Nuclear SDH2-1 and SDH2-2 Genes, Encoding the Iron-Sulfur Subunit of Mitochondrial Complex II in Arabidopsis, Have Distinct Cell-Specific Expression Patterns and Promoter Activities

Abstract

Three different nuclear genes encode the essential iron-sulfur subunit of mitochondrial complex II (succinate dehydrogenase) in Arabidopsis (Arabidopsis thaliana), raising interesting questions about their origin and function. To find clues about their role, we have undertaken a detailed analysis of their expression. Two genes (SDH2-1 and SDH2-2) that likely arose via a relatively recent duplication event are expressed in all organs from adult plants, whereas transcripts from the third gene (SDH2-3) were not detected. The tissue-and cell-specific expression of SDH2-1 and SDH2-2 was investigated by in situ hybridization. In flowers, both genes are regulated in a similar way. Enhanced expression was observed in floral meristems and sex organ primordia at early stages of development. As flowers develop, SDH2-1 and SDH2-2 transcripts accumulate in anthers, particularly in the tapetum, pollen mother cells, and microspores, in agreement with an essential role of mitochondria during anther development. Interestingly, in contrast to the situation in flowers, only SDH2-2 appears to be expressed at a significant level in root tips. Strong labeling was observed in all cell layers of the root meristematic zone, and a cell-specific pattern of expression was found with increasing distance from the root tip, as cells attain their differentiated state. Analysis of transgenic Arabidopsis plants carrying SDH2-1 and SDH2-2 promoters fused to the b-glucuronidase reporter gene indicate that both promoters have similar activities in flowers, driving enhanced expression in anthers and/or pollen, and that only the SDH2-2 promoter is active in root tips. These b-glucuronidase staining patterns parallel those obtained by in situ hybridization, suggesting transcriptional regulation of these genes. Progressive deletions of the promoters identified regions important for SDH2-1 expression in anthers and/or pollen and for SDH2-2 expression in anthers and/or pollen and root tips. Interestingly, regions driving enhanced expression in anthers are differently located in the two promoters.The mitochondrial electron transport chain of eukaryotes consists of four major multimeric enzyme complexes, one of which is succinate:ubiquinone oxidoreductase (succinate dehydrogenase; EC 1.3.5.1), commonly referred to as complex II. This important membrane-associated complex is a functional part of both the citric acid cycle and the aerobic respiratory chain, catalyzing the oxidation of succinate to fumarate and the reduction of ubiquinone to ubiquinol.Complex II has been well characterized in bacteria, fungi, and mammals and is known to be the simplest of all the complexes of the electron transport chain, with four subunits (Lemire and Oyedotun, 2002;Yankovskaya et al., 2003). It contains two peripheral membrane proteins, a flavoprotein (SDH1) and an iron-sulfur protein (SDH2), and two small integral membrane proteins (SDH3 and SDH4). The succinatebinding site is formed by the SDH1 polypeptide, which is linked covalently to a FAD molecule acting as acceptor o...

A Nuclear Gene Encoding the Iron-Sulfur Subunit of Mitochondrial Complex II Is Regulated by B3 Domain Transcription Factors during Seed Development in Arabidopsis

“…Homozygous transgenic plants carrying a SDH2-3 promoter fragment (1,603 or 391 bp) and the SDH2-3 5# untranslated region (UTR) fused to GUS (Elorza et al, 2004) were crossed with homozygous abi3-5 and 35S::ABI3 plants and their respective controls (Ler and C24). F1 seeds from the abi3-5 cross were germinated on hygromycin to select for the pSDH2-3::GUS construct, and homozygous abi3-5 F2 seeds were identified by their green phenotype.…”

“…A, Embryos manually dissected from seeds were stained for GUS activity. Plants carrying the promoters of SDH2-1, SDH2-2, and SDH2-3 fused to the GUS gene have been described (Elorza et al, 2004). Walkingstick to dry seed embryos were stained for 60 h (pSDH2-2::GUS) or 14 h (pSDH2-1::GUS and pSDH2-3::GUS).…”

“…To analyze their expression during embryo development, we used plants transformed with fusions of the SDH2-1 and SDH2-2 promoters to the GUS reporter gene (Elorza et al, 2004). Embryos from the early maturation stage to the desiccation stage were dissected and stained for GUS activity (Fig.…”

“…Mutagenesis was performed by PCR, using a construct containing the SDH2-3 promoter and 5# UTR fused to GUS as a template (construct P5; Elorza et al, 2004). Two PCRs were carried out for each mutant with the same template.…”

“…This is supported by the completely different exonintron structure of SDH2-3, which encodes a protein only 67% similar to SDH2-1 and SDH2-2. Moreover, whereas SDH2-1 and SDH2-2 have similar expression patterns, being expressed in all organs from adult plants (Figueroa et al, 2001;Elorza et al, 2004), SDH2-3 is highly expressed in the embryo during the maturation phase of seed development and SDH2-3 transcripts are abundant in dry seeds and decline during germination (Elorza et al, 2006). These data suggest that SDH2-1 and SDH2-2 are probably redundant and that SDH2-3 may have a specific role as the complex II iron-sulfur protein-coding gene during embryo maturation and/or germination.…”