Unveiling the crucial roles of abscisic acid in plant physiology: implications for enhancing stress tolerance and productivity

Abstract

Abscisic acid (ABA), one of the six major plant hormones, plays an essential and irreplaceable role in numerous physiological and biochemical processes during normal plant growth and in response to abiotic stresses. It is a key factor in balancing endogenous hormones and regulating growth metabolism in plants. The level of ABA is intricately regulated through complex mechanisms involving biosynthesis, catabolism, and transport. The functionality of ABA is mediated through a series of signal transduction pathways, primarily involving core components such as the ABA receptors PYR/PYL/RCAR, PP2C, and SnRK2. Over the past 50 years since its discovery, most of the genes involved in ABA biosynthesis, catabolism, and transport have been characterized, and the network of signaling pathways has gradually become clearer. Extensive research indicates that externally increasing ABA levels and activating the ABA signaling pathway through molecular biology techniques significantly enhance plant tolerance to abiotic stresses and improve plant productivity under adverse environmental conditions. Therefore, elucidating the roles of ABA in various physiological processes of plants and deciphering the signaling regulatory network of ABA can provide a theoretical basis and guidance for addressing key issues such as improving crop quality, yield, and stress resistance.

Keywords: abscisic acid; biosynthesis; catabolism; molecular mechanisms; signaling.

Figure 1

Structure of abscisic acid (ABA). ABA has a chemical formula of C₁₅H₂₀O₄ and features a molecular structure comprising an acrylic acid group and an isopentenyl alcohol group. Additionally, it contains a carboxyl group (–COOH) and a hydroxyl group (–OH). The biologically active form primarily exists as dextrorotatory (+)-S-ABA. Overall, the structure of abscisic acid is relatively straightforward, yet it plays a crucial regulatory role in plant growth and adaptation to the environment.

Figure 2

The history of abscisic acid (ABA) research development. 1950s to 1960s: discovery and early research on ABA. Hemberg discovered a plant growth inhibitor that was soluble in water and ether, initiating preliminary studies on physiological effects of ABA (Hemberg, 1949a, 1949b). It was found that ABA plays a crucial regulatory role in processes such as seed dormancy and germination, root growth, water regulation, flowering, and fruit ripening (Eagles and Wareing, 1963; Ohkuma et al., 1963). 1970s to 1990s: research on aba biosynthesis and metabolism. Following the discovery of Arabidopsis mutants defective in ABA biosynthesis pathways (Koornneef et al., 1982), a combination of molecular biology, genetic engineering, and forward and reverse genetics gradually revealed the pathways involved in ABA biosynthesis (Guiltinan et al., 1990; Anderberg and Walker-Simmons, 1992; Leung et al., 1994; Meyer et al., 1994; Tan et al., 1997). Until now: advancements in molecular biology and beyond. Since the 2000s, significant progress has been made in the study of ABA biosynthesis, metabolism, receptors, and signaling pathways, owing to advancements in molecular biology and bioinformatics technologies. Particularly, the identification of ABA receptors has linked plant perception of ABA to core signaling components for the first time (Ma et al., 2009; Park et al., 2009; Kuromori et al., 2010). Through techniques such as genetic engineering, genomics, and proteomics, the complexity and diversity of ABA signaling pathways have been unveiled. Researchers have also explored the regulatory mechanisms of ABA in plant growth, development, stress responses, and stress tolerance, providing crucial theoretical foundations for plant biology and agricultural production.

Figure 3

Biosynthesis and catabolism metabolic pathways of abscisic acid (ABA) in higher plants. The biosynthesis and catabolism pathways of ABA represent pivotal aspects of plant physiology research. ABA biosynthesis predominantly occurs in plastids and cytosol, with C40 β-carotenoids serving as the primary precursors. 9-cis-Cyclocarotenoid dioxygenase (NCED) acts as a crucial rate-limiting enzyme in the synthesis pathway. Additionally, hydrolysis of ABA-GE provides free ABA, constituting another essential mechanism for regulating ABA concentration in plants. ABA catabolism primarily involves two pathways: hydroxylation and glucosylation. With the assistance of enzymes, ABA is metabolized into inactive metabolites, which no longer exhibit biological activity in plant growth and development.

Figure 4

The abscisic acid (ABA)-dependent ABA receptor “Gate-Latch-Lock” model. The “Gate-Latch-Lock” model elegantly describes how ABA binding induces structural changes in its receptor, facilitating precise regulation of the ABA signaling pathway. Gate: In the absence of ABA, the receptor’s gate remains closed, preventing interaction with downstream signaling components. This gate is maintained in a closed position by the structural configuration of the receptor. Latch: When ABA binds to the receptor, it acts like a key that releases the latch. The binding of ABA induces a conformational change in the receptor, causing the latch to release and allowing the gate to open. This conformational change is essential for the activation of the receptor. Lock: Once the gate is open and the latch is released, the receptor can interact with downstream proteins, such as protein phosphatases. The binding of ABA effectively “locks” the receptor into an active conformation, ensuring that the signaling pathway remains activated as long as ABA is present.

Figure 5

Abscisic acid (ABA) transport system in leaf. During stress conditions, ABCG25 and ABCG40, as key ABA transport proteins, play crucial roles. ABCG25 is responsible for transporting ABA from the vascular bundle to the extracellular region surrounding the guard cells. By transporting ABA to this area, ABCG25 facilitates subsequent uptake by the guard cells. ABCG40, however, is responsible for the uptake of ABA from the extracellular space into the guard cells. By absorbing ABA, ABCG40 promotes the accumulation of ABA within the guard cells. The concerted action of ABCG25 and ABCG40 transports ABA from the vascular bundle to the extracellular region surrounding the guard cells, ultimately leading to stomatal closure. Stomatal closure is a critical physiological response of plants to stress, as it helps conserve water by reducing transpiration.