Genetic correlation of the orf224/atp6 gene region with Polima CMS in Brassica somatic hybrids

Male sterility in plants: an overview of advancements from natural CMS to genetically manipulated systems for hybrid seed production

KEY MESSAGE

The male sterility system in plants has traditionally been utilized for hybrid seed production. In last three decades, genetic manipulation for male sterility has revolutionized this area of research related to hybrid seed production technology. Here, we have surveyed some of the natural cytoplasmic male sterility (CMS) systems that existed/ were developed in different crop plants for developing male sterility-fertility restoration systems used in hybrid seed production and highlighted some of the recent biotechnological advancements in the development of genetically engineered systems that occurred in this area. We have indicated the possible future directions toward the development of engineered male sterility systems. Cytoplasmic male sterility (CMS) is an important trait that is naturally prevalent in many plant species, which has been used in the development of hybrid varieties. This is associated with the use of appropriate genes for fertility restoration provided by the restorer line that restores fertility on the corresponding CMS line. The development of hybrids based on a CMS system has been demonstrated in several different crops. However, there are examples of species, which do not have usable cytoplasmic male sterility and fertility restoration systems (Cytoplasmic Genetic Male Sterility Systems-CGMS) for hybrid variety development. In such plants, it is necessary to develop usable male sterile lines through genetic engineering with the use of heterologous expression of suitable genes that control the development of male gametophyte and fertile male gamete formation. They can also be developed through gene editing using the recently developed CRISPR-Cas technology to knock out suitable genes that are responsible for the development of male gametes. The present review aims at providing an insight into the development of various technologies for successful production of hybrid varieties and is intended to provide only essential information on male sterility systems starting from naturally occurring ones to the genetically engineered systems obtained through different means.

Mechanism and Utilization of Ogura Cytoplasmic Male Sterility in Cruciferae Crops

Hybrid production using lines with cytoplasmic male sterility (CMS) has become an important way to utilize heterosis in vegetables. Ogura CMS, with the advantages of complete pollen abortion, ease of transfer and a progeny sterility rate reaching 100%, is widely used in cruciferous crop breeding. The mapping, cloning, mechanism and application of Ogura CMS and fertility restorer genes in Brassica napus, Brassica rapa, Brassica oleracea and other cruciferous crops are reviewed herein, and the existing problems and future research directions in the application of Ogura CMS are discussed.

Biotechnological approach for improvement of Crambe species as valuable oilseed plants for industrial purposes

Boosting technological innovation for a sustainable and circular bioeconomy encompasses the use of renewable materials and development of highly effective biotechnological approaches to improve the quality of oilseed crops and facilitate their industrial deployment. The interest in cultivating Crambe as a potential crop is steadily growing due to its low propensity to crossbreeding with other oilseed crops, valuable seed oil composition and a high yield capacity. The main focus is located on Crambe abyssinica as the most adapted into the agriculture and well-studied Crambe species. At the same time, the Crambe genus is one of the most numerous of the Brassicaceae family featuring several underestimated (orphaned) species with useful traits (abiotic stress tolerance, wide range of practical applications). This review features progress in the biotechnological improvement of well-adapted and wild Crambe species starting with aseptic culture establishment and plant propagation in vitro reinforced with the use of genetic engineering and breeding techniques. The aim of the paper is to highlight and review the existing biotechnological methods of both underestimated and well-adapted Crambe species improvment, including the establishment of aseptic culture, in vitro cultivation, plant regeneration and genetic transformation to modify seed oil content and morphological traits of valuable species.

Elucidating Mitochondrial DNA Markers of Ogura-Based CMS Lines in Indian Cauliflowers (Brassica oleracea var. botrytis L.) and Their Floral Abnormalities Due to Diversity in Cytonuclear Interactions

Mitochondrial markers can be used to differentiate diverse mitotypes as well as cytoplasms in angiosperms. In cauliflower, cultivation of hybrids is pivotal in remunerative agriculture and cytoplasmic male sterile lines constitute an important component of the hybrid breeding. In diversifying the source of male sterility, it is essential to appropriately differentiate among the available male sterile cytoplasms in cauliflower. PCR polymorphism at the key mitochondrial genes associated with male sterility will be instrumental in analyzing, molecular characterization, and development of mitotype-specific markers for differentiation of different cytoplasmic sources. Presence of auto- and alloplasmic cytonuclear combinations result in complex floral abnormalities. In this context, the present investigation highlighted the utility of organelle genome-based markers in distinguishing cytoplasm types in Indian cauliflowers and unveils the epistatic effects of the cytonuclear interactions influencing floral phenotypes. In PCR-based analysis using a set of primers targeted to orf-138, 76 Indian cauliflower lines depicted the presence of Ogura cytoplasm albeit the amplicons generated exhibited polymorphism within the ofr-138 sequence. The polymorphic fragments were found to be spanning over 200-280 bp and 410-470 bp genomic regions of BnTR4 and orf125, respectively. Sequence analysis revealed that such cytoplasmic genetic variations could be attributed to single nucleotide polymorphisms and insertion or deletions of 31/51 nucleotides. The cytoplasmic effects on varying nuclear-genetic backgrounds rendered an array of floral abnormalities like reduction in flower size, fused flowers, splitted style with the exposed ovule, absence of nonfunctional stamens, and petaloid stamens. These floral malformations caused dysplasia of flower structure affecting female fertility with inefficient nectar production. The finding provides an important reference to ameliorate understanding of mechanism of cytonuclear interactions in floral organ development in Brassicas. The study paves the way for unraveling developmental biology of CMS phenotypes in eukaryotic organisms and intergenomic conflict in plant speciation.

Current understanding of male sterility systems in vegetable Brassicas and their exploitation in hybrid breeding

Overview of the current status of GMS and CMS systems available in Brassica vegetables, their molecular mechanism, wild sources of sterile cytoplasm and exploitation of male sterility in hybrid breeding. The predominantly herbaceous family Brassicaceae (crucifers or mustard family) encompasses over 3700 species, and many of them are scientifically and economically important. The genus Brassica is an economically important genus within the tribe Brassicaceae that comprises important vegetable, oilseed and fodder crops. Brassica vegetables display strong hybrid vigor, and heterosis breeding is the integral part in their improvement. Commercial production of F₁ hybrid seeds in Brassica vegetables requires an effective male sterility system. Among the available male sterility systems, cytoplasmic male sterility (CMS) is the most widely exploited in Brassica vegetables. This system is maternally inherited and studied intensively. A limited number of reports about the genic male sterility (GMS) are available in Brassica vegetables. The GMS system is reported to be dominant, recessive and trirecessive in nature in different species. In this review, we discuss the available male sterility systems in Brassica vegetables and their potential use in hybrid breeding. The molecular mechanism of mt-CMS and causal mitochondrial genes of CMS has been discussed in detail. Finally, the exploitation of male sterility system in heterosis breeding of Brassica vegetables, future prospects and need for further understanding of these systems are highlighted.

A Novel Cytoplasmic Male Sterility in Brassica napus (inap CMS) with Carpelloid Stamens via Protoplast Fusion with Chinese Woad

A novel cytoplasmic male sterility (CMS) in Brassica napus (inap CMS) was selected from the somatic hybrid with Isatis indigotica (Chinese woad) by recurrent backcrossing. The male sterility was caused by the conversion of tetradynamous stamens into carpelloid structures with stigmatoid tissues at their tips and ovule-like tissues in the margins, and the two shorter stamens into filaments without anthers. The feminized development of the stamens resulted in the complete lack of pollen grains, which was stable in different years and environments. The pistils of inap CMS displayed normal morphology and good seed-set after pollinated by B. napus. Histological sections showed that the developmental alteration of the stamens initiated at the stage of stamen primordium differentiation. AFLP analysis of the nuclear genomic composition with 23 pairs of selective primers detected no woad DNA bands in inap CMS. Twenty out of 25 mitochondrial genes originated from I. indigotica, except for cox2-2 which was the recombinant between cox2 from woad and cox2-2 from rapeseed. The novel cox2-2 was transcribed in flower buds of inap CMS weakly and comparatively with the fertile B. napus addition line Me harboring one particular woad chromosome. The restorers of other autoplasmic and alloplasmic CMS systems in rapeseed failed to restore the fertility of inap CMS and the screening of B. napus wide resources found no fertility restoration variety, showing its distinct origin and the related mechanism of sterility. The reasons for the mitochondrial rearrangements and the breeding of the restorer for the novel CMS system were discussed.

The role of mitochondria in plant development and stress tolerance

Eukaryotic cells require orchestrated communication between nuclear and organellar genomes, perturbations in which are linked to stress response and disease in both animals and plants. In addition to mitochondria, which are found across eukaryotes, plant cells contain a second organelle, the plastid. Signaling both among the organelles (cytoplasmic) and between the cytoplasm and the nucleus (i.e. nuclear-cytoplasmic interactions (NCI)) is essential for proper cellular function. A deeper understanding of NCI and its impact on development, stress response, and long-term health is needed in both animal and plant systems. Here we focus on the role of plant mitochondria in development and stress response. We compare and contrast features of plant and animal mitochondrial genomes (mtDNA), particularly highlighting the large and highly dynamic nature of plant mtDNA. Plant-based tools are powerful, yet underutilized, resources for enhancing our fundamental understanding of NCI. These tools also have great potential for improving crop production. Across taxa, mitochondria are most abundant in cells that have high energy or nutrient demands as well as at key developmental time points. Although plant mitochondria act as integrators of signals involved in both development and stress response pathways, little is known about plant mtDNA diversity and its impact on these processes. In humans, there are strong correlations between particular mitotypes (and mtDNA mutations) and developmental differences (or disease). We propose that future work in plants should focus on defining mitotypes more carefully and investigating their functional implications as well as improving techniques to facilitate this research.

Characterization and classification of one new cytoplasmic male sterility (CMS) line based on morphological, cytological and molecular markers in non-heading Chinese cabbage (Brassica rapa L.)

A new non-heading Chinese cabbage CMS line M119A was characterized and specific molecular markers were developed to classify different CMS types. One new non-heading Chinese cabbage (Brassica rapa L.) cytoplasmic male sterile (CMS) line M119A was obtained by interspecific crosses between the recently discovered hau CMS line of Brassica juncea and B. rapa. Furthermore, the line was characterized and compared with other five isonuclear-alloplasmic CMS lines. The M119A line produced six stamens without pollen and only two stamen fused together in fewer flowers. Tissue section indicated that anther abortion in M119A may have occurred during differentiation of the archesporial cells without pollen sac. All the six CMS lines were grouped into three types based on the presence of three PCR fragments of 825, 465 and 772 bp amplified with different mitochondrial genes specific primers. The 825-bp fragment was amplified both in 09-10A and H201A using the specific primer pair P-orf224-atp6, and showed 100 % identity with the mitochondrial gene of pol CMS. The 465-bp fragment was amplified in 30A and 105A using the primer pair P-orf138 and shared 100 % identity with the mitochondrial gene of ogu CMS. The 772-bp fragment was amplified in M119A and H203A using the primer pair P-orf288 and showed 100 % identity with the mitochondrial gene of hau CMS. Therefore, these markers could efficiently distinguish different types of isonuclear-alloplasmic CMS lines of non-heading Chinese cabbage, which were useful for improving the efficiency of cross-breeding and heterosis utilization in cruciferous vegetables.

Expression of kenaf mitochondrial chimeric genes HM184 causes male sterility in transgenic tobacco plants

Chimeric genes resulting from the rearrangement of a mitochondrial genome were generally thought to be a causal factor in the occurrence of cytoplasmic male sterility (CMS). In the study, earlier we reported that identifying a 47 bp deletion at 3'- flanking of atp9 that was linked to male sterile cytoplasm in kenaf. The truncated fragment was fused with atp9, a mitochondrial transit signal (MTS) and/or GFP, comprised two chimeric genes MTS-HM184-GFP and MTS-HM184. The plant expression vector pBI121 containing chimeric genes were then introduced to tobacco plants by Agrobacterium-mediated T-DNA transformation. The result showed that certain transgenic plants were male sterility or semi-sterility, while some were not. The expression analysis further demonstrated that higher level of expression were showed in the sterility plants, while no expression or less expression in fertility plants, the levels of expression of semi-sterility were in between. And the sterile plant (containing MTS-HM184-GFP) had abnormal anther produced malformed/shriveled pollen grains stained negative that failed to germinate (0%), the corresponding fruits was shrunken, the semi-sterile plants having normal anther shape produced about 10-50% normal pollen grains, the corresponding fruits were not full, and the germination rate was 58%. Meanwhile these transgenic plants which altered on fertility were further analyzed in phenotype. As a result, the metamorphosis leaves were observed in the seedling stage, the plant height of transgenic plants was shorter than wild type. The growth duration of transgenic tobacco was delayed 30-45 days compared to the wild type. The copy numbers of target genes of transgenic tobacco were analyzed using the real-time quantitative method. The results showed that these transgenic plants targeting-expression in mitochondrial containing MTS-HM184-GFP had 1 copy and 2 copies, the other two plants containing MTS-HM184 both had 3 copies, but 0 copy in wild type. In summary, the two manual chimeric genes might be related to male sterility in kenaf.

Cytoplasmic male sterility in Brassicaceae crops

Brassicaceae crops display strong hybrid vigor, and have long been subject to F1 hybrid breeding. Because the most reliable system of F1 seed production is based on cytoplasmic male sterility (CMS), various types of CMS have been developed and adopted in practice to breed Brassicaceae oil seed and vegetable crops. CMS is a maternally inherited trait encoded in the mitochondrial genome, and the male sterile phenotype arises as a result of interaction of a mitochondrial CMS gene and a nuclear fertility restoring (Rf) gene. Therefore, CMS has been intensively investigated for gaining basic insights into molecular aspects of nuclear-mitochondrial genome interactions and for practical applications in plant breeding. Several CMS genes have been identified by molecular genetic studies, including Ogura CMS from Japanese radish, which is the most extensively studied and most widely used. In this review, we discuss Ogura CMS, and other CMS systems, and the causal mitochondrial genes for CMS. Studies on nuclear Rf genes and the cytoplasmic effects of alien cytoplasm on general crop performance are also reviewed. Finally, some of the unresolved questions about CMS are highlighted.

A restorer-of-fertility like pentatricopeptide repeat gene directs ribonucleolytic processing within the coding sequence of rps3-rpl16 and orf240a mitochondrial transcripts in Arabidopsis thaliana

The pentatricopeptide repeat (PPR) proteins represent a large family of RNA-binding proteins that have many roles in post-transcriptional RNA processes within plant organelles. Among the PPR proteins that target plant mitochondria, the restorer-of-fertility (Rf) proteins are characterized by their inhibitory action on mitochondrion-localized cytoplasmic male sterility (CMS) genes in various crop species. Close homologs to known Rfs from radish, petunia, and rice can be identified in most higher plant species and these proteins define the recognized subgroup of Rf-like (RFL) PPR proteins. In this paper we describe the function of the RFL9 gene from Arabidopsis thaliana, and show that it is associated with ribonucleolytic cleavages within the coding sequences of rps3-rpl16 and orf240a mitochondrial transcripts in the Col-0 accession. RFL9 therefore represents an Rf-like PPR gene that has the potential to compromise the function of an essential mitochondrial gene and whose function is also associated with a mitochondrial orf sharing significant homology with a proven CMS-causing orf. We observe that RFL9 is active in only a few Arabidopsis accessions genetically close to Col-0, which supports the idea that the genetic fixation of this gene represents a regional event in the recent evolution of Arabidopsis. Additionally, RFL9 counts among the RFL genes that are probably controlled by short regulatory RNAs, and our results provides a potential explanation for such control, which in the case of RFL9 might have evolved to limit its detrimental effect on rps3 expression.

Accelerated evolution of the mitochondrial genome in an alloplasmic line of durum wheat

BACKGROUND

Wheat is an excellent plant species for nuclear mitochondrial interaction studies due to availability of large collection of alloplasmic lines. These lines exhibit different vegetative and physiological properties than their parents. To investigate the level of sequence changes introduced into the mitochondrial genome under the alloplasmic condition, three mitochondrial genomes of the Triticum-Aegilops species were sequenced: 1) durum alloplasmic line with the Ae. longissima cytoplasm that carries the T. turgidum nucleus designated as (lo) durum, 2) the cytoplasmic donor line, and 3) the nuclear donor line.

RESULTS

The mitochondrial genome of the T. turgidum was 451,678 bp in length with high structural and nucleotide identity to the previously characterized T. aestivum genome. The assembled mitochondrial genome of the (lo) durum and the Ae. longissima were 431,959 bp and 399,005 bp in size, respectively. The high sequence coverage for all three genomes allowed analysis of heteroplasmy within each genome. The mitochondrial genome structure in the alloplasmic line was genetically distant from both maternal and paternal genomes. The alloplasmic durum and the Ae. longissima carry the same versions of atp6, nad6, rps19-p, cob and cox2 exon 2 which are different from the T. turgidum parent. Evidence of paternal leakage was also observed by analyzing nad9 and orf359 among all three lines. Nucleotide search identified a number of open reading frames, of which 27 were specific to the (lo) durum line.

CONCLUSIONS

Several heteroplasmic regions were observed within genes and intergenic regions of the mitochondrial genomes of all three lines. The number of rearrangements and nucleotide changes in the mitochondrial genome of the alloplasmic line that have occurred in less than half a century was significant considering the high sequence conservation between the T. turgidum and the T. aestivum that diverged from each other 10,000 years ago. We showed that the changes in genes were not limited to paternal leakage but were sufficiently significant to suggest that other mechanisms, such as recombination and mutation, were responsible. The newly formed ORFs, differences in gene sequences and copy numbers, heteroplasmy, and substoichiometric changes show the potential of the alloplasmic condition to accelerate evolution towards forming new mitochondrial genomes.

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.

Towards positional cloning in Brassica napus: generation and analysis of doubled haploid B. rapa possessing the B. napus pol CMS and Rfp nuclear restorer gene

The Polima (pol) system of cytoplasmic male sterility (CMS) and its fertility restorer gene Rfp are used in hybrid rapeseed production in Brassica napus. To facilitate map-based cloning of the Rfp gene, we have successfully transferred the pol cytoplasm and Rfp from the amphidiploid B. napus to the diploid species B. rapa and generated a doubled haploid pol cytoplasm B. rapa population that segregates for the Rfp gene. This was achieved through interspecific crosses, in vitro rescue of hybrid embryos, backcrosses, and microspore culture. Male fertility conditioned by Rfp was shown to co-segregate in this population with Rfp-specific mitochondrial transcript modifications and with DNA markers previously shown to be linked to Rfp in B. napus. The selfed-progeny of one doubled haploid plant were confirmed to be characteristic B. rapa diploids by cytogenetic analysis. Clones recovered from a genomic library derived from this plant line using the RFLP probe cRF1 fell into several distinct physical contigs, one of which contained Rfp-linked polymorphic restriction fragments detected by this probe. This indicates that chromosomal DNA segments anchored in the Rfp region can be recovered from this library and that the library may therefore prove to be a useful resource for the eventual isolation of the Rfp gene.

Pistillody, homeotic transformation of stamens into pistil-like structures, caused by nuclear-cytoplasm interaction in wheat

Homeotic transformation of stamens into pistil-like structures (pistillody) has been observed in a cytoplasmic substitution (alloplasmic) line of wheat (Triticum aestivum L.) cv. Norin 26, which has the cytoplasm of a wild relative species, Aegilops crassa L. On the other hand, an alloplasmic line of wheat cv. Chinese Spring (CS) with Ae. crassa cytoplasm has normal flowers. This is due to the presence in the CS nucleus of a fertility-restoring gene, Rfd1. Deletion mapping analysis revealed that Rfd1 is located on the middle part of the long arm of chromosome 7B. To investigate the function of the Rfd1 gene by a loss-of-function strategy, we produced alloplasmic lines of CS ditelosomic 7BS [(cr)-CSdt7BS] and CS monotelodisomic 7BS [(cr)-CSmd7BS] with the Ae. crassa cytoplasm, and characterized their phenotypes. The line (cr)-CSdt7BS without Rfd1 exhibited pistillody in all florets, and also female sterility. Scanning electron microscopy of the young spikes revealed that the pistillody was induced at an early stage of stamen development. The pistillate stamens often developed incomplete ovule-like structures with integuments instead of tapetum and pollen grains. It is possible that MADS box genes are associated with the induction of pistillody, because the expression of wheat APETALA3 homologue (WAP3) was reduced in the young spikes of (cr)-CSdt7BS. In addition, a histological study indicated that the female sterility in (cr)-CSdt7BS is due to the abnormality of the ovule, which fails to form an inner epidermis and integuments in the chalaza region. The line (cr)-CSmd7BS, hemizygous for Rfd1, showed partial pistillody (51%) and restored female fertility up to 72%. These results suggest that the induction of both pistillody and ovule deficiency caused by the Ae. crassa cytoplasm is inhibited by the Rfd1 gene in a dose-dependent manner.

Post-transcriptional and developmental regulation of a CMS-associated mitochondrial gene region by a nuclear restorer gene

Transcripts of the mitochondrial gene region orf224/atp6, which is associated with the Polima or pol cytoplasmic male sterility (CMS) of Brassica napus, differ among fertile, sterile and nuclear-restored plants. We show here that the effects of the restorer gene Rfp on orf224/atp6 transcripts varies among different floral organs. Relative to monocistronic atp6 transcripts, levels of the dicistronic transcripts spanning orf224 and atp6 are dramatically reduced in petals, stamens and carpels, but not sepals, of restored flowers. In pol CMS plants, the relative levels of different orf224/atp6 transcripts are similar among the floral organs. Analysis of guanylyltransferase-labeled mtRNA indicates that only the dicistronic 2.2 and 1.9 kb orf224/atp6 transcripts carry an initiator 5' terminus; hence the 1.4 and 1.3 kb transcripts of restored plants, as well as the 1.1 kb atp6 transcript common to all genotypes, are generated by RNA processing and not de novo initiation. Although steady-state levels of dicistronic transcripts in flower buds are lower in restored than in sterile plants, run-on transcription experiments show that these transcripts are synthesized at the same rate in both types of flowers. These findings imply that the restorer gene acts by conditioning the removal of sequences from the 5' end of dicistronic transcripts in a developmentally regulated manner. Run-on transcription experiments indicate that the single 1.1 kb atp6 transcript of nap cytoplasm is also generated by removal of sequences from the 5' end of a precursor. We suggest that specific endonucleolytic cleavage of a precursor RNA, followed by non-specific 3' to 5' exonuclease action, may represent a common mechanism for tailoring transcripts in plant mitochondria.

Nuclear genes associated with a single Brassica CMS restorer locus influence transcripts of three different mitochondrial gene regions

Previous studies have shown that the mitochondrial orf224/atp6 gene region is correlated with the Polima (pol) cytoplasmic male sterility (CMS) of Brassica napus. We now extend this correlation by showing that the effects of nuclear fertility restoration on orf224/atp6 transcripts cosegregate with the pol restorer gene Rfp1 in genetic crosses. We also show, however, that the recessive rfp1 allele, or a very tightly linked gene, acts as a dominant gene, designated Mmt (modifier of mitochondrial transcripts), in controlling the presence of additional smaller transcripts of the nad4 gene and a gene possibly involved in cytochrome c biogenesis. A common sequence, TTGTGG, maps immediately downstream of the 5' termini of both of the transcripts specific to plants with the Mmt gene and may serve as a recognition motif in generation of these transcripts. A similar sequence, TTGTTG, that may be recognized by the product of the alternate allele (or haplotype), Rfp1, is found within orf224 just downstream of the major 5' transcript terminus specific to fertility restored plants. Our results suggest that Rfp1/ Mmt is a novel nuclear genetic locus that affects the expression of multiple mitochondrial gene regions, with different alleles or haplotypes exerting specific effects on different mitochondrial genes.