Triticeae Genomics and Genetics, 2024, Vol.15, No.3, 162-171 http://cropscipublisher.com/index.php/tgg 165 3.3.2 Transgenic approaches Transgenic approaches involve the introduction of foreign genes into the wheat genome to confer new traits. This method has been used to develop wheat lines with enhanced resistance to biotic and abiotic stresses. For example, transgenic SHW lines have been created to express antifungal enzymes and pathogen-related genes, providing resistance to leaf rust and other diseases (Truong et al., 2020). These transgenic lines also exhibit improved yield and quality traits, making them valuable resources for wheat breeding programs (Truong et al., 2020). 4 Impacts of Hexaploid Genetics on Breeding Outcomes 4.1 Yield improvement 4.1.1 Genetic basis of yield traits Hexaploid wheat (Triticum aestivum) has a complex genome that provides a rich source of genetic variation for yield improvement. The genetic basis of yield traits in hexaploid wheat involves numerous quantitative trait loci (QTLs) that influence grain weight and number. Key genes such as TaGNI, TaGW2, TaCKX6, TaGS5, TaDA1, WAPO1, and TaRht1 have been identified to impact these traits, with some genes increasing grain weight while others affect grain number, highlighting the trade-offs in yield components (Tillett et al., 2022). Additionally, synthetic hexaploid wheat (SHW) derived from crossing tetraploid wheat with Aegilops tauschii has been shown to enhance early biomass traits, which are crucial for yield improvement (Yang et al., 2020). 4.1.2 Breeding for high yield varieties Breeding strategies utilizing SHW have led to the development of high-yield wheat cultivars. For instance, the 'large population with limited backcrossing method' and the 'recombinant inbred line-based breeding method' have been employed to pyramid yield-related QTLs/genes from SHW into new cultivars, resulting in record-breaking high-yield wheat in southwestern China (Wan et al., 2023). The use of SHW has also facilitated the transfer of desirable traits such as early vigour and enhanced resource utilization efficiency, contributing to yield (Yang et al., 2020). 4.2 Disease resistance 4.2.1 Genetic resistance to major wheat diseases Hexaploid wheat possesses a diverse array of resistance genes that provide protection against major wheat diseases. Advances in genomics have enabled the identification and characterization of these resistance genes, such as those involved in resistance to Fusarium head blight (FHB) and leaf rust. For example, the Sm1 gene associated with insect resistance and other nucleotide-binding leucine-rich repeat proteins have been identified in hexaploid wheat (Walkowiak et al., 2020). Additionally, SHW lines have been developed with resistance to leaf rust and heat stress, demonstrating the potential of SHW in enhancing disease resistance (Truong et al., 2020). 4.2.2 Incorporating resistance genes through breeding Breeding programs have successfully incorporated resistance genes from hexaploid wheat into new cultivars. The creation of a wheat resistance gene atlas has been proposed to facilitate the rapid deployment of resistance genes against a wide range of pathogens, ensuring durable resistance in wheat crops (Hafeez et al., 2021). Furthermore, the development of synthetic hexaploid wheat lines with resistance to diseases such as powdery mildew and FHB has been achieved through the pyramiding of resistance genes, resulting in cultivars with both high yield potential and disease resistance (Zhu et al., 2022; Han et al., 2023). 4.3 Abiotic stress tolerance 4.3.1 Drought and heat tolerance Hexaploid wheat has been a valuable resource for breeding drought-tolerant varieties. SHW lines possess numerous genes for drought tolerance, and QTL mapping has identified specific loci associated with root traits that contribute to drought resistance (Liu et al., 2020). Additionally, SHW lines have been developed with enhanced heat tolerance, as demonstrated by the rapid expression of heat shock proteins under stress conditions (Truong et al., 2020).
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