Maize Genomics and Genetics 2025, Vol.16, No.2, 98-107 http://cropscipublisher.com/index.php/mgg 101 It is worth noting that the genetic background of maize not only determines their disease resistance genes, but also affects the expression and stability of these genes. The main disease resistance genes of NCLB, such as Ht2, Ht3 and Htn1, show different performances in different maize lines because they have allelic differences (Yang et al., 2021). Therefore, when breeding, it is very important not to focus on a certain disease resistance gene and ignore the genetic background of the entire variety. 3.3 Application of genetic markers in genetic diversity assessment Evaluating maize genetic diversity is now inseparable from genetic marker technology, such as simple sequence repeats (SSR) and single nucleotide polymorphisms (SNP). These markers can provide us with more detailed information on the genetic structure of disease resistance. In particular, high-density SNP genotyping is used in GWAS to find SNPs significantly associated with NCLB resistance, which is very helpful for marker-assisted selection in breeding (Van Inghelandt et al., 2012; Rashid et al., 2020). In QTL positioning, the use of SNP markers plays an even more important role. For example, multiple QTLs that confer NCLB resistance were found on chromosomes 2, 5, and 8, one of which can even explain 16.34% of the phenotypic differences (Figure 2) (Ranganatha et al., 2021). Although SSR markers are technically a little older, they are still widely used to describe the genetic diversity and resistance loci of corn populations (Welz and Geiger, 2000), indicating that these genetic marker tools are still indispensable in genetic research. Figure 2 LOD peak for QTL conditioning resistance to northern corn leaf blight on chromosomes 2, 5, and 8 in rainy season (Kharif) of 2013, 2014, and pooled analysis over seasons (Adopted from Ranganatha et al., 2021) 4 Disease Resistance Traits in Maize and Their Genetic Basis 4.1 Classification and function of major disease resistance genes (such as Ht gene) Resistance of maize to northern corn leaf spot (NCLB) is controlled by the combined action of multiple genes. Generally speaking, resistance is divided into two categories: qualitative and quantitative. Qualitative resistance is often attributed to several major genes, such as Ht1, Ht2, Ht3, and Htn1, which can show strong resistance to specific pathogens. The proteins they encode are involved in the plant immune response, including nucleotide-binding leucine-rich repeat (NLR) receptors and cell wall-related receptor kinases (RLKs) (Van Inghelandt et al., 2012; Ranganatha et al., 2021; Thatcher et al., 2022). Taking Ht1 as an example, the NLR receptor it encodes can recognize specific pathogen effectors and activate defense responses (Thatcher et al., 2022). In contrast, the kinase encoded by Htn1 mainly relies on the pathogen recognition mechanism of the cell wall to activate plant innate immunity (Ranganatha et al., 2021).
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