Arabidopsis CKA4Gene Of Plastid Casein Kinase2Involved In ABA And Heat Signaling

In recent years, it has been reported that protein phosphorylation regulatesimportant chloroplast processes such as photosynthesis or transcription. However,only very few chloroplast-localized protein kinases have been identified to date.Plastid casein kinase2(CK2) is a major Ser/Thr-specific enzyme accounting forprotein phosphorylation activity in the stroma of the chloroplast and its kinaseactivity is regulated by redox signals. To understand the role of CK2phosphor ylationof chloroplast proteins in abiotic stress signaling, the Arabidopsis plastid CK2(CKA4)knockout mutants were investigated for alterations in plant response to abscisic acid(ABA) and heat stress. CKA4expression was upregulated by ABA and heat treatments.The cka4mutant showed the increased stomatal aperture size，reduced ABA contentand leaf water loss rate and the reduced sensitivity to ABA, salt and osmotic stressduring seed germination. The cka4mutant was more sensitive to heat stress thanwild-type Col-0. The expression levels of a number of genes in the ABA regulatorynetwork were reduced in the cka4mutant. Many heat-upregulated genes (heat shockfactors and heat shock proteins) were also reduced in the cka4mutant. The cka4mutant showed the reduced expression levels of plastid-encoded RNA polymerasetarget genes (atpB and psbA). CKA4knockout mutation also resulted in reduction inexpression of some critical genes (PTM, ABI4and PRS1) involved in retrogradesignaling from the chloroplast to the nucleus. Similar results were observed in mutantplants with the knockout mutation in both CKA4and CKA3that encodes a nuclearCK2a3subunit. CKA3expression was not responsive to ABA and heat stress. Theseresults suggest that CKA4is an enhancing factor in abiotic stress signaling throughmodulating the expression of some molecular players in retrograde signaling.