Sorghum mitochondrial orf25 and a related chimeric configuration of a male-sterile cytoplasm

A Systematic Review and Developmental Perspective on Origin of CMS Genes in Crops

Cytoplasmic male sterility (CMS) arises from the incompatibility between the nucleus and cytoplasm as typical representatives of the chimeric structures in the mitochondrial genome (mitogenome), which has been extensively applied for hybrid seed production in various crops. The frequent occurrence of chimeric mitochondrial genes leading to CMS is consistent with the mitochondrial DNA (mtDNA) evolution. The sequence conservation resulting from faithfully maternal inheritance and the chimeric structure caused by frequent sequence recombination have been defined as two major features of the mitogenome. However, when and how these chimeric mitochondrial genes appear in the context of the highly conserved reproduction of mitochondria is an enigma. This review, therefore, presents the critical view of the research on CMS in plants to elucidate the mechanisms of this phenomenon. Generally, distant hybridization is the main mechanism to generate an original CMS source in natural populations and in breeding. Mitochondria and mitogenomes show pleomorphic and dynamic changes at key stages of the life cycle. The promitochondria in dry seeds develop into fully functioning mitochondria during seed imbibition, followed by massive mitochondria or mitogenome fusion and fission in the germination stage along with changes in the mtDNA structure and quantity. The mitogenome stability is controlled by nuclear loci, such as the nuclear gene Msh1. Its suppression leads to the rearrangement of mtDNA and the production of heritable CMS genes. An abundant recombination of mtDNA is also often found in distant hybrids and somatic/cybrid hybrids. Since mtDNA recombination is ubiquitous in distant hybridization, we put forward a hypothesis that the original CMS genes originated from mtDNA recombination during the germination of the hybrid seeds produced from distant hybridizations to solve the nucleo-cytoplasmic incompatibility resulting from the allogenic nuclear genome during seed germination.

From TgO/GABA-AT, GABA, and T-263 Mutant to Conception of Toxoplasma

Toxoplasma gondii causes morbidity, mortality, and disseminates widely via cat sexual stages. Here, we find T. gondii ornithine aminotransferase (OAT) is conserved across phyla. We solve TgO/GABA-AT structures with bound inactivators at 1.55 Å and identify an inactivator selective for TgO/GABA-AT over human OAT and GABA-AT. However, abrogating TgO/GABA-AT genetically does not diminish replication, virulence, cyst-formation, or eliminate cat's oocyst shedding. Increased sporozoite/merozoite TgO/GABA-AT expression led to our study of a mutagenized clone with oocyst formation blocked, arresting after forming male and female gametes, with "Rosetta stone"-like mutations in genes expressed in merozoites. Mutations are similar to those in organisms from plants to mammals, causing defects in conception and zygote formation, affecting merozoite capacitation, pH/ionicity/sodium-GABA concentrations, drawing attention to cyclic AMP/PKA, and genes enhancing energy or substrate formation in TgO/GABA-AT-related-pathways. These candidates potentially influence merozoite's capacity to make gametes that fuse to become zygotes, thereby contaminating environments and causing disease.

Mitochondrial regulation of flower development

Flower development in plants depends not only on a set of nuclear genes but also on the coordinate action of the mitochondrion. Certain mitochondrial genomes in combination with certain nuclear genomes lead to the expression of cytoplasmic male-sterility (CMS). Both mitochondrial genes that determine male-sterility and nuclear Restorer-of-fertility genes that suppress the male-sterile phenotype have been cloned. Lately, the interactions between mitochondrial and nuclear genes through retrograde signalling in CMS-systems have been dissected. Of special interest are the altered expression patterns of floral homeotic genes in certain CMS-systems. Here, we review the mitochondrial influence on flower development and give examples from CMS-systems developed in Brassica, Daucus carota, Nicotiana tabacum and Triticum aestivum.

Modelling the maintenance of male-fertile cytoplasm in a gynodioecious population

Gynodioecy is the co-occurrence of females and hermaphrodites in populations. It is usually due to the combined action of cytoplasmic male sterility (CMS) genes and nuclear genes that restore male fertility. According to previous theoretical studies, it is very difficult to explain the maintenance of gynodioecy with CMS and male-fertile cytotypes, although it has been observed in some species. However, only very specific situations have been investigated so far. We present a model to investigate the conditions that promote the maintenance of this breeding system in the case of an outcrossed species when CMS and male-fertile (non-CMS) cytotypes are present in an infinite panmictic population. We show that the type of cost of restoration strongly affects the conditions for stable maintenance of gynodioecy. Stable nuclear-cytoplasmic gynodioecy requires a female advantage, which is a classical condition for gynodioecy, but also a cost of CMS for female fitness, which had been rarely investigated. A cost of restoration is also needed, which could affect either pollen or seeds. Finally, we found that gynodioecy was attainable for a large set of parameter values, including low differences in fitness among genotypes and phenotypes. Our theoretical predictions are compared with previous theoretical work and with results of empirical studies on various gynodioecious species.

The products of the mitochondrial orf25 and orfB genes are FO components in the plant F1FO ATP synthase

The F(O) portion of the mitochondrial ATP synthase contains a range of different subunits in bacteria, yeast and mammals. A search of the Arabidopsis genome identified sequence orthologs for only some of these subunits. Blue native polyacrylamide gel electrophoresis separation of Arabidopsis mitochondrial respiratory chain complexes revealed intact F(1)F(O), and separated F(1) and F(O) components. The subunits of each complex were analysed by mass spectrometry and matched to Arabidopsis genes. In the F(1)F(O) complex a series of nine known subunits were identified along with two additional proteins matching the predicted products of the mitochondrial encoded orfB and orf25 genes. The F(1) complex contained the five well-characterised F(1) subunits, while four subunits in the F(O) complex were identified: subunit 9, d subunit, and the orfB and orf25 products. Previously, orfB has been suggested as the plant equivalent of subunit 8 based on structural and sequence similarity. We propose that orf25 is the plant b subunit based on structural similarity and its presence in the F(O) complex. Chimerics of orf25, orfB, subunit 9 and subunit 6 have been associated with cytoplasmic male sterility in a variety of plant species, our additional findings now place all these proteins in the same protein complex.

Cosegregation of single genes associated with fertility restoration and transcript processing of sorghum mitochondrial orf107 and urf209

Defective nuclear-cytoplasmic interactions leading to aberrant microgametogenesis in sorghum carrying the IS1112C male-sterile cytoplasm occur very late in pollen maturation. Amelioration of this condition, the restoration of pollen viability, involves a novel two-gene gametophytic system, wherein genes designated Rf3 and Rf4 are required for viability of individual gametes. Rf3 is tightly linked to, or represents, a single gene that regulates a transcript processing activity that cleaves transcripts of orf107, a chimeric mitochondrial open reading frame specific to IS1112C. The mitochondrial gene urf209 is also subject to nucleus-specific enhanced transcript processing, 5' to the gene, conferred by a single dominant gene designated Mmt1. Examinations of transcript patterns in F2 and two backcross populations indicated cosegregation of the augmented orf107 and urf209 processing activities in IS1112C. Several sorghum lines that do not restore fertility or confer orf107 transcript processing do exhibit urf209 transcript processing, indicating that the activities are distinguishable. We conclude that the nuclear gene(s) conferring enhanced orf107 and urf209 processing activities are tightly linked in IS1112C. Alternatively, the similarity in apparent regulatory action of the genes may indicate allelic differences wherein the IS1112C Rf3 allele may differ from alleles of maintainer lines by the capability to regulate both orf107 and urf209 processing activities.