The LRR receptor-like kinase OsXIAO regulates rice root growth by interacting with auxin transporter OsPIN1a

Root system plays a crucial role in plant survival and normal growth. Identifying molecular determinants that optimize root system is an important strategy to improve yield production in crops. Here, we demonstrated an LRR-RLK (Leucine-Rich Repeat Receptor-Like Kinase), OsXIAO, which has an important function in rice root growth. The high expression level of OsXIAO was observed in rice roots, which was increased by IAA. Mutation of OsXIAO caused partially agravitropic root growth phenotype with short and curled roots, and severely repressed plant growth and grain production, while overexpressing of OsXIAO significantly promoted root growth and grain production. OsXIAO mutant showed reduced sensitivity to IAA and significantly lower IAA level in the root tips, while the overexpressing lines showed higher IAA level in the root tips. RNAseq analysis showed that 37 genes involved in auxin biosynthesis, signal transduction, and transmembrane transport were differentially expressed, and phosphoproteomic analyses revealed that the phosphorylation levels of 284th Thr and 288th Ser residues of OsPIN1a were significantly down-regulated in the roots of mutant. Moreover, Y2H (Yeast Two-Hybrid, LUC (Luciferase), and BIFC (Bimolecular Fluorescence Complementation) assays confirmed that OsXIAO could interact with OsPIN1a on the plasma membrane. Similar to OsXIAO, OsPIN1a was highly expressed in rice roots and induced by IAA, and the root growth was significantly inhibited in OsPIN1a mutants. Taken together, OsXIAO interacts with OsPIN1a on the plasma membrane and promoted auxin transport in rice roots, which improves root growth and elevates yield production.