Exploration and utilization of maize male sterility resources.

Male sterility refers to the defective development of male reproductive organs, which led to plants incapable of producing normal and functional pollens. Maize (Zea mays L.) is one of the most important food crops, as well as one of the earliest crops to utilize heterosis in breeding. Single cross hybrid has been the main type of maize heterosis utilization for a long time. The planting area of maize hybrid in China has been stable at about 620 million mu. More than one billion kilograms of commercial hybrid seeds are needed each year, and the annual seed production area has been stable at about 2.5 million mu in recent years. So far, manual emasculation has been the major way of maize hybrid seed production in China, which is laborious and time consuming. Generally, spatial isolation is necessary for maize hybrid seed production, this requirement results in only some regions in the country suitable for maize hybrid seed production. Manual emasculation requires seasonal demand of labors. At present, with the urbanization of a large number of rural laborers, the seed production regions experience a serious labor shortage. Accordingly, the cost of seed production increases with the rising of land rent and labor costs. In addition, it is difficult to guarantee the seed purity with manual or mechanical emasculation for hybrid seed production. However, incorporating male sterility into maize hybrid seed production could reduce its cost and ensure hybrid seed purity. It can also avoid the difficulties of manual or mechanical emasculation in field operation under extreme weather conditions. Therefore, it is the inevitable trend of development in the maize seed industry. In this review, we summarize the exploitation and creation of maize cytoplasmic male sterility (CMS), maize genic male sterility (GMS) resources in China, and the developing process from natural discovery to targeted creation of male sterility resources in plants, and the research progress of maize male sterility. We then analyze the application status and existing problems of maize male sterility, based on the development trend of maize seed industry, as well as the research and application status of male sterility in China. We also identify seven aspects that need to be further strengthen, thereby providing the reference for the creation, research and utilization of maize male sterility in the future.

[Genetics of fertility restoration in the isocytoplasm allonuclear C-group of cytoplasmic male sterility in maize].

Maize is one of the first crops to produce hybrids using cytoplasmic male sterile lines. The C-type cytoplasmic male sterile (CMS-C) line is vital for hybrid seed production, and the fertility-restoration reaction along with its stability has a direct bearing on its applications. However, fertility restoration mechanism of CMS-C is complex and is still not clear so far. To further explore the factors affecting the fertility restoration of maize CMS, a series of test crosses were carried out by pollinating the isocytoplasm allonuclear CMS-C lines C48-2, Chuangzaosi and C478 with the test lines 18 bai, zi 330, 5022 and the restorer line A619, respectively. Four F2 populations and six double-cross combinations were obtained from the self-cross of fertility restored F1 and pollinating male-sterility-maintained F1 with the male-fertility-restored F1, respectively. Meanwhile, we developed the incomplete diallel-cross combinations using the isocytoplasm allonuclear male sterile lines as maternal parents and their respective maintainer lines 48-2, huangzaosi and 478 as paternal parents. All the F1, F2 and double-cross populations were planted at distinct locations in different years, and the fertility-restoration reaction was scored by field investigation and pollen staining with I2-IK. The results were as follows: 1) The same test line could restore the CMS-C line at a certain genetic background, but failed to restore the CMS-C line at the other genetic backgrounds, suggesting that the genetic background of CMS-C lines plays an important role in the fertility restoration. 2) The fertile-to-sterile segregation ratio of (C48-2×A619) F2 population planted in both Sichuan and Yunnan fited well to 15:1 by the χ 2 test. However, the fertility level of individuals in Yunnan mainly belonged to the 3 and 4grades, but which in Sichuan mainly belonged to the 5 grade, indicating the environmental factors had effect on the fertility-restoration reaction of (C48-2×A619) F2. 3) In our study, 18 bai could not restore C478, and 48-2 could not restore C478, but the fertile and sterile segregated plants were unexpectedly found in their double-cross population [(C478×18 bai) F1S×(C48-2×18 bai) F1F]. The similar case was also observed in the double-cross population [(C48-2 × zi 330) F1S × (C478 × zi 330) F1F]. Therefore, we speculated that there are minor fertility restorer genes not only in the nuclear background of C48-2 and C478, but also in zi 330 and 18 bai, and when these minor genes were gathered by hybridization, they could restore the fertility of C478 and C48-2. This conforms to the restorer genes dose-effect for fertility restoration in the plant CMS system. These results not only contribute to our understanding of the complexity and diversity of CMS-C restoration mechanism, but also provide an important reference for the practical applications about maize CMS-C.

Research progress on mechanisms of male sterility in plants based on high-throughput RNA sequencing.

Male sterility is defined as failing to produce functional pollen during stamen development in plants, and it plays a crucial role in plant reproductive research and hybrid seed production in utilization of crop heterosis. High throughput RNA sequencing (RNA-seq) has been used widely in the study of different fields of life science, as it readily detects all the mRNA and non-coding RNA in cells. Recently, RNA-seq has been reported to be applied in different species and kinds of pollen abortion types in plants, which has contributed to the understanding of the molecular mechanism and metabolic networks of male sterility at the transcription level. In this review, we summarize research progress on the mechanisms of male sterility in plants, focusing on RNA-seq analysis encompassing strategies of RNA library construction, differentially expressed genes and functional characteristics of noncoding RNAs involved in stamen abortion. Furthermore, we also discuss application of transcriptome sequencing technology to elucidate pollen abortion mechanisms and map fertility-related genes. We hope to provide references to the study of male sterility in plants.