Expression of mitochondrial gene fragments within the tapetum induce male sterility by limiting the biogenesis of the respiratory machinery in transgenic tobacco

Plant mitochondrial genomes (mtDNAs) are large and undergo frequent recombination events. A common phenotype that emerges as a consequence of altered mtDNA structure is cytoplasmic-male sterility (CMS). The molecular basis for CMS remains unclear, but it seems logical that altered respiration activities would result in reduced pollen production. Analysis of tobacco (Nicotiana tabacum) mtDNAs indicated that CMS-associated loci often contain fragments of known organellar genes. These may assemble with organellar complexes and thereby interfere with normal respiratory functions. Here, we analyzed whether the expression of truncated fragments of mitochondrial genes (i.e. atp4, cox1 and rps3) may induce male sterility by limiting the biogenesis of the respiratory machinery. cDNA fragments corresponding to atp4f, cox1f and rps3f were cloned in-frame to a mitochondrial localization signal and a C-termini HA-tag under a tapetum-specific promoter and introduced to tobacco plants by Agrobacterium-mediated transformation. The constructs were then analyzed for their effect on mitochondrial activity and pollen fertility. Atp4f, Cox1f and Rps3f plants demonstrated male sterility phenotypes, which were tightly correlated with the expression of the recombinant fragments in the floral meristem. Fractionation of native organellar extracts showed that the recombinant ATP4f-HA, COX1f-HA and RPS3f-HA proteins are found in large membrane-associated particles. Analysis of the respiratory activities and protein profiles indicated that organellar complex I was altered in Atp4f, Cox1f and Rps3f plants.