Maize Genomics and Genetics 2025, Vol.16, No.2, 98-107 http://cropscipublisher.com/index.php/mgg 102 However, qualitative resistance is sometimes susceptible to environmental influences, while quantitative resistance is usually more stable. This resistance is regulated by multiple small effect gene loci, distributed on different chromosomes, and the related quantitative trait loci (QTL) have a certain resistance to multiple pathogens (Welz and Geiger, 2000; Poland et al., 2011; Yang et al., 2021). Studies have shown that some QTLs related to NCLB resistance were found on chromosomes 1, 3, 5 and 8, and these loci were consistent in different maize populations (Welz and Geiger, 2000; Poland et al., 2011). It is worth noting that these QTLs often cluster with genes for resistance to other fungal diseases and pests, indicating that the genetic structure of maize disease resistance is relatively complex (Welz and Geiger, 2000). 4.2 Effects of gene expression and regulation on disease resistance mechanisms Not only the gene itself, but also the expression level and regulation mechanism of the disease resistance gene are critical. For example, the disease resistance effect of the Ht1 gene is closely related to its expression, and the susceptible allele with low expression usually lacks resistance. In the experiment, by introducing the Ht1 gene into susceptible varieties, its disease resistance can be significantly enhanced, which shows that the gene expression level is crucial for resistance (Thatcher et al., 2022). Similarly, the expression pattern of the Htn1 gene in resistant and susceptible corn is also different, further reflecting the influence of gene regulation on disease resistance (Ranganatha et al., 2021). In addition, RNA sequencing analysis showed that the expression of related defense genes in disease-resistant corn lines changed more significantly when infected with pathogens, especially genes involved in pathogenesis proteins and secondary metabolites (Ranganatha et al., 2021; Thatcher et al., 2022). The regulation of these genes is also affected by environmental factors, resulting in differences in resistance stability. Quantitative resistance genes are often less sensitive to environmental changes and show greater stability because they act on multiple sites (Welz and Geiger, 2000; Galiano-Carneiro and Miedaner, 2017). 4.3 Phenotypic evaluation and standard measurement of disease resistance traits When evaluating maize disease resistance traits, it is usually necessary to measure a variety of traits that reflect the plant's response to pathogens. Among them, the incubation period (IP) and the area under the disease progression curve (AUDPC) are the core indicators for evaluating NCLB quantitative resistance (Welz and Geiger, 2000). These traits have high heritability and are often used in research to locate genomic regions related to disease resistance. For example, in studies involving three mapping populations, IP and AUDPC-related QTLs on chromosomes 3, 5, and 8 performed well (Welz and Geiger, 2000; Feng, 2024). Field trials are an important means of evaluating disease resistance, especially artificial inoculation in multiple high-incidence locations, scoring according to the severity of lesions, and obtaining reliable phenotypic data (Wang et al., 2018; Rashid et al., 2020). With the development of high-density SNP markers and genotyping technology, the accuracy of disease resistance phenotypic evaluation has been greatly improved, which has also promoted the association analysis of significant SNPs and haplotypes with resistance traits (Rashid et al., 2020). In addition, chromosome segment substitution lines (CSSLs) are widely used for QTL verification. This method effectively confirms the genetic basis of disease resistance and supports the implementation of marker-assisted breeding (Wang et al., 2018). 5 Summary of the Results of the Meta-analysis of Genetic Diversity and Disease Resistance in Maize 5.1 Results of the study on the relationship between genetic diversity and disease resistance in different maize germplasms The relationship between genetic diversity and disease resistance is very complex in maize germplasm. Genome-wide association studies (GWAS) have found multiple single nucleotide polymorphisms (SNPs) associated with resistance to northern corn leaf spot (NCLB) in different maize populations. For example, in the high-resolution GWAS of tropical maize lines, 22 SNPs were significantly associated with NCLB resistance, many of which fell in some regions on chromosomes that contain well-known resistance genes, such as Ht2, Ht3, and Htn1 (Rashid et al., 2020). This suggests that genetic diversity near these genes may play a role in disease resistance.
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