Plant Physiology Preview
Published on May 12, 2006; 10.1104/pp.106.080069
Received March 8, 2006
Returned for revision April 2, 2006
Accepted May 4, 2006
The SCABRA3 nuclear gene encodes the plastid RpoTp RNA polymerase, which is required for chloroplast biogenesis and mesophyll cell proliferation in Arabidopsis
Andrea Hricová , Victor Quesada , and José Luis Micol *
División de Genética and Instituto de Bioingeniería, Universidad Miguel Hernández, Campus de Elche, 03202 Elche, Spain
* Corresponding author; email: jlmicol{at}umh.es.
In many plant species, a subset of the genes of the chloroplast genome is transcribed by RpoTp, a nuclear-encoded plastid-targeted RNA polymerase. Here, we describe the positional cloning of the SCABRA3 (SCA3) gene, which was found to encode RpoTp in 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 a quarter of which encoded chloroplast-targeted proteins. Quantitative RT-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.
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