Nitric Oxide as a Mediator of Aba Signalling in Stomatal Guard Cells

Water shortage is likely to be one of the major global environmental stresses of the 21st century. A key plant response to declining soil water potential is increased biosynthesis and subsequent action of abscisic acid (ABA). ABA is an endogenous anti-transpirant that induces stomatal closure, thereby leading to water conservation. The signal transduction processes commencing with guard cell ABA recognition and culminating in transport of ions and water across the vacuolar and plasma membranes, resulting in guard cell shrinkage and stomatal closure, have not yet been completely resolved. Nitric oxide (NO) has recently been identified as a mediator of guard cell ABA signalling (Neill et al., 2002. Plant Physiol. 128, 13-16). ABA induces the synthesis of NO in guard cells, NO induces stomatal closure and either scavenging of NO or inhibition of NO synthesis reduces ABA-induced stomatal closure. These data indicate that NO synthesis is a critical component of ABA-induced stomatal closure. Aspects of NO synthesis and action are discussed.