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Auxin is considered one of the cardinal hormones in plant growth anddevelopment. It regulates a wide range of processes throughout the plant.Synthetic auxins exploit the auxin-signalling pathway and are valuable asherbicidal agrochemicals. Currently, despite a diversity of chemical scaffoldsall synthetic auxins have a carboxylic acid as the active core group. Byapplying bio-isosteric replacement we discovered that indole-3-tetrazole wasactive by surface plasmon resonance (SPR) spectrometry, showing that thetetrazole could initiate assembly of the TIR1 auxin co-receptor complex. Wethen tested the tetrazole's efficacy in a range of whole plant physiologicalassays and in protoplast reporter assays which all confirmed auxin activity,albeit rather weak. We then tested indole-3-tetrazole against the AFB5homologue of TIR1, finding that binding was selective against TIR1, absent withAFB5. The kinetics of binding to TIR1 are contrasted to those for the herbicidepicloram, which shows the opposite receptor preference as it binds to AFB5 withfar greater affinity than to TIR1. The basis of the preference ofindole-3-tetrazole for TIR1 was revealed to be a single residue substitutionusing molecular docking, and assays using tir1 and afb5 mutant lines confirmedselectivity in vivo. Given the potential that a TIR1-selective auxin might havefor unmasking receptor-specific actions, we followed a rational design, leadoptimisation campaign and a set of chlorinated indole-3-tetrazoles wassynthesised. Improved affinity for TIR1 and the preference for binding to TIR1was maintained for 4- and 6-chloroindole-3-tetrazoles, coupled with improvedefficacy in vivo. This work expands the range of auxin chemistry for the designof receptor-selective synthetic auxins.

The tetrazole analogue of the auxin indole-3-acetic acid binds preferentially to TIR1 and not AFB5