Biology – Homeostasis in plants | e-Consult
Homeostasis in plants (1 questions)
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During water stress, plants accumulate abscisic acid (ABA), a plant hormone that plays a crucial role in stomatal closure. ABA's primary function is to trigger a cascade of events leading to the efflux of potassium ions (K+) from guard cells. This K+ efflux is a key step in initiating stomatal closure. Here's a detailed breakdown of the process:
- ABA Perception: When ABA levels rise, it enters the guard cell and binds to specific ABA receptors located on the plasma membrane. These receptors are part of a signaling complex.
- Activation of Phospholipase C (PLC): ABA binding activates a receptor-mediated signaling pathway that triggers the activation of phospholipase C (PLC).
- Inositol Trisphosphate (IP3) and Diacylglycerol (DAG) Production: PLC hydrolyzes phosphatidylinositol bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG).
- IP3-mediated Calcium Release: IP3 binds to IP3 receptors on the endoplasmic reticulum (ER), causing the release of calcium ions (Ca2+) from the ER stores into the cytoplasm. This increase in cytosolic calcium is a critical second messenger.
- Calcium-Dependent Signaling: The increase in cytosolic Ca2+ activates various downstream signaling pathways. Calcium ions bind to specific calcium-binding proteins, including calcium-dependent protein kinases (CDPKs).
- Potassium Efflux: CDPKs phosphorylate proteins involved in regulating ion channels in the plasma membrane of guard cells. This phosphorylation leads to the activation of anion channels (e.g., Cl-) and the inhibition of potassium channels. The inhibition of potassium channels reduces the potassium gradient across the plasma membrane, leading to K+ efflux.
- Water Potential Changes and Stomatal Closure: The efflux of potassium ions creates an electrochemical gradient that drives water out of the guard cells. This loss of water reduces the water potential inside the guard cells, causing them to lose turgor pressure. As turgor pressure decreases, the guard cells become flaccid, and the stomatal pore closes.
Therefore, ABA initiates a signaling cascade that ultimately leads to the efflux of potassium ions from guard cells, causing a decrease in water potential and stomatal closure. The increase in cytosolic calcium acts as a crucial second messenger, amplifying the initial ABA signal and activating downstream signaling pathways.