The grain filling of inferior spikelets is much less complete than that of superior spikelets in rice cultivars with large panicles and numerous spikelets and is promoted by moderate soil drying (MD) post-anthesis. A growing body of evidence has shown that microRNAs function in regulating grain development. However, little is known about the mechanism of microRNA control of grain filling of inferior spikelets in response to MD. In this study, grain filling of inferior spikelets was promoted by MD treatment in Nipponbare. Small-RNA profiling at the most active grain-filling stage was conducted in inferior spikelets under control (CK) and MD treatment. Of 521 known and 128 novel miRNAs, 38 known and 9 novel miRNAs were differentially expressed between the CK and MD treatments. Target genes of differentially expressed miRNAs were involved in multiple developmental and signaling pathways associated with catalytic activity, carbohydrate metabolism, and other functions. Both miR1861 and miR397 were upregulated by MD, leading to a decrease in OsSBDCP1 and OsLAC, two negative regulators of SSIIIa activity and BR signaling, respectively. In contrast, miR1432 abundance was reduced by MD, resulting in upregulation of OsACOT and thus an elevated content of both ABA and IAA. These results suggest that both starch synthesis and phytohormone biosynthesis are regulated by differentially expressed miRNAs in inferior spikelets in response to MD treatment. Our results suggest the molecular mechanisms by which miRNAs regulate grain filling in inferior spikelets of rice under moderate soil drying, providing potential application in agriculture to increase rice yields by genetic approaches.

Crop yield loss due to soil salinization is an increasing threat to agriculture worldwide. Salt stress drastically affects the growth, development, and grain productivity of rice (Oryza sativa L.), and the improvement of rice tolerance to salt stress is a desirable approach for meeting increasing food demand. The main contributors to salt toxicity at a global scale are Na⁺ and Cl⁻ ions, which affect up to 50% of irrigated soils. Plant responses to salt stress occur at the organismic, cellular, and molecular levels and are pleiotropic, involving (1) maintenance of ionic homeostasis, (2) osmotic adjustment, (3) ROS scavenging, and (4) nutritional balance. In this review, we discuss recent research progress on these four aspects of plant physiological response, with particular attention to hormonal and gene expression regulation and salt tolerance signaling pathways in rice. The information summarized here will be useful for accelerating the breeding of salt-tolerant rice.