OsABCB4 disruption impairs auxin homeostasis, delaying heading and compromising agronomic traits in rice.

Members of the ABCB transporter subfamily are essential for various aspects of plant growth and development; however, a large number of ABCB proteins are functionally uncharacterized. Here, we report the functional characterization of a rice ABCB member, OsABCB4. Tissue-specific expression analysis of 27 ABCB genes in rice identified a cluster with seed-specific expression, among which OsABCB4 was most highly expressed in developing panicles and seeds. CRISPR/Cas9-generated osabcb4 mutants exhibited a significant delay in heading date. Furthermore, the mutants displayed severe yield-related defects, including dwarfism, reduced panicle length, and a sharp decrease in seed-setting rate, primarily attributable to significantly impaired pollen fertility. Hormone quantification indicated a substantial reduction in indole-3-acetic acid (IAA) content in the panicles of mutant plants. Transcriptome analysis revealed global changes in gene expression, with differentially expressed genes significantly enriched in plant hormone signal transduction and starch/sucrose metabolism pathways. Consistent with these findings, the mutants showed abnormal accumulation of grain storage substances, characterized by significantly decreased starch content and increased protein content, consequently slowing down both seed germination and early seedling growth. Taken together, our results suggest that OsABCB4 may act as a key regulator that influences heading date, pollen fertility, and grain filling by modulating auxin homeostasis in rice.