ABA Biosynthesis and Signaling

ABA regulates a variety of divergent processes in plants from seed development and prevention of viviparity to plants’ response to osmotic stress. The latter is the best known of all the functions attributed to ABA, where ABA causes stomatal closure to reduce transpirational water loss. Since ABA is essential for mediating plants’ response to drought stress, which is crucial for growth and survival under limited water, ABA would have played a pivotal evolutionary role in plants’ ability to colonize land. Dissecting the role of ABA in perception and response to osmotic stress is fundamental to breeding crop varieties that remain productive under increased incidence of drought, and is highly relevant given the current scarcity of arable land in the midst of climate change.

In the current Plant Reactome beta version we have incorporated a curated pathway of ABA biosynthesis and signaling in rice. As illustrated therein ABA is synthesized via the isoprenoid pathway where a carotenoid deoxygenase enzyme, located in the plastids cleave the C40 neoxanthin to form Xanthoxin, a carotenoid dedicated to ABA biosynthesis. Xanthoxin then moves to the cytosol and via couple more enzymatic catalytic steps to form ABA.

ABA receptors initiate the relay of protein-protein interactions that provide the mechanistic connection between ABA and the ABA induced gene expression, which in turn mediates plants physiological response. ABA signaling can be conceptually reduced to four components. At the default state, (1) serine-threonine kinases such as OST1/SnRK2E are kept inactive by the protein phosphatases PP2C, (2) after ABA perception by a cytosolic ABA receptor (RCAR /pyrabactin resistance-like protein (PYL)-family ), the phosphatase activity of the PP2C is suppressed, (3) leading to, autophosphorylation and activation of OST1/SnRK2E (4) consequently, OST1/SNRK2E phosphorylate the basic leucine zipper transcription factors (ABA-responsive binding factors) which binds to ABA responsive promoter elements resulting in ABA induced gene expression, furthermore, the first three components above leading to de-repression of the OST1 can activate the slow anion channel 1 (SLAC1) and the potassium channel KAT1 by phosphorylation, leading to stomatal closure.

An interesting point to ponder here is that PP2C is a negative regulator of ABA response and therefore would have evolved to strike a balance to between ‘complete shut-down’ and adaptation, under for e.g drought stress, and since the members of PP2C gene family exhibit both tissue specificity and specificity in its response to the level of ABA signal, PP2C could be a potential target for finding candidate variants in crop improvement for drought tolerance.