Mitochondrial transcript processing and restoration of male fertility in T-cytoplasm maize

Discovery of a novel mitochondrial DNA molecule associated with tetrad pollen sterility in potato

BACKGROUND

Tetrad sterility in potato is caused by a specific cytoplasm, called TSC_sto, derived from the Mexican wild tetraploid species Solanum stoloniferum. Different S. stoloniferum accessions crossed as females with S. tuberosum resulted in 12 fertile hybrids and 27 sterile hybrids exhibiting tetrad sterility.

RESULTS

Whole-mitochondrial-genome sequencing was performed for two fertile hybrids and three hybrids exhibiting tetrad sterility. Two to seven contigs, with the total assembly lengths ranging from 462,716 to 535,375 bp, were assembled for each hybrid. Unlike for the reference mitochondrial genome (cv. Désirée), two different recombinant-type contigs (RC-I and RC-II) were identified. RC-I featured by the rpl5-ψrps14 gene joined to the nad6 gene, generating a novel intergenic region. Using a PCR marker (P-3), we found that this intergenic region occurred exclusively in interspecific hybrids exhibiting tetrad sterility and in their parental S. stoloniferum accessions. A part of this intergenic sequence was expressed in the pollen. From a large survey in which P-3 was applied to 129 accessions of 27 mostly Mexican wild species, RC-I was found in diploid S. verrucosum and polyploid species. From eight accessions of S. verrucosum used as females, 92 interspecific hybrids were generated, in which only those carrying RC-I exhibited tetrad sterility.

CONCLUSIONS

RC-I was clearly associated with tetrad sterility, and the RC-I-specific intergenic region likely contains a causal factor of tetrad sterility.

Genetic Structure and Molecular Mechanisms Underlying the Formation of Tassel, Anther, and Pollen in the Male Inflorescence of Maize (Zea mays L.)

Maize tassel is the male reproductive organ which is located at the plant’s apex; both its morphological structure and fertility have a profound impact on maize grain yield. More than 40 functional genes regulating the complex tassel traits have been cloned up to now. However, the detailed molecular mechanisms underlying the whole process, from male inflorescence meristem initiation to tassel morphogenesis, are seldom discussed. Here, we summarize the male inflorescence developmental genes and construct a molecular regulatory network to further reveal the molecular mechanisms underlying tassel-trait formation in maize. Meanwhile, as one of the most frequently studied quantitative traits, hundreds of quantitative trait loci (QTLs) and thousands of quantitative trait nucleotides (QTNs) related to tassel morphology have been identified so far. To reveal the genetic structure of tassel traits, we constructed a consensus physical map for tassel traits by summarizing the genetic studies conducted over the past 20 years, and identified 97 hotspot intervals (HSIs) that can be repeatedly mapped in different labs, which will be helpful for marker-assisted selection (MAS) in improving maize yield as well as for providing theoretical guidance in the subsequent identification of the functional genes modulating tassel morphology. In addition, maize is one of the most successful crops in utilizing heterosis; mining of the genic male sterility (GMS) genes is crucial in developing biotechnology-based male-sterility (BMS) systems for seed production and hybrid breeding. In maize, more than 30 GMS genes have been isolated and characterized, and at least 15 GMS genes have been promptly validated by CRISPR/Cas9 mutagenesis within the past two years. We thus summarize the maize GMS genes and further update the molecular regulatory networks underlying male fertility in maize. Taken together, the identified HSIs, genes and molecular mechanisms underlying tassel morphological structure and male fertility are useful for guiding the subsequent cloning of functional genes and for molecular design breeding in maize. Finally, the strategies concerning efficient and rapid isolation of genes controlling tassel morphological structure and male fertility and their application in maize molecular breeding are also discussed.

Comparative proteomic analysis of mitochondrial proteins from maize CMS-C sterile, maintainer and restorer anthers

The maize C system of cytoplasmic male sterility (CMS) and its fertility restoration gene Rf4 have been widely used for maize hybrid production; however, the underlying mechanism is still uncertain. The sterility factor functions in mitochondria, where it interacts directly or indirectly with the restorer. Mitoproteomics can capture all participants involved in CMS and restoration at the organelle level. In the present study, we identified and quantified anther mitochondrial proteins from CMS, maintainer and restorer lines. We obtained 14,528 unique peptides belonging to 3,369 proteins. Comparative analysis of 1840 high-confidence proteins revealed 68 were differentially accumulated proteins likely involved in CMS or its restoration within mitochondria. These proteins were mainly associated with fatty acid metabolism, amino acid metabolism and protein-processing pathways. These results suggest that an energy deficiency caused by the sterility factor hinders other proteins or protein complexes required for pollen development through nuclear-mitochondrial interaction. The restorer factor may boost the energy generation by activating alternative metabolic pathways and by improving the post-translation processing efficiency of proteins in energy-producing complexes to restore pollen fertility. Our findings may aid detailed molecular analysis and contribute to a better understanding of maize CMS-C restoration and sterility.

Molecular Approaches for Manipulating Male Sterility and Strategies for Fertility Restoration in Plants

Usable pollination control systems have proven to be effective system for the development of hybrid crop varieties, which are important for optimal performance over varied environments and years. They also act as a biocontainment to check horizontal transgene flow. In the last two decades, many genetic manipulations involving genes controlling the production of cytotoxic products, conditional male sterility, altering metabolic processes, post-transcriptional gene silencing, RNA editing and chloroplast engineering methods have been used to develop a proper pollination control system. In this review article, we outline the approaches used for generating male sterile plants using an effective pollination control system to highlight the recent progress that occurred in this area. Furthermore, we propose possible future directions for biotechnological improvements that will allow the farmers to buy hybrid seed once for many generations in a cost-effective manner.

Comparative transcriptome analysis of isonuclear-alloplasmic lines unmask key transcription factor genes and metabolic pathways involved in sterility of maize CMS-C

Although C-type cytoplasmic male sterility (CMS-C) is one of the most attractive tools for maize hybrid seed production, the detailed regulation network of the male sterility remains unclear. In order to identify the CMS-C sterility associated genes and/or pathways, the comparison of the transcriptomes between the CMS-C line C48-2 and its isonuclear-alloplasmic maintainer line N48-2 at pollen mother cell stage (PS), an early development stage of microspore, and mononuclear stage (MS), an abortive stage of microspore, were analyzed. 2,069 differentially expressed genes (DEGs) between the two stages were detected and thought to be essential for the spikelet development of N48-2. 453 of the 2,069 DEGs were differentially expressed at MS stage between the two lines and thought to be participated in the process or the causes of microspore abortion. Among the 453 DEGs, 385 (84.99%) genes were down-regulated and only 68 (15.01%) genes were up-regulated in C48-2 at MS stage. The dramatic decreased expression of the four DEGs encoding MYB transcription factors and the DEGs involved in "polyamine metabolic process", "Cutin, suberine and wax biosynthesis", "Fatty acid elongation", "Biosynthesis of unsaturated fatty acids" and "Proline metabolism" might play an important role in the sterility of C48-2. This study will point out some directions for detailed molecular analysis and better understanding of sterility of CMS-C in maize.

Allelism analysis of BrRfp locus in different restorer lines and map-based cloning of a fertility restorer gene, BrRfp1, for pol CMS in Chinese cabbage (Brassica rapa L.)

In Chinese cabbage, there are two Rf loci for pol CMS and one of them was mapped to a 12.6-kb region containing a potential candidate gene encoding PPR protein. In Chinese cabbage (Brassica rapa), polima cytoplasmic male sterility (pol CMS) is an important CMS type and is widely used for hybrid breeding. By extensive test crossing in Chinese cabbage, four restorer lines (92s105, 01s325, 00s109, and 88s148) for pol CMS were screened. By analyzing the allelism of the four restorer lines, it was found that 92s105, 01s325, and 00s109 had the same "restorers of fertility" (Rf) locus (designated as BrRfp1), but 88s148 had a different Rf locus (designated as BrRfp2). For fine mapping the BrRfp1 locus of 92s105, a BC₁F₁ population with 487 individuals and a BC₁F₂ population with 2485 individuals were successively constructed. Using simple sequence repeat (SSR) markers developed from Brassica rapa reference genome and InDel markers derived from whole-genome resequencing data of 94c9 and 92s105, BrRfp1 was mapped to a 12.6-kb region containing a potential candidate gene encoding pentatricopeptide repeat-containing protein. Based on the nucleotide polymorphisms of the candidate gene sequence between the restoring and nonrestoring alleles, a co-segregating marker SC718 was developed, which would be helpful for hybrid breeding by marker-assisted screening and for detecting new restorer lines.

Genomic Conflicts that Cause Pollen Mortality and Raise Reproductive Barriers in Arabidopsis thaliana

Species differentiation and the underlying genetics of reproductive isolation are central topics in evolutionary biology. Hybrid sterility is one kind of reproductive barrier that can lead to differentiation between species. Here, we analyze the complex genetic basis of the intraspecific hybrid male sterility that occurs in the offspring of two distant natural strains of Arabidopsis thaliana, Shahdara and Mr-0, with Shahdara as the female parent. Using both classical and quantitative genetic approaches as well as cytological observation of pollen viability, we demonstrate that this particular hybrid sterility results from two causes of pollen mortality. First, the Shahdara cytoplasm induces gametophytic cytoplasmic male sterility (CMS) controlled by several nuclear loci. Second, several segregation distorters leading to allele-specific pollen abortion (pollen killers) operate in hybrids with either cytoplasm. The complete sterility of the hybrid with the Shahdara cytoplasm results from the genetic linkage of the two causes of pollen mortality, i.e., CMS nuclear determinants and pollen killers. Furthermore, natural variation at these loci in A. thaliana is associated with different male-sterility phenotypes in intraspecific hybrids. Our results suggest that the genomic conflicts that underlie segregation distorters and CMS can concurrently lead to reproductive barriers between distant strains within a species. This study provides a new framework for identifying molecular mechanisms and the evolutionary history of loci that contribute to reproductive isolation, and possibly to speciation. It also suggests that two types of genomic conflicts, CMS and segregation distorters, may coevolve in natural populations.

Cytoplasmic male sterility (CMS) in hybrid breeding in field crops

A comprehensive understanding of CMS/Rf system enabled by modern omics tools and technologies considerably improves our ability to harness hybrid technology for enhancing the productivity of field crops. Harnessing hybrid vigor or heterosis is a promising approach to tackle the current challenge of sustaining enhanced yield gains of field crops. In the context, cytoplasmic male sterility (CMS) owing to its heritable nature to manifest non-functional male gametophyte remains a cost-effective system to promote efficient hybrid seed production. The phenomenon of CMS stems from a complex interplay between maternally-inherited (mitochondrion) and bi-parental (nucleus) genomic elements. In recent years, attempts aimed to comprehend the sterility-inducing factors (orfs) and corresponding fertility determinants (Rf) in plants have greatly increased our access to candidate genomic segments and the cloned genes. To this end, novel insights obtained by applying state-of-the-art omics platforms have substantially enriched our understanding of cytoplasmic-nuclear communication. Concomitantly, molecular tools including DNA markers have been implicated in crop hybrid breeding in order to greatly expedite the progress. Here, we review the status of diverse sterility-inducing cytoplasms and associated Rf factors reported across different field crops along with exploring opportunities for integrating modern omics tools with CMS-based hybrid breeding.

The Propensity of Pentatricopeptide Repeat Genes to Evolve into Restorers of Cytoplasmic Male Sterility

Cytoplasmic male sterility (CMS) is a widespread phenotype in plants, which present a defect in the production of functional pollen. The male sterilizing factors usually consist of unusual genes or open reading frames encoded by the mitochondrial genome. CMS can be suppressed by specific nuclear genes called restorers of fertility (Rfs). In the majority of cases, Rf genes produce proteins that act directly on the CMS conferring mitochondrial transcripts by binding them specifically and promoting processing events. In this review, we explore the wide array of mechanisms guiding fertility restoration. PPR proteins represent the most frequent protein class among identified Rfs and they exhibit ideal characteristics to evolve into restorer of fertility when the mechanism of restoration implies a post-transcriptional action. Here, we review the literature that highlights those characteristics and help explain why PPR proteins are ideal for the roles they play as restorers of fertility.

Mitochondrion role in molecular basis of cytoplasmic male sterility

Cytoplasmic male sterility and its fertility restoration via nuclear genes offer the possibility to understand the role of mitochondria during microsporogenesis. In most cases rearrangements in the mitochondrial DNA involving known mitochondrial genes as well as unknown sequences result in the creation of new chimeric open reading frames, which encode proteins containing transmembrane domains. So far, most of the CMS systems have been characterized via restriction fragment polymorphisms followed by transcript analysis. However, whole mitochondrial genome sequence analyses comparing male sterile and fertile cytoplasm open options for deeper insights into mitochondrial genome rearrangements. We more and more start to unravel how mitochondria are involved in triggering death of the male reproductive organs. Reduced levels of ATP accompanied by increased concentrations of reactive oxygen species, which are produced more under conditions of mitochondrial dysfunction, seem to play a major role in the fate of pollen production. Nuclear genes, so called restorer-of-fertility are able to restore the male fertility. Fertility restoration can occur via pentatricopeptide repeat (PPR) proteins or via different mechanisms involving non-PPR proteins.

Transcriptome analysis of cytoplasmic male sterility and restoration in CMS-D8 cotton

A global view of differential expression of genes in CMS-D8 of cotton was presented in this study which will facilitate the understanding of cytoplasmic male sterility in cotton. Cytoplasmic male sterility (CMS) is a maternally inherited trait in higher plants which is incapable of producing functional pollen. However, the male fertility can be restored by one or more nuclear-encoded restorer genes. A genome-wide transcriptome analysis of CMS and restoration in cotton is currently lacking. In this study, Affymetrix GeneChips© Cotton Genome Array containing 24,132 transcripts was used to compare differentially expressed (DE) genes of flower buds at the meiosis stage between CMS and its restorer cotton plants conditioned by the D8 cytoplasm. A total of 458 (1.9 %) of DE genes including 127 up-regulated and 331 down-regulated ones were identified in the CMS-D8 line. Quantitative RT-PCR was used to validate 10 DE genes selected from seven functional categories. The most frequent DE gene group was found to encode putative proteins involved in cell wall expansion, such as pectinesterase, pectate lyase, pectin methylesterase, glyoxal oxidase, polygalacturonase, indole-3-acetic acid-amino synthetase, and xyloglucan endo-transglycosylase. Genes in cytoskeleton category including actin, which plays a key role in cell wall expansion, cell elongation and cell division, were also highly differentially expressed between the fertile and CMS plants. This work represents the first study in utilizing microarray to identify CMS-related genes by comparing overall DE genes between fertile and CMS plants in cotton. The results provide evidence that many CMS-associated genes are mainly involved in cell wall expansion. Further analysis will be required to elucidate the molecular mechanisms of male sterility which will facilitate the development of new hybrid cultivars in cotton.

The mechanism of ORFH79 suppression with the artificial restorer fertility gene Mt-GRP162

The restoration fertility complex (RFC) was previously identified in Honglian (HL)-cytoplasmic male sterility (CMS) rice (Oryza sativa), and glycine-rich protein 162 (GRP162) is responsible for binding to the CMS-associated transcript atp6-orfH79. Here, we engineered a recombinant GRP162 containing the mitochondrial transit peptide, termed Mt-GRP162, as an artificial restorer of fertility (Rf) gene. Mt-GRP162 was confirmed to bind to CMS-associated RNA and to localize to the mitochondria. The transgenic plants showed restored fertility with partially functional pollen. We found that the expression of ORFH79 decreased in transgenic plants, while the expression of atp6-orfH79 was not changed. These findings indicate that Mt-GRP162 restores fertility by suppressing the expression of the cytotoxic protein ORFH79 at the post-transcriptional level rather than via the cleavage of atp6-orfH79 in the presence of RFC. These findings contribute to our understanding of the mechanisms of restoration through diverse pathways.

Genetical and molecular analysis reveals a cooperating relationship between cytoplasmic male sterility- and fertility restoration-related genes in Oryza species

Although the characterization of genes associated with cytoplasmic male sterility (CMS) and fertility restoration (Rf) has been well documented, the evolutionary relationship between nuclear Rf and CMS factors in mitochondria in Oryza species is still less understood. Here, 41 accessions from 7 Oryza species with AA genome were employed for analyzing the evolutionary relationships between the CMS factors and Rf candidates on chromosome 10. The phylogenetic tree based on restriction fragment length polymorphism patterns of CMS-associated mitochondrial genes showed that these 41 Oryza accessions fell into 3 distinct groups. Another phylogenetic tree based on PCR profiles of the nuclear Rf candidates on chromosome 10 was also established, and three groups were distinctively grouped. The accessions in each subgroup/group of the two phylogenetic trees are well parallel to each other. Furthermore, the 41 investigated accessions were test-crossed with Honglian (gametophytic type) and Wild-abortive (sporophytic type) CMS, and 5 groups were classified according to their restoring ability. The accessions in the same subgroup of the two phylogenetic trees shared similar fertility restoring pattern. Therefore, we conclude that the CMS-associated mitotypes are compatible to the Rf candidate-related nucleotypes, CMS and Rf have a parallel evolutionary relation in the Oryza species.

Computational finishing of large sequence contigs reveals interspersed nested repeats and gene islands in the rf1-associated region of maize

The architecture of grass genomes varies on multiple levels. Large long terminal repeat retrotransposon clusters occupy significant portions of the intergenic regions, and islands of protein-encoding genes are interspersed among the repeat clusters. Hence, advanced assembly techniques are required to obtain completely finished genomes as well as to investigate gene and transposable element distributions. To characterize the organization and distribution of repeat clusters and gene islands across large grass genomes, we present 961- and 594-kb contiguous sequence contigs associated with the rf1 (for restorer of fertility1) locus in the near-centromeric region of maize (Zea mays) chromosome 3. We present two methods for computational finishing of highly repetitive bacterial artificial chromosome clones that have proved successful to close all sequence gaps caused by transposable element insertions. Sixteen repeat clusters were observed, ranging in length from 23 to 155 kb. These repeat clusters are almost exclusively long terminal repeat retrotransposons, of which the paleontology of insertion varies throughout the cluster. Gene islands contain from one to four predicted genes, resulting in a gene density of one gene per 16 kb in gene islands and one gene per 111 kb over the entire sequenced region. The two sequence contigs, when compared with the rice (Oryza sativa) and sorghum (Sorghum bicolor) genomes, retain gene colinearity of 50% and 71%, respectively, and 70% and 100%, respectively, for high-confidence gene models. Collinear genes on single gene islands show that while most expansion of the maize genome has occurred in the repeat clusters, gene islands are not immune and have experienced growth in both intragene and intergene locations.

From Arabidopsis to rice: pathways in pollen development

The control of male fertility is of vital importance for crop breeding, hybrid generation, and the control of pollen release. Recent development in the analysis of Arabidopsis male sterile mutants has meant that there is a greater understanding of the gene regulatory networks controlling maternal development of the anther and the resultant sporophytes. With the advent of the genome sequence and tools to allow the analysis of gene function, this knowledge base is now extending into the monocot crop rice. This has shown high levels of similarity between the networks of pollen development in Arabidopsis and rice, which will serve as valuable tools to understand and manipulate this developmental pathway further in plants.

When gametophytic self-incompatibility meets gynodioecy

The occurrence of gynodioecy among angiosperms appears to be associated with self-compatibility. We use individual-based simulations to investigate the conditions for breakdown of a gametophytic self-incompatibility system in gynodioecious populations and make a comparison with hermaphroditic populations where the conditions are well known. We study three types of mutations causing self-compatibility. We track the fate of these mutations in both gynodioecious and hermaphroditic populations, where we vary the number of S-alleles, inbreeding depression and selfing rate. We find that the conditions for breakdown are less stringent if the population is gynodioecious and that the breakdown of self-incompatibility tends to promote stability of gynodioecious populations since it results in a higher frequency of females. We also find that fecundity selection has a large effect on the probability of breakdown of self-incompatibility, in particular if caused by a mutation destroying the female function of the S-locus.

RFLP analysis for mitochondrial genome of CMS-rice

Restriction fragment length polymorphism (RFLP) was used to analyze mitochondrial (mt) genome of cytoplasmic male sterility (CMS) rice. Differences were observed among mitochondrial genomes of the sterile line (A) and maintain line (B) of nine types of CMS rice; Mitochondrial genomic differences were also detected between A and B in many functional gene regions. Even the materials with the same nucleic background have differences in their mtDNA. This provides molecular evidence for the cytoplasmic heterogeneity and the CMS mechanism research.

Sex inheritance in gynodioecious species: a polygenic view

Gynodioecy is defined as the coexistence of two different sexual morphs in a population: females and hermaphrodites. This breeding system is found among many different families of angiosperms and is usually under nucleo-cytoplasmic inheritance, with maternally inherited genes causing male sterility and nuclear factors restoring male fertility. Numerous theoretical models have investigated the conditions for the stable coexistence of females and hermaphrodites. To date, all models rest on the assumption that restoration of a given male sterile genotype is controlled by a single Mendelian factor. Here, we review data bearing on the genetic determinism of sex inheritance in three gynodiecious plant species. We suggest that restoration of male fertility is probably best viewed as a quantitative trait controlled by many loci. We develop a threshold model that accommodates an underlying polygenic trait, which is resolved at the phenotypic level in discrete sexual morphs. We use this model to reanalyse data in Thymus vulgaris, Silene vulgaris and Plantago coronopus. A simple Mendelian inheritance of sex determinism is unlikely in all three species. We discuss how our model can shed additional light on the genetics of restoration and point towards future efforts in the modelling of gynodioecy.

Positional cloning of the rice Rf-1 gene, a restorer of BT-type cytoplasmic male sterility that encodes a mitochondria-targeting PPR protein

The combination of cytoplasmic male sterility (CMS) in one parent and a restorer gene ( Rf) to restore fertility in another are indispensable for the development of hybrid varieties. We have found a rice Rf-1 gene that restores BT-type CMS by applying a positional cloning strategy. Using linkage analysis in combination with 6,104 BC(1)F(3) progeny derived from a cross between two near-isogenic lines (NILs) differing only at the Rf-1 locus, we delimited the Rf-1 gene to a 22.4-kb region in the rice genome. Duplicate open reading frames ( Rf-1A and Rf-1B) with a pentatricopeptide (PPR) motif were found in this region. Since several insertions and/or deletions were found in the regions corresponding to both the Rf-1A and Rf-1B genes in the maintainer's allele, they may have lost their function. Rf-1A protein had a mitochondria-targeting signal, whereas Rf-1B did not. The Rf-1B gene encoded a shorter polypeptide that was determined by a premature stop codon. Based on the function of the Rf-1 gene, its product is expected to target mitochondria and may process the transcript from an atp6/orf79 region in the mitochondrial genome. Since the Rf-1A gene encodes a 791-amino acid protein with a signal targeting mitochondria and has 16 repeats of the PPR motif, we concluded that Rf-1A is the Rf-1 gene. Nine duplications of Rf-1A homologs were found around the Rf-1 locus in the Nipponbare genome. However, while some of them encoded proteins with the PPR motif, they do not restore BT-type CMS based on the lack of co-segregation with the restoration phenotype. These duplicates may have played diversified roles in RNA processing and/or recombination in mitochondria during the co-evolution of these genes and the mitochondrial genome.

A pentatricopeptide repeat-containing gene restores fertility to cytoplasmic male-sterile plants

Known in over 150 species, cytoplasmic male sterility is encoded by aberrant mitochondrial genes that prevent pollen development. The RNA- or protein-level expression of most of the mitochondrial genes encoding cytoplasmic male sterility is altered in the presence of one or more nuclear genes called restorers of fertility that suppress the male-sterile phenotype. Cytoplasmic male sterility/restorer systems have been proven to be an invaluable tool in the production of hybrid seeds. Despite their importance for both the production of major crops such as rice and sunflower and the study of organelle/nuclear interactions in plants, none of the nuclear fertility-restorer genes that reduce the expression of aberrant mitochondrial proteins have previously been cloned. Here we report the isolation of a gene directly involved in the control of the expression of a cytoplasmic male sterility-encoding gene. The Petunia restorer of fertility gene product is a mitochondrially targeted protein that is almost entirely composed of 14 repeats of the 35-aa pentatricopeptide repeat motif. In a nonrestoring genotype we identified a homologous gene that exhibits a deletion in the promoter region and is expressed in roots but not in floral buds.

Microscopy of a cytoplasmic male-sterile soybean from an interspecific cross between Glycine max and G. soja (Leguminosae)

Cytoplasmic male sterility has been found independently in soybean three times since 1995, but no microscopic investigation has been published. The purpose of this microscopic study was to establish the developmental sequence leading to sterility in a cytoplasmic male-sterile soybean line that has been found to be stable under all environmental conditions tested and to demarcate the temporal and spatial parameters that result in degeneration of the microspores and pollen grains. Light microscopy showed an abnormal development and/or premature degeneration of the tapetum after meiosis II, but some pollen grains persisted until after microspore mitosis. The pollen grains never completely filled with reserves. Premature formation of the endothecium also was evident. Histochemical staining for water-insoluble carbohydrates revealed an abnormal pattern of starch deposition in anther walls that coincided with lack of pollen filling. Electron microscopy showed degeneration of the inner mitochondrial membrane in the tapetal cells as the first detectable change leading to cell degeneration. Subsequently, the tapetal endoplasmic reticulum exhibited atypical concentric rings. Pollen grains displayed mitochondria with unusually enlarged inner mitochondrial spaces, degraded plastids, a rudimentary intine, and no starch or lipid reserves. Results link mitochondrial degeneration, premature formation of the endothecium, and energy deprivation to male sterility.

Sensitivity to Alternaria alternata toxin in citrus because of altered mitochondrial RNA processing

Specificity in the interaction between rough lemon (Citrus jambhiri Lush.) and the fungal pathogen Alternaria alternata rough lemon pathotype is determined by a host-selective toxin, ACR-toxin. Mitochondria from rough lemon are sensitive to ACR-toxin whereas mitochondria from resistant plants, including other citrus species, are resistant. We have identified a C. jambhiri mitochondrial DNA sequence, designated ACRS (ACR-toxin sensitivity gene), that confers toxin sensitivity to Escherichia coli. ACRS is located in the group II intron of the mitochondrial tRNA-Ala and is translated into a SDS-resistant oligomeric protein in C. jambhiri mitochondria but is not translated in the toxin-insensitive mitochondria. ACRS is present in the mitochondrial genome of both toxin-sensitive and -insensitive citrus. However, in mitochondria of toxin-insensitive plants, the transcripts from ACRS are shorter than those in mitochondria of sensitive plants. These results demonstrate that sensitivity to ACR-toxin and hence specificity of the interaction between A. alternata rough lemon pathotype and C. jambhiri is due to differential posttranscriptional processing of a mitochondrial gene.

The RNA world of plant mitochondria

Mitochondria are well known as the cellular power factory. Much less is known about these organelles as a genetic system. This is particularly true for mitochondria of plants, which subsist with respect to attention by the scientific community in the shadow of the chloroplasts. Nevertheless the mitochondrial genetic system is essential for the function of mitochondria and thus for the survival of the plant. In plant mitochondria the pathway from the genetic information encoded in the DNA to the functional protein leads through a very diverse RNA world. How the RNA is generated and what kinds of regulation and control mechanisms are operative in transcription are current topics in research. Furthermore, the modes of posttranscriptional alterations and their consequences for RNA stability and thus for gene expression in plant mitochondria are currently objects of intensive investigations. In this article current results obtained in the examination of plant mitochondrial transcription, RNA processing, and RNA stability are illustrated. Recent developments in the characterization of promoter structure and the respective transcription apparatus as well as new aspects of RNA processing steps including mRNA 3' processing and stability, mRNA polyadenylation, RNA editing, and tRNA maturation are presented. We also consider new suggestions concerning the endosymbiont hypothesis and evolution of mitochondria. These novel considerations may yield important clues for the further analysis of the plant mitochondrial genetic system. Conversely, an increasing knowledge about the mechanisms and components of the organellar genetic system might reveal new aspects of the evolutionary history of mitochondria.