Differential screening of mitochondrial cDNA libraries from male-fertile and cytoplasmic male-sterile sugar-beet reveals genome rearrangements at atp6 and atpA loci

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.

OsABCG15 encodes a membrane protein that plays an important role in anther cuticle and pollen exine formation in rice

An ABC transporter gene ( OsABCG15 ) was proven to be involved in pollen development in rice. The corresponding protein was localized on the plasma membrane using subcellular localization. Wax, cutin, and sporopollenin are important for normal development of the anther cuticle and pollen exine, respectively. Their lipid soluble precursors, which are produced in the tapetum, are then secreted and transferred to the anther and microspore surface for polymerization. However, little is known about the mechanisms underlying the transport of these precursors. Here, we identified and characterized a member of the G subfamily of ATP-binding cassette (ABC) transporters, OsABCG15, which is required for the secretion of these lipid-soluble precursors in rice. Using map-based cloning, we found a spontaneous A-to-C transition in the fourth exon of OsABCG15 that caused an amino acid substitution of Thr-to-Pro in the predicted ATP-binding domain of the protein sequence. This osabcg15 mutant failed to produce any viable pollen and was completely male sterile. Histological analysis indicated that osabcg15 exhibited an undeveloped anther cuticle, enlarged middle layer, abnormal Ubisch body development, tapetum degeneration with a falling apart style, and collapsed pollen grains without detectable exine. OsABCG15 was expressed preferentially in the tapetum, and the fused GFP-OsABCG15 protein was localized to the plasma membrane. Our results suggested that OsABCG15 played an essential role in the formation of the rice anther cuticle and pollen exine. This role may include the secretion of the lipid precursors from the tapetum to facilitate the transfer of precursors to the surface of the anther epidermis as well as to microspores.

The ABORTED MICROSPORES regulatory network is required for postmeiotic male reproductive development in Arabidopsis thaliana

The Arabidopsis thaliana ABORTED MICROSPORES (AMS) gene encodes a basic helix-loop-helix (bHLH) transcription factor that is required for tapetal cell development and postmeiotic microspore formation. However, the regulatory role of AMS in anther and pollen development has not been fully defined. Here, we show by microarray analysis that the expression of 549 anther-expressed genes was altered in ams buds and that these genes are associated with tapetal function and pollen wall formation. We demonstrate that AMS has the ability to bind in vitro to DNA containing a 6-bp consensus motif, CANNTG. Moreover, 13 genes involved in transportation of lipids, oligopeptides, and ions, fatty acid synthesis and metabolism, flavonol accumulation, substrate oxidation, methyl-modification, and pectin dynamics were identified as direct targets of AMS by chromatin immunoprecipitation. The functional importance of the AMS regulatory pathway was further demonstrated by analysis of an insertional mutant of one of these downstream AMS targets, an ABC transporter, White-Brown Complex homolog, which fails to undergo pollen development and is male sterile. Yeast two-hybrid screens and pull-down assays revealed that AMS has the ability to interact with two bHLH proteins (AtbHLH089 and AtbHLH091) and the ATA20 protein. These results provide insight into the regulatory role of the AMS network during anther development.

Tapetum degeneration retardation is critical for aliphatic metabolism and gene regulation during rice pollen development

As a complex wall system in flowering plants, the pollen outer wall mainly contains aliphatic sporopollenin; however, the mechanism for synthesizing these lipidic precursors during pollen development remains less well understood. Here, we report on the function of the rice tapetum-expressing TDR (Tapetum Degeneration Retardation) gene in aliphatic metabolism and its regulatory role during rice pollen development. The observations of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses suggested that pollen wall formation was significantly altered in the tdr mutant. The contents of aliphatic compositions of anther were greatly changed in the tdr mutant revealed by GC-MS (gas chromatography-mass spectrometry) testing, particularly less accumulated in fatty acids, primary alcohols, alkanes and alkenes, and an abnormal increase in secondary alcohols with carbon lengths from C29 to C35 in tdr. Microarray data revealed that a group of genes putatively involved in lipid transport and metabolism were significantly altered in the tdr mutant, indicating the critical role of TDR in the formation of the pollen wall. Also, a wide range of genes (236 in total-154 up-regulated and 82 down-regulated) exhibited statistically significant expressional differences between wild-type and tdr. In addition to its function in promoting tapetum PCD, TDR possibly plays crucial regulatory roles in several basic biological processes during rice pollen development.

Relationship between cytoplasmic male sterility and SPL-like gene expression in stem mustard

We studied how mitochondria-nuclear interactions may give rise to cytoplasmic male sterility (CMS) in stem mustard exhibiting abnormal microsporogenesis. In this system, expression of SPL-like, the counterpart of the Arabidopsis nuclear gene SPOROCYTELESS, is specifically lost in buds of CMS plants. When mitochondrial-specific inhibitors were applied to wild-type fertile stem mustard plants, expression of SPL-like was repressed to some extent. As a consequence, the shape and vigor of pollen grains were severely affected, whereas the fertility of pistils remained unaltered. Thereby, we suggest that a probable pathway responsible for CMS in stem mustard involves mitochondrial retrograde regulation, with SPL-like as a target nuclear gene for a mitochondrial signal.

Mitochondrial nad2 gene is co-transcripted with CMS-associated orfB gene in cytoplasmic male-sterile stem mustard (Brassica juncea)

The transcriptional patterns of mitochondrial respiratory related genes were investigated in cytoplasmic male-sterile and fertile maintainer lines of stem mustard, Brassica juncea. There were numerous differences in nad2 (subunit 2 of NADH dehydrogenase) between stem mustard CMS and its maintainer line. One novel open reading frame, hereafter named orfB gene, was located at the downstream of mitochondrial nad2 gene in the CMS. The novel orfB gene had high similarity with YMF19 family protein, orfB in Raphanus sativus, Helianthus annuus, Nicotiana tabacum and Beta vulgaris, orfB-CMS in Daucus carota, atp8 gene in Arabidopsis thaliana, 5' flanking of orf224 in B. napus (nap CMS) and 5' flanking of orf220 gene in CMS Brassica juncea. Three copies probed by specific fragment (amplified by primers of nad2F and nad2R from CMS) were found in the CMS line following Southern blotting digested with HindIII, but only a single copy in its maintainer line. Meanwhile, two transcripts were shown in the CMS line following Northern blotting while only one transcript was detected in the maintainer line, which were probed by specific fragment (amplified by primers of nad2F and nad2R from CMS). Meanwhile, the expression of nad2 gene was reduced in CMS bud compared to that in its maintainer line. We thus suggested that nad2 gene may be co-transcripted with CMS-associated orfB gene in the CMS. In addition, the specific fragment that was amplified by primers of nad2F and nad2R just spanned partial sequences of nad2 gene and orfB gene. Such alterations in the nad2 gene would impact the activity of NADH dehydrogenase, and subsequently signaling, inducing the expression of nuclear genes involved in male sterility in this type of cytoplasmic male sterility.

Mitochondrial atpA gene is altered in a new orf220-type cytoplasmic male-sterile line of stem mustard (Brassica juncea)

The purpose of this research is to identify the probable mitochondrial factor associated with cytoplasmic male sterility (cms) by comparative analysis of cms and its isogenic maintainer lines in stem mustards. Dramatic variations in the morphology of floral organs were observed in cms stem mustard. Mitochondrial atpA gene was shown to be altered in cms compared with that in its maintainer line, of which mitochondrial atpA gene from its maintainer line was sequenced to encode 507 amino acids. It was indicative of high homology with mitochondrial atpA genes from other species, even as high as 94% in similarity with Oryza sativa in terms of amino acid constituents. However, only 429 amino acids were deduced in cms showing 83% similarity with atpA gene from its maintainer line. Two copies were observed in its maintainer line, but only one was found in cms. Such numerous differences of mitochondrial atpA gene between cms and its maintainer lines may not be the results of evolutionary divergence but the rearrangements of mitochondria. Expression of mitochondrial atpA gene was shown to be down-regulated in cms by using Northern blot. Consequently, mitochondrial ATP synthesis was severely decreased more than one fold in cms stem mustard indicating deficiency in mitochondrial ATP synthesis in this type of cms. Therefore, we deduced that mitochondrial atpA gene altered in cms could be associated with male-sterility in this type of cms.

The cytoplasmic male-sterile type and normal type mitochondrial genomes of sugar beet share the same complement of genes of known function but differ in the content of expressed ORFs

The complete nucleotide sequence (501,020 bp) of the mitochondrial genome from cytoplasmic male-sterile (CMS) sugar beet was determined. This enabled us to compare the sequence with that previously published for the mitochondrial genome of normal, male-fertile sugar beet. The comparison revealed that the two genomes have the same complement of genes of known function. The rRNA and tRNA genes encoded in the CMS mitochondrial genome share 100% sequence identity with their respective counterparts in the normal genome. We found a total of 24 single nucleotide substitutions in 11 protein genes encoded by the CMS mitochondrial genome. However, none of these seems to be responsible for male sterility. In addition, several other ORFs were found to be actively transcribed in sugar beet mitochondria. Among these, Norf246 was observed to be present in the normal mitochondrial genome but absent from the CMS genome. However, it seems unlikely that the loss of Norf246 is causally related to the expression of CMS, because previous studies on mitochondrial translation products failed to detect the product of this ORF. Conversely, the CMS genome contains four transcribed ORFs (Satp6presequence, Scox2-2 , Sorf324 and Sorf119) which are missing from the normal genome. These ORFs, which are potential candidates for CMS genes, were shown to be generated by mitochondrial genome rearrangements.

Mapping of a chloroplast RFLP marker associated with the CMS cytoplasm of sugar beet (Beta vulgaris)

The Owen cytoplasm of male-sterile sugar beet is associated with several alterations of mitochondrial DNA and one additional HindIII site of chloroplast DNA. The region of this HindIII site has been cloned and sequenced. The site maps in a small reading frame (orf32) close to the ycf7 (orf31) gene in the petG-psbE region of chloroplast DNA. Possible functional implications of the results are discussed. The chloroplast RFLP marker described could be useful for studies on chloroplast-mitochondrial interactions, CMS of sugar beet, and the origin of the Owen cytoplasm.

Physical and gene organization of mitochondrial DNA from the fertile cytoplasm of sugarbeet (Beta vulgaris L.)

We have constructed a complete physical map of the mitochondrial genome from the male-fertile cytoplasm of sugarbeet. The entire sequence complexity can be represented on a single circular master chromosome of 358 kb. This master chromosome contains three copies of one recombinationally active repeat sequence, with two copies in direct orientation and the other in inverted orientation. The positions of the rRNA genes and of 23 polypeptide genes, determined by filter hybridization, are scattered throughout the genome, with triplicate rrn26 genes located partially or entirely within the recombination-repeat elements. Three ribosomal-protein genes (rps1A, rps14 and rps19) were found to be absent from sugarbeet mtDNA. Our results also reveal that at least six regions homologous with cDNA are dispersed in the mitochondrial genome.

How do alterations in plant mitochondrial genomes disrupt pollen development?

Cytoplasmic male sterility arises when mitochondrial activities are disrupted that are essential for pollen development. Rearrangements in the mitochondrial genome that create novel open reading frames are strongly correlated with CMS phenotypes in a number of systems. The morphological aberrations which indicate CMS-associated degeneration are frequently restricted to the male sporogenous tissue and a limited number of vegetative tissues. In several cases, this tissue specificity may result from interactions between the mitochondrial genome and nuclear genes that regulate mitochondrial gene expression. A molecular mechanism by which CMS might be caused has not been conclusively demonstrated for any system. Several hypotheses for general mechanisms by which mitochondrial dysfunction might disrupt pollen development are discussed, based on similarities between the novel CMS-associated genes from a number of systems.