The isoleucic acid triad: distinct impacts on plant defense, root growth, and formation of reactive oxygen species.

Isoleucic acid (ILA ), a branched-chain amino acid-related 2-hydroxy carboxylic acid, occurs ubiquitously in plants. ILA enhances pathogen resistance and inhibits root growth of Arabidopsis thaliana. The salicylic acid (SA) glucosyltransferase UGT76B1 is able to conjugate ILA as well. Here, we investigate the role of ILA in planta and reveal a triad of distinct responses to this small molecule. ILA synergistically cooperates with SA to activate SA-responsive gene expression and resistance in a UGT76B1-dependent manner in agreement with the observed competitive ILA-dependent repression of SA glucosylation by UGT76B1. However, ILA also shows an SA-independent stress response. NBT staining and pharmacological experiments indicate that ILA induces superoxide formation of the wild type and of an SA-deficient (NahG sid2) line. In contrast, the inhibitory effect of ILA on root growth is independent of both SA and superoxide induction. These effects of ILA are specific and distinct from its isomeric compound leucic acid and from the amino acid isoleucine. Leucic acid and isoleucine do not induce expression of defense marker genes or superoxide production, whereas both compounds inhibit root growth. All three responses to ILA are also observed in Brassica napus.