Abscisic acid (ABA), as one of the major phytohormones, participates in myriad physiological processes, including plant growth and development, biotic and abiotic stress responses. The increased level of ABA induced by adverse environmental stress helps plant to response and deal with the environmental challenges. ABA perception is a key step to activate ABA signaling.
ABA receptor proteins are recently studied to be degraded by the 26S proteasome pathway and by vacuolar degradation after processing via endosomal sorting complex required for transport (ESCRT) proteins. Despite their known essentiality in ABA signaling, the upstream regulators of ESCRTs in plants remain unknown.
Recently, a research team led by Prof. XIE Qi from the Institute of Genetics and Developmental Biology of the Chinese Academy of Sciences revealed how the ESCRT-I component VPS23A was controlled by upstream factors to finely modulate the protein level of ABA receptors in ABA signaling.
In this study, the researchers found that the ESCRT-I component VPS23A is an unstable protein that is degraded via the ubiquitin-proteasome system. VPS23A can be modified by K48-linked ubiquitin chains, and K48-linked ubiquitin chains modification often leads protein degradation through 26S proteasome.
XBAT35, a RING type E3 ubiquitin ligase, was identified by IP/LC-MS-MS as an interacting candidate of VPS23A. Phenotype analysis indicated that XBAT35 is a positive regulator of responses to ABA and drought in plants, which is opposite to VPS23A, suggesting XBAT35 may negatively regulate VPS23A by ubiquitination.
A series of biochemical experiments proved that XBAT35 accelerates the degradation of VPS23A, and directly ubiquitinates VPS23A. This indicates that VPS23A is a degradative substrate of the E3 ubiquitin ligase XBAT35. Further experiments analysis revealed that ABA promotes the degradation of VPS23A by XBAT35, thus release the inhibition of ABA receptor PYL4 by VPS23A to activate ABA signaling.
Beyond offering the first demonstration of how an ESCRT component is regulated in plants, this study deepens our understanding of plant stress responses by illustrating a mechanism wherein crosstalk between the ubiquitin-proteasome system (UPS) and endosome-vacuole mediated degradation pathways controls ABA signaling.
This research was supported by the National Key R&D Program of China and the National Natural Science Foundation of China.
XBAT35 Alters ABA Signaling via the VPS23A/PYL4 Complex (Image by IGDB)