Many heat-upregulated genes (heat-shock factors and heat-shock proteins) were also reduced in the cka4 mutant. The expression levels of a number of genes in the ABA regulatory network were reduced in the cka4 mutant. The cka4 mutant was more sensitive to heat stress than the wild-type Columbia-0. The cka4 mutant showed reduced sensitivity to ABA during seed germination and seedling growth, and increased stomatal aperture and leaf water loss with a slightly reduced leaf ABA level.
CKA4 expression was upregulated by ABA and heat treatment. To understand the role of CK2 phosphorylation of chloroplast proteins in abiotic stress signalling, an Arabidopsis plastid CK2 ( CKA4) knockout mutant was investigated in terms of the plant response to abscisic acid (ABA) and heat stress. Plastid casein kinase 2 (CK2) is a major Ser/Thr-specific enzyme for protein phosphorylation in the chloroplast stroma and its kinase activity is regulated by redox signals.
