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

Breeding Brassica juncea hybrids with higher seed weight and oil content: Defining criteria for selection of parents

Most of the released high-yielding hybrids of Brassica juncea have a low 1000-seed weight (TSW) with no increment in the percent oil content (OC), and, therefore, these hybrids have poor acceptance among the farmers in India. It is, thus, imperative to understand the genetic basis of these traits and deploy them in commercial hybrid breeding programs. The present study utilized a set of 15 diverse B. juncea genotypes with TSW and OC ranging from 1.32 to 8.26 g and 31.93-43.39 %, respectively, to generate 210 hybrids following a full diallel mating scheme. These hybrids along with their parents, were evaluated in three different environments. Inheritance of TSW suggests the predominance of additive gene action, whereas non-additive gene action was observed to regulate OC. Further, TSW and OC were reported to be influenced by maternal and non-maternal effects, respectively. Parents with bold seeds viz., NPJ 253, RH 761 and EC 223389 were identified as good general combiners for both the traits. Hybrid generated from the cross NPJ 253 x NPJ 161, with both parents having high seed weights, exhibited the highest mean values (8.43 g) and heterobeltiosis (17.2 %) for TSW. Whereas, hybrid between parents NPJ 253 and IC 426372, possessing high and low seed weights, respectively, observed the highest mean value (44.95 %) and heterobeltiosis (14.89 %) for OC. Keeping both TSW and OC together, hybrids viz., NPJ 253 x EC 223389 (H1), NPJ 253 x NPJ 161 (H2) and NPJ 253 x Pusa Tarak (H4) were identified as promising using the Multi-trait Genotype-Ideotype Distance Index. Lines with higher TSW and better combining ability for OC shall be converted to male and/or female lines for generating commercial hybrids. The scheme for deploying higher seed weight and improved oil content in B. juncea hybrids is discussed.

Nucleo-cytoplasmic interactions affecting biological performance of Lipaphis erysimi in Brassica juncea

Hybrids have been successfully used to improve crop productivity, including Brassicas. Nucleo-cytoplasmic interactions have been reported to influence the expression of resistance to insect pests in several crops. We studied the effects of Cytoplasmic Male Sterility (CMS) in Brassica juncea carrying alien cytoplasms and their respective maintainer (B) lines on the antibiosis mechanism of resistance, involving development, survival, reproduction potential and population build-up of mustard aphid, Lipaphis erysimi, and the levels of defense phyto-chemicals. Present findings revealed that the numbers of aphids/plant, aphid multiplication rate and aphid resistance index were lower on ber CMS under natural, mori CMS under artificial infestation conditions, and juncea under both the test conditions indicating nucleo-cytoplasmic interactions for aphid reaction. Across cytoplasms, nymphal, reproductive and total developmental periods were significantly longer on SEJ 8, NPJ 161, LES 39, and NPJ 93, while the reproductive potential and survival were lower on PM 30, Pusa Tarak and SEJ 8 as compared to other nuclear backgrounds. Across nuclear backgrounds, nymphal, reproductive and total developmental periods were significantly longer on ber CMS, while reproductive potential and survival were lower on ber and mori CMS as compared to other cytoplasms. Total glucosinolates were significantly greater and myrosinase lower in Pusa Agrani, SEJ 8, LES 39, PM 30, NPJ 112, and Pusa Tarak as compared to the other nuclear backgrounds. Furthermore, total glucosinolates were significantly greater and myrosinase lower in ber CMS and juncea as compared to other cytoplasms. The studies suggest that CMS as well as cytoplasmic and nuclear gene interactions regulate the expression of defense compounds such as glucosinolates and determine the expression of resistance/susceptibility to L. erysimi. These findings shall help in identification of suitable L. erysimi tolerant nucleo-cytoplasmic combinations for their deployment in B. juncea hybrid breeding program.

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.

Mitochondrial genome and transcriptome analysis of five alloplasmic male-sterile lines in Brassica juncea

Background

Alloplasmic lines, in which the nuclear genome is combined with wild cytoplasm, are often characterized by cytoplasmic male sterility (CMS), regardless of whether it was derived from sexual or somatic hybridization with wild relatives. In this study, we sequenced and analyzed the mitochondrial genomes of five such alloplasmic lines in Brassica juncea.

Results

The assembled and annotated mitochondrial genomes of the five alloplasmic lines were found to have virtually identical gene contents. They preserved most of the ancestral mitochondrial segments, and the same candidate male sterility gene (orf108) was found harbored in mitotype-specific sequences. We also detected promiscuous sequences of chloroplast origin that were conserved among plants of the Brassicaceae, and found the RNA editing profiles to vary across the five mitochondrial genomes.

Conclusions

On the basis of our characterization of the genetic nature of five alloplasmic mitochondrial genomes, we speculated that the putative candidate male sterility gene orf108 may not be responsible for the CMS observed in Brassica oxyrrhina and Diplotaxis catholica. Furthermore, we propose the potential coincidence of CMS in alloplasmic lines. Our findings lay the foundation for further elucidation of male sterility gene.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5721-2) contains supplementary material, which is available to authorized users.

Mitochondrial dynamics and the cell cycle

Nuclear-mitochondrial (NM) communication impacts many aspects of plant development including vigor, sterility, and viability. Dynamic changes in mitochondrial number, shape, size, and cellular location takes place during the cell cycle possibly impacting the process itself and leading to distribution of this organelle into daughter cells. The genes that underlie these changes are beginning to be identified in model plants such as Arabidopsis. In animals disruption of the drp1 gene, a homolog to the plant drp3A and drp3B, delays mitochondrial division. This mutation results in increased aneuploidy due to chromosome mis-segregation. It remains to be discovered if a similar outcome is observed in plants. Alloplasmic lines provide an opportunity to understand the communication between the cytoplasmic organelles and the nucleus. Examples of studies in these lines, especially from the extensive collection in wheat, point to the role of mitochondria in chromosome movement, pollen fertility and other aspects of development.

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.

Evidence of restoration cost in the annual gynodioecious Phacelia dubia

A negative pleiotropic effect on fitness of nuclear sex-determining genes (cost of restoration) could explain nuclear-cytoplasmic gynodioecy but rarely has been demonstrated empirically. In a gynodioecious Phacelia dubia population, maternal lineages produce only hermaphroditic progenies irrespective of the pollen parent (N) or can segregate females (S). Natural progenies of N maternal plants had lower seed viability than that of S. Full-sib progenies of unrelated hermaphrodites from all possible matings between N and S lineages had similar pollen filling but differed in sporophyte performance, mainly at seed germination stage. A discrete multivariate analysis reveals that the performance of N(female symbol) x S(male symbol) progeny at early stages of development was significantly lower than that of the other three types of mating in agreement with the silent-cost-of-restoration hypothesis, affecting the sporophyte. The restoration cost and male sterility appear to be dominant and consequence of nuclear-cytoplasmic incompatibilities that may maintain nuclear-cytoplasmic polymorphism by frequency-dependent selection.

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.

Arabidopsis thaliana chromosome III restores fertility in a cytoplasmic male-sterile Brassica napus line with A. thaliana mitochondrial DNA

Somatic Brassica napus (+) Arabidopsis thaliana hybrids with a cytoplasmic male sterility (CMS)-inducing cytoplasm were screened for fertility-restored plants. One line was selected and recurrently backcrossed with the maintainer line, B. napus, resulting in fertile/sterile segregating populations. Restriction fragment length polymorphism mapping showed the co-segregation of A. thaliana chromosome (chr) III markers with the fertility trait. As it was not possible to stabilise the fertility trait via selfings, a dihaploidisation strategy was assessed. Ninety haploid plants were regenerated and analysed with numerous simple sequence length polymorphism (SSLP) markers. Markers covering both arms of A. thaliana chr III were present in two plants, whereas no A. thaliana DNA could be detected in the other plants. Following colchicine-induced chromosome doubling only these two plants with A. thaliana DNA produced fertile offspring. In one of the two lines, however, the A. thaliana-specific DNA markers and fertility were lost in subsequent generations. The other line remained fertile after repeated selfings. Using genomic in situ hybridisation (GISH) we were able to demonstrate that this latter line possessed a disomic addition of the A. thaliana chromosome. The restored line was comparable to the maintainer line with respect to flower morphology, but the petals and stamens were slightly reduced in size. The homeotic conversion of stamens to pistil-like structures, which is typical for the CMS line, was reversed, and stamens with a normal appearance with viable pollen appeared. Flowering time was as in the CMS line-in both lines it was delayed in comparison to the maintainer line. The introgressed chromosome also contributes to several pleiotropic effects, such as reduced leaf crinkling and shorter stems. The ability to restore fertility through the introgression of nuclear genes from the main cytoplasmic donor species indicates that the CMS trait in this system mainly is due to B. napus/ A. thaliana alloplasmic incompatibility and not mitochondrial DNA rearrangements. Further exploitation of the material is discussed.

Intergeneric hybridization between Erucastrum cardaminoides and two diploid crop Brassica species

Two intergeneric hybrids involving wild species Erucastrum cardaminoides (2 n=18, E(cd) E(cd)) and two crop brassica species, Brassica rapa (2 n=20, AA) and B. nigra (2 n=16, BB), were synthesized through in vitro sequential ovary culture. Morphological, molecular and cytological studies were conducted to establish their hybridity. Both hybrids, though morphologically distinct, were intermediate phenotypically between their respective parents. Cytological analysis of the E. cardaminoides x B. rapa hybrid (2 n=19), revealed the occurrence of 17 I+1 II at diakinesis/metaphase in the majority (28%) of the pollen mother cells (PMCs), whereas in E. cardaminoides x B. nigra hybrid (2 n=17), 13 I+2 II was the predominant (32%) meiotic configuration. A maximum of 5 II was recorded in both hybrids, indicating homoeologous pairing in the respective combined genomes. Chromosome doubling by colchicine application gave rise to two new amphiploids (AA E(cd)E(cd) and BB E(cd)E(cd)) having normal chromosome pairing and pollen fertility. The occasional occurrence of one quadrivalent in the amphiploids confirmed partial homoeology between the E(c) and A/B genomes. The E. cardaminoides x B. nigra hybrid and amphiploid appeared to be tolerant to alternaria blight under field conditions.