CYP79B2 and CYP79B3 contribute to root branching through production of the auxin precursor indole-3-acetonitrile

Lateral root placement, outgrowth and density are influenced by environmental changes, including salinity stress. CYP79B2 and B3 are two cytochrome P450 enzymes previously identified as required for root architecture remodeling in salt. They produce iAOx, a metabolite that can be converted into indole glucosinolates (IGs), camalexin and indole-3-acetic acid (IAA), a type of auxin. We report here that lateral root appearance, induced by an auxin maximum in the bending zone after gravistimulation, is delayed in the absence of CYP79B2/B3. This delay traces back to a decrease in early lateral root growth after emergence, taking place before lateral roots are macroscopically visible. We measured gene transcripts and abundance of metabolites in the iAOx pathway in root segments that are forming lateral roots. Genes involved in tryptophane and IG biosynthesis were upregulated in cyp79b2/b3 mutants, suggesting a transcriptional feedback-loop. Salt stress was found to increase the expression of genes involved in IAN biosynthesis, a precursor of both IAA and camalexin, in the root during lateral root formation. Moreover, salt increases the concentration of IAN in tissue forming lateral roots in a CYP79B2/B3 dependent manner, but these changes in IAN did not coincide with altered IAA levels. Both the reduction in lateral root density under salt and the delayed lateral root appearance in cyp79b2/b3 knock-out mutants can be complemented by exogenous application of IAN. Our results reveal a role for the iAOx pathway in regulating the timing of lateral root appearance, allowing the modulation of lateral root density under salt stress.