OsAux1 is required for nutritropism in rice roots

Nutritropism is a directional root growth response to nutrient gradients, and rice (Oryza sativa L.) roots show nutritropism toward sources containing ammonium. Here, we report characterization of an EMS-mutagenized rice plant that shows a complete defect in nutritropism. Genetic analysis of the mutant identified a nucleotide substitution as a candidate for the causal mutation; this mutation leads to an amino acid substitution (E111V) in an auxin influx carrier protein, OsAUX1 (LOC_Os01g63770, Os01g0856500). We generated novel allelic mutants using CRISPR/Cas9, and these independent mutant lines showed defects in nutritropism. We concluded that OsAUX1 is the gene required for nutritropism based on this result. Analysis of RFP fluorescence in root tips expressing the DR5::RFP reporter suggested an asymmetric distribution of auxin in nutritropic root bending, and the asymmetry was similar to that in gravitropism. Nutritropism of aux1 mutants was recovered by exogenous application of membrane-permeable 1-naphthaleneacetic acid (NAA) but not by impermeable 2,4-dichlorophenoxyacetic acid, indicating that passive diffusion of NAA into the cytosol from the medium is sufficient to imitate the OsAUX1 functions. Our results establish the involvement of auxin in nutritropism, indicating the similarity to gravitropism but not to hydrotropism. This provides insights into the mechanisms for integration of the complex tropic stimulations from environments.