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 have been unclear.

A collaborative research led by Dr. Jian-Kang Zhu of the Shanghai Center for Plant Stress Biology revealed that the TOR kinase phosphorylates ABA receptors to prevent stress response inun-stressed conditions and to promote growth recovery after stresses subside. Under stress conditions, SnRK2s, downstream effectors of PYLs, phosphorylate Raptor, a regulatory component in the TOR complex, to inhibit growth by impeding TOR activity. The findings were published in the latest issue of Molecular Cell (2017).

In this study, scientists found that TOR kinase inhibits stress defense responses under unstressed conditions by phosphorylating ABA receptors at a conserved serine residue. The phosphorylation inhibits ABA perception and signaling by disrupting the interaction ofABA receptors withABA, and with downstream effector PP2C phosphatases. They also show that ABA-activated SnRK2 protein kinases phosphorylate Raptor, a component of the TOR complex. The phosphorylation promotes the disassociation of Raptor from TOR complex and therefore inhibits TOR activity under stress conditions. This study reveals a new function for the TOR complex: 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. The phosphorylation loop between the ABA core signaling components and the TOR complex represents a critical mechanism that balances stress and growth responses for plants to adapt to continuously changing environments (Figure).

Figure: Model Illustrating How TOR Kinase and PYL Phosphorylation Balances Growth and Stress Response in Plants

Contact:
Jian-Kang Zhu, Ph.D.
Senior Principal Investigator
Shanghai Center for Plant Stress Biology, Chinese Academy of Sciences
3888 Chenhua Road,
Research Center 325, Shanghai, China 201602
Email: jkzhu@sibs.ac.cn
Phone: (86) 21-57078201
http://www.psc.ac.cn