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 thalianaPAP 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.
Subject(s)
Arabidopsis/genetics , Cytokinins/pharmacology , Gene Expression Regulation, Plant/drug effects , Genes, Chloroplast , Arabidopsis/drug effects , Cell Nucleus/drug effects , Cell Nucleus/genetics , Chloroplasts/drug effects , Chloroplasts/genetics , Mutation/genetics , Photosynthesis/drug effects , RNA, Messenger/genetics , RNA, Messenger/metabolism , Signal Transduction/drug effects , Transcription, Genetic/drug effects , Zeatin/pharmacologyABSTRACT
Heat shock is one of the major abiotic factors that causes severe retardation in plant growth and development. To dissect the principal effects of hyperthermia on chloroplast gene expression, we studied the temporal dynamics of transcript accumulation for chloroplast-encoded genes in Arabidopsis thaliana and genes for the chloroplast transcription machinery against a background of changes in physiological parameters. A marked reduction in the transcript amounts of the majority of the genes at the early phases of heat shock (HS) was followed by a return to the baseline levels of rbcL and the housekeeping genes clpP, accD, rps14 and rrn16. The decline in the mRNA levels of trnE (for tRNAglu) and the PSI genes psaA and psaB was opposed by the transient increase in the transcript accumulation of ndhF and the PSII genes psbA, psbD, and psbN and their subsequent reduction with the development of stress. However, the up-regulation of PSII genes in response to elevated temperature was absent in the heat stress-sensitive mutants abi1 and abi2 with the impaired degradation of D2 protein. The expression of rpoA and rpoB, which encode subunits of PEP, was strongly down-regulated throughout the duration of the heat treatment. In addition, heat stress-induced PEP deficiency caused the compensatory up-regulation of the genes for the nuclear-encoded RNA polymerases RPOTp and RPOTmp, the PEP-associated proteins PAP6 and PAP8, the Ser/Thr protein kinase cPCK2, and the stress-inducible sigma factor gene SIG5. Thus, heat stress differentially modulates the transcript accumulation of plastid-encoded genes in A. thaliana at least in part via the expression of HS-responsive nuclear genes for the plastid transcription machinery.
Subject(s)
Gene Expression Regulation, Plant , Genes, Chloroplast/physiology , Arabidopsis/genetics , Arabidopsis/physiology , Carotenoids/metabolism , Chlorophyll/metabolism , Genes, Chloroplast/genetics , Heat-Shock Response , Photosystem II Protein Complex/metabolism , Real-Time Polymerase Chain ReactionABSTRACT
KEY MESSAGE: Cytokinin membrane receptors of the Arabidopsis thaliana AHK2 and AHK3 play opposite roles in the expression of plastid genes and genes for the plastid transcriptional machinery during leaf senescence Loss-of-function mutants of Arabidopsis thaliana were used to study the role of cytokinin receptors in the expression of chloroplast genes during leaf senescence. Accumulation of transcripts of several plastid-encoded genes is dependent on the ÐÐÐ2/ÐÐÐ3 receptor combination. ÐÐÐ2 is particularly important at the final stage of plant development and, unlike ÐÐÐ3, a positive regulator of leaf senescence. Cytokinin-dependent up-regulation of the nuclear encoded genes for chloroplast RNA polymerases RPOTp and RPOTmp suggests that the hormone controls plastid gene expression, at least in part, via the expression of nuclear genes for the plastid transcription machinery. This is further supported by cytokinin dependent regulation of genes for the nuclear encoded plastid σ-factors, SIG1-6, which code for components of the transcriptional apparatus in chloroplasts.
