The SCABRA3 Nuclear Gene Encodes the Plastid RpoTp RNA Polymerase, Which Is Required for Chloroplast Biogenesis and Mesophyll Cell Proliferation in Arabidopsis

Abstract

In many plant species, a subset of the genes of the chloroplast genome is transcribed by RpoTp, a nuclear-encoded plastidtargeted RNA polymerase. Here, we describe the positional cloning of the SCABRA3 (SCA3) gene, which was found to encode RpoTp in Arabidopsis (Arabidopsis thaliana). We studied one weak (sca3-1) and two strong (sca3-2 and sca3-3) alleles of the SCA3 gene, the latter two showing severely impaired plant growth and reduced pigmentation of the cotyledons, leaves, stem, and sepals, all of which were pale green. The leaf surface was extremely crumpled in the sca3 mutants, although epidermal cell size and morphology were not perturbed, whereas the mesophyll cells were less densely packed and more irregular in shape than in the wild type. A significant reduction in the size, morphology, and number of chloroplasts was observed in homozygous sca3-2 individuals whose photoautotrophic growth was consequently perturbed. Microarray analysis showed that several hundred nuclear genes were differentially expressed in sca3-2 and the wild type, about one-fourth of which encoded chloroplast-targeted proteins. Quantitative reverse transcription-PCR analyses showed that the sca3-2 mutation alters the expression of the rpoB, rpoC1, clpP, and accD plastid genes and the SCA3 paralogs RpoTm and RpoTmp, which respectively encode nuclear-encoded mitochondrion or dually targeted RNA polymerases. Double-mutant analysis indicated that RpoTmp and SCA3 play redundant functions in plant development. Our findings support a role for plastids in leaf morphogenesis and indicate that RpoTp is required for mesophyll cell proliferation.

Arabidopsis Phage-Type RNA Polymerases: Accurate in Vitro Transcription of Organellar Genes

“…This suggested function is not supported by our previous study demonstrating that RpoTm and RpoTmp genes exhibit fully overlapping expression patterns in different tissues and at different developmental stages in Arabidopsis (Emanuel et al, 2006). From a phenotypical comparison of RpoTp-defective, RpoTmp-defective, and double mutant plants, Hricova et al (2006) concluded RpoTp and RpoTmp to have redundant functions in plant development.…”

Periodic root branching in Arabidopsis requires synthesis of an uncharacterized carotenoid derivative

“…1 E and F). Mutation of a nuclearencoded, plastid-targeted RNA polymerase SCABRA 3 (SCA3) (32), which is required for chloroplast biogenesis and has an indirect impact on carotenoid content, also showed similarly reduced LR capacity ( Fig. 1 E and F).…”

Intraplastidial trafficking of a phage-type RNA polymerase is mediated by a thylakoid RING-H2 protein

“…However, only two genes (RPOTp and RPOTm) have been detected in monocotyledons (4,7,8), thus raising the question of the function of RPOTmp in plastids of dicotyledon plants. Recent results have shown that RPOTp can be regarded as the principal NEP enzyme in chloroplasts playing an important role in chloroplast transcription, biogenesis, and mesophyll cell proliferation (9,10). Results obtained with RPOTmp indicate partial replacement of RPOTp in RPOTp mutants (10,11), an early function in lightinduced accumulation of several plastid mRNAs (12,13), and specific transcription of the rrn operon of Arabidopsis from the PC promoter during seed imbibition and germination (10).…”