Molecular mapping of 2 environmentally sensitive male-sterile mutants in soybean

Male sterility and hybrid breeding in soybean

Hybrid breeding can help us to meet the challenge of feeding a growing world population with limited agricultural land. The demand for soybean is expected to grow; however, the hybrid soybean is still in the process of commercialization even though considerable progress has been made in soybean genome and genetic studies in recent years. Here, we summarize recent advances in male sterility-based breeding programs and the current status of hybrid soybean breeding. A number of male-sterile lines with cytoplasmic male sterility (CMS), genic-controlled photoperiod/thermo-sensitive male sterility, and stable nuclear male sterility (GMS) have been identified in soybean. More than 40 hybrid soybean varieties have been bred using the CMS three-line hybrid system and the cultivation of hybrid soybean is still under way. The key to accelerating hybrid soybean breeding is to increase the out-crossing rate in an economical way. This review outlines current problems with the hybrid soybean breeding systems and explores the current efforts to make the hybrid soybean a commercial success.

Comparative transcriptome analysis provides insight into the important pathways and key genes related to the pollen abortion in the thermo-sensitive genic male sterile line 373S in Brassica napus L

The thermo-sensitive genic male sterility (TGMS) system plays a key role in the production of two-line hybrids in rapeseed (Brassica napus). To uncover key cellular events and genetic regulation associated with TGMS, a combined study using cytological methods and RNA-sequencing analysis was conducted for the rapeseed TGMS line 373S. Cytological studies showed that microspore cytoplasm of 373S plants was condensed, the microspore nucleus was degraded at an early stage, the exine was irregular, and the tapetum developed abnormally, eventually leading to male sterility. RNA-sequencing analysis identified 430 differentially expressed genes (298 upregulated and 132 downregulated) between the fertile and sterile samples. Gene ontology analysis demonstrated that the most highly represented biological processes included sporopollenin biosynthetic process, pollen exine formation, and extracellular matrix assembly. Kyoto encyclopedia of genes and genomes analysis indicated that the enriched pathways included amino acid metabolism, carbohydrate metabolism, and lipid metabolism. Moreover, 26 transcript factors were identified, which may be associated with abnormal tapetum degeneration and exine formation. Subsequently, 19 key genes were selected, which are considered to regulate pollen development and even participate in pollen exine formation. Our results will provide important insight into the molecular mechanisms underlying TGMS in rapeseed.

GmIAA27 Encodes an AUX/IAA Protein Involved in Dwarfing and Multi-Branching in Soybean

Soybean plant height and branching affect plant architecture and yield potential in soybean. In this study, the mutant dmbn was obtained by treating the cultivar Zhongpin 661 with ethylmethane sulfonate. The dmbn mutant plants were shorter and more branched than the wild type. The genetic analysis showed that the mutant trait was controlled by a semi-dominant gene. The candidate gene was fine-mapped to a 91 kb interval on Chromosome 9 by combining BSA-seq and linkage analysis. Sequence analysis revealed that Glyma.09g193000 encoding an Aux/IAA protein (GmIAA27) was mutated from C to T in the second exon of the coding region, resulting to amino acid substitution of proline to leucine. Overexpression of the mutant type of this gene in Arabidopsis thaliana inhibited apical dominance and promoted lateral branch development. Expression analysis of GmIAA27 and auxin response genes revealed that some GH3 genes were induced. GmIAA27 relies on auxin to interact with TIR1, whereas Gmiaa27 cannot interact with TIR1 owing to the mutation in the degron motif. Identification of this unique gene that controls soybean plant height and branch development provides a basis for investigating the mechanisms regulating soybean plant architecture development.

Molecular mapping of a novel male-sterile gene msNJ in soybean [Glycine max (L.) Merr.]

Nuclear male sterility (NMS) is a potential characteristic in crop recurrent selection and hybrid breeding. Mapping of nuclear male-sterile genes is key to utilizing NMS. Previously, we discovered a spontaneous soybean (Glycine max [L.] Merr.) male-sterile female-fertile mutant NJS-13H, which was conferred by a single recessive gene, designated msNJ. In this study, the msNJ was mapped to Chromosome 10 (LG O), and narrowed down between two SSR (simple sequence repeats) markers, BARCSOYSSR_10_794 and BARCSOYSSR_10_819 using three heterozygote-derived segregating populations, i.e., (NJS-13H × NN1138-2)F2, (NJS-13H × N2899)F2 and (NJS-13H)SPAG (segregating populations in advanced generations). This region spans approximately 1.32 Mb, where 27 genes were annotated according to the soybean reference genome sequence (Wm82.a2.v1). Among them, four genes were recognized as candidate genes for msNJ. Comparing to the physical locations of all the known male-sterile loci, msNJ is demonstrated to be a new male-sterile locus. This result may help the utilization and cloning of the gene.

Mutation in a PHD-finger protein MS4 causes male sterility in soybean

BACKGROUND

Male sterility has tremendous scientific and economic importance in hybrid seed production. Identification and characterization of a stable male sterility gene will be highly beneficial for making hybrid seed production economically feasible. In soybean, eleven male-sterile, female-fertile mutant lines (ms1, ms2, ms3, ms4, ms5, ms6, ms7, ms8, ms9, msMOS, and msp) have been identified and mapped onto various soybean chromosomes, however the causal genes responsible for male sterility are not isolated. The objective of this study was to identify and functionally characterize the gene responsible for the male sterility in the ms4 mutant.

RESULTS

The ms4 locus was fine mapped to a 216 kb region, which contains 23 protein-coding genes including Glyma.02G243200, an ortholog of Arabidopsis MALE MEIOCYTE DEATH 1 (MMD1), which is a Plant Homeodomain (PHD) protein involved in male fertility. Isolation and sequencing of Glyma.02G243200 from the ms4 mutant line showed a single base insertion in the 3rd exon causing a premature stop codon resulting in truncated protein production. Phylogenetic analysis showed presence of a homolog protein (MS4_homolog) encoded by the Glyma.14G212300 gene. Both proteins were clustered within legume-specific clade of the phylogenetic tree and were likely the result of segmental duplication during the paleoploidization events in soybean. The comparative expression analysis of Ms4 and Ms4_homologs across the soybean developmental and reproductive stages showed significantly higher expression of Ms4 in early flowering (flower bud differentiation) stage than its homolog. The functional complementation of Arabidopsis mmd1 mutant with the soybean Ms4 gene produced normal stamens, successful tetrad formation, fertile pollens and viable seeds, whereas the Ms4_homolog was not able to restore male fertility.

CONCLUSIONS

Overall, this is the first report, where map based cloning approach was employed to isolate and characterize a gene responsible for the male-sterile phenotype in soybean. Characterization of male sterility genes may facilitate the establishment of a stable male sterility system, highly desired for the viability of hybrid seed production in soybean. Additionally, translational genomics and genome editing technologies can be utilized to generate new male-sterile lines in other plant species.

Molecular mapping of three male-sterile, female-fertile mutants and generation of a comprehensive map of all known male sterility genes in soybean

In soybean, an environmentally stable male sterility system is vital for making hybrid seed production commercially viable. Eleven male-sterile, female-fertile mutants (ms1, ms2, ms3, ms4, ms5, ms6, ms7, ms8, ms9, msMOS, and msp) have been identified in soybean. Of these, eight (ms2, ms3, ms5, ms7, ms8, ms9, msMOS, and msp) have been mapped to soybean chromosomes. The objectives of this study were to (i) locate the ms1, ms4, and ms6 genes to soybean chromosomes; (ii) generate genetic linkage maps of the regions containing these genes; and (iii) develop a comprehensive map of all known male-sterile, female-fertile genes in soybean. The bulked segregant analysis technique was used to locate genes to soybean chromosomes. Microsatellite markers from the corresponding chromosomes were used on F2 populations to generate genetic linkage maps. The ms1 and ms6 genes were located on chromosome 13 (molecular linkage group F) and ms4 was present on chromosome 2 (molecular linkage group D1b). Molecular analyses revealed markers Satt516, BARCSOYSSR_02_1539, and AW186493 were located closest to ms1, ms4, and ms6, respectively. The ms1 and ms6 genes, although present on the same chromosome, were independently assorting with a genetic distance of 73.7 cM. Using information from this study and compiled information from previously published male sterility genes in soybean, a comprehensive genetic linkage map was generated. Eleven male sterility genes were present on seven soybean chromosomes. Four genes were present in two regions on chromosome 2 (molecular linkage group D1b) and two genes were present on chromosome 13 (molecular linkage group F).

Molecular Mapping of D₁, D₂ and ms5 Revealed Linkage between the Cotyledon Color Locus D₂ and the Male-Sterile Locus ms5 in Soybean

In soybean, genic male sterility can be utilized as a tool to develop hybrid seed. Several male-sterile, female-fertile mutants have been identified in soybean. The male-sterile, female-fertile ms5 mutant was selected after fast neutron irradiation. Male-sterility due to ms5 was associated with the “stay-green” cotyledon color mutation. The cotyledon color trait in soybean is controlled by two loci, D₁ and D₂. Association between cotyledon color and male-sterility can be instrumental in early phenotypic selection of sterility for hybrid seed production. The use of such selection methods saves time, money, and space, as fewer seeds need to be planted and screened for sterility. The objectives of this study were to compare anther development between male-fertile and male-sterile plants, to investigate the possible linkages among the Ms5, D₁ and D₂ loci, and to determine if any of the d₁ or d₂ mutations can be applied in hybrid seed production. The cytological analysis during anther development displayed optically clear, disintegrating microspores and enlarged, engorged pollen in the male-sterile, female-fertile ms5ms5 plants, a common characteristic of male-sterile mutants. The D₁ locus was mapped to molecular linkage group (MLG) D1a and was flanked by Satt408 and BARCSOYSSR_01_1622. The ms5 and D₂ loci were mapped to MLG B1 with a genetic distance ~12.8 cM between them. These results suggest that use of the d₂ mutant in the selection of male-sterile line may attenuate the cost hybrid seed production in soybean.