Molecular Plant Breeding 2024, Vol.15, No.4, 167-177 http://genbreedpublisher.com/index.php/mpb 173 Two photoperiod- and thermo-sensitive genic male sterile lines of PMeS lines, PS006 and PS012, were bred and found to have desirable agronomic traits, high outcrossing rates, and good combining abilities. These lines show strong heterosis in their hybrids and have great potential for increasing rice productivity and quality, providing valuable resources for further research into polyploidy and hybrid vigor in rice (Zhang et al., 2017). Autotetraploid rice hybrids also exhibit enhanced stress tolerance and disease resistance. The development of neo-tetraploid lines with high fertility and heterosis has been a significant breakthrough. These lines display strong heterosis when crossed with other autotetraploid rice lines, resulting in hybrids with improved stress tolerance and disease resistance (Bei et al., 2019; Chen et al., 2022). For instance, the use of CRISPR/Cas9 technology to develop thermo-sensitive genic male sterile lines in neo-tetraploid rice has led to hybrids with high levels of hybrid vigor and resilience to environmental stresses (Chen et al., 2022). These autotetraploid rice hybrids hold great promise for future rice breeding programs, as they collectively underscore the potential to achieve higher yields, improved fertility, and better stress tolerance, thereby paving the way for their broader application in rice breeding programs. 6 Future Prospects and Challenges 6.1 Potential applications in rice breeding China's research on utilizing the heterosis of rice hybrids is at the forefront globally, making significant contributions to food security, particularly in China and worldwide. However, with the continuous global population growth and reduction in arable land, increasing rice yields has become increasingly important (Zhang et al., 2017). Furthermore, global climate warming, extreme weather events, droughts, floods, soil degradation, and frequent pest and disease outbreaks pose severe threats to rice growth and harvest (Chen et al., 2021). Therefore, there is an urgent need to develop new high-yielding rice varieties that can enhance yield and improve resilience to climate change and disasters. To address these challenges, scientists have proposed two approaches: First, the polyploidization of cultivated rice, focusing on the development of tetraploid rice, to leverage its high resistance and strong heterosis; Second, the acceleration of the domestication of allotetraploid wild rice using molecular biology techniques to create new rice species (Figure 2) (Yu et al., 2021). Figure 2 The novel approaches for creating superior new germplasms in rice, MAS: Molecular marker assisted selection (Adapted from Yu et al., 2021; Liu et al., 2023) The high-fertility tetraploid rice hybrids exhibit distinct advantages that can be maintained across multiple generations, offering significant potential for breakthroughs in polyploid rice breeding. To harness the heterosis of high-fertility tetraploid rice in agricultural production, three bottlenecks need to be addressed: (1) Development of efficient autotetraploid rice sterile lines. In addition to traditional hybridization methods, CRISPR/Cas9 gene editing technology can be used to knockout the TMS5 gene in novel tetraploid rice to efficiently breed thermo-sensitive sterile lines (Song et al., 2021; Chen et al., 2022).
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