Dehydration rapidly induces expression of NCED genes from a single subclade in diverse eudicots

During dehydration, some plants can rapidly synthesise the stress hormone abscisic acid (ABA) in leaves within 20 min, triggering the closure of stomata and limiting further water loss. This response is associated with significant transcriptional upregulation of Nine-cis-Epoxycarotenoid Dioxygenase (NCED) genes, which encode the enzyme considered to be rate-limiting in ABA biosynthesis. However, most embryophyte species possess multiple NCED genes, and it is not currently known whether there is any phylogenetic pattern to which NCED genes are involved in this response. We tested transcriptional responses to dehydration for all NCED genes present in three diverse eudicot species—Arabidopsis thaliana (Arabidopsis), pea and tomato—over both the timeframe of stomatal responses (< 20 min) and in response to sustained dehydration (hours). We found that there is a single NCED gene per species, AtNCED3, PsNCED2, and SlNCED1, respectively, that is rapidly upregulated by dehydration. Using a null mutant, we confirmed that the rapidly responsive gene identified in Arabidopsis is important for physiological responses to a sudden drop in humidity. Analysis of the relationships and the evolutionary history of NCED genes using available sequence data from diverse land plant species revealed that the identified genes in each species all belong to the same subclade within the gene family, suggesting a conserved role for this subclade in rapid dehydration responses in eudicots. These findings enable future phylogenetically-informed prediction of genes of interest for rapid dehydration responses within this important multigene family in eudicot species.