain starch synthesis was higher, and expression of ethylene receptors and signal transducer genes was lower in the low- compared to high-sterile cultivars. Thus, the quantum of grain set in the post-anthesis stage determining actual panicle yield was not dependent on the expression of genetic factors in the pre-anthesis stage defining potential grain yield. It was concluded that the default expression of ethylene synthesis and transducer genes at anthesis cause spikelet degeneration and deride the yield advantage of a high spikelet number in some cylinder-shaped rice panicles. The genomic studies controlling ethylene evolution and action in rice panicles must be an inalienable part of any constructive contribution to the yield potential.