Abscisic Acid (ABA)

Abstract

Abscisic acid (ABA) is a phytohormone that mediates responses to abiotic stress conditions and controls stomatal aperture. It is also involved in seed development and is associated with seed dormancy. ABA is biosynthesised in higher plants from carotenoid precursors and early steps take place in the chloroplast. ABA binds to PYRABACTIN RESISTANCE (PYR)/PYR1-like (PYL) receptor proteins that are localised in the cytosol and nucleus. The ABA–receptor complex binds to and inhibits specific PP2C protein phosphatases. Inhibition of these PP2Cs results in activation of SnRK2 protein kinases that in turn activate downstream signalling proteins resulting in both short-term changes in ion-channel activity and long-term effects on gene expression. ABA effects depend on influx and efflux carriers that facilitate movement between cells and through conductive tissue to coordinate responses in shoots and roots.

Key Concepts

• ABA reduces water use and causes adaptive responses to water stress.
• One of the most important effects of ABA is to reduce stomatal aperture.
• ABA acts by binding to PYR/PYL/RCAR receptors located in the cytoplasm and nucleus.
• ABA causes rapid changes in ion-channel activity and longer term changes in transcription factor activity leading to changes in gene expression.
• Movement of ABA between cells and tissues is regulated by influx and efflux carriers on cell surfaces.
• ABA is synthesised from carotenoid precursors and early steps in the pathway occur in the chloroplast.