Abstract
The plant hormone abscisic acid (ABA) regulates many key processes involved in plant development and adaptation to biotic and abiotic stresses. Under stress conditions, plants synthesize ABA in various organs and initiate defense mechanisms, such as the regulation of stomatal aperture and expression of defense-related genes conferring resistance to environmental stresses. The regulation of stomatal opening and closure is important to pathogen defense and control of transpirational water loss. Recent studies using a combination of approaches, including genetics, physiology, and molecular biology, have contributed considerably to our understanding of ABA signal transduction. A number of proteins associated with ABA signaling and responses—especially ABA receptors—have been identified. ABA signal transduction initiates signal perception by ABA receptors and transfer via downstream proteins, including protein kinases and phosphatases. In the present review, we focus on the function of ABA in stomatal defense against biotic and abiotic stresses, through analysis of each ABA signal component and the relationships of these components in the complex network of interactions. In particular, two ABA signal pathway models in response to biotic and abiotic stress were proposed, from stress signaling to stomatal closure, involving the pyrabactin resistance (PYR)/PYR-like (PYL) or regulatory component of ABA receptor (RCAR) family proteins, 2C-type protein phosphatases, and SnRK2-type protein kinases.
Highlights
The plant hormone abscisic acid (ABA) regulates many key processes involved in plant development and adaptation to biotic and abiotic stresses
We focus on recent research into ABA responses to stomatal defense, and crosstalk of biotic and abiotic responses through the regulation of stomatal movement
The interactions between pyrabactin resistance (PYR)/PYL/regulatory component of ABA receptor (RCAR) proteins and their target PP2Cs induce the activation of downstream targets of the group A PP2Cs, including SnRK2 protein kinases (SnRK2.2, SnRK2.3, SnRK2.6) and the S-type anion channel (SLAC1); these in turn play key roles in the regulation of transcriptional response and stomatal closure [26,56,58]
Summary
Biotic and abiotic stresses adversely affect plant growth and induce severe losses in agricultural crop production. Plants produce and accumulate increased amounts of ABA in the guard cells, and this induces stomatal closure to conserve water. ABA biosynthesis and catabolism are known to be major determinants of endogenous ABA levels in plant cells [14,15]. The NCED3 gene is induced by drought stress and it upregulates endogenous ABA levels in overexpressed transgenic plants, thereby leading to lower transpiration rates [16,17,18,19,20]. Under conditions of biotic and abiotic stresses, ABA functions as a chemical messenger that induces stomatal closure through the activation and inactivation of ion channels by. The entire sequence of ABA signal transduction, from ABA receptors to stomatal closure, will be discussed later in this review
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