Arabidopsis RUGOSA2 encodes an mTERF family member required for mitochondrion, chloroplast and leaf development

Abstract

SUMMARYLittle is known about the mechanisms that control transcription of the mitochondrial and chloroplastic genomes, and their interplay within plant cells. Here, we describe the positional cloning of the Arabidopsis RUG2 gene, which encodes a protein that is dual-targeted to mitochondria and chloroplasts, and is homologous with the metazoan mitochondrial transcription termination factors (mTERFs). In the loss-offunction rug2 mutants, most organs were pale and showed reduced growth, and the leaves exhibited both green and pale sectors, with the latter containing sparsely packed mesophyll cells. Chloroplast and mitochondrion development were strongly perturbed in the rug2-1 mutant, particularly in pale leaf sectors, in which chloroplasts were abnormally shaped and reduced in number, thereby impairing photoautotrophic growth. As expected from the pleiotropic phenotypes caused by its loss-of-function alleles, the RUG2 gene was ubiquitously expressed. In a microarray analysis of the mitochondrial and chloroplastic genomes, 56 genes were differentially expressed between rug2-1 and the wild type: most mitochondrial genes were downregulated, whereas the majority of the chloroplastic genes were upregulated. Quantitative RT-PCR analyses showed that the rug2-1 mutation specifically increases expression of the RpoTp nuclear gene, which encodes chloroplastic RNA polymerase. Therefore, the RUG2 nuclear gene seems to be crucial for the maintenance of the correct levels of transcripts in the mitochondria and chloroplasts, which is essential for optimized functions of these organelles and proper plant development. Our results highlight the complexity of the functional interaction between these two organelles and the nucleus.

Leaf Development

“…A retrograde signal from the chloroplast to the nuclei may be involved in this mechanism. Mutational analyses also indicate that mitochondrion-and/or chloroplast-dependent signals play roles in the development of leaf architecture in Arabidopsis (e.g., Wetzel et al 1994;Hricová et al 2006;Quesada et al 2011). But, even in plants treated with norflurazon, the onset of cell expansion in leaves is delayed although not cancelled (Andriankaja et al 2012), suggesting that other components are also important in the phase shift.…”

Influence of plastids on light signalling and development

“…For example, the yellow and white sectors of variegated mutants had abnormal palisade cells [10,61,62] and an attenuated cell division and cell expansion response of palisade cells to excess light [63]. The green sectors of variegated leaves resembled wild-type leaves grown in optimal conditions [62] or resembled wild-type leaves exposed to excess light [64]. Additionally, immutans (im) had variegated leaves, attenuated biogenesis of non-photosynthetic plastids and short roots.…”

“…Thus, these mutant alleles were suggested to disrupt plastid-to-nucleus signalling mechanisms that contribute to leaf development [10,61]. For example, the yellow and white sectors of variegated mutants had abnormal palisade cells [10,61,62] and an attenuated cell division and cell expansion response of palisade cells to excess light [63]. The green sectors of variegated leaves resembled wild-type leaves grown in optimal conditions [62] or resembled wild-type leaves exposed to excess light [64].…”

Nucleoid-Enriched Proteomes in Developing Plastids and Chloroplasts from Maize Leaves: A New Conceptual Framework for Nucleoid Functions

“…The Arabidopsis genome encodes 35 mTERF proteins, and GFP fusion studies suggested that 11 of these are plastid localized, 17 are localized to mitochondria, while the localization of the remaining seven has not been determined (Babiychuk et al, 2011). Several plastid-localized mTERF proteins (At4g02990, BSM; At2g03050, SOLDAT10; At2g21710, EMB2219; At4g02990, RUGOSA2) have been genetically characterized in Arabidopsis; the null mutants have pale-green or embryo-lethal phenotypes with defects in plastid gene expression, but the precise molecular functions are unclear (Tzafrir et al, 2004;Meskauskiene et al, 2009;Babiychuk et al, 2011;Quesada et al, 2011). We identified 10 mTERF proteins in the maize nucleoid samples; eight of these have Arabidopsis orthologs based on reciprocal BLAST analysis, all of which were shown to be chloroplast localized in GFP fusion assays (Babiychuk et al, 2011).…”