The auxin precursor indole-3-acetamide controls a specific subset of stress-related target genes

Auxins, including indole-3-acetic acid (IAA) as the most common naturally occurring phytohormone of the auxin class in plants, are well-known for their plant growth promoting activities. Previous work of our laboratory provided compelling evidence of an independent role for one of the IAA metabolic precursors, indole-3-acetamide (IAM), as a trigger for plant stress responses and repressor of plant growth. This process was demonstrated to proceed via the induction of abscisic acid (ABA) biosynthesis upon IAM accumulation. However, little information is available on the molecular mechanisms by which IAM drives ABA formation and plant growth repression. This work shows that IAA and IAM drive the expression of distinct subgroups of genes and that a small group of environmental stress-related genes respond quickly to the treatment with IAM. Among the identified target genes, we focussed our attention on the myeloblastosis (MYB) transcription factor MYB74. MYB transcription factors are reported to be involved, among other things, in plant development, secondary metabolism, and abiotic stress tolerance. Here, we demonstrate that IAM directly drives the expression of MYB74, and that the conditional overexpression of MYB74 translates into a considerable reduction of plant growth. It is noteworthy that the genome-wide transcriptomics analysis of the MYB74 gain-of-function mutant revealed the differential expression of several genes associated with abiotic stresses, including water and osmotic stress. Finally, we characterised the role of MYB74 in conferring increased stress tolerance to Arabidopsis thaliana plants by taking reverse genetics approach and found that an increased expression of MYB74 enhances the osmotic stress tolerance. In summary, our study provides comprehensive evidence of the role of MYB74 as negative plant growth regulator during abiotic stress responses. In addition, we were able to show that IAM, which was previously shown to accumulate under stress conditions in plants, triggers the gene expression of MYB74 which, in turn, improves the osmotic stress tolerance in Arabidopsis.