APICAL SPIKELET ABORTION (ASA) Controls Apical Panicle Development in Rice by Regulating Salicylic Acid Biosynthesis.

Panicle degradation causes severe yield reduction in rice. There are two main types of panicle degradation: apical spikelet abortion and basal degeneration. In this study, we isolated and characterized the apical panicle abortion mutant apical spikelet abortion (asa), which exhibits degeneration and defects in the apical spikelets. This mutant had a pleiotropic phenotype, characterized by reduced plant height, increased tiller number, and decreased pollen fertility. Map-based cloning revealed that OsASA encodes a boric acid channel protein that showed the highest expression in the inflorescence, peduncle, and anther. RNA-seq analysis of the asa mutant vs wild-type (WT) plants revealed that biological processes related to reactive oxygen species (ROS) homeostasis and salicylic acid (SA) metabolism were significantly affected. Furthermore, the asa mutants had an increased SA level and H2O2 accumulation in the young panicles compared to the WT plants. Moreover, the SA level and the expression of OsPAL3, OsPAL4, and OsPAL6 genes (related to SA biosynthesis) were significantly increased under boron-deficient conditions in the asa mutant and in OsASA-knockout plants. Collectively, these results suggest that the boron distribution maintained by OsASA is required for normal panicle development in a process that involves modulating ROS homeostasis and SA biosynthesis.

Control of grain size and rice yield by GL2-mediated brassinosteroid responses.

Given the continuously growing population and decreasing arable land, food shortage is becoming one of the most serious global problems in this century(1). Grain size is one of the determining factors for grain yield and thus is a prime target for genetic breeding(2,3). Although a number of quantitative trait loci (QTLs) associated with rice grain size have been identified in the past decade, mechanisms underlying their functions remain largely unknown(4,5). Here we show that a grain-length-associated QTL, GL2, has the potential to improve grain weight and grain yield up to 27.1% and 16.6%, respectively. We also show that GL2 is allelic to OsGRF4 and that it contains mutations in the miR396 targeting sequence. Because of the mutation, GL2 has a moderately increased expression level, which consequently activates brassinosteroid responses by upregulating a large number of brassinosteroid-induced genes to promote grain development. Furthermore, we found that GSK2, the central negative regulator of rice brassinosteroid signalling, directly interacts with OsGRF4 and inhibits its transcription activation activity to mediate the specific regulation of grain length by the hormone. Thus, this work demonstrates the feasibility of modulating specific brassinosteroid responses to improve plant productivity.