An Update on Abscisic Acid Signaling in Plants and More …

Abstract

The mode of abscisic acid (ABA) action, and its relations to drought adaptive responses in particular, has been a captivating area of plant hormone research for much over a decade. The hormone triggers stomatal closure to limit water loss through transpiration, as well as mobilizes a battery of genes that presumably serve to protect the cells from the ensuing oxidative damage in prolonged stress. The signaling network orchestrating these various responses is, however, highly complex. This review summarizes several significant advances made within the last few years. The biosynthetic pathway of the hormone is now almost completely elucidated, with the latest identification of the ABA4 gene encoding a neoxanthin synthase, which seems essential for de novo ABA biosynthesis during water stress. This leads to the interesting question on how ABA is then delivered to perception sites. In this respect, regulated transport has attracted renewed focus by the unexpected finding of a shoot-to-root translocation of ABA during drought response, and at the cellular level, by the identification of a beta-galactosidase that releases biologically active ABA from inactive ABA-glucose ester. Surprising candidate ABA receptors were also identified in the form of the Flowering Time Control Protein A (FCA) and the Chloroplastic Magnesium Protoporphyrin-IX Chelatase H subunit (CHLH) in chloroplast-nucleus communication, both of which have been shown to bind ABA in vitro. On the other hand, the protein(s) corresponding to the physiologically detectable cell-surface ABA receptor(s) is (are) still not known with certainty. Genetic and physiological studies based on the guard cell have reinforced the central importance of reversible phosphorylation in modulating rapid ABA responses. Sucrose Non-Fermenting Related Kinases (SnRK), Calcium-Dependent Protein Kinases (CDPK), Protein Phosphatases (PP) of the 2C and 2A classes figure as prominent regulators in this single-cell model. Identifying their direct in vivo targets of regulation, which may include H(+)-ATPases, ion channels, 14-3-3 proteins and transcription factors, will logically be the next major challenge. Emerging evidence also implicates ABA as a repressor of innate immune response, as hinted by the highly similar roster of genes elicited by certain pathogens and ABA. Undoubtedly, the most astonishing revelation is that ABA is not restricted to plants and mosses, but overwhelming evidence now indicates that it also exists in metazoans ranging from the most primitive to the most advance on the evolution scale (sponges to humans). In metazoans, ABA has healing properties, and plays protective roles against both environmental and pathogen related injuries. These cross-kingdom comparisons have shed light on the surprising ancient origin of ABA and its attendant mechanisms of signal transduction.

Abscisic Acid Has a Key Role in Modulating Diverse Plant-Pathogen Interactions

“…Recently, several studies have demonstrated that ABA may also be widely involved in plant responses to biotic stresses caused by a broad range of plant pathogens (MauchMani and Mauch, 2005;Asselbergh et al, 2008a) or insect herbivores (Thaler and Bostock, 2004;Bodenhausen and Reymond, 2007). Many early studies relied on the application of exogenous ABA to increase plant ABA levels (Asselbergh et al, 2008a), although exogenous ABA may not exert the same physiological effects as endogenous ABA (Christmann et al, 2005), and this may potentially complicate the interpretations of the roles of ABA in plant-pathogen interactions (Wasilewska et al, 2008). Our work on the cds2-1D mutant, which was identified from a forward genetic screen, showed that activation of NCED5, a key enzyme regulating de novo ABA biosynthesis, led to elevated endogenous ABA levels and to enhanced susceptibility to infection by P. syringae.…”

The multifaceted role of ABA in disease resistance

“…The first ABA precursor is zeaxanthin (see Figure 3 in the main text), which is converted into xanthoxin by a series of enzyme-mediated epoxidation and isomerization steps and a final dioxygenation reaction that cleaves the compound from the C 40 carotenoid. Xanthoxin is transported into the cytoplasm, where it is further oxidized to ABA ( Figure I) [51].…”

“…Under conditions of drought and high salinity, it promotes tolerance of the plants to desiccation [65], enabling them to survive under adverse conditions or to colonize areas with scarse water availability. ABA also regulates developmental processes, such as seed germination, vegetative growth and bud dormancy [51]. It therefore enables plants to adapt optimally to their environment by inhibiting germination under sub-optimal conditions, partaking in developmental processes and protecting the plant against biotic and abiotic stress during its vegetative growth phase.…”

“…The ABA biosynthetic pathway and its interaction with the ascorbate-glutathione cycle. The biosynthesis of ABA involves multiple redox-dependent reactions that occur in the plastid and the cytoplasm [51]. The enzyme zeaxanthin epoxidase (ZEP) converts zeaxanthin into violaxanthin through an epoxidation reaction that occurs under low light and relatively alkaline conditions.…”

Functional Convergence of Oxylipin and Abscisic Acid Pathways Controls Stomatal Closure in Response to Drought

“…We selected Col-0, a natural loss-of-function mutant in hpl (Duan et al, 2005), together with the other two ecotypes that produce HPL-and AOS-derived metabolites. As control, we expanded the oxylipin profiling and measured the levels of the classical droughtresponsive hormone ABA (Wasilewska et al, 2008;Kim et al, 2010).…”