NAD meets ABA: connecting cellular metabolism and hormone signaling

NAD is a ubiquitous metabolic coenzyme. Although the role of NAD as a central redox shuttle remains of critical interest in plant metabolism, recent evidence indicates that NAD serves additional functions in signaling and regulation. A link with the plant stress hormone abscisic acid (ABA) has emerged on the basis of similar plant phenotypes following interference with NAD or ABA, especially in stomatal development, stomatal movements, responses to pathogens and abiotic stress insults, and seed germination. The association between NAD and ABA regulation appears specific and cannot be accounted for by pleiotropic interference. Here, we review the current picture of the NAD - ABA relationship, discuss emerging candidate mechanisms, and assess avenues to dissect interaction mechanisms.