Plant breeding: Revealing the secrets of cytoplasmic male sterility in wheat

Insights into cellular crosstalk regulating cytoplasmic male sterility and fertility restoration

Cytoplasmic male sterility has been a popular genetic tool in development of hybrids. The molecular mechanism behind maternal sterility varies from crop to crop. An understanding of underlying mechanism can help in development of new functional CMS gene in crops which lack effective and stable CMS systems. In crops where seed or fruit is the commercial product, fertility must be recovered in F1 hybrids so that higher yield gains can be realized. This necessitates the presence of fertility restorer gene (Rf) in nucleus of male parent to overcome the effect of sterile cytoplasm. Fertility restoring genes have been identified in crops like wheat, maize, sunflower, rice, pepper, sugar beet, pigeon pea etc. But in crops like eggplant, bell pepper, barley etc. unstable fertility restorers hamper the use of Cytoplasmic genic male sterility (CGMS) system. Stability of CGMS system is influenced by environment, genetic background or interaction of these factors. This review thus aims to understand the genetic mechanisms controlling mitochondrial-nuclear interactions required to design strong and stable restorers without any pleiotropic effects in F1 hybrids.

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'.

Integrated transcriptome and proteome analysis provides insights into the mechanism of cytoplasmic male sterility (CMS) in tobacco (Nicotiana tabacum L.)

Cytoplasmic male sterility (CMS) is critical in maximizing crop yield and quality by utilizing tobacco heterosis. However, the mechanism of tobacco CMS formation remains unknown. Using paraffin section observation, transcriptome sequencing, and TMT proteomic analysis, this study describes the differences in expression profiles in morphology, transcription, and translation between the sua-CMS tobacco line (MSYY87) and its corresponding maintainer line (YY87). According to the microspore morphology, MSYY87 began to exhibit abnormal microspore development during the early stages of germination and differentiation (androgynous primordium differentiation stage). According to transcriptomic and proteomic analyses, 17 genes/proteins involved in lipid transport/binding and phenylpropane metabolism were significantly down-regulated at both the mRNA and protein levels. Through further analysis, we identified some key genes that may be involved in tobacco male sterility, including β-GLU related to energy metabolism, 4CL and bHLHs related to anther wall formation, nsLTPs related to pollen germination and anther cuticle, and bHLHs related to pollen tapetum degradation. We speculate that the down-regulation of these genes affects the normal physiological metabolism, making tobacco plants show male sterility. SIGNIFICANCE: Cytoplasmic male sterility (CMS) plays a vital role in utilizing tobacco heterosis and enhancing crop yield and quality. We observed paraffin sections and conducted transcriptome sequencing and mitochondrial proteomics to examine the tobacco CMS line Yunyan 87 (MSYY87) and its maintainer line Yunyan 87 (YY87). The down-regulation expression of β-GLU resulted in insufficient ATP supply, which resulted in disordered energy metabolism. The down-regulation expression of 4CL, nsLTPs and bHLHs may affect the formation of anther wall and anther cuticle, pollen germination, as well as the degradation of pollen tapetum. These various abnormal physiological processes, the male sterility of tobacco is finally caused. The findings shed light on the molecular mechanisms of tobacco CMS and serve as a model for fertility research in other flowering plants.

Plant non-coding RNAs function in pollen development and male sterility

Male sterility is classified as either cytoplasmic male sterility (CMS) or genic male sterility (GMS). Generally, CMS involves mitochondrial genomes interacting with the nuclear genome, while GMS is caused by nuclear genes alone. Male sterility is regulated by multilevel mechanisms in which non-coding RNAs (ncRNAs), including microRNAs (miRNAs), long non-coding RNAs (lncRNAs), and phased small interfering RNAs (phasiRNAs), which have been proven to be critical elements. The development of high-throughput sequencing technology offers new opportunities to evaluate the genetic mechanism of ncRNAs in plant male sterility. In this review, we summarize the critical ncRNAs that regulate gene expression in ways dependent on or independent of hormones, which involve the differentiation of the stamen primordia, degradation of the tapetum, formation of microspores, and the release of pollen. In addition, the key mechanisms of the miRNA-lncRNA-mRNA interaction networks mediating male sterility in plants are elaborated. We present a different perspective on exploring the ncRNA-mediated regulatory pathways that control CMS in plants and create male-sterile lines through hormones or genome editing. A refined understanding of the ncRNA regulatory mechanisms in plant male sterility for the development of new sterile lines would be conducive to improve hybridization breeding.

Triticeae genome sequences reveal huge expansions of gene families implicated in fertility restoration

Breakthroughs in assembly of whole-genome sequencing and targeted sequence capture data have accelerated comparative genomics analyses in cereals with big and complex genomes such as wheat. This newly acquired information has revealed unexpected expansions in two large gene families linked to restoration of fertility in species that exhibit cytoplasmic male sterility. Extreme levels of copy-number and structural variation detected within and between species illustrate the genetic diversity among the family members and reveal the evolutionary mechanisms at work. This new knowledge will greatly facilitate the development of hybrid production strategies in wheat and related species.