The auxin transporter OsAUX1 regulates tillering in rice (Oryza sativa)

Abstract

Tillering is an important agronomic trait of rice (Oryza sativa) that affects the number of effective panicles, thereby affecting yields. The phytohormone auxin plays a key role in tillering. Here we identified the high tillering and semi-dwarf 1 (htsd1) mutant with auxin-deficiency root characteristics, such as shortened lateral roots, reduced lateral root density, and enlarged root angles. htsd1 showed reduced sensitivity to auxin, but the external application of indole-3-acetic acid (IAA) inhibited its tillering. We identified the mutated gene in htsd1 as AUXIN1 (OsAUX1, LOC_Os01g63770), which encodes an auxin influx transporter. The promoter sequence of OsAUX1 contains many SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) binding sites, and we demonstrated that SPL7 binds to the OsAUX1 promoter. TEOSINTE BRANCHED1 (OsTB1), a key gene that negatively regulates tillering, was significantly downregulated in htsd1. Tillering was enhanced in the OsTB1 knockout mutant, and the external application of IAA inhibited tiller elongation in this mutant. Overexpressing OsTB1 restored the multi-tiller phenotype of htsd1. These results suggest that SPL7 directly binds to the OsAUX1 promoter and regulates tillering in rice by altering OsTB1 expression to modulate auxin signaling.