Abscisic acid (ABA) regulates plant stress responses and development. However, how the ABA signal is transmitted in response to stresses remains largely unclear, especially in monocots. In this study, we found that rice (Oryza sativa) OsPM1 (PLASMA MEMBRANE PROTEIN 1), encoded by a gene of AWPM-19 like family, mediates ABA influx through the plasma membrane. OsPM1 is predominantly expressed in vascular tissues, guard cells and mature embryos. Phenotypic analysis of overexpression, RNA interference (RNAi), and knock out (KO) lines showed that OsPM1 is involved in drought responses and seed germination regulation. 3H-(±)ABA transport activity and FRET (Fluorescence Resonance Energy Transfer) assays both demonstrated that OsPM1 facilitates ABA uptake into cells. The physiological isomer of ABA, (+)-ABA, is the preferred substrate of OsPM1. Higher ABA accumulation and faster stomatal closure in response to ABA treatment were observed in the overexpression lines compared to the wild-type control. Many ABA-responsive genes were up-regulated more in the OsPM1-overexpression lines but less in the RNAi lines compared to wild-type plants. Further investigation revealed that OsPM1 expression is regulated by the AREB/ABF family transcription factor OsbZIP46. Our results thus revealed that OsPM1 is an ABA influx carrier that plays an important role in drought responses.

Summary: Drought during rice reproductive development results in yield loss. It is important to understand the functions of drought‐responsive genes in reproductive tissues for improving rice yield under water‐deficit conditions. We show here that MID1 (MYB Important for Drought Response1), encoding a putative R‐R‐type MYB‐like transcription factor, can improve rice yield under drought. MID1 was primarily expressed in root and leaf vascular tissues, with low level in the tapetum, and was induced by drought and other abiotic stresses. Compared with wild type, MID1‐overexpressing plants were more tolerant to drought at both vegetative and reproductive stages and produced more grains under water stress. MID1‐overexpressing plants exhibited less severe anther defects such as deformed anther locules, abnormal tapetum, degenerated microspores and expanded middle layer, with improved pollen fertility and higher seed setting rate. MID1 was localized to the nucleus and could activate gene expression in yeast, and its homologs were identified in many other plants with high levels sequence similarity. In addition, candidate MID1‐regulated genes were analyzed using RNA‐seq and qRT‐PCR, including genes crucial for stress responses and anther development, with altered expressions in the florets of MID1‐overexpressing plants and RNAi lines. Furthermore, MID1 could bind to the promoters of two drought‐related genes (Hsp17.0 and CYP707A5) and one anther developmental gene (KAR) according to ChIP‐qPCR data. Our findings suggest that MID1 is a transcriptional regulator that promotes rice male development under drought by modulating the expressions of drought‐related and anther developmental genes and provide valuable information for crop improvement.