Auxin branches out

Plant roots require extensive branching to gather nutrients or water from the environment. When roots lose contact with soil and hit an air gap, an adaptive response called xerobranching suppresses lateral root branching until contact with water is re-established. Abscisic acid is a key hormone that mediates xerobranching by decreasing auxin levels and is produced under drought conditions. However, abscisic acid production occurs at a later stage of the xerobranching response, suggesting the requirement of an earlier signal. Given that reactive oxygen species (ROS) are involved in various plant stresses, Roy, Mehra et al. tested whether ROS signaling was involved in mediating the initial response of xerobranching. The expression of the ROS-producing enzymes respiratory burst oxidase homologs (RBOHs) was increased in the roots at the induction of xerobranching. In addition, the use of nuclear-localized H₂O₂ sensors showed rapid oxidation with xerobranching, indicative of a ROS burst in the nucleus that declined after root contact with water. rboh combinatorial homolog mutants exhibited continued lateral root growth in an air gap. The group also connected ROS signaling to auxin regulation, with the auxin transcriptional repressor IAA3 being a key intermediary. IAA3 contains a large number of modifiable cysteine residues, which mediate formation of multimers under non-reducing conditions. Mutation of four critical cysteine residues in IAA3, which were predicted to form disulfide bridges, blocked H₂O₂-mediated formation of multimers and resembled iaa3 mutants with extensive branching into air gaps. Loss of IAA3 multimer formation prevented interaction with the co-repressor TOPLESS, which prevented auxin target gene repression. Overall, the findings from Roy et al. have now identified a ROS–IAA3-mediated pathway that enables the early step of xerobranching.

Original reference: Science https://doi.org/10.1126/science.adu1470 (2025)