Current insights and advances into plant male sterility: new precision breeding technology based on genome editing applications

Advances in gene editing-led route for hybrid breeding in crops

With the global demand for sustainable agriculture on the rise, RNA-guided nuclease technology offers transformative applications in crop breeding. Traditional hybrid breeding methods, like three-line and two-line systems, are often labor-intensive, transgenic, and economically burdensome. While chemical mutagens facilitate these systems, they not only generate weak alleles but also produce strong alleles that induce permanent sterility through random mutagenesis. In contrast, RNA-guided nuclease system, such as clustered regularly interspaced short palindromic repeats (CRISPR)- associated protein (Cas) system, facilitates more efficient hybrid production by inducing male sterility through targeted genome modifications in male sterility genes, such as MS8, MS10, MS26, and MS45 which allows precise manipulation of pollen development or pollen abortion in various crops. Moreover, this approach allows haploid induction for the rapid generation of recombinant and homozygous lines from hybrid parents by editing essential genes, like CENH3, MTL/NLD/PLA, and DMP, resulting in high-yield, transgene-free hybrids. Additionally, this system supports synthetic apomixis induction by employing the MiMe (Mitosis instead of Meiosis) strategy, coupled with parthenogenesis in hybrid plants, to create heterozygous lines and retain hybrid vigor in subsequent generations. RNA-guided nuclease-induced synthetic apomixis also enables genome stacking for autopolyploid progressive heterosis via clonal gamete production for trait maintenance to enhance crop adaptability without compromising yield. Additionally, CRISPR-Cas-mediated de novo domestication of wild relatives, along with recent advances to circumvent tissue culture- recalcitrance and -dependency through heterologous expression of morphogenic regulators, holds great promise for incorporating diversity-enriched germplasm into the breeding programs. These approaches aim to generate elite hybrids adapted to dynamic environments and address the anticipated challenges of food insecurity.

Genetic and molecular mechanisms underlying the male sterility in rice

Male reproductive development is a complex and highly ordered phenomenon which demands comprehensive understandings of underlying molecular mechanisms to expand its scope for crop improvement. Genetic manipulation of male fertility/sterility is critical for crop hybrid breeding. Although male sterility is not a good trait for the plant itself, its wider application in hybrid rice breeding has made it valuable. The currently widely used male sterile line breeding systems mainly include the following: three-line hybrid rice based on cytoplasmic male sterility and two-line hybrid rice based on environmentally sensitive gene male sterility. The study of male sterility is an excellent thoroughfare to critically understand the regulatory mechanisms essential for the complicated male reproductive developmental process. The unique trait of male sterility also provides valuable resources and convenience for the genetic improvement of rice hybrids. Therefore, deeper and broader understandings about the genetic causes of male sterility are necessary for both basic studies and rice genetic improvement.

Circular cell clusters and calcium oxalate crystals: critical players in Solanaceae anther dehiscence

MAIN CONCLUSION

Circular cell clusters in Solanaceae anthers, crucial for pollen release, influence anther opening through coordinated development, programmed cell death, and calcium oxalate crystal dynamics. Circular cell clusters (CCCs), which are specialised crystal idioblasts, represent unique anatomical structures in Solanaceae anthers. These clusters may impact anther dehiscence through precisely coordinated developmental processes and programmed cell death (PCD). The dynamic metabolism of calcium oxalate (CaOx) crystals within CCCs likely contributes to PCD signalling. Although many factors, such as phytohormones, anther wall mechanics, and dehydration processes, influence anther dehiscence, it is increasingly clear that the CaOx crystals in CCCs are important for the timely degradation of CCCs and stomium cells. This review summarises the current understanding of the functions of CCCs in Solanaceae, highlighting their multifaceted roles in plant reproduction. A deeper comprehension of these mechanisms may provide insights for innovative crop improvement strategies. Similar structures in other plant families indicate a conserved evolutionary strategy for anther dehiscence across angiosperms.

Unlocking male sterility in horticultural crops through gene editing technology for precision breeding applications: presentation of a case study in tomato

Plant male sterility (MS) refers to the failure of the production of functional anthers, viable pollen grains and/or fertile sperm cells. This feature has great potential in horticultural crops for the exploitation of heterosis through the development of F1 hybrid varieties. MS in plants can occur spontaneously or can be induced artificially by exploiting biotechnological tools, such as the editing of genes involved in spore formation or pollen development. The success of such an approach strongly depends both on preliminary knowledge of the involved genes and on effective procedures for in vitro transfection/regeneration of whole plants. Furthermore, according to previous studies based on CRISPR/Cas9 technology, the efficacy of targeting and the resulting mutation profile are critically influenced by intrinsic factors, such as the CRISPR target primary sequence sites and chromatin signatures, which are often associated with varying levels of chromatin accessibility across different genomic regions. This relationship underscores the complexity of CRISPR-based genome editing and highlights the need to identify a precise suitable target. Our paper reports the results obtained for site-specific in vivo mutagenesis via a CRISPR/Cas9-mediated strategy applied to the MYB80 gene, which is a promising target for implementing male sterility in horticultural crops. We highlight the main steps that play a key role in the whole experimental pipeline, which aims at the generation of CRISPR/Cas-edited DNA-free tomato plants. This goal was achieved via protoplast-based technology and by directly delivering a ribonucleoprotein complex consisting of the Cas9 protein and in vitro synthesized single guide RNAs that can target different positions of the gene under investigation. Overall findings and insights are presented and critically discussed.

Molecular advances in research and applications of male sterility systems in tomato

Tomato, belonging to the nightshade family, is globally considered as a model system for classical and molecular genetics, genomics, and reproductive developmental studies. In the current scenario of climate change, hybrid development is among the crucial elements in the genetic improvement of crop plants. The phenomenon of male sterility is a viable approach for ensuring hybrid seed purity and reducing the cost of hybrid seed production. This review aims to shed light on the use of neoteric genomics and genome editing tools in understanding the genetics and molecular regulation of male sterility in tomato. Plant male gametophyte development is highly susceptible to environmental stress. Abnormalities at any stage of male reproductive development, such as premature or delayed tapetal cell degradation triggered by oxidative stress and programmed cell death (PCD) leads to male sterility in tomato. In tomato, more than 55 sporogenous, structural, and functional male sterile mutants, which are mainly under the control of recessive nuclear genes, have been reported. Recently, the role of open reading frames (ORFs) in governing cytoplasmic male sterility in tomato has also been documented. This review highlights the genetic and genomic progress in the investigation of underlying molecular pathways and practical application of potential male sterile mutants in tomato breeding. The applications and future prospects of genome engineering with CRISPR/Cas9 and mitoTALEN in the generation of novel male sterile systems to expedite tomato breeding is discussed.

Past and future of cytoplasmic male sterility and heterosis breeding in crop plants

Plant breeding needs to embrace genetic innovations to ensure stability in crop yields under fluctuating climatic conditions. Development of commercial hybrid varieties has proven to be a sustainable and economical alternative to deliver superior yield, quality and resistance with uniformity in a number of food crops. Cytoplasmic male sterility (CMS), a maternally inherited inability to produce functional pollen, facilitates a three-line system for efficient hybrid seed production strategies in crops. The CMS system has illustrated its potential as a robust pollination control mechanism to support the billion-dollar seed industry. In plants, CMS arises due to a genomic conflict between mitochondrial open reading frames (orfs) and nuclear-encoding restoration-of-fertility (Rf) genes, leading to floral abnormalities and pollen sterility. Research on pollen sterility and fertility restoration provides deeper insights into cytoplasmic-nuclear interplay in plants and elucidates key molecular targets for hybrid breeding in crops. More recently, programmable gene editing (e.g., TALEN, CRISPR-Cas) has emerged as a promising tool to functionally validate CMS and Rf genes and obviate the need for pollen donors or Rf-genes for hybrid breeding. Modern genomic prediction models have allowed establishment of high-performing heterotic groups and patterns for sustaining long-term gain in hybrid breeding. This article reviews latest discoveries elucidating the molecular mechanisms behind CMS and fertility restoration in plants. We then present our perspective on how evolving genetic technologies are contributing to advance fundamental knowledge of the CMS-Rf genetic system for producing crop hybrids with high heterosis.

Optimizing Brassica oleracea L. Breeding Through Somatic Hybridization Using Cytoplasmic Male Sterility (CMS) Lines: From Protoplast Isolation to Plantlet Regeneration

The Brassica oleracea L. species embrace important horticultural crops, such as broccoli, cauliflower, and cabbage, which are highly valued for their beneficial nutritional effects. However, the complexity of flower emasculation in these species has forced breeders to adopt biotechnological approaches such as somatic hybridization to ease hybrid seed production. Protoplasts entail a versatile tool in plant biotechnology, supporting breeding strategies that involve genome editing and hybridization. This review discusses the use of somatic hybridization in B. oleracea L. as a biotechnological method for developing fusion products with desirable agronomic traits, particularly cytoplasmic male sterile (CMS) condition. These CMS lines are critical for implementing a cost-effective, efficient, and reliable system for producing F1 hybrids. We present recent studies on CMS systems in B. oleracea L. crops, providing an overview of established models that explain the mechanisms of CMS and fertility restoration. Additionally, we emphasize key insights gained from protoplast fusion applied to B. oleracea L. breeding. Key steps including pre-treatments of donor plants, the main tissues used as sources of parental protoplasts, methods for obtaining somatic hybrids and cybrids, and the importance of establishing a reliable plant regeneration method are discussed. Finally, the review explores the incorporation of genome editing technologies, such as CRISPR-Cas9, to introduce multiple agronomic traits in Brassica species. This combination of advanced biotechnological tools holds significant promise for enhancing B. oleracea breeding programs in the actual climate change context.

Fertile grounds: exploring male sterility in cotton and its marker development

The high cost of producing conventional hybrid cotton seeds led to more research efforts on cotton male sterility systems. There is a lack of studies on cytology, histology, morphological variation, yield, and altered restorer backgrounds to identify and develop male sterility markers in cotton hybrids. Hybrid cotton can be efficiently produced by exploiting genetic male sterility. Among the 19 Genetic Male Sterility (GMS) genes discovered, the lines with ms5ms6 genes are mostly utilised to establish successful hybrid cotton in India. Molecular markers closely associated with the MS alleles are identified to facilitate the efficient and rapid backcrossing of male-sterility genes into elite lines or cultivars by marker-assisted backcrossing. The majority of the markers which are random DNA markers (RDMs), are probably lost, when recombination occurs. In contradiction, molecular markers (functional markers, or FMs) within the genic region can be identified and employed in crops for diverse traits, if prospective characteristic genes are known. In this review, the mechanism of male sterility, its gene expression level, and the need for functional markers for the male sterility trait in cotton have been put forward.

Assessment of Cassava Pollen Viability and Ovule Fertilizability under Red-Light, 6-Benzyl Adenine, and Silver Thiosulphate Treatments

Understanding pollen and ovule fertility as factors influencing fruit and seed set is important in cassava breeding. Extended daylength with red light (RL) and plant growth regulators (PGRs) have been used to induce flowering and fruit set in cassava without any reference to effects on pollen viability or ovule fertilizability. This study investigated the effects of field-applied RL and PGR on pollen viability and ovule fertilizability. Panels of cassava genotypes with early or moderate flowering responses were used. RL was administered from dusk to dawn. Two PGRs, 6-benzyl adenine (BA), a cytokinin and silver thiosulphate (STS), an anti-ethylene, were applied. Pollen viability was assessed based on pollen grain diameter, in vitro stainability, in vivo germinability, ovule fertilizability, and ploidy level. Treating flowers with RL increased the pollen diameter from 145.6 in control to 148.5 µm in RL, 78.5 to 93.0% in stainability, and 52.0 to 56.9% in ovule fertilizability in treated female flowers. The fruit set also increased from 51.5 in control to 71.8% in RL-treated female flowers. The seed set followed a similar trend. The ploidy level of pollen from RL-treated flowers increased slightly and was positively correlated with pollen diameter (R2 = 0.09 *), ovule fertilization (R2 = 0.20 *), fruit set (R2 = 0.59 *), and seed set (R2 = 0.60 *). Treating flowers with PGR did not affect pollen diameter but increased stainability from 78.5% in control to 82.1%, ovule fertilizability from 42.9 to 64.9%, and fruit set from 23.2 to 51.9% in PGR-treated female flowers. Combined BA + STS application caused the highest ovule fertilizability, fruit, and seed set efficiency. These results show that RL and PGR treatments increase pollen viability and ovule fertilizability. This is important for planning pollination strategies in cassava breeding programmes.

Genetic characterization and fine mapping of a recessive genic male-sterile gene in flowering Chinese cabbage (Brassica rapa var. parachinensis)

Brassica vegetables exhibit pronounced heterosis; nevertheless, investigations on fertility-related genes are scarce. The present study scrutinized a recessive genic male-sterile line, 7-3A, capable of generating a completely sterile population, holding significant promise for flowering Chinese cabbage breeding. By whole-genome resequencing of sterile and fertile plants, the male-sterile gene was confined to approximately 185 kb on chromosome A07, situated between markers C719 and NP10 in Brassica rapa var. Chiifu-401. Notably, substantial structural variation was identified within this region across diverse Brassica rapa reference genomes. Despite discernible expression level disparities of a homologous gene, Bnams4b, between male sterile and fertile plants, no sequence divergence was detected. Further elucidation is required to pinpoint a novel sterile gene within the candidate interval. This investigation contributes to the advancement of both the molecular-assisted breeding scheme for flowering Chinese cabbage and the comprehension of male sterility mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-04005-7.

MADS-Box Family Genes in Lagerstroemia indica and Their Involvement in Flower Development

MADS-box is a key transcription factor regulating the transition to flowering and flower development. Lagerstroemia indica 'Xiang Yun' is a new cultivar of crape myrtle characterized by its non-fruiting nature. To study the molecular mechanism underlying the non-fruiting characteristics of 'Xiang Yun', 82 MADS-box genes were identified from the genome of L. indica. The physicochemical properties of these genes were examined using bioinformatics methods, and their expression as well as endogenous hormone levels at various stages of flower development were analyzed. The results showed that LiMADS genes were primarily classified into two types: type I and type II, with the majority being type II that contained an abundance of cis-acting elements in their promoters. By screening nine core proteins by predicted protein interactions and performing qRT-PCR analysis as well as in combination with transcriptome data, we found that the expression levels of most MADS genes involved in flower development were significantly lower in 'Xiang Yun' than in the wild type 'Hong Ye'. Hormonal analysis indicated that 'Xiang Yun' had higher levels of iP, IPR, TZR, and zeatin during its early stages of flower development than 'Hong Ye', whereas the MeJA content was substantially lower at the late stage of flower development of 'Hong Ye'. Finally, correlation analysis showed that JA, IAA, SA, and TZR were positively correlated with the expression levels of most type II genes. Based on these analyses, a working model for the non-fruiting 'Xiang Yun' was proposed. During the course of flower development, plant hormone response pathways may affect the expression of MADS genes, resulting in their low expression in flower development, which led to the abnormal development of the stamen and embryo sac and ultimately affected the fruiting process of 'Xiang Yun'.

Newly Developed Restorer Lines of Sorghum [Sorghum bicolor (L.) Moench] Resistant to Greenbug

Eight lines of grain sorghum [Sorghum bicolor (L.) Moench], which can be used as a promising source material in heterotic hybrid breeding as pollen fertility restorers and donors of resistance to the greenbug (Schizaphis graminum Rondani), are characterized. The new restorer lines (R-lines) were developed by crossing the maternal sterile line Nizkorosloe 81s (CMS A1) with two lines selected from the grain sorghum collection accessions VIR-928 and VIR-929 as the paternal forms. The R-lines were genotyped using PCR markers, and also characterized by height, duration of the seedling-flowering period, and some of the technological properties of flour. With the use of microsatellite markers linked to the Rf genes and by hybridological analysis, it was shown that the new lines carry the dominant allele of the gene Rf2. The PCoA analysis demonstrated clear differences of each R-line from the parents. The genotypes of the new lines and their parental forms for the Rf2 locus were confirmed by applying three allele-specific codominant CAPS markers which detected SNPs in the candidate Rf2 gene. All new lines were highly fertile, as demonstrated by cytological analysis of acetocarmine-stained pollen preparations. A high resistance to the greenbug was demonstrated for each new R-line both in the laboratory and field conditions against a severe aphid infestation. Grain quality parameters such as protein content and dough rheological properties varied widely and were quite satisfactory in some R-lines. Characteristics common to all eight sorghum lines studied, such as the ability to restore pollen fertility in the F1 generation, good pollen quality, greenbug resistance, early ripening, spreading panicle, and low stature, allow us to recommend them for producing commercial F1 hybrids with satisfactory grain quality for both fodder and food purposes.