The auxin precursor indole-3-acetamide (IAM) is conventionally considered a potential intermediate in the biosynthesis of the primary plant growth hormone, indole-3-acetic acid (IAA). The latter compound plays a crucial role in regulating plant development, including root growth and root system architecture. However, the researchers observed that an excess of IAM resulted in reduced root growth, prompting an investigation into how a precursor to a growth hormone could function as an inhibitory factor. The study found that IAM likely serves not only as a building block for IAA but also acts as an emergency alarm signal under osmotic, drought, or cold stress conditions.

Until now, the molecular basis of this relationship linking plant growth with abiotic stress responses remained uncertain. A recent study, published in New Phytologist and led by Stephan Pollmann and Jesús Vicente-Carbajosa from CBGP, elucidates how IAM triggers the formation and signalling of the plant stress hormone ABA, thereby enabling the plant to adapt to environmental changes.

A novel plant hormone-like compound

The research team utilized natural variation within a population of over 150 Arabidopsis thaliana accessions from the Iberian Peninsula as a molecular tool to identify novel components associated with the reduced primary root growth observed in plants treated with the auxin precursor molecule IAM. Initially, a Genome-wide Association Study (GWAS) was conducted to examine the differential root growth responses among the various Arabidopsis accessions, aiming to identify genetic regions linked to the phenotype. This advanced technique enabled the team to identify and prioritize potential target genes. Subsequent experiments conclusively identified ABA3, an enzyme involved in the biosynthesis of ABA, as a contributor to the observed phenotype. These studies demonstrated that IAM is implicated in the transcriptional regulation of the ABA3 gene and, more significantly, that IAM, rather than the end product of the biosynthetic pathway, IAA, is involved in the induction of ABA formation and signalling.

The conclusions were facilitated by the pivotal discovery that IAM does not influence the root growth responses of ABA biosynthesis and signalling mutants, while it activates the response of ABA signalling reporter lines at concentrations as low as 2.5 µM with a brief incubation period of only two hours. These findings suggest that IAM is likely a plant hormone-like compound on its own right that integrates plant growth with stress responses.

Towards Climate Change-adapted Crops

This discovery not only enhances our comprehension of abiotic stress responses in plants but also elucidates the role of the cross-talk between two antagonistic plant hormones in adapting plant responses to osmotic and water stress. A comprehensive understanding of these processes is a crucial step towards the potential development of elite crop lines that are better adapted to the effects associated with climate change.

Translating our findings into the development of such crops could ultimately ensure plant productivity under anticipated unfavourable environmental conditions, thereby paving the way for crops that contribute to increased food security and, hopefully, a more sustainable agriculture.