Novel auxin transport inhibitors phenocopy the auxin influx carrier mutation aux1

The hormone auxin is transported in plants through the combined actions of diffusion and specific auxin influx and efflux carriers. In contrast to aux in efflux, for which there are well documented inhibitors, understanding th e developmental roles of carrier-mediated auxin influx has been hampered by the absence of specific competitive inhibitors. However, several molecules that inhibit auxin influx in cultured cells have been described recently. The physiological effects of two of these novel influx carrier inhibitors, 1-naphthoxyacetic acid (1-NOA) and 3-chloro-4-hydroxyphenylacetic acid (CHP AA), have been investigated in intact seedlings and tissue segments using c lassical and new auxin transport bioassays. Both molecules do disrupt root gravitropism, which is a developmental process requiring rapid auxin redist ribution. Furthermore, the auxin-insensitive and agravitropic root-growth c haracteristics of aux1 plants were phenocopied by 1-NOA and CHPAA. Similarl y, the agravitropic phenotype of inhibitor-treated seedlings was rescued by the auxin 1-naphthaleneacetic acid, but not by 2,4-dichlorophenoxyacetic a cid, again resembling the relative abilities of these two auxins to rescue the phenotype of aux1. Further investigations have shown that none of these compounds block polar auxin transport, and that CHPAA exhibits some auxin- like activity at high concentrations. Whilst results indicate that 1-NOA an d CHPAA represent useful tools for physiological studies addressing the rol e of auxin influx in planta, 1-NOA is likely to prove the more useful of th e two compounds.