Abstract
The Arabidopsis genome is methylated in CG and non-CG (CHG, and CHH in which H stands for A, T, or C) sequence contexts. DNA methylation has been suggested to be critical for seed development, and CHH methylation patterns change during stratification and germination. In plants, CHH methylation occurs mainly through the RNA-directed DNA methylation (RdDM) pathway. To test for an involvement of the RdDM pathway in chloroplast development, we analyzed seedling greening and the maximum quantum yield of photosystem II (Fv/Fm) in Arabidopsis thaliana seedlings perturbed in components of that pathway. Neither seedling greening nor Fv/Fm in seedlings and adult plants were affected in this comprehensive set of mutants, indicating that alterations in the RdDM pathway do not affect chloroplast development. Application of inhibitors like lincomycin or norflurazon inhibits greening of seedlings and represses the expression of photosynthesis-related genes including LIGHT HARVESTING CHLOROPHYLL A/B BINDING PROTEIN1.2 (LHCB1.2) in the nucleus. Our results indicate that the LHCB1.2 promoter is poorly methylated under both control conditions and after inhibitor treatment. Therefore no correlation between LHCB1.2 mRNA transcription and methylation changes of the LHCB1.2 promoter could be established. Moreover, we conclude that perturbations in the RdDM pathway do not interfere with gun signaling.
Highlights
Chloroplasts are the hallmark of algae and plants
According to the current view, the canonical RNA-directed DNA methylation (RdDM) pathway is initiated by RNA polymerase IV (POL IV)-mediated transcription of noncoding RNAs, which is followed by RNA-DEPENDENT RNA POLYMERASE 2 (RDR2)-dependent copying of the transcript to generate a double-stranded RNA
One such proposal concerns the potential relationship between DNA methylation changes, chloroplast development and the expression of nucleus-encoded photosynthesis genes
Summary
Chloroplasts are the hallmark of algae and plants. In addition to their most prominent role as organelles of photosynthesis, they host numerous metabolic reactions involving amino acids, lipids, sulfur, isoprenoids, vitamins, as well as the production of some hormones or their precursors[1]. The expression of plastid-encoded genes is not sensitive to DNA m ethylation[15], but the nuclear genome of Arabidopsis is methylated in both CG and non-CG (CHG, and CHH in which H stands for A, T, or C) sequence contexts, and the global DNA methylation state is defined by de novo methylation, maintenance of methylation and active d emethylation[16]. Despite the critical significance of chloroplast biogenesis for plant development and performance, our understanding of how functional chloroplasts are initially established remains incomplete, and new layers of control are still being considered This holds and especially true for the biogenic type of genomes uncoupled (gun) signaling. The results described below show that RdDM-mediated DNA methylation changes have no influence on chloroplast development or the expression of nuclear marker genes of retrograde signaling
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