6. Conclusions

In this chapter, we focus on the molecular cloning, functional confirmation, and application value assessment of GMS genes as well as their application in hybrid seed production via several BMS systems in cereal crops, such as rice, maize, and wheat. With the rapid development of the next-generation sequencing technology, more genome information of cereal crops are available, leading to plenty GMS genes cloned and characterized in crop plants. As shown in Table 1, there are more than 70 GMS genes cloned in cereal crops, and most of them (57/73) are identified by using forward genetic approach, including 38 genes isolated by map-based cloning, 15 genes identified by T-DNA/Transposon tagging and 4 genes isolated by MutMap method. Whereas the rest of GMS genes are identified via reverse genetic approach, including 7 genes isolated through homology-based cloning, 5 genes identified by anther-specific expression gene screening and 4 genes cloned by other reverse genetic methods. Among them, there are 49 GMS genes cloned in rice, 17 GMS genes in maize, 6 GMS genes in wheat, and 1 GMS gene in barley. From these data, we conclude that the forward genetic approaches, especially map-based cloning, are the most popular method for GMS gene cloning in crops; most GMS genes have been cloned in rice and maize, whereas only a few GMS genes are cloned in wheat and barley. Consider that the conserved role of GMS genes in different species and the sequence information of GMS genes in rice and maize can be used for cloning of the orthologs in wheat and barley through reverse genetic approaches. For example, the functions of TaMs26 and TaMs45, the wheat homologs of maize Ms26 and Ms45, were confirmed via a custom-designed homing endonuclease and CRISPR-Cas9-targeted mutagenesis in wheat, respectively [76, 77], while the role of HvMs1, the barley homolog of Arabidopsis Ms1 and rice PTC1, was analyzed by using RNAi silencing in barley [93].

Although there are a lot of GMS genes identified in cereal crops up to now, less than 10 GMS genes are assessed for the value in heterosis utilization and hybrid seed production (Table 1). For example, ZmMs7, ZmMs30, and ZmMs33 are tested in maize MCS system [9, 18, 21]; ZmMs26, Zmms44, and ZmMs45 are tested in maize SPT (or SPT-like) system [16, 22, 62]; OsNP1 is tested in rice SPT-like system [69]; and TaMs1 is tested in wheat SPT-like system [49]. All these BMS systems belong to transgenic construct-driven non-transgenic product strategies, leading to potential application of these systems in commercial hybrid seed production, especially in the countries and/or regions with strict regulatory policy. These systems have many advantages, such as non-transgenic final products, environment-friendly without application of herbicide in hybrid seed production fields, and deregulated by the regulatory authority in some countries, whereas they are limited by using

Molecular Cloning of Genic Male-Sterility Genes and Their Applications for Plant Heterosis… DOI: http://dx.doi.org/10.5772/intechopen.86976

transgenic maintainer line based on completely male-sterile mutants and the male-fertility genes and requirement of fluorescent seed color-sorting machine. Therefore, the transgenic male-sterility systems, such as RHS system based on glyphosate-mediated male sterility, have also been developed and used in commercial hybrid seed production in maize [101, 102]. This system is independent of male-sterility mutants and male-fertility genes, no need for transgenic maintainer line and seed color sorting, and the herbicide-resistant male-sterility lines are helpful to highly efficient and mechanized hybrid seed production. However, this transgenic male-sterility system is limited by the "zero tolerance" regulatory policy preventing transgenic planting in many countries, need for application of herbicide in hybrid seed production fields, and potential risk of gene flow. In summary, both SPT and RHS systems have advantages and disadvantages compared with each other.

With the advance of molecular cloning methods including both forward and reverse genetic approaches, especially the next-generation sequencing technology and genome-editing technology (e.g., CRISPR-Cas9), more GMS genes in cereal crops with large and complex genome (e.g., wheat and barley) will be identified and characterized by using multiple strategies as described in this chapter. At the same time, the application value of the putative GMS genes should be assessed systemically in genetic stability of male sterility, effects on heterosis performance, and potential linkage with detrimental traits. This will not only boost our understanding in the molecular mechanism of anther and pollen development but also give great opportunity to develop novel BMS systems for commercial hybrid seed production in cereal crops.
