Auxin modulates ABI3 expression as part of a negative feedback loop during lateral root development in Arabidopsis

Lateral rooting confers essential plasticity to root system architecture, ensuring efficient growth and stress adaptations. Auxin has long been associated with lateral root development, but the intricate regulatory mechanism remains elusive. Here, we show that ABI3, a B3-domain transcription factor, controls lateral root formation by intercepting auxin response in Arabidopsis thaliana. ABI3 deletion promotes lateral rooting, characterised by higher primordia number, density, accelerated emergence and enhanced auxin response, compared to wild-type. While optimum auxin is known to promote lateral rooting, excess auxin inhibits it. We show that auxin concentrations that impair lateral root development augment ABI3 expression. Transactivation assay and ChIP-based results indicate that the auxin-responsive factor ARF19 weakly induces ABI3 expression, probably as part of a secondary auxin response, coupled with ABI3-mediated autoactivation, resulting in higher transcriptional output. ABI3 functions as a transcriptional repressor of MYC2. MYC2 affects auxin biosynthesis and signalling through ERF109 and ASA1, followed by the downstream module of ARF7/19-LBD29, to ensure efficient lateral root development. ABI3-mediated repression of MYC2 acts to attenuate this pathway, restricting lateral root formation. Absence of ABI3 thus augments auxin signalling through MYC2, effectively promoting enhanced lateral root proliferation. ABI3, therefore, functions as a critical regulator that fine-tunes auxin signalling to balance lateral root growth.