OsPIN9, an auxin efflux carrier, is required for the regulation of rice tiller bud outgrowth by ammonium.

The degree of rice tillering is an important agronomic trait that can be remarkably affected by nitrogen supply. However, less is known about how nitrogen-regulated rice tillering is related to polar-auxin transport. Compared with nitrate, ammonium induced tiller development and was paralleled with increased 3 H-IAA transport and greater auxin into the junctions. OsPIN9, an auxin efflux carrier, was selected as the candidate gene involved in ammonium-regulated tillering based on the GeneChip data. Compared to wild-type plants, ospin9 mutants had fewer tillers, and OsPIN9 overexpression increased the tiller number. Additionally, OsPIN9 was mainly expressed in vascular tissue of the junction and tiller buds, and encodes a membrane-localized protein. Heterologous expression in Xenopus oocytes and yeast demonstrated that OsPIN9 is a functional auxin-efflux transporter. More importantly, its RNA and protein levels were induced by ammonium but not by nitrate, and tiller numbers in mutants didn't respond to nitrogen forms. Furthermore, greater advantages, including increased tiller number and grain yield, were observed in overexpression lines grown in the paddy field at a low-nitrogen rate than at a high-nitrogen rate. Our data reveal that ammonium supply and an auxin efflux transporter coordinately control tiller bud elongation in rice.