Identification of Auxin Activity Like 1, a chemical with weak functions in auxin signaling pathway.

KEY MESSAGE: A new synthetic auxin AAL1 with new structure was identified. Different from known auxins, it has weak effects. By AAL1, we found specific amino acids could restore the effects of auxin with similar structure. Auxin, one of the most important phytohormones, plays crucial roles in plant growth, development and environmental response. Although many critical regulators have been identified in auxin signaling pathway, some factors, especially those with weak fine-tuning roles, are still yet to be discovered. Through chemical genetic screenings, we identified a small molecule, Auxin Activity Like 1 (AAL1), which can effectively inhibit dark-grown Arabidopsis thaliana seedlings. Genetic screening identified AAL1 resistant mutants are also hyposensitive to indole-3-acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D). AAL1 resistant mutants such as shy2-3c and ecr1-2 are well characterized as mutants in auxin signaling pathway. Genetic studies showed that AAL1 functions through auxin receptor Transport Inhibitor Response1 (TIR1) and its functions depend on auxin influx and efflux carriers. Compared with known auxins, AAL1 exhibits relatively weak effects on plant growth, with 20 µM and 50 µM IC50 (half growth inhibition chemical concentration) in root and hypocotyl growth respectively. Interestingly, we found the inhibitory effects of AAL1 and IAA could be partially restored by tyrosine and tryptophan respectively, suggesting some amino acids can also affect auxin signaling pathway in a moderate manner. Taken together, our results demonstrate that AAL1 acts through auxin signaling pathway, and AAL1, as a weak auxin activity analog, provides us a tool to study weak genetic interactions in auxin pathway.