Drought is the most important environmental stress limiting agricultural productivity and threatening world food security. Plants respond to drought stress by rapidly accumulating the phytohormone abscisic acid (ABA), which binds the PYR1/PYL/RCAR family of receptors to activate defense responses and inhibit growth.

When the stress subsides, plants must cease the defense responses and rapidly activate mechanisms to promote growth and recovery. However, the mechanisms regulating the physiological switch between these defense and growth states in plants are still unclear.

A research group led by Dr. ZHU Jiankang at Shanghai Center for Plant Stress Biology of Chinese Academy of Sciences revealed that the Target of Rapamycin (TOR) kinase phosphorylates ABA receptors to prevent stress response in un-stressed conditions and to promote growth recovery after stresses subside.

Researchers found that TOR kinase inhibits stress defense responses under unstressed conditions by phosphorylating ABA receptors at a conserved serine residue, and the phosphorylation inhibits ABA perception and signaling by disrupting the interaction of ABA receptors with ABA, and with downstream effector PP2C phosphatases.

They also showed that ABA-activated SnRK2 protein kinases phosphorylate Raptor, a component of the TOR complex, and the phosphorylation promotes the disassociation of Raptor from TOR complex and therefore inhibits TOR activity under stress conditions.

This study revealed a new function for the TOR complex that it acts as a negative modulator of ABA signaling and inhibitor of the stress response, in addition to its known role in growth enhancing processes such as protein synthesis regulation, metabolism, and proliferation.

Besides, the phosphorylation loop between the ABA core signaling components and the TOR complex was found to represent a critical mechanism that balances stress and growth responses for plants to adapt to continuously changing environments. The findings were published in Molecular Cell.