The Glycosyltransferase QUA1 Regulates Chloroplast-Associated Calcium Signaling During Salt and Drought Stress in Arabidopsis

摘要

Cytoplasmic Ca2+ (Ca2+(cyt)) elevation induced by various signals is responsible for appropriate downstream responses. Through a genetic screen of Arabidopsis thaliana mutants defective in stress-induced Ca2+(cyt) elevation, the glycosyltransferase QUASIMODO1 (QUA1) was identified as a regulator of Ca2+ (cyt) in response to salt stress. Compared with the wild type, the qua1-4 mutant exhibited a dramatically greater increase in Ca2+(cyt) under NaCl treatment. Functional analysis showed that QUA1 is a novel chloroplast protein that regulates cytoplasmic Ca2+ signaling. QUA1 was detected in chloroplast thylakoids, and the qua1-4 mutant exhibited irregularly stacked grana. The observed greater increase in Ca2+(cyt) was inhibited upon recovery of chloroplast function in the qua1-4 mutant. Further analysis showed that CAS, a thylakoid-localized calcium sensor, also displayed irregularly stacked grana, and the chloroplasts of the qua1-4 cas-1 double mutant were similar to those of cas-1 plants. In QUA1-overexpressing plants, the protein level of CAS was decreased, and CAS was readily degraded under osmotic stress. When CAS was silenced in the qua1-4 mutant, the large Ca2+(cyt) increase was blocked, and the higher expression of PLC3 and PLC4 was suppressed. Under osmotic stress, the qua1-4 mutant showed an even greater elevation in Ca2+(cyt) and was hypersensitive to drought stress. However, this sensitivity was inhibited when the increase in Ca2+(cyt) was repressed in the qua1-4 mutant. Collectively, our data indicate that QUA1 may function in chloroplast-dependent calcium signaling under salt and drought stresses. Additionally, CAS may function downstream of QUA1 to mediate these processes.