Plant Mitochondrial Genome Evolution and Cytoplasmic Male Sterility

Mitochondria are responsible for providing energy currency to life processes in the molecular form of ATP and are therefore typically referred to as the power factories of cells. Plant mitochondria are also relevant to the common phenomenon of cytoplasmic male sterility, which is agronomically important in various crop species. Cytoplasmic male sterility (CMS) is a complex trait that may be influenced by patterns of mitochondrial genome evolution, and by intergenomic gene transfer among the organellar and nuclear compartments of plant cells. Here, we review patterns and processes that shape plant mitochondrial genomes, some relevant interactions between organelles, and the general features shared by the majority of cytoplasmic male-sterile genes in plants to further the goal of understanding CMS.     

Tissue-Specific Protein Expression in Plant Mitochondria

Although the physiological role of plant mitochondria is thought to vary in different tissues at progressive stages of development, there has been little documentation that the complement of mitochondrial proteins is altered in different plant organs. Because the phenomenon of cytoplasmic male sterility suggests an unusual function for mitochondria in floral buds, we examined the tissue-specific expression of mitochondrial proteins in petunia buds at several stages of development, using both fertile and cytoplasmic male sterile plants. On tissue prints of cryostat-sectioned buds, antibodies recognizing subunit A of the mitochondrial ATPase (ATPA) localized very differently from antibodies recognizing subunit II of the cytochrome oxidase (COXII), which indicated that mitochondria in the same tissue could differentially express mitochondrially encoded proteins. The petunia cytoplasmic male sterility-associated fused (pcf) gene encodes a protein that colocalized with ATPA and the nuclear-encoded mitochondrial alternative oxidase (AOA) in sporogenous tissues, where little COXII protein was found. These overlapping and differential localization patterns may provide clues to the molecular mechanism of cytoplasmic male sterility.     

Cytoplasmic male sterility: a window to the world of plant mitochondrial-nuclear interactions

Mitochondrial function depends on the coordinate action of nuclear and mitochondrial genomes. The genetic dissection of these interactions presents special challenges in obligate aerobes, because the viability of these organisms depends on mitochondrial respiration. The plant trait cytoplasmic male sterility (CMS) is determined by the mitochondrial genome and is associated with a pollen sterility phenotype that can be suppressed or counteracted by nuclear genes known as restorer-of-fertility genes. Here, I review the nature and the origin of the genes that determine CMS, together with recent investigations that have exploited CMS to provide new insights into plant mitochondrial-nuclear communication. These studies have implicated mitochondrial signaling pathways, including those involved in regulating cell death and nuclear gene expression, in the elaboration of CMS. The molecular cloning of nuclear genes that restore fertility (i.e. restorer-of-fertility genes) has identified genes encoding pentatricopeptide-repeat proteins as key regulators of plant mitochondrial gene expression.     

Mitochondrial genome organization and cytoplasmic male sterility in plants

Plant mitochondrial genomes are much larger and more complex than those of other eukaryotic organisms. They contain a very active recombination system and have a multipartite genome organization with a master circle resolving into two or more subgenomic circles by recombination through repeated sequences. Their protein coding capacity is very low and is comparable to that of animal and fungal systems. Several subunits of mitochondrial functional complexes, a complete set of tRNAs and 26S, 18S and 5S rRNAs are coded by the plant mitochondrial genome. The protein coding genes contain group II introns. The organelle genome contains stretches of DNA sequences homologous to chloroplast DNA. It also contains actively transcribed DNA sequences having open reading frames. Plasmid like DNA molecules are found in mitochondria of some plants Cytoplasmic male sterility in plants, characterized by failure to produce functional pollen grains, is a maternally inherited trait. This phenomenon has been found in many species of plants and is conveniently used for hybrid plant production. The genetic determinants for cytoplasmic male sterility reside in the mitochondrial genome. Some species of plants exhibit more than one type of cytoplasmic male sterility. Several nuclear genes are known to control expression of cytoplasmic male sterility. Different cytoplasmic male sterility types are distinguished by their specific nuclear genes(rfs) which restore pollen fertility. Cytoplasmic male sterility types are also characterized by mitochondrial DNA restriction fragment length polymorphism patterns, variations in mitochondrial RNAs, differences in protein synthetic profiles, differences in sensitivity to fungal toxins and insecticides, presence of plasmid DNAs or RNAs and also presence of certain unique sequences in the genome. Recently nuclear male sterility systems based on (i) over expression of agrobacterialrol C gene and (ii) anther specific expression of an RNase gene have been developed in tobacco andBrassica by genetic engineering methods.     

植物胞质雄性不育及育性恢复的分子机制研究进展(综述)

本文从与雄性不育有关的线粒体基因引起雄性不育的机理、雄性不育育性恢复机制以及育性恢复基因的克隆等方面,介绍国内外对植物细胞质雄性不育分子机理的研究进展,并对今后的研究进行讨论。     

The nucleo-mitochondrial conflict in cytoplasmic male sterilities revisited

Cytoplasmic male sterility (CMS) in plants is a classical example of genomic conflict, opposing maternally-inherited cytoplasmic genes (mitochondrial genes in most cases), which induce male sterility, and nuclear genes, which restore male fertility. In natural populations, this type of sex control leads to gynodioecy, that is, the co-occurrence of female and hermaphroditic individuals within a population. According to theoretical models, two conditions may maintain male sterility in a natural population: (1) female advantage (female plants are reproductively more successful than hermaphrodites on account of their global seed production); (2) the counter-selection of nuclear fertility restorers when the corresponding male-sterility-inducing cytoplasm is lacking. In this review, we re-examine the model of nuclear-mitochondrial conflict in the light of recent experimental results from naturally occurring CMS, alloplasmic CMS (appearing after interspecific crosses resulting from the association of nuclear and cytoplasmic genomes from different species), and CMS plants obtained in the laboratory and carrying mitochondrial mutations. We raise new hypotheses and discuss experimental models that would take physiological interactions between cytoplasmic and nuclear genomes into account.     

The molecular biology of cytoplasmically inherited male sterility and prospects for its engineering

Nucleocytoplasmic male sterilities are binary genetic systems driven by mitochondrial, maternally inherited genes that induce male sterility and a female phenotype and which are overcome by nuclear restorers of fertility. They contribute to the reproductive biology and evolution of natural populations and are valuable tools for the commercial production of hybrid seeds in crops. For species with no natural form of cytoplasmic male sterility, such sterility can in some cases be introduced from different, but related, species through sexual crosses or somatic hybridisation. Somatic hybridisation is the only technique currently available for manipulating plant mitochondrial genomes. Recent successes in plastid transformation have opened up entirely new perspectives for the engineering of cytoplasmic male sterilities in transplastomic plants.     

Mitochondrial DNA polymorphism induced by protoplast fusion in Cruciferae

Summary The mitochondrial genomes of five rapeseed somatic hybrid plants, which combine in a first experimentBrassica napus chloroplasts and a cytoplasmic male sterility trait coming fromRaphanus sativus, and in a second experiment chloroplasts of a triazine resistantB. compestris and a cytoplasmic male sterility trait fromR. sativus, were analyzed by restriction endonucleases. Restriction fragment patterns indicate that these genomes differ from each other and from both parents. The presence of new bands in the somatic hybrid mitochondrial DNA restriction patterns is evidence of mitochondrial recombination in somatic hybrid cells. In both parental and somatic hybrid plants large quantitative variations in a mitochondrial plasmid-like DNA have been observed. Our results suggest that the cytoplasmic support for male sterility is located in the chromosomal mitochondrial DNA instead of the plasmid-like DNA.     

Mitochondrially-targeted expression of a cytoplasmic male sterility-associated <Emphasis Type="Italic">orf220</Emphasis> gene causes male sterility in <Emphasis Type="Italic">Brassica juncea</Emphasis>

Background  

The novel chimeric open reading frame (orf) resulting from the rearrangement of a mitochondrial genome is generally thought to be a causal factor in the occurrence of cytoplasmic male sterility (CMS). Both positive and negative correlations have been found between CMS-associated orfs and the occurrence of CMS when CMS-associated orfs were expressed and targeted at mitochondria. Some orfs cause male sterility or semi-sterility, while some do not. Little is currently known about how mitochondrial factor regulates the expression of the nuclear genes involved in male sterility. The purpose of this study was to investigate the biological function of a candidate CMS-associated orf220 gene, newly isolated from cytoplasmic male-sterile stem mustard, and show how mitochondrial retrograde regulated nuclear gene expression is related to male sterility.     

Transfer of wild abortive cytoplasmic male sterility through protoplast fusion in rice

Wild abortive cytoplasmic male sterility has been extensively used in hybrid seed production in the tropics. Using protoplast fusion between cytoplasmic male sterile and fertile maintainer lines; we report here, transfer of wild abortive cytoplasmic male sterility to the nuclear background of RCPL1-2C, an advance breeding line which also served as maintainer of this cytoplasm. In total, 27 putative cybrids between V20A and RCPL1-2C and 23 lines between V20A and V20B were recovered and all of them were sterile. DNA blots prepared from the mitochondrial DNA of the cybrid lines from both the sets were probed with orf155 that is known to exhibit polymorphism between the mitochondrial DNA of the male-sterile and fertile maintainer lines. Hybridization of orf155 to 1.3 kb HindIII-digested mitochondrial DNA fragment of the cybrids showed transfer of mitochondrial DNA from wild abortive cytoplasmic male-sterile line to the maintainers, viz. RCPL 1-2C and V20B. Expression of male sterility was confirmed by the presence of sterile pollen grains and the lack of seed setting due to selfing in all the cybrid lines. These cybrids, on crossing with respective fertile maintainers set seeds that in turn, produced sterile BC1 plants. DNA blots from HindIII-digested mitochondrial DNA of these BC1 plants when probed with orf155 again exhibited localization of orf155 in wild abortive cytoplasm-specific 1.3 kb HindIII-digested mitochondrial DNA fragments. This demonstrated that the cytoplasmic male sterility transferred through protoplast fusion retained intact female fertility and was inherited and expressed in BC1 plants. Fusion-derived CMS lines, on pollination with pollen grains from restorer, showed restoration of fertility in all the lines. The results demonstrate that protoplasts fusion can be used for transferring maternally inherited traits like cytoplasmic male sterility to the desired nuclear background which can, in turn, be used in hybrid seed production programme of rice in the tropical world.     

高等植物胞质雄性不育及育性恢复的分子生物学研究进展

综述了几种植物胞质雄性不育的分子机理研究进展,着重介绍了与细胞质雄性不育相关的线粒体连锁位点的分析及育性恢复的几种假说,并对今后的研究进行了讨论。     

Emergence of gynodioecy in wild beet (Beta vulgaris ssp. maritima L.): a genealogical approach using chloroplastic nucleotide sequences

Gynodioecy is a breeding system where both hermaphroditic and female individuals coexist within plant populations. This dimorphism is the result of a genomic interaction between maternally inherited cytoplasmic male sterility (CMS) genes and bi-parentally inherited nuclear male fertility restorers. As opposed to other gynodioecious species, where every cytoplasm seems to be associated with male sterility, wild beet Beta vulgaris ssp. maritima exhibits a minority of sterilizing cytoplasms among numerous non-sterilizing ones. Many studies on population genetics have explored the molecular diversity of different CMS cytoplasms, but questions remain concerning their evolutionary dynamics. In this paper we report one of the first investigations on phylogenetic relationships between CMS and non-CMS lineages. We investigated the phylogenetic relationships between 35 individuals exhibiting different mitochondrial haplotypes. Relying on the high linkage disequilibrium between chloroplastic and mitochondrial genomes, we chose to analyse the nucleotide sequence diversity of three chloroplastic fragments (trnK intron, trnD-trnT and trnL-trnF intergenic spacers). Nucleotide diversity appeared to be low, suggesting a recent bottleneck during the evolutionary history of B. vulgaris ssp. maritima. Statistical parsimony analyses revealed a star-like genealogy and showed that sterilizing haplotypes all belong to different lineages derived from an ancestral non-sterilizing cytoplasm. These results suggest a rapid evolution of male sterility in this taxon. The emergence of gynodioecy in wild beet is confronted with theoretical expectations, describing either gynodioecy dynamics as the maintenance of CMS factors through balancing selection or as a constant turnover of new CMSs.     

植物雄性不育基因的研究进展

本文概述了植物雄性核不育基因的分子标记及其定位,综述了植物细胞质雄性不育中不育系与保持系在叶绿体和线粒体基因组的结构、转录和翻译产物方面的差异以及和雄性不育之间的可能关系,以及恢复系中的恢复基因分子水平的研究现状;讨论了环境条件如光周期和温度对雄性不育的影响在分子水平上的研究现状,指出了植物雄性不育基因研究方面存在的问题和解决的思路。     

Isolation of mitochondrial DNA sequences that distinguish male-sterility-inducing cytoplasms in Sorghum bicolor (L.) Moench

We have demonstrated that sorghum DNA sequences of mitochondrial origin can be used to distinguish different male-sterility-inducing cytoplasms. Six DNA clones containing single-copy mitochondrial sequences were hybridized on Southern blots to restriction enzyme-digested DNA of 28 sorghum lines representing sources of different cytoplasmic male-sterility (CMS) groups. Four cytoplasmic types were defined on the basis of the pattern of DNA fragments detected. Similar analyses of 50 additional diverse sorghum accessions suggested that three of the four cytoplasmic types may be diagnostic for CMS. Also, three other cytoplasmic types were discovered. These and other mitochondrial DNA clones may be useful molecular tools for “fingerprinting” sterility-inducing cytoplasms in breeding programs, determining cytoplasmic diversity among germ plasm accessions, and identifying new sources of cytoplasm that induce male sterility.     

植物细胞质雄性不育与线粒体

细胞质雄性不育(cytoplasmic male sterility, CMS)是由于核基因组与细胞质基因组之间不协调互作导致的一种雄性器官异常而雌器官可以接受外来花粉正常结实的自然现象。在生产上,CMS是植物杂交制种的有力工具和杂种优势利用的重要途径。对CMS分子机制的解析是其有效利用的基础,一直以来都是研究的热点,然而其机制研究却相对滞后。该文从线粒体嵌合基因(orfs)形成、特征及分类,基因转录后修饰(RNA编辑)的特点及与CMS的关系,基因翻译产物特征、分类及其与CMS的关系等3个方面综述了近年来植物胞质雄性不育的机制研究进展,以期为进一步深入解析其分子机制提供理论参考。     

Identification of a mitochondrial protein associated with cytoplasmic male sterility in petunia.

The petunia fused gene (pcf), which is associated with cytoplasmic male sterility (CMS), is composed of sequences derived from atp9, coxII, and an unidentified reading frame termed urfS. To determine whether the pcf gene is expressed at the protein level, we produced antibodies to synthetic peptides specified by the coxII and urfS portions of the pcf gene. Anti-COXII peptide antibodies recognized petunia COXII but no other mitochondrial proteins. Anti-URF-S peptide antibodies recognized a 20-kilodalton protein present in both cytoplasmic male sterile and fertile lines and a protein with an apparent molecular mass of 25 kilodaltons present only in cytoplasmic male sterile lines. The 25-kilodalton protein was found to be synthesized by isolated mitochondria and to fractionate into both the soluble and membrane portions of disrupted mitochondria, whereas the 20-kilodalton protein was found only in the membrane fraction. The abundance of the 25-kilodalton protein was much lower in fertile plants carrying the cytoplasmic male sterile cytoplasm and a single dominant nuclear fertility restorer gene, Rf. Thus, the pcf gene is correlated with cytoplasmic male sterility not only by its co-segregation with the phenotype in somatic hybrids, but also by the modification of its expression at the protein level through the action of a nuclear gene that confers fertility.     

Cytoplasmic male sterility and restoration of pollen fertility in higher plants

The review deals with cytoplasmic male sterility (CMS) in higher plants: impairment of the pollen formation resulting from interaction of the nuclear and mitochondrial genomes. The information on the known nuclear restorer-of-fertility genes and their effects on the expression of CMS-associated mitochondrial loci are considered. Heteroplasmy of mtDNA in plants and its potential association with CMS inheritance, as well as possible mechanisms of the observed direct and reverse association between altered expression of the CMS-inducing mitochondrial genome, metabolic defects, and pollen sterility are discussed.     

Cytoplasmic male sterility and restoration of pollen fertility in higher plants

The review deals with cytoplasmic male sterility (CMS) in higher plants: impairment of the pollen formation resulting from interaction of the nuclear and mitochondrial genomes. The information on the known nuclear restorer-of-fertility genes and their effects on the expression of CMS-associated mitochondrial loci are considered. Heteroplasmy of mtDNA in plants and its potential association with CMS inheritance, as well as possible mechanisms of the observed direct and reverse association between altered expression of the CMS-inducing mitochondrial genome, metabolic defects, and pollen sterility are discussed.