Targeting the NAD7 Subunit to Mitochondria Restores a Functional Complex I and a Wild Type Phenotype in the Nicotiana sylvestris CMS II Mutant Lacking nad7

Abstract

The mitochondrial DNA of the Nicotiana sylvestris CMSII mutant carries a 72-kb deletion comprising the single copy nad7 gene that encodes the NAD7 subunit of the respiratory complex I (NADH-ubiquinone oxidoreductase). CMSII plants lack rotenone-sensitive complex I activity and are impaired in physiological and phenotypical traits. To check whether these changes directly result from the deletion of nad7, we constructed CMS transgenic plants (termed as CMSnad7) carrying an edited nad7 cDNA fused to the CAMV 35 S promoter and to a mitochondrial targeting sequence. The nad7 sequence was transcribed and translated and the NAD7 protein directed to mitochondria in CMSnad7 transgenic plants, which recovered both wild type morphology and growth features. Blue-native/SDS gel electrophoresis and enzymatic assays showed that, whereas fully assembled complex I was absent from CMSII mitochondria, a functional complex was present in CMSnad7 mitochondria. Furthermore, a supercomplex involving complex I and complex III was present in CMSnad7 as in the wild type. Taken together, these data demonstrate that lack of complex I in CMSII was indeed the direct consequence of the absence of nad7. Hence, NAD7 is a key element for complex assembly in plants. These results also show that allotopic expression from the nucleus can fully complement the lack of a mitochondrialencoded complex I gene.

Inactivation of genes coding for mitochondrial Nd7 and Nd9 complex I subunits in Chlamydomonas reinhardtii. Impact of complex I loss on respiration and energetic metabolism

“…In tobacco (Nicotiana sylvestris), CMSI/II mutants are impaired in the expression of chloroplastic nad7 gene (Lelandais et al, 1998;Pla et al, 1995). As a consequence, rotenone-sensitive oxygen uptake is decreased and there is no complex I assembly (Gutierres et al, 1997;Pineau et al, 2005). The Chlamydomonas amc14 mutant isolated in this work bears an insertion in the promoter region of NUO9 gene, which consequently reduces gene expression and lowers in parallel complex I activity and assembly (data not shown), suggesting that the amount of fully assembled complex I is at least controlled by the amount of Nd9 subunit.…”

Miniaturized mitogenome of the parasitic plant Viscum scurruloideum is extremely divergent and dynamic and has lost all nad genes

“…Although the evolutionary loss of complex I is unprecedented in plants, mutant lines are informative for understanding the potential consequences of the loss of the nad genes in Viscum. In particular, three mutant lines, Nicotiana sylvestris CMSII lacking the mitochondrial nad7 gene (35,36), Arabidopsis thaliana ndufs4 lacking a nuclear nad gene (37), and NCS2 maize plants lacking functional nad4, are (37,38,40), albeit with constitutively lower mitochondrial phosphorylation efficiencies. Plants in general have the ability to oxidize NADH and NADPH via several nuclear-encoded alternative dehydrogenases (41).…”

“…The plant complex I mutants grow more slowly than their respective wild types and have other deficiencies: N. sylvestris CMSII mutants exhibit male sterility and slow growth (36); A. thaliana ndufs4 shows depressed germination and growth (37); and homoplasmic NC2 maize plants exhibit sterility and severely depressed growth (42). Because Viscum does not exhibit noticeably defective germination or growth, regardless of any deleterious effects caused by loss of complex I in Viscum, the organism has been able to adapt and thrive.…”

AtnMat2, a nuclear-encoded maturase required for splicing of group-II introns in Arabidopsis mitochondria

“…Complex I dysfunctions have been previously associated with plants having growth to flowering developmental defects (Gutierres et al 1997;Brangeon et al 2000;Sabar et al 2000;Pineau et al 2005;Garmier et al 2008). Accordingly, nMat2 plants demonstrated slow growth, late flowering, and were partially sterile ( Fig.…”