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Regulatory mechanism of abscisic acid signaling

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ABSTRACT

Abscisic acid (ABA) is a major phytohormone that mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Studies on ABA signaling have progressed rapidly since the recent discovery of PYR/PYL receptor proteins as soluble ABA receptors. In plant cells, the receptor receives ABA to inhibit the phosphatase activity of type 2C protein phosphatase (PP2C), which is the major negative regulator in ABA signaling. SNF1-related protein kinase 2 (SnRK2) is then released from negative regulation by PP2C, turning on ABA signals by the phosphorylation of downstream factors. Insights into the regulation of PYR/PYL receptor proteins is therefore required in order to control drought-stress tolerance in plants. This article reviews the regulatory mechanism of the ABA receptor by ABA and its selective agonist. Structural analyses of PYR/PYL receptors have clearly elucidated the mechanism of ABA perception of the receptor or the mechanism of interaction with PP2C that leads to inhibition of its phosphatase activity. Moreover, the structures of PYR/PYL receptors complexed with pyrabactin, a selective ABA agonist, have provided the structural basis of ABA agonism and antagonism.

No MeSH data available.


Structural basis of pyrabactin selectivity. (a) Chemical structure of pyrabactin. (b) Pyrabactin-induced gate closure of PYL1 (ABA agonism). The arrangement of pyrabactin in the ABA-binding cavity is defined by a contact between the pyridine ring and PYL1 I137 and a water-mediated hydrogen bond between the pyridyl nitrogen and PYL1 K86. The closed gate loop contacts the naphthalene ring of pyrabactin by van der Waals attractions. This panel was created using PDB coordinates of pyrabactin-bound PYL1 (3NEF). (c) Pyrabactin-binding mode of PYL2 with an open gate loop (ABA antagonism). Pyrabactin is located away from the gate loop by a water-mediated hydrogen bond between the sulfonamide group and PYL2 K64 in the ABA-binding cavity, which makes the gate loop stay in an open conformation. This panel was created using PDB coordinates of pyrabactin-bound PYL2 (3NS2).
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f5-7_123: Structural basis of pyrabactin selectivity. (a) Chemical structure of pyrabactin. (b) Pyrabactin-induced gate closure of PYL1 (ABA agonism). The arrangement of pyrabactin in the ABA-binding cavity is defined by a contact between the pyridine ring and PYL1 I137 and a water-mediated hydrogen bond between the pyridyl nitrogen and PYL1 K86. The closed gate loop contacts the naphthalene ring of pyrabactin by van der Waals attractions. This panel was created using PDB coordinates of pyrabactin-bound PYL1 (3NEF). (c) Pyrabactin-binding mode of PYL2 with an open gate loop (ABA antagonism). Pyrabactin is located away from the gate loop by a water-mediated hydrogen bond between the sulfonamide group and PYL2 K64 in the ABA-binding cavity, which makes the gate loop stay in an open conformation. This panel was created using PDB coordinates of pyrabactin-bound PYL2 (3NS2).

Mentions: Pyrabactin is a synthetic ABA agonist that has been used to identify PYR/PYL receptors8. The chemical structure of pyrabactin (Fig. 5a) is not similar to that of (+)ABA (Fig. 2c). ABA activates all PYR/PYL receptors to inhibit PP2C, whereas pyrabactin activates several receptors, such as PYR1 and PYL1, and inhibits others, such as PYL28. In other words, pyrabactin acts as a selective agonist for PYR1 and PYL1. As a result of pyrabactin’s selectivity, it inhibits seed germination but shows no transcriptional activation of an ABA-responsive gene in vegetative tissues8. Therefore, a robust framework is necessary for the design of new agonists capable of regulating ABA signaling globally. Several groups have reported the structures of ABA receptors bound to pyrabactin25–28, providing new insights into ABA agonism and antagonism to aid the rational design of ABA agonists.


Regulatory mechanism of abscisic acid signaling
Structural basis of pyrabactin selectivity. (a) Chemical structure of pyrabactin. (b) Pyrabactin-induced gate closure of PYL1 (ABA agonism). The arrangement of pyrabactin in the ABA-binding cavity is defined by a contact between the pyridine ring and PYL1 I137 and a water-mediated hydrogen bond between the pyridyl nitrogen and PYL1 K86. The closed gate loop contacts the naphthalene ring of pyrabactin by van der Waals attractions. This panel was created using PDB coordinates of pyrabactin-bound PYL1 (3NEF). (c) Pyrabactin-binding mode of PYL2 with an open gate loop (ABA antagonism). Pyrabactin is located away from the gate loop by a water-mediated hydrogen bond between the sulfonamide group and PYL2 K64 in the ABA-binding cavity, which makes the gate loop stay in an open conformation. This panel was created using PDB coordinates of pyrabactin-bound PYL2 (3NS2).
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Related In: Results  -  Collection

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f5-7_123: Structural basis of pyrabactin selectivity. (a) Chemical structure of pyrabactin. (b) Pyrabactin-induced gate closure of PYL1 (ABA agonism). The arrangement of pyrabactin in the ABA-binding cavity is defined by a contact between the pyridine ring and PYL1 I137 and a water-mediated hydrogen bond between the pyridyl nitrogen and PYL1 K86. The closed gate loop contacts the naphthalene ring of pyrabactin by van der Waals attractions. This panel was created using PDB coordinates of pyrabactin-bound PYL1 (3NEF). (c) Pyrabactin-binding mode of PYL2 with an open gate loop (ABA antagonism). Pyrabactin is located away from the gate loop by a water-mediated hydrogen bond between the sulfonamide group and PYL2 K64 in the ABA-binding cavity, which makes the gate loop stay in an open conformation. This panel was created using PDB coordinates of pyrabactin-bound PYL2 (3NS2).
Mentions: Pyrabactin is a synthetic ABA agonist that has been used to identify PYR/PYL receptors8. The chemical structure of pyrabactin (Fig. 5a) is not similar to that of (+)ABA (Fig. 2c). ABA activates all PYR/PYL receptors to inhibit PP2C, whereas pyrabactin activates several receptors, such as PYR1 and PYL1, and inhibits others, such as PYL28. In other words, pyrabactin acts as a selective agonist for PYR1 and PYL1. As a result of pyrabactin’s selectivity, it inhibits seed germination but shows no transcriptional activation of an ABA-responsive gene in vegetative tissues8. Therefore, a robust framework is necessary for the design of new agonists capable of regulating ABA signaling globally. Several groups have reported the structures of ABA receptors bound to pyrabactin25–28, providing new insights into ABA agonism and antagonism to aid the rational design of ABA agonists.

View Article: PubMed Central - PubMed

ABSTRACT

Abscisic acid (ABA) is a major phytohormone that mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Studies on ABA signaling have progressed rapidly since the recent discovery of PYR/PYL receptor proteins as soluble ABA receptors. In plant cells, the receptor receives ABA to inhibit the phosphatase activity of type 2C protein phosphatase (PP2C), which is the major negative regulator in ABA signaling. SNF1-related protein kinase 2 (SnRK2) is then released from negative regulation by PP2C, turning on ABA signals by the phosphorylation of downstream factors. Insights into the regulation of PYR/PYL receptor proteins is therefore required in order to control drought-stress tolerance in plants. This article reviews the regulatory mechanism of the ABA receptor by ABA and its selective agonist. Structural analyses of PYR/PYL receptors have clearly elucidated the mechanism of ABA perception of the receptor or the mechanism of interaction with PP2C that leads to inhibition of its phosphatase activity. Moreover, the structures of PYR/PYL receptors complexed with pyrabactin, a selective ABA agonist, have provided the structural basis of ABA agonism and antagonism.

No MeSH data available.