Dual ubiquitin signalling by SLOMO controls AUX1 activity and turnover during root gravitropism

Gravity-directed growth ensures that shoots and roots grow upwards and downwards, respectively. To achieve this, the organ's angle with respect to gravity drives the asymmetric redistribution of the plant hormone auxin and consequently differential elongation, thus creating a curvature of the organ. In addition to efflux transporters, the auxin influx transporter AUXIN RESISTANT 1 (AUX1) is essential for auxin redistribution during root gravitropism. Here, we show that the F-box protein SLOMO regulates AUX1 via two distinct mechanisms. First, SLOMO promotes proteolytic degradation of AUX1, possibly in an indirect manner. Second, SLOMO controls the ubiquitination of K261, K264, and K266, thus potentially affecting AUX1 auxin transport properties that are regulated by these residues. This reveals a dual mode of SLOMO-mediated regulation of AUX1, including a novel, non-proteolytic role for SLOMO-mediated ubiquitination in addition to SLOMO-mediated degradation of AUX1.