Medicinal Plant Research 2024, Vol.14, No.6, 345-357 http://hortherbpublisher.com/index.php/mpr 347 3 Genetic Resources and Resistance Traits in Hangbaiju 3.1 Germplasm diversity and resistant gene pools Hangbaiju has significant genetic diversity, which provides an important basis for breeding disease-resistant and insect-resistant varieties. Studies using ISSR and SSR molecular markers have shown that there is a high degree of polymorphism between local varieties, wild relatives and breeding lines of Hangbaiju, showing rich resistance allele resources (Luo et al., 2018; Hodaei et al., 2019). For instance, ISSR analysis of 20 Chrysanthemum materials found that they could be classified into three major genetic groups, and the leaf spot resistance performance of these groups was highly consistent with field observations (Guo et al., 2014). SSR-based studies further confirmed the genetic differences between wild species, large-flowered varieties and local varieties, supporting the rational use of diverse germplasm resources in resistance breeding (Luo et al., 2018; Hao et al., 2022). Wild relatives and traditional landraces, such as ‘Hangbaiju’, ‘Gongju’, and ‘Chuju’, are particularly valuable for introducing novel resistance traits into breeding programs (Hodaei et al., 2019; Hao et al., 2022). Molecular markers have become an important tool for screening resistant genotypes. For example, ISSR markers have been successfully used for early identification of leaf spot resistant varieties (Guo et al., 2014); while SSR and SNP markers are associated with aphid resistance and white rust resistance, providing technical support for marker-assisted selection (MAS) in resistance breeding (Luo et al., 2018; Fu et al., 2018; Sumitomo et al., 2021). Besides, transcriptome analysis has identified multiple differentially expressed genes (DEGs) and candidate resistance genes, most of which are involved in secondary metabolite synthesis, plant hormone signal transduction, and plant-pathogen interaction processes, and are significantly upregulated in resistant materials or under stress conditions (Zhang et al., 2019; Li et al., 2024a; Wang et al., 2024; Zhang et al., 2025). 3.2 Inheritance patterns of resistance traits The resistance traits of chrysanthemum can be controlled by a single gene or regulated by multiple genes. For example, the resistance of the ‘Southern Pegasus’ variety to white rust (Puccinia horiana) is controlled by a single gene, and linkage analysis of SNP markers has verified this (Sumitomo et al., 2021). In contrast, resistance to aphids and wilt caused by Fusarium oxysporum often belongs to a polygenic inheritance pattern, involving multiple loci and their regulatory pathways. Association analysis found that multiple markers were associated with the aphid resistance trait, which can explain a large proportion of phenotypic variation, and have high heritability and moderate genetic gain, indicating that the trait has the genetic characteristics of a quantitative trait (Fu et al., 2018). Transcriptome studies also revealed that resistance to Fusarium wilt and black spot caused by Alternaria alternata involves complex transcription factor regulatory networks (like WRKY, bHLH, MYB), plant hormone signaling pathways, and metabolic pathways, further supporting its multi-gene control characteristics (Zhao et al., 2020; Li et al., 2024a; Ding et al., 2023; Miao et al., 2023a; b). The expression of resistance traits is also affected by the interaction between genotype and environment. Field and greenhouse studies have shown that environmental factors such as temperature, humidity, and soil conditions can modulate the actual effectiveness of resistance genes (Guo et al., 2014; Hodaei et al., 2019). Taking aphid resistant varieties as an example, they exhibit stability in different seasons, but certain traits such as persistent metabolite accumulation during flowering are influenced by both genotype and environment (Hodaei et al., 2019; Long et al., 2022). 3.3 Screening indicators for disease and pest resistance The resistance screening of Hangbaiju usually combines phenotype evaluation under field and control conditions, mainly examining indicators such as lesion size, disease severity, pest infection level, and plant growth potential (Guo et al., 2014; Fu et al., 2018; Li et al., 2024a). The results of grouping germplasm resources based on field performance often match molecular marker data, indicating that combining molecular screening with phenotype screening is a reliable method (Guo et al., 2014; Luo et al., 2018; Hodaei et al., 2019). In the evaluation of aphid resistance, commonly used indicators include aphid damage index and physiological and biochemical indicators (such as flavonoid content, enzyme activity, etc.) (Zhang et al., 2019; Wang et al., 2024) (Figure 1).
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