Abstract
Mitochondrial complex II (succinate dehydrogenase [SDH]) is part of the tricarboxylic acid cycle and the respiratory electron transport chain. Its flavoprotein subunit is encoded by two nuclear genes, SDH1-1 and SDH1-2, in Arabidopsis (Arabidopsis thaliana). The SDH1-2 gene is significantly expressed only in roots, albeit at very low level, and its disruption has no effect on growth and development of homozygous mutant plants. In contrast, SDH1-1 transcripts are ubiquitously expressed, with highest expression in flowers. Disruption of the SDH1-1 gene results in alterations in gametophyte development. Indeed, heterozygous SDH1-1/sdh1-1 mutant plants showed normal vegetative growth, yet a reduced seed set. In the progeny of selfed SDH1-1/sdh1-1 plants, distorted segregation ratios were observed, and no homozygous mutant plants were obtained. Reciprocal test crosses with the wild type demonstrated that the mutated sdh1-1 allele is not transmitted through the male gametophyte and is only partially transmitted through the female gametophyte. Consistently, microscopic analysis showed that mutant microspores develop normally until the vacuolated microspore stage, but fail to undergo mitosis I, and then cell structures are degraded and cell content disappears. On the other hand, half the mutant embryo sacs showed arrested development, either at the two-nucleate stage or before polar nuclei fusion. Down-regulation of SDH1-1 by RNA interference results in pollen abortion and a reduced seed set, as in the insertional mutant. Altogether, our results show that SDH1-1, and therefore complex II, are essential for gametophyte development.
Highlights
Mitochondrial complex II is part of the tricarboxylic acid cycle and the respiratory electron transport chain
SDH1-1 and SDH1-2 appear to be expressed at very different levels: the SDH1-1 mRNA was detected by northern-blot hybridization in all tissues examined, whereas SDH1-2 transcripts were only detected in reverse transcriptionPCR and 3# RACE experiments (Figueroa et al, 2002)
Our results reveal that SDH1-2 is dispensable and that the sdh1-1 null allele behaves as a general gametophytic mutation, demonstrating that complex II plays an essential role in gametophyte development
Summary
Nucleo Milenio de Biologıa Celular Vegetal, Centro de Biotecnologıa Vegetal, Universidad Andres Bello, Republica 217, Santiago, Chile. SDH1-1 (At5g66760) and SDH1-2 (At2g18450) have similar structures and encode highly similar proteins that would be functional as complex II flavoprotein since they are highly conserved when compared to their homologs in other organisms (Figueroa et al, 2002). They contain the residues known to be involved in FAD binding and substrate binding, and in proton transfer during catalysis, and they are actively imported into isolated plant mitochondria (Figueroa et al, 2002). Our results reveal that SDH1-2 is dispensable and that the sdh null allele behaves as a general gametophytic mutation, demonstrating that complex II plays an essential role in gametophyte development
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