Maize Genomics and Genetics 2025, Vol.16, No.3, 119-128 http://cropscipublisher.com/index.php/mgg 122 3 Heat-Resistant Breeding Strategies 3.1 Conventional breeding methods As one of the most important food crops in the world, the yield and quality of corn are often seriously affected by high temperature stress. Against the background of the increasingly intensified trend of global warming, heat-resistant breeding has become an important direction to ensure stable and high yields of corn. Conventional breeding methods, as the basic means of corn improvement, still play an irreplaceable role in the current breeding system, especially in the selection of heat-resistant varieties, which have wide application value (Dowd and Johnson, 2018). Hybrid breeding is the most widely used method in conventional heat-resistant breeding. This strategy uses hybrid vigor to select hybrids that perform well under high temperature conditions by hybridizing inbred lines with strong heat resistance with inbred lines with high yield, good quality but poor heat resistance. Studies have shown that hybrids are usually more heat-resistant than their parents because most heat stress-related traits are controlled by dominant genes. Therefore, hybrid breeding not only helps to improve heat resistance, but also enhances the adaptability of varieties while ensuring yield and quality (El-Sappah et al., 2022). However, this method also has certain limitations, such as a long combining ability screening cycle, a large workload of field trials, and strong dependence on environmental conditions. Pedigree selection is also a common heat-resistant breeding method. This method accumulates heat-resistant genes from generation to generation through continuous self-pollination and systematic selection, thereby cultivating heat-resistant strains with stable genetic backgrounds. This method is of great significance in breeding basic parent materials, but because heat stress traits are controlled by multiple genes and are easily affected by the environment, pure phenotypic selection is often inefficient and the improvement progress is relatively slow (Mukaro et al., 2023). Backcross breeding has also gradually shown its application prospects in improving maize heat resistance. This method uses excellent main varieties as receptors to introduce excellent genes from heat-resistant donors into target strains to achieve precise transfer of heat-resistant genes. Backcross breeding improves its adaptability to high temperature environments while maintaining the excellent traits of the original varieties. However, since heat stress-related traits often involve multiple gene loci, the introduction of a single gene is difficult to obtain significant improvement effects, and the background recovery process is relatively complicated (Prasanna et al., 2021; Hill and Li, 2022). Although conventional breeding methods have a solid foundation and broad application prospects in corn heat-resistant breeding, they also face problems such as low genetic improvement efficiency, long cycle, and great environmental interference. Therefore, in future research, in heat-resistant breeding work, we should integrate modern technical means such as molecular marker-assisted selection (MAS), genome-wide association analysis (GWAS) and genomic selection (GS) on the basis of conventional methods, build a "conventional+molecular" collaborative breeding system, and promote corn heat-resistant breeding towards high efficiency, precision and intelligence. 3.2 Utilizing the genetic diversity of local varieties and wild germplasm resources In the current corn breeding system, improving the stress resistance and adaptability of crops has become a research focus. As a rich genetic diversity pool, local varieties and wild germplasm resources provide an important foundation for modern corn genetic improvement. Compared with modern cultivated varieties, these resources have accumulated unique stress resistance traits under long-term natural selection and farmer selection, especially in terms of heat resistance, drought resistance, disease and insect resistance. Wild corn (such as Mexican wild corn Teosinte) is considered to be an important ancestor of modern corn. During the evolution process, it has retained a large number of genes that have not been used by modern breeding, especially in terms of genes related to environmental stress response, which has potential utilization value. Studies have shown that some wild corn germplasms have important genetic diversity in terms of heat resistance, root
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