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

Panicle size is a critical determinant of grain yield in rice () 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 (). 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 mutant undergo programmed cell death, accompanied by nuclear DNA fragmentation and accumulation of higher levels of HO and malondialdehyde. Molecular cloning revealed that harbors a mutation in , 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 mutant contained less malate than the wild type, particularly at the apical portions, and injection of malate into the 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.