Transfer of Ogu cytoplasmic male sterility to Brassica juncea and improvement of the male sterile line through somatic cell fusion

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.

Electrophysiological, Morphologic, and Transcriptomic Profiling of the Ogura-CMS, DGMS and Maintainer Broccoli Lines

To better serve breeding of broccoli, the electrophysiological, morphological and transcriptomic profiling of the isogenic Ogura-CMS, DGMS and their maintainer fertile lines, were carried out by scanning electron microscopy, investigation of agronomic traits and RNA-sequencing analysis. The agronomic traits of plant height, length of the largest leaf, plant spread angle, single head weight, head width and stem diameter showed stronger performance in Ogura-CMS broccoli than in DGMS line or maintainer fertile line. However, the Ogura-CMS broccoli was poorer in the seed yield and seed germination than in the DGMS line and maintainer fertile line. Additionally, the DGMS broccoli had longer maturation and flowering periods than the Ogura-CMS and maintainer fertile lines. There were obvious differences in the honey gland, happening in the male sterility and fertile lines of broccoli. Additionally, the mechanism regulating Ogura-CMS and DGMS in broccoli was investigated using florets transcriptome analyses of the Ogura-CMS, DGMS and maintainer fertile lines. As a result, a total of 2670 differentially expressed genes (DEGs) were detected, including 1054 up- and 1616 downregulated genes in the Ogura-CMS and DGMS lines compared to the maintainer fertile line. A number of functionally known genes involved in plant hormones (auxin, salicylic acid and brassinosteroid), five Mitochondrial Oxidative Phosphorylation (OXPHOS) genes of atp8, LOC106319879, LOC106324734, LOC106314622 and LOC106298585, and three upregulated genes (Lhcb1, Lhcb3 and Lhcb5) associated with the photosynthesis-antenna protein pathway, were obviously detected to be highly associated with reproductive development including flowering time, maturity and reproductive period in the Ogura-CMS and DGMS broccoli comparing to their maintainer fertile line. Our research would provide a comprehensive foundation for understanding the differences of electrophysiological, morphological and transcriptomic profiles in the Ogura-CMS, DGMS and maintainer broccoli, and as well as being beneficial to exploring the mechanism of male sterility in Brassica crops.

Exploiting sterility and fertility variation in cytoplasmic male sterile vegetable crops

Cytoplasmic male sterility (CMS) has long been used to economically produce hybrids that harness growth vigor through heterosis. Yet, how CMS systems operate within commercially viable seed production strategies in various economically important vegetable crops, and their underlying molecular mechanisms, are often overlooked details that could expand the utility of CMS as a cost-effective and stable system. We provide here an update on the nature of cytoplasmic-nuclear interplay for pollen sterility and fertility transitions in vegetable crops, based on the discovery of components of nuclear fertility restoration and reversion determinants. Within plant CMS systems, pollen fertility can be rescued by the introduction of nuclear fertility restorer genes (Rfs), which operate by varied mechanisms to countermand the sterility phenotype. By understanding these systems, it is now becoming feasible to achieve fertility restoration with Rfs designed for programmable CMS-associated open reading frames (ORFs). Likewise, new opportunities exist for targeted disruption of CMS-associated ORFs by mito-TALENs in crops where natural Rfs have not been readily identified, providing an alternative approach to recovering fertility of cytoplasmic male sterile lines in crops. Recent findings show that facultative gynodioecy, as a reproductive strategy, can coordinate the sterility and fertility transition in response to environmental cues and/or metabolic signals that reflect ecological conditions of reproductive isolation. This information is important to devising future systems that are more inherently stable.

Characterization of tribenuron-methyl-induced male sterility in Brassica juncea L

Significant heterosis has been documented in Brassica juncea L. that are grown as agriculturally important oilseeds, vegetables and condiments crops. Male sterility induced by chemical hybridizing agents is an important pollination control system in hybrid crop breeding. Herein, we show that tribenuron-methyl (TBM), a sulfonylurea herbicide, is an effective male gametocide in B. juncea when used at a very low dosage. In the present study, foliar application of various rates of TBM induced a significant increase in pollen sterility in B. juncea (90.57-100%). TBM-treated plants exhibited reductions in size of floral organ and yield components; however, lower dose of TBM (0.075 g a.i. ha^-1) did not cause a significant reduction in seed yield per plant. Tapetum cells of TBM-treated plants were hypertrophied and degenerated earlier, and abnormal meiosis was observed at the meiotic stage. A significant decrease of acetohydroxyacid synthase (AHAS) activities was detected in buds of plants treated with 0.10 g a.i. ha^-1 TBM, and RT-qPCR analysis showed that TBM exposure perturbed AHAS expression in small buds, which support that TBM induces male sterility in B. juncea by targeting AHAS expression. Our results suggest that TBM could be used as an efficient chemical hybridization agent in B. juncea, which has practical implications for the application of hybrid breeding in B. juncea.

Analysis of mitochondrial recombination in the male sterile Brassica juncea cybrid Og1 and identification of the molecular basis of fertility reversion

Recombinations between the parental genomes produced a novel mitochondrial genome in the cytoplasmic male sterile Brassica juncea cybrid Og1. A mitochondrial stoichiometric shift greatly reduced the molecule containing male-sterility-inducing orf138 gene leading to reversion to male fertility. An improved, chlorosis-corrected, cytoplasmic male sterile Brassica juncea cybrid Og1 derived from Ogura cytoplasm shows frequent reversion to male fertility. To determine the nature of mitochondrial recombination in the cybrid and to uncover the molecular mechanism of male fertility reversion, we sequenced the mitochondrial genomes of Og1, its isonuclear parental lines (OgRLM and Brassica juncea RLM198) and the revertant line (Og1-rt). Assembly of Og1 mitochondrial genome gave two circular molecules, Og1a (250.999 kbp) and Og1b (96.185 kbp) sharing two large direct repeat regions capable of recombining to form a single circular molecule. Og1a contains all essential mitochondrial genes, but the male-sterility-causing orf138 was uniquely present in Og1b along with 16 other complete or partial genes already represented in Og1a. Eleven and four recombinations between the parental mitochondrial genomes produced the Og1a and the Og1b molecules, respectively. Five genes were duplicated within Og1a, of which trnfM was inherited from both the parents while the other four genes, atp4, cox1 nad4L and trnM, were inherited from RLM198. RFLP analysis revealed that orf138-containing molecules were less abundant than Og1a in the male-sterile plants while og1b bearing molecules were undetectable in the revertant line. However, orf138 transcripts were amplified in RT-PCR and were also detected in northern blots revealing that Og1b molecules are not completely lost in the revertant plants. This is the first report where the mitochondrial genome of a cybrid is compared with its actual parents. The findings are discussed in the light of previous reports on mitochondrial genome recombination in cybrids.

Utilization of Ogura CMS germplasm with the clubroot resistance gene by fertility restoration and cytoplasm replacement in Brassica oleracea L

Clubroot disease, a major plant root disease caused by Plasmodiophora brassicae, has become one of the most destructive diseases among cultivated cruciferous vegetables. However, clubroot-resistant Brassica oleracea materials are rare. A few clubroot-resistant cabbage varieties are available on the market, but all are Ogura cytoplasmic male sterile (CMS) types. Therefore, in this study, to reutilize the clubroot-resistant Ogura CMS germplasm of cabbage, a new fertility-restored Ogura CMS material, 16Q2-11, was used as a bridge to transfer the clubroot resistance (CR) gene from the Ogura CMS cytoplasm to the normal cytoplasm by a two-step method (a fertility restoration and cytoplasm replacement method). In the first cross for fertility restoration of Ogura CMS clubroot-resistant cabbage (FRCRC), 16Q2-11 was used as a restorer to cross with Ogura CMS materials containing the CR gene CRb2. Eleven Rfo-positive progenies were generated, of which four contained CRb2: F8-514, F8-620, F8-732 and F8-839. After inoculation with race 4 of P. brassicae, these four CRb2-positive individuals showed resistance. Furthermore, F8-514 and F8-839 were then used as male parents in the second cross of FRCRC to cross with cabbage inbred lines, resulting in the successful introgression of the CRb2 gene into the inbred lines. All offspring produced from this step of cross, which had a normal cytoplasm, showed a high resistance to race 4 of P. brassicae and could be utilized for the breeding of clubroot-resistant cabbage varieties in the future. This is the first time that the Ogura CMS restorer has been used to restore the fertility of Ogura CMS clubroot-resistant cabbages, which could improve germplasm diversity in cabbage and provide a reference method for using CMS germplasm in Brassica crops.

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.

Transcript levels of orf288 are associated with the hau cytoplasmic male sterility system and altered nuclear gene expression in Brassica juncea

Cytoplasmic male sterility (CMS) is primarily caused by chimeric genes located in the mitochondrial genomes. In Brassica juncea, orf288 has been identified as a CMS-associated gene in the hau CMS line; however, neither the specific abortive stage nor the molecular function of the gene have been determined. We therefore characterized the hau CMS line, and found that defective mitochondria affect the development of archesporial cells during the L2 stage, leading to male sterility. The expression level of the orf288 transcript was higher in the male-sterility line than in the fertility-restorer line, although no significant differences were apparent at the protein level. The toxicity region of ORF288 was found to be located near the N-terminus and repressed growth of Escherichia coli. However, transgenic expression of different portions of ORF288 indicated that the region that causes male sterility resides between amino acids 73 and 288, the expression of which in E. coli did not result in growth inhibition. Transcriptome analysis revealed a wide range of genes involved in anther development and mitochondrial function that were differentially expressed in the hau CMS line. This study provides new insights into the hau CMS mechanism by which orf288 affects the fertility of Brassica juncea.

Development of male sterile Eruca sativa carrying a Raphanus sativus/Brassica oleracea cybrid cytoplasm

KEY MESSAGE

Alloplasmic male sterile breeding lines of Eruca sativa were developed by intergeneric hybridization with CMS- Brassica oleracea, followed by recurrent backcrosses and determination of the breeding value.

ABSTRACT

Male sterile breeding lines of rocket salad (Eruca sativa) were developed by intergeneric hybridization with cytoplasmic male sterile (CMS) cauliflower (Brassica oleracea) followed by recurrent backcrosses. Five amphidiploid F1 plants (2n = 2x = 20, CE), achieved by manual crosses and embryo rescue, showed an intermediate habit. The plants were completely male sterile and lacked seed set after pollination with the Eruca parent. Allotetraploid F1-hybrid plants (4n = 4x = 40, CCEE) obtained after colchicine treatment were backcrossed six times with pollen of the Eruca parent to select alloplasmic diploid E. sativa lines. The hybrid status and the nucleo-cytoplasmic constellation were continuously controlled by RAPD and Southern analysis during subsequent backcrosses. The ploidy level was investigated by flow cytometry and chromosome analysis. Premeiotic (sporophytic) and postmeiotic (pollen abortive) defects during the anther development were observed in the alloplasmic E. sativus plants in comparison to the CMS-cauliflower donor. No further incompatibilities were noticed between the CMS-inducing cybrid cytoplasm and the E. sativa nuclear genome. The final alloplasmic E. sativa lines were diploid with 2n = 2x = 22 chromosomes and revealed complete male sterility and restored female fertility. Plant vigor and yield potential of the CMS-E. sativa BC5 lines were comparable to the parental E. sativus line. In conclusion, the employed cybrid-cytoplasm has been proven as a vital source of CMS for E. sativa. The developed lines are directly applicable for hybrid breeding of rocket salad.

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.

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.

A duplicated coxI gene is associated with cytoplasmic male sterility in an alloplasmic Brassica juncea line derived from somatic hybridization with Diplotaxis catholica

A cytoplasmic male sterile (CMS) line of Brassica juncea was derived by repeated backcrossing of the somatic hybrid (Diplotaxis catholica + B. juncea) to B. juncea. The new CMS line is comparable to euplasmic lines for almost all characters, except for flowers which bear slender, needle-like anthers with aborted pollen. Detailed Southern analysis revealed two copies of coxI gene in the CMS line. One copy, coxI-1 is similar to the coxI gene of B. juncea, whereas the second copy, coxI-2 is present in a novel rearranged region. Northern analysis with eight mitochondrial gene probes showed altered transcript pattern only for the coxI gene. Two transcripts of 2.0 and 2.4 kb, respectively, were detected in the CMS line. The novel 2.4 kb transcript was present in floral bud tissue but absent in the leaf tissue. In plants where male sterility broke down under high temperature during the later part of the growing season, the 2.4 kb coxI transcript was absent, which suggested its association with the CMS. The two coxI genes from the CMS line showed two amino acid changes in the coding region. The novel coxI gene showed unique repeats in the 5' region suggesting recombination of mitochondrial genomes of the two species. The possible role of the duplicated coxI gene in causing male sterility is discussed.

Homeotic-like modification of stamens to petals is associated with aberrant mitochondrial gene expression in cytoplasmic male sterile Ogura Brassica juncea

We have previously reported correction of severe leaf chlorosis in the cytoplasmic male sterile Ogura (also called Ogu) Brassica juncea line carrying Ogura cytoplasm by plastid substitution via protoplast fusion. Two cybrids obtained from the fusion experiment, Og1 and Og2, were green and carried the plastid genome of B. juncea cv. RLM198. While Og1 displayed normal flower morphology comparable to that of its euplasmic B. juncea counterpart except for sterile anthers, Og2 retained homeotic-like floral modification of stamens to petal-like structures and several other floral deformities observed in the chlorotic (Ogu) B. juncea cv. RLM198 (or OgRLM). With respect to the mitochondrial genome, Og1 showed 81% genetic similarity to the fertile cultivar RLM while Og2 showed 93% similarity to OgRLM. In spite of recombination and rearrangements in the mitochondrial genomes in the cybrids, expression patterns of 10 out of 11 mitochondrial genes were similar in all the three CMS lines; the only exception was atp6, whose expression was altered. While Og1 showed normal atp6 transcript similar to that in RLM, in Og2 and OgRLM weak expression of a longer transcript was detected. These results suggest that the homeotic-like changes in floral patterning leading to petaloid stamens in Og2 and OgRLM may be associated with aberrant mitochondrial gene expression.

A new cytoplasmic male sterility system for hybrid seed production in Indian oilseed mustard Brassica juncea

We report a novel cytoplasmic male sterility (CMS) system in Brassica juncea (oilseed mustard) which could be used for production of hybrid seed in the crop. A male sterile plant identified in a microspore derived doubled haploid population of re-synthesized B. napus line ISN 706 was found to be a CMS as the trait was inherited from the female parent. This CMS, designated '126-1', was subsequently transferred to ten different B. juncea varieties and lines through inter-specific crosses followed by recurrent backcrossing. The F(1)s of inter-specific crosses were invariably partially fertile, but irrespective of the variety/line used, the recipient lines became progressively male sterile over five to seven generations and could be maintained by crossing the male sterile lines with their normal counterparts. The male sterile lines were found to be stable for the trait under both long and short day conditions. CMS lines when crossed with lines other than the respective maintainer line were restored for fertility, implying that any variety could act as a restorer for '126-1' cytoplasm in B. juncea. These unique features in maintenance and restoration of CMS lines coupled with near normal floral morphology of the CMS lines have allowed the use of '126-1' cytoplasm for hybrid seed production. The uniqueness of '126-1' has been further established by Southern hybridization with mitochondrial DNA probes and by a histological study of the development of male sterile anthers.

Brassica napus lines with rearranged Arabidopsis mitochondria display CMS and a range of developmental aberrations

Numerous Brassica napus (+) Arabidopsis thaliana somatic hybrids were screened for male sterility and aberrant flower phenotypes. Nine hybrids were selected and backcrossed recurrently to B. napus. The resulting lines displayed stable maternal inheritance of flower phenotypes. Nuclear and organellar genomes were characterized molecularly using RFLP analysis. No DNA from A. thaliana was found in the nuclear genome after six back-crosses, whilst the mitochondrial genomes contained rearranged DNA from both A. thaliana and B. napus. Each line tested had a unique RFLP pattern of the mitochondrial DNA (mtDNA) that remained unchanged between the BC(3) and BC(6) generation. The plastid genomes consisted of B. napus DNA. Five lines of the BC(5) generation were subjected to more comprehensive investigations of growth, morphology and fertility. On the basis of these investigations, the five CMS lines could be assigned to two groups, one represented by three lines displaying reduced vegetative development, complete male sterility, and homeotic conversions of stamens into feminized structures. The second group, represented by the other two lines, were not completely male-sterile but still displayed severely affected flower morphologies. These two lines did not display any reduction in vegetative development. For both groups only stamens and petals suffered from the morphological and functional aberrations, while the sepals and pistils displayed normal morphology. All plants were fully female-fertile. Different rearrangements of the mitochondrial genome disturbed nuclear-mitochondrial interactions and led to various types of aberrant growth and flower development. The existence of numerous CMS lines with different mitochondrial patterns involving a species with a sequenced genome offers new opportunities to investigate the genetic regulation of CMS and its associated developmental perturbations.

Alloplasmic male-sterile Brassica juncea with Enarthrocarpus lyratus cytoplasm and the introgression of gene(s) for fertility restoration from cytoplasm donor species

A new cytoplasmic male sterility (CMS) source in Brassica juncea (2n = 36; AABB) was developed by substituting its nucleus into the cytoplasm of Enarthrocarpus lyratus (2n = 20; E(l)E(l)). Male sterility was complete, stable and manifested in either petaloid- or rudimentary-anthers which were devoid of fertile pollen grains. Male sterile plants resembled the euplasmic B. juncea except for slight leaf yellowing and delayed maturity. Leaf yellowing was due mainly to higher level of carotenoids rather than a reduction in chlorophyll pigments. Female fertility in male-sterile plants varied; it was normal in lines having rudimentary anthers but poor in those with petaloid anthers. Each of the 62 evaluated germplasm lines of B. juncea was a functional maintainer of male sterility. The gene(s) for male-fertility restoration ( Rf) were introgressed from the cytoplasm donor species through homoeologous pairing between A and E(l) chromosomes in monosomic addition plants (2n = 18II+1E(l)). The percent pollen fertility of restored F(1) ( lyr CMS x putative restorer) plants ranged from 60 to 80%. This, however, was sufficient to ensure complete seed set upon by bag selfing. The CMS ( lyr) B. juncea compared favourably with the existing CMS systems for various productivity related characteristics. However, the reduced transmission frequency of the Rf gene(s) through pollen grains, which was evident from the sporadic occurrence of male-sterile plants in restored F(1) hybrids, remains a limitation.