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

Tillering is an important agronomic trait of rice (Oryza sativa) that affect the number of seffective panicle, thereby affecting yields. The phytohormone auxin plays a key role in tillering. Here we identified the high tillering and semi-dwarf 1 (htsd1) mutant, whose roots exhibit auxin-deficiency 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 auxin inhibited its tillering. We identified the mutated gene in htsd1 as AUXIN1 (OsAUX1, LOC_Os01g63770), encoding an auxin influx transporter. The promoter sequence of OsAUX1 contained many SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) binding sites. We demonstrate 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 auxin 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.