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

Mutations in nuclear genes encoding mitochondrial ribosome proteins restore pollen fertility in S male-sterile maize

The interaction of plant mitochondrial and nuclear genetic systems is exemplified by mitochondria-encoded cytoplasmic male sterility (CMS) under the control of nuclear restorer-of-fertility genes. The S type of CMS in maize is characterized by a pollen collapse phenotype and a unique paradigm for fertility restoration in which numerous nuclear restorer-of-fertility lethal mutations rescue pollen function but condition homozygous-lethal seed phenotypes. Two nonallelic restorer mutations recovered from Mutator transposon-active lines were investigated to determine the mechanisms of pollen fertility restoration and seed lethality. Mu Illumina sequencing of transposon-flanking regions identified insertion alleles of nuclear genes encoding mitochondrial ribosomal proteins RPL6 and RPL14 as candidate restorer-of-fertility lethal mutations. Both candidates were associated with lowered abundance of mitochondria-encoded proteins in developing maize pollen, and the rpl14 mutant candidate was confirmed by independent insertion alleles. While the restored pollen functioned despite reduced accumulation of mitochondrial respiratory proteins, normal-cytoplasm plants heterozygous for the mutant alleles showed a significant pollen transmission bias in favor of the nonmutant Rpl6 and Rpl14 alleles. CMS-S fertility restoration affords a unique forward genetic approach to investigate the mitochondrial requirements for, and contributions to, pollen and seed development.

Stigma and Glume Characteristics Synergistically Determine the Stigma Exsertion Rate in Thermo-Photo-Sensitive Genic Male Sterile Wheat

Wheat hybrids have been widely demonstrated to have remarkable heterosis or hybrid vigor in increasing yield potential and stability since the 1960s. Two-line hybrid wheat can achieve higher yields than local varieties, especially in marginal environments. However, the commercial application of hybrid wheat is hindered by higher seed costs, primarily due to lower yields in hybrid seed production. Stigma exsertion has been verified as a decisive factor in increasing rice's hybrid seed yield, but more investigation is needed in hybrid wheat breeding and production. In this study, four thermo-photo-sensitive genic male sterile lines, including K41S, K64S, K66S, and K68S, with different stigma exsertion rates (SERs) were used to compare the differences in floral architecture relative to stigma exsertion over two growing seasons. The results revealed that the K41S and K64S exhibited a relatively higher SER at 21.87% and 22.81%, respectively. No exserted stigma was observed in K66S, and K68S had an SER of only 0.82%. This study found that the stigma length, glume width and the length-width ratio of the glume were significantly correlated with the SER, with correlation coefficients of 0.46, -0.46 and 0.60, respectively. Other stigma features such as the branch angle, stretch width and hairbrush length, as well as the glume length, also had a weakly positive correlation with SER (r = 0.09-0.27). For K41S and K64S, the SER was significantly affected by the differences in the stigma branch angle and stigma stretch width among florets. A cross-pollination survey showed that the out-crossing ability of florets with an exserted stigma was about three times as high as that of florets with a non-exserted stigma. As a result, the stigma-exserted florets that accounted for 21.87% and 22.81% of the total florets in K41S and K64S produced 46.80% and 48.53% of the total cross-pollinated seeds in both sterile lines. These findings suggest that a longer stigma combined with a slender glume appears to be the essential floral feature of stigma exsertion in sterile wheat lines. It is expected that breeding and utilizing sterile lines with a higher SER would be a promising solution to cost-effective hybrid wheat seed production.

Factors Influencing the Emergence of Heterogeneous Populations of Common Bean (Phaseolus vulgaris L.) and Their Potential for Intercropping

The common bean is an important legume valued for its protein-rich seeds and its ability to fix nitrogen, making it a key element of crop rotation. In conventional agriculture, the emphasis is on uniformity and genetic purity to optimize crop performance and maximize yields. This is due to both the legal obligations to register varieties and the challenges of implementing breeding programs to create genetically diverse varieties. This paper focuses on the factors that influence the occurrence of heterogeneous common bean populations. The main factors contributing to this diversity have been described, including local adaptations, variable weather conditions, different pollinator species, and intricate interactions between genes controlling seed coat colour. We also discuss the benefits of intercropping common beans for organic farming systems, highlighting the improvement in resistance to diseases, and adverse environmental conditions. This paper contributes to a better understanding of common bean seed heterogeneity and the legal obligation to use heterogeneous populations.

One-step creation of CMS lines using a BoCENH3-based haploid induction system in Brassica crop

Heterosis utilization in a large proportion of crops depends on the use of cytoplasmic male sterility (CMS) tools, requiring the development of homozygous fertile lines and CMS lines1. Although doubled haploid (DH) technology has been developed for several crops to rapidly generate fertile lines2,3, CMS lines are generally created by multiple rounds of backcrossing, which is time consuming and expensive4. Here we describe a method for generating both homozygous fertile and CMS lines through in vivo paternal haploid induction (HI). We generated in-frame deletion and restored frameshift mutants of BoCENH3 in Brassica oleracea using the CRISPR/Cas9 system. The mutants induced paternal haploids by outcrossing. We subsequently generated HI lines with CMS cytoplasm, which enabled the generation of homozygous CMS lines in one step. The BoCENH3-based HI system provides a new DH technology to accelerate breeding in Brassica and other crops.

Analysis of the Candidate Genes and Underlying Molecular Mechanism of P198, an RNAi-Related Dwarf and Sterile Line

The genome-wide long hairpin RNA interference (lhRNAi) library is an important resource for plant gene function research. Molecularly characterizing lhRNAi mutant lines is crucial for identifying candidate genes associated with corresponding phenotypes. In this study, a dwarf and sterile line named P198 was screened from the Brassica napus (B. napus) RNAi library. Three different methods confirmed that eight copies of T-DNA are present in the P198 genome. However, only four insertion positions were identified in three chromosomes using fusion primer and nested integrated polymerase chain reaction. Therefore, the T-DNA insertion sites and copy number were further investigated using Oxford Nanopore Technologies (ONT) sequencing, and it was found that at least seven copies of T-DNA were inserted into three insertion sites. Based on the obtained T-DNA insertion sites and hairpin RNA (hpRNA) cassette sequences, three candidate genes related to the P198 phenotype were identified. Furthermore, the potential differentially expressed genes and pathways involved in the dwarfism and sterility phenotype of P198 were investigated by RNA-seq. These results demonstrate the advantage of applying ONT sequencing to investigate the molecular characteristics of transgenic lines and expand our understanding of the complex molecular mechanism of dwarfism and male sterility in B. napus.