  Auxin is considered one of the cardinal hormones in plant growth and
development. It regulates a wide range of processes throughout the plant.
Synthetic auxins exploit the auxin-signalling pathway and are valuable as
herbicidal agrochemicals. Currently, despite a diversity of chemical scaffolds
all synthetic auxins have a carboxylic acid as the active core group. By
applying bio-isosteric replacement we discovered that indole-3-tetrazole was
active by surface plasmon resonance (SPR) spectrometry, showing that the
tetrazole could initiate assembly of the TIR1 auxin co-receptor complex. We
then tested the tetrazole's efficacy in a range of whole plant physiological
assays and in protoplast reporter assays which all confirmed auxin activity,
albeit rather weak. We then tested indole-3-tetrazole against the AFB5
homologue of TIR1, finding that binding was selective against TIR1, absent with
AFB5. The kinetics of binding to TIR1 are contrasted to those for the herbicide
picloram, which shows the opposite receptor preference as it binds to AFB5 with
far greater affinity than to TIR1. The basis of the preference of
indole-3-tetrazole for TIR1 was revealed to be a single residue substitution
using molecular docking, and assays using tir1 and afb5 mutant lines confirmed
selectivity in vivo. Given the potential that a TIR1-selective auxin might have
for unmasking receptor-specific actions, we followed a rational design, lead
optimisation campaign and a set of chlorinated indole-3-tetrazoles was
synthesised. Improved affinity for TIR1 and the preference for binding to TIR1
was maintained for 4- and 6-chloroindole-3-tetrazoles, coupled with improved
efficacy in vivo. This work expands the range of auxin chemistry for the design
of receptor-selective synthetic auxins.
