OsALMT7 Maintains Panicle Size and Grain Yield in Rice by Mediating Malate Transport

Abstract

Panicle size is a critical determinant of grain yield in rice (<i>Oryza sativa</i>) and other grain crops. During rice growth and development, spikelet abortion often occurs at either the top or the basal part of the panicle under unfavorable conditions, causing a reduction in fertile spikelet number and thus grain yield. In this study, we report the isolation and functional characterization of a panicle abortion mutant named <i>panicle apical abortion1-1</i> (<i>paab1-1</i>). <i>paab1-1</i> exhibits degeneration of spikelets on the apical portion of panicles during late stage of panicle development. Cellular and physiological analyses revealed that the apical spikelets in the <i>paab1-1</i> mutant undergo programmed cell death, accompanied by nuclear DNA fragmentation and accumulation of higher levels of H₂O₂ and malondialdehyde. Molecular cloning revealed that <i>paab1-1</i> harbors a mutation in <i>OsALMT7</i>, which encodes a putative aluminum-activated malate transporter (OsALMT7) localized to the plasma membrane, and is preferentially expressed in the vascular tissues of developing panicles. Consistent with a function for OsALMT7 as a malate transporter, the panicle of the <i>paab1-1</i> mutant contained less malate than the wild type, particularly at the apical portions, and injection of malate into the <i>paab1-1</i> panicle could alleviate the spikelet degeneration phenotype. Together, these results suggest that OsALMT7-mediated transport of malate into the apical portion of panicle is required for normal panicle development, thus highlighting a key role of malate in maintaining the sink size and grain yield in rice and probably other grain crops.