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

Abstract

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 <i>apical spikelet abortion</i> (<i>asa</i>), 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 <i>OsASA</i> encodes a boric acid channel protein that showed the highest expression in the inflorescence, peduncle, and anther. RNA-seq analysis of the <i>asa</i> 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 <i>asa</i> 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 <i>OsPAL3</i>, <i>OsPAL4</i>, and <i>OsPAL6</i> genes (related to SA biosynthesis) were significantly increased under boron-deficient conditions in the <i>asa</i> mutant and in <i>OsASA-</i>knockout plants. Collectively, these results suggest that the boron distribution maintained by <i>OsASA</i> is required for normal panicle development in a process that involves modulating ROS homeostasis and SA biosynthesis.