Hybrid sorghum breeding in China: A historical review and perspectives.

The exploitation of male sterility systems has enabled the commercialization of heterosis in rice, with greatly increased yield and total production of this major staple food crop. Hybrid rice, which was adopted in the 1970s, now covers nearly 13.6 million hectares each year in China alone. Various types of cytoplasmic male sterility (CMS) and environment-conditioned genic male sterility (EGMS) systems have been applied in hybrid rice production. In this paper, recent advances in genetics, biochemistry, and molecular biology are reviewed with an emphasis on major male sterility systems in rice: five CMS systems, i.e., BT-, HL-, WA-, LD- and CW- CMS, and two EGMS systems, i.e., photoperiod- and temperature-sensitive genic male sterility (P/TGMS). The interaction of chimeric mitochondrial genes with nuclear genes causes CMS, which may be restored by restorer of fertility (Rf) genes. The PGMS, on the other hand, is conditioned by a non-coding RNA gene. A survey of the various CMS and EGMS lines used in hybrid rice production over the past three decades shows that the two-line system utilizing EGMS lines is playing a steadily larger role and TGMS lines predominate the current two-line system for hybrid rice production. The findings and experience gained during development and application of, and research on male sterility in rice not only advanced our understanding but also shed light on applications to other crops.Electronic supplementary materialThe online version of this article (doi:10.1186/s12284-014-0013-6) contains supplementary material, which is available to authorized users.

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.

Fertility restoration in three CMS systems of eggplant by the Rf genes of each other's systems and their SCAR marker

The cause of Cytoplasmic Genetic Male Sterility (CGMS) is specific nuclear and mitochondrial interactions. Almost all commercial sorghum hybrids were developed using the A1 cytoplasmic genetic male sterility system. Understanding the inheritance of fertility restoration in sorghum for A1 cytoplasm, for example, can improve the selection efficiency of restorer lines for increased seed production. In a cross of male sterile line 296A with A1 cytoplasm and restorer lines comprised of a set of Recombinant Inbred Lines (RILs), the inheritance pattern of fertility restoration of sorghum was studied. The F1 hybrid was completely fertile, revealing the dominant nature of fertility restoration, which is controlled by one or two major genes with modifiers. In this study, the genetics of fertility restoration of the A1 cytoplasmic nuclear male sterility system (CGMS) in sorghum were investigated in segregating F2 and BC1 populations of A1 cytoplasm crosses. Fertility restoration was governed by a monogenic inheritance (3F:1S) mechanism represented by a single dominant gene responsible for fertility restoration in all of the crosses studied.

Wide crosses were made to identify new cytoplasmic male sterility (CMS) systems in faba beans, based on the interaction of cytoplasm with restorer and maintainer alleles. A total of 330 F1 hybrids were produced in both reciprocal forms. Male sterile segregates were observed in one reciprocal version in the F2 generation of six crosses. Two of these crosses with female parents originating from Afghanistan and Egypt expressed stable male sterility in subsequent backcross generations. Based on the female parents of the two crosses, these two CMS systems were designated CMS 199 and CMS 297. CMS 199 was more stable than CMS 297 during backcross generations and across different environments. Maintainer and restorer lines for both CMS systems were identified. Lower expression of male sterility occurred in CMS 297 in the greenhouse during the winter generations than in isolation cages during the summer generations, which may be utilized to maintain male sterile lines by selfing. Regarding practical applications, the CMS 199 shows great promise for hybrid breeding in faba beans.

Food Security Under The Era Of Climate Change Threat

Sorghum bicolor (L. Moench) is a cereal grain and forage crop that is grown across tropical and temperate regions of the world. Sorghum has a complete flower, resulting in self-pollination as the primary form of reproduction, but it is also grown commercially as a hybrid. Consequently, methods of cross-pollination for both breeding and hybrid seed production are important. In sorghum breeding, current methods of cross-pollination are effective, but they have limitations in terms of achieving complete and temporal male sterility. With the development of new breeding approaches, such as doubled haploids, temporal male sterility is essential. Temporal male sterility would also be useful in testing new seed parent lines prior to an investment in sterilization of the line in the cytoplasmic male sterility system. The objective of this study was to evaluate the efficacy of trifluoromethanesulfonamide (TFMSA) as a sorghum male gametocide under field conditions. TFMSA was foliarly applied to three male-fertile parental lines, in two environments, using a pipette and a sprayer, respectively, in dosages ranging from 5 to 30 mg/plant. Repeated applications over time for the 10 and 15 mg dosage rates were conducted on a subset of individual plants. The results indicate that once a minimum dosage threshold (between 10 and 15 mg) was reached, panicles became male sterile. Additional dosages and number of applications had little overall effect, and both hand-applied and sprayer-applied TFMSA had similar male sterility induction capability. From these studies, it appears that TFMSA can be used as an effective chemical male gametocide on sorghum under field conditions.

The genetic relationship among three cytoplasmic male sterility (CMS) systems, consisting of WA, Dissi, and Gambiaca, was studied. The results showed that the maintainers of one CMS system can also maintain sterility in other cytoplasmic backgrounds. The F1 plants derived from crosses involving A and R lines of the respective cytoplasm and their cross-combination with other CMS systems showed similar pollen and spikelet fertility values, indicating that similar biological processes govern fertility restoration in these three CMS systems. The results from an inheritance study showed that the pollen fertility restoration in all three CMS systems was governed by two independent and dominant genes with classical duplicate gene action. Three F2 populations, generated from the crosses between the parents of good-performing rice hybrids, that possess WA, Dissi, and Gambiaca CMS cytoplasm, were used to map the Rf genes. For the WA-CMS system, Rf3 was located at a distance of 2.8 cM from RM490 on chromosome 1 and Rf4 was located at 1.6 cM from RM1108 on chromosome 10. For the Dissi-CMS system, Rf3 was located on chromosome 1 at 1.9 cM from RM7466 and Rf4 on chromosome 10 was located at 2.3 cM from RM6100. The effect of Rf3 on pollen fertility appeared to be stronger than the effect of Rf4. In the Gambiaca-CMS system, only one major locus was mapped on chromosome 1 at 2.1 cM from RM576. These studies have led to the development of marker-assisted selection (MAS) for selecting putative restorer lines, new approaches to alloplasmic line breeding, and the transfer of Rf genes into adapted cultivars through a backcrossing program in an active hybrid rice breeding program.

Hybrid sorghum [Sorghum bicolor (L.) Moench] seed production relies exclusively on cytoplasmic male sterility (CMS) systems and almost all hybrid sorghum seed is produced using the A1 CMS system. However, the reliance on a single CMS system increases the vulnerability of the crop to diseases and stresses that may attack that particular CMS system. Alternative CMS systems have been described and even used on a limited basis for hybrid seed production, but a direct comparison of the agronomic effects of different cytoplasms has not been possible because male‐sterile lines with a common genetic background (and different cytoplasm) were not available. The recent development of isocytoplasmic A‐lines allows more direct comparison of cytoplasmic effect on agronomic performance. The purpose of this study was to determine by means of a set of isocytoplasmic hybrids if cytoplasm per se influences agronomic performance. Twelve hybrid genotypes were created in three different cytoplasms (A1, A2, and A3 for a total of 36 hybrids), and they were evaluated for plant height, days to anthesis, and grain yield at Weslaco and College Station, TX, in 1998 and 1999. As expected, significant differences existed among hybrids for plant height, days to anthesis, and grain yield. Cytoplasm type had no effect on plant height and was of minimal practical effect on days to anthesis, but a significant reduction in yield was observed in A3 cytoplasm hybrids as compared with A1 and A2 cytoplasm hybrids. The specific reason for the reduced yield of A3 hybrids is not known, but seed set data indicated that it was not associated with fertility restoration. The results indicate that hybrids created in A2 cytoplasm yield comparably to the commonly used A1 cytoplasm and therefore, the A2 system will provide a suitable alternative for hybrid seed production should problems be encountered in the A1 CMS system.

Cytoplasmic male sterile system in ridge gourd has been converted to cytoplasmic genetic male sterile (CGMS) system through the development of analogues of male sterile (MS) line, maintainer line and fertility restorer line. These lines were developed by crossing the MS mutant, regenerated through in vitro culture, with monoecious pollen parents Deepthi, Haritham, LA 101, CO 2, IC 92761 and IC 92685. All hybrids and the BC1 generation developed by crossing with the recurring pollen parents Deepthi, Haritham and LA 101 were male sterile. Male sterile BC1 plants have been advanced to BC6 generation and the parental line LA 101 was proved to be a successful maintainer line, producing male sterile progeny in successive back cross generations. Analogue of cytoplasmic male sterile line, MS LA 101, was developed through back crossing and on crossing with fertility restorer lines Arka Sumeet and LA 102, this line excelled as female parent, resulting heterotic combinations. Mitochondrial marker rpS14 and SCAR Tm-53 were identified to yield male sterility specific markers whereas SSR marker 18956 has generated the male fertility specific marker. These primers are recommended for marker assisted selection of ridge gourd, for utilizing male sterility for hybrid seed production and for developing A, B and C lines in CGMS system.

Sorghum [Sorghum bicolor (L.) Moench] germplasm lines Tx3489 (Reg. no. GP‐943, PI 698649) and Tx3490 (Reg. no GP‐944, PI 698650) with yellow seed, favorable agronomics, and popping attributes in hybrid combinations were developed by the Texas A&M AgriLife Research sorghum breeding and genetics program in 2020. Compared with a grain sorghum hybrid check, these lines produced hybrids with similar agronomic performance and superior popping performance. In hybrid combinations, the two lines produced hybrids comparable to the agronomic productivity of standard grain sorghum hybrids. Additionally, these two lines produced hybrids with 84 and 78% popping efficiency, 9:1 and 7.4:1 expansion ratios and 0.35‐ and 0.34‐cm3 flake sizes. In contrast, the check hybrid produced grain with 74% popping efficiency, 6.3:1 expansion ratio and a 0.25‐cm3 flake. Ultimately these two lines produce grain sorghum hybrids with comparable agronomic productivity and superior popping performance. While these lines can be used as pollinator parents to produce grain sorghum hybrids for popping, they may also be a parent for the development of new pop sorghum parental lines.

Heterosis and combining ability for floral and yield characters in rice using cytoplasmic male sterility system

Six cytoplasmic male sterility (CMS) systems, viz. moricandia, ogura, oxyrrhina, siifolia, tournefortii and trachystoma of Indian mustard (Brassica juncea L.) were characterized for agronomic and floral characteristics. Introgression of alien cytoplasm caused alterations in different floral traits in ogura, siifolia, tournefortii and trachystoma CMS systems. Varied response to different genetic backgrounds of CMS lines indicated the presence of cytoplasmic–nuclear interaction in alteration of floral traits. On the basis of floral characteristics, CMS systems could be grouped into distinct classes. Siifolia, tournefortii and trachystoma CMS lines had narrow petals, while moricandia, ogura and oxyrrhina had wider petals, which were distinguishable on the basis of visual observations. The ratio between length and width of petals were >2.0 in wide petal group but <2.0 in narrow petal group. Further, the relative position of anther and stigma, which was estimated as the ratio between stamen and style length could differentiate the CMS systems. Stamens were longer than styles in oxyrrhina, equal in moricandia and shorter in ogura, siifolia, tournefortii and trachystoma male sterile lines. Non‐viable pollen grains were present in moricandia and oxyrrhina systems, but absent in other systems. In tournefortii and trachystoma, few flowers showed petaloid corolla. All male sterile lines, except trachystoma, which showed crooked siliqua formation were at par with their respective maintainers for flower initiation, plant height, primary branches, seeds per siliqua, seed yield, harvest index, oil and protein content. In general, flower senescence and maturity occurred earlier in male sterile lines than in their respective maintainer lines.

A cytoplasmic male-sterility system has been developed in mustard (Brassica juncea) following repeated backcrossings of the somatic hybrid Moricandia arvensis (2n=28, MM)+B. juncea (2n=36, AABB), carrying mitochondria and chloroplasts from M. arvensis, to Brassica juncea. Cytoplasmic male-sterile (CMS) plants are similar to normal B. juncea; however, the leaves exhibit severe chlorosis resulting in delayed flowering. Flowers are normal with slender, non-dehiscent anthers and excellent nectaries. CMS plants show regular meiosis with pollen degeneration occurring during microsporogenesis. Female fertility was normal. Genetic information for fertility restoration was introgressed following the development of a M. arvensis monosomic addition line on CMS B. juncea. The additional chromosome paired allosyndetically with one of the B. juncea bivalents and allowed introgression. The putative restorer plant also exhibited severe chlorosis similar to CMS plants but possessed 89% and 73% pollen and seed fertility, respectively, which subsequently increased to 96% and 87% in the selfed progeny. The progeny of the cross of CMS line with the restorer line MJR-15, segregated into 1 fertile : 1 sterile. The CMS (Moricandia) B. juncea, the restorer (MJR-15), and fertility restored F1 plants possess similar cytoplasmic organellar genomes as revealed by ‘Southern’ analysis.

The Polima cytoplasmic male sterility (CMS) system has been successfully used in three/two-line hybrid production in rapeseed (Brassica napus L.). However, the sterility of the Polima (pol) CMS lines is sensitive to temperature fluctuations. Also, traces of pollen can cause self-pollination within the CMS lines, which results in reduced levels of F1 hybrid seed purity and leads to a significant yield loss. Self-incompatibility (SI) is another important approach for hybrid seed production in rapeseed. Despite having a wide range of restorers and being easily selected in a breeding program, SI system has some drawbacks. In this study, SI genes from a self-incompatible line of Brassica napus were transferred to a pol CMS line and S372A, a novel line of combined cytoplasmic male sterility with self-incompatibility was bred. Due to the SI genes, this line produced very few seeds when it was selfed at low temperature and no seeds at high temperature. This suggested that the line with CMS + SI had combined the advantages and overcome the disadvantages of both the pol CMS and SI systems. Furthermore, our results showed that most of the maintainers and all the restorers of the pol CMS system were also maintainers and restorers of the CMS + SI line, respectively. This indicates that the CMS + SI system can be easily used to establish three-line hybrids of rapeseed, and we believe this novel system could be extended to other species of Brassica.