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Leaf "commander" —— dynamic regulation, signal transduction and function of abscisic acid (ABA) in plants
Release time:
2025-03-11
In the early 1960s, researchers discovered abscisic acid (ABA), an important plant hormone, in cotton bolls that were about to abscise. As research on ABA continued to expand, its mysterious nature was gradually unveiled. Scientists found that ABA not only regulates plant growth and development but also plays a crucial role in plant responses to biotic and abiotic stresses.
Currently, research on ABA mainly focuses on four key areas: ABA biosynthesis and degradation, ABA transport, ABA signal transduction, and ABA function. This article will introduce ABA based on these four aspects.
ABA Biosynthesis and Degradation
The biosynthesis and metabolic degradation of ABA determine its cellular concentration, thereby influencing the strength of ABA signaling.
1. ABA Biosynthesis
Indirect ABA Biosynthesis Pathway (Higher Plants): ABA synthesis begins with methylerythritol phosphate (MEP) as the precursor. Through a series of complex steps, MEP is converted into an important intermediate, β-carotene. β-carotene then undergoes oxidation, epoxidation, and isomerization, ultimately forming the key intermediate xanthoxin. Xanthoxin is further oxidized in two steps to produce ABA.
Glycosyl Hydrolysis (ABA Release): ABA-glucose ester (ABA-GE) can be rapidly hydrolyzed by β-glucosidase (βG), releasing free ABA and thereby regulating the local ABA concentration within the cell.
2. ABA Degradation
Hydroxylation: ABA is hydroxylated by cytochrome P450 monooxygenases (CYP707A), forming 8′-hydroxy-ABA. This compound spontaneously isomerizes into phaseic acid (PA) in plants. PA is then further converted into dihydrophaseic acid (DPA) under the action of the ABH1 enzyme.
Glycosylation (ABA Storage): Under the catalysis of UDP-glucosyltransferases (UGTs), the carboxyl group of free ABA covalently binds to a glucose moiety, forming ABA-glucose ester (ABA-GE), thereby rendering it inactive.
Figure 1: ABA Metabolism in Higher Plants
Enzymes are marked in red.
ZEP: Zeaxanthin epoxidase;NSY: Neoxanthin synthase;NCED: 9-cis-epoxycarotenoid dioxygenase;XD: Xanthoxin dehydrogenase;ABAO: Abscisic aldehyde oxidase;CYP707A: ABA 8'-hydroxylase;ABH1: Phaseic acid reductase 1;ABAGT: ABA glucosyltransferase;βG: β-glucosidase;
Inhibitors of enzymes are marked in blue.
(+)-9'-AABA: (+)-9'-Acetylenic ABA;AHI4: ABA 8'-hydroxylase inhibitor 4;(+)-8'-MABA: (+)-8'-Methylidene ABA;NDGA: Nordihydroguaiaretic acid;SLCCD13: Sesquiterpene-like carotenoid cleavage dioxygenase inhibitor 13
ABA Transport
Studies have shown that ABA undergoes both short-distance and long-distance transport within plants. This includes local ABA distribution in roots and leaves, as well as long-distance transport between roots and shoot apices, shoot apices and roots, and leaves and seeds.
At the molecular level, ABA is transported between cells and tissues via transporter proteins, including the ABCG transporter family, NRT1/PTR family, MATE-DTX family, and AWPM-19 family proteins. Since the mechanism of ABA transport is not yet fully understood, this process has received increasing research attention in recent years.
In 2021, a study published in Science Advances (IF = 11.7), titled "Leaf-derived ABA regulates rice seed development via a transporter-mediated and temperature-sensitive mechanism," identified a novel ABA transporter protein, DG1, in rice using forward genetics. The dg1 mutant exhibited significantly lower ABA levels in the spikelet compared to the wild type. Based on this, the researchers conducted a series of genetic, physiological, biochemical, and ABA isotope-labeling experiments, confirming that ABA in the spikelet originates from the leaves. They further elucidated the molecular mechanism by which the DG1 protein regulates long-distance ABA transport (see Figure 2).
Molecular Mechanism of DG1-Mediated Long-Distance ABA Transport (Figure 2)
The DG1 protein transports ABA synthesized in the leaves into the phloem. ABA then moves through the phloem along with the transpiration stream to reach the enlarged vascular bundles (EVB). From there, ABA is transferred to the parenchyma cells via unknown transporters.
Within parenchyma cells, ABA is further transported by DG1 and other minorABA transporters to the diffuse vascular bundles (DVB) and the stem. In the stem, ABA continues its movement via the transpiration stream, reaching the pedicel (spikelet base) region. Here, DG1 facilitates the transfer of ABA into the spikelet.
Once leaf-derived ABA reaches the spikelet, it directly activates the expressionof key genes required for starch biosynthesis, thereby regulating seed development. The efficiency of this transport system increases with rising temperatures.
ABA Signal Transduction
Upon ABA perception, ABA receptors (PYR/PYL/RCAR) bind to and inhibit Group A 2C-type protein phosphatases (PP2C), thereby suppressing their activity. This inhibition leads to the activation of kinases previously repressed by PP2C, such as Subclass III Sucrose Non-Fermenting-1-Related Protein Kinases 2 (SnRK2s) (Figure 3).
SnRK2 targets can be categorized into two main types: membrane channel proteins and transcription factors.
Membrane channel proteins include slow anion channel 1 (SLAC1), Arabidopsis potassium channel 1 (KAT1), and NADPH oxidase respiratory burst oxidase homolog F (RBOHF).
Transcription factors include basic leucine zipper (bZIP) domain-containing transcription factors, such as ABF, AREB, and ABI5.
Figure 3: Model of the ABA Signal Transduction Mechanism
A 2021 study published in Nature Communications (IF = 16.6), titled "Initiation and amplification of SnRK2 activation in ABA signaling," investigated the initiation and amplification mechanisms of SnRK2 activation in ABA signaling.
To explore this, researchers constructed multiple mutants targeting the B2 and B3 subgroups of RAF kinases. They then conducted ABA sensitivity assays, germination and growth studies under high ABA concentrations, as well as phenotypic analyses related to stomatal movement, growth and development, and stress responses using the B2 and B3 subgroup RAF multi-mutant (OK100-nonu), the SnRK2.2/3/6 triple mutant, and pyl multi-mutants.
Through these experiments, they confirmed that the B2 and B3 subgroup RAF kinases are core components of the ABA signaling pathway and proposed a model for the activation-amplification mechanism of ABA signaling (Figure 4).
Figure 4: Model of the Activation-Amplification Mechanism in ABA Signaling
Left Panel: Under normal growth conditions, PP2C binds to and inhibits SnRK2 activity through dephosphorylation. Since there is no stress, the B2 and B3 subgroup RAF kinases exhibit only basal levels of phosphorylation and activity, mainly functioning to maintain SnRK2 activity and ABA signal transduction.
Middle Panel: In the presence of ABA but under non-stress conditions, ABA binds to its receptors PYR/PYL/RCARs (PYL), forming an ABA-PYL complex, which then binds to and inhibits PP2C activity. As a result, SnRK2 is released from PP2C inhibition and rapidly phosphorylated by RAF kinases with basal activity, thereby initiating ABA signal transduction. Under stress conditions, RAF kinases are further activated through an unknown mechanism, leading to increased phosphorylation and activity levels.
Right Panel: Activated SnRK2 undergoes self-phosphorylation, further enhancing its activity. Additionally, SnRK2 can phosphorylate other SnRK2 molecules, amplifying the ABA signaling response. Meanwhile, activated SnRK2 phosphorylates downstream targets, regulating the expression of ABA-responsive genes.
Functions of Abscisic Acid (ABA)
Abscisic acid (ABA) plays multiple roles in plants, with its most prominent functions being the regulation of plant growth and development, as well as the response to abiotic stress. ABA is involved in seed dormancy maintenance, inhibition of seed germination, vegetative growth, bud dormancy, stomatal movement, embryo maturation, leaf senescence, and responses to salt, cold, and osmotic stress. Below, we introduce ABA's functions through two high-quality research articles.
1. Seed Germination
A 2023 study published in Plant Communications (IF = 9.4), titled “ABI5-FLZ13 Module Transcriptionally Represses Growth-related Genes to Delay Seed Germination in Response to ABA”, investigated the role of ABA in seed germination.
Using protein labeling techniques, the researchers identified neighboring proteins of ABI5, leading to the discovery of a novel ABI5-interacting protein, FLZ13.
Functional analysis revealed that FLZ13 positively regulates ABA signaling.
Transcriptomic analysis showed that ABI5 and FLZ13 jointly regulate the expression of numerous ABA-repressed genes.
Further genetic studies indicated that ABI5 and FLZ13 are partially functionally dependent on each other, collectively regulating ABA response and ABA-inhibited gene expression.
This study elucidated the transcriptional regulatory mechanism of ABI5 in seed germination and early seedling development, as well as the biological function of FLZ13 (Figure 5).
These findings enhance our understanding of how ABA inhibits plant growth and development, providing a theoretical basis for plant genetic improvement via the ABA signaling pathway.
Figure 5: Mechanistic Model of ABI5-FLZ13 in Seed Germination and Seedling Establishment
Under favorable conditions, after seed germination, ABA levels and ABI5 activity decrease, reducing the repression of downstream growth-related genes, thereby promoting normal germination and seedling establishment.
However, if germinating seeds are exposed to unfavorable conditions, increased ABA levels activate ABI5, and the ABI5-FLZ13 module suppresses the transcription of growth-related genes, preventing seedling establishment.
2. Abiotic Stress
A 2024 study published in Journal of Hazardous Materials (IF = 12.2), titled “Ammonium mitigates cadmium toxicity by activating the bZIP20-APX2/CATA transcriptional module in rice seedlings in an ABA-dependent manner”, explored how different nitrogen fertilizers (nitrate vs. ammonium) influence cadmium (Cd) toxicity in rice roots.
By analyzing root length, cadmium content, peroxide levels, ABA biosynthesis gene expression, and ABA content (ABA content was measured by VIBARAY), the study found that ammonium fertilization promotes ABA biosynthesis, alleviates cadmium toxicity, and enhances Cd tolerance.
Using transcriptomic analysis, the researchers identified OsbZIP20 as a key transcription factor that is upregulated in response to ABA.
Protein interaction and genetic transformation experiments revealed that OsbZIP20 directly binds to and activates antioxidant-related genes.
The study also demonstrated that OsSAPK9-mediated phosphorylation of OsbZIP20 is a critical step for its transcriptional activation function.
Phosphorylated OsbZIP20 more effectively activates the expression of downstream antioxidant genes.
Finally, through protein interaction and genetic transformation techniques, the study identified interacting proteins of OsbZIP20, providing molecular insights into how NH₄⁺ alleviates Cd²⁺ toxicity (Figure 6).
Figure 6: Molecular Mechanism of NH₄⁺ Mitigating Rice Response to Cd²⁺ Stress by Activating ABA-Dependent Signaling Pathway
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