Why is abscisic acid also known as stress hormone?

Abscisic acid (ABA) is often referred to as the "stress hormone" in plants due to its critical role in mediating responses to various environmental stresses. Several factors contribute to this characterization:

1. Drought Stress: ABA levels in plants increase significantly in response to drought stress. It helps plants cope with water scarcity by promoting stomatal closure, reducing water loss through transpiration, and maintaining cellular water balance. Stomatal closure induced by ABA conserves water but also reduces CO2 uptake for photosynthesis, which is a trade-off mechanism to balance water conservation with the need for carbon assimilation.

2. Salt Stress: High soil salinity can disrupt the osmotic balance of plant cells, leading to water loss and cellular dehydration. ABA helps plants respond to salt stress by regulating ion uptake and transport, promoting ion homeostasis, and minimizing water loss through stomatal closure.

3. Cold Stress: Exposure to low temperatures can cause cellular damage and impair physiological processes in plants. ABA accumulation in response to cold stress helps plants acclimate to cold conditions by inducing the expression of genes involved in cold tolerance, adjusting membrane fluidity, and enhancing antioxidant defense mechanisms.

4. Heat Stress: High temperatures can also pose significant challenges to plant growth and survival. ABA plays a role in heat stress responses by regulating stomatal closure, preventing excessive water loss, and modulating the expression of heat shock proteins and other stress-responsive genes.

5. Oxidative Stress: Environmental factors such as pollutants, pathogens, and excess light can generate reactive oxygen species (ROS) in plant cells, causing oxidative damage to biomolecules. ABA helps plants mitigate oxidative stress by regulating antioxidant enzyme activities, scavenging ROS, and modulating the expression of stress-responsive genes involved in antioxidant defense pathways.

Overall, abscisic acid acts as a signaling molecule that integrates environmental cues and coordinates adaptive responses to various stresses encountered by plants. Its role in mediating stress responses has earned it the designation of "stress hormone" in the context of plant physiology.