Abstract
Cytokinins (CKs) are known to regulate the biogenesis of chloroplasts under changing environmental conditions and at different stages of plant ontogenesis. However, the underlying mechanisms are still poorly understood. Apparently, the mechanisms can be duplicated in several ways, including the influence of nuclear genes that determine the expression of plastome through the two-component CK regulatory circuit. In this study, we evaluated the role of cytokinins and CK signaling pathway on the expression of nuclear genes for plastid RNA polymerase-associated proteins (PAPs). Cytokinin induced the expression of all twelve Arabidopsis thaliana PAP genes irrespective of their functions via canonical CK signaling pathway but this regulation might be indirect taking into consideration their different functions and versatile structure of promoter regions. The disruption of PAP genes contributed to the abolishment of positive CK effect on the accumulation of the chloroplast gene transcripts and transcripts of the nuclear genes for plastid transcription machinery as can be judged from the analysis of pap1 and pap6 mutants. However, the CK regulatory circuit in the mutants remained practically unperturbed. Knock-out of PAP genes resulted in cytokinin overproduction as a consequence of the strong up-regulation of the genes for CK synthesis.
Highlights
Chloroplast development and function requires the coordinated transcriptional regulation of genes encoded by the nuclear and chloroplast genomes
Genome-wide transcript analyses revealed in Arabidopsis thaliana more than 100 cytokinin-responsive genes related to photosynthesis, in addition to a list of genes implicated in cytokinin-regulated chlorophyll biosynthesis and the expression of plastid genes [1,2,3]
To identify polymerase-associated proteins (PAPs) genes potentially regulated by CK, their promoter sequences were examined for the occurrence of CK-related cis-elements
Summary
Chloroplast development and function requires the coordinated transcriptional regulation of genes encoded by the nuclear and chloroplast genomes. Phytohormones, in particular cytokinins (CK), are among the main factors affecting differential expression of the genes encoding plastid or plastid-related proteins encoded by the nucleus. Genome-wide transcript analyses revealed in Arabidopsis thaliana more than 100 cytokinin-responsive genes related to photosynthesis, in addition to a list of genes implicated in cytokinin-regulated chlorophyll biosynthesis and the expression of plastid genes [1,2,3]. Cytokinins act on plastids via a multistep signaling system consisting of histidine kinases, histidine phosphotransfer proteins (AHP) and type-A and type-B response regulators (RRs). The system includes a subset of cytokinin response factors (CRFs) which are induced either directly by phosphorylated AHPs or by type-B RRs. The activated type-B RRs together with CRFs mediate CK-regulated gene expression through directly binding to specific sites on the promoter regions of CK-regulated genes [4]
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