Molecular Pathogens, 2025, Vol.16, No.4, 182-192 http://microbescipublisher.com/index.php/mp 185 3.2 Research progress on resistance-related quantitative trait sites (QTLs) Grape downy mildew resistance is usually a quantitative trait that is jointly operated by multiple genes, so using quantitative trait loci (QTL) analysis is an effective method to analyze its genetic basis. Historically, the first reported downy mildew resistance QTL was Rpv1: Merdinoglu et al. used the disease-resistant round leaf grape variety 28-8-28 to hybridize with the susceptible variety Syrah, and located on the 12th chromosome 28-8-28 with a main effect site that significantly enhances resistance, named Rpv1. Since then, German scholar Welter et al. have discovered another main-effect resistant QTL on the 18th chromosome through the multi-year identification of the ‘Regent’× ‘Lemberger’F1 population, later called Rpv3, which can explain about 37% of the resistance phenotypic variants. When offspring plants carrying Rpv3 sites were infected with downy mildew, lesions were limited in expansion and cyst yields were significantly reduced (Eisenmann et al., 2019). With more disease-resistant germplasms applied, researchers have successively detected new resistance sites on other chromosomes. The currently published downy mildew resistance QTLs are distributed on chromosomes 4, 5, 6, 7, 8, 9, 10, 12, 14, 17 and 18, with a total of 28 main or micro-resistance resistance sites (Marsan-Prado et al., 2025). The interpretation rates of these QTL adversarial phenotypes range from 3.5% to up to 86.3%, and their sources include Eurasian species and a variety of wild species. With the construction of the genetic map of high-density molecular markers, candidate genes in some resistant QTL regions are gradually locked. Advances in QTL research have also provided marker tools for molecular breeding: For multiple major Rpv loci, researchers have developed linkage SSR or SNP molecular markers for early screening of offspring individuals carrying resistance genes. 3.3 Case analysis: genetic differences in disease resistance of different grape varieties Different species and varieties in the genus Variety vary greatly to downy mildew resistance, which is an important factor to consider in breeding and disease management. Most cultivars of traditional Eurasian grapes (V. vinifera) are highly susceptible to downy mildew, while many wild species or European and American hybrids show medium and high levels of resistance and even immunity. This difference stems from the long-term co-evolutionary history of species: European grapes are not stressed from downy mildew, so they lack corresponding disease-resistant genes; while wild grapes in North America and East Asia have evolved a resistance genetic mechanism under the long-term stress of downy mildew bacteria (Possamai and Wiedemann-Merdinoglu, 2022). Case studies show that the downy mildew resistance abilities of different varieties (germplasms) can be compared through vaccination tests. Experts optimized and established an indoor ex vivo leaf inoculation method for identification of downy mildew disease resistance of grapes, and conducted disease resistance measurements on 32 different types of grape varieties (strains). The results showed that the differences in downy mildew resistance between varieties were significant. The resistance of wild grape resources sometimes varies. For example, some real mutations in hairy grapes (V. davidii) can also occur in high humidity environments, but the symptoms are mild; M. rotundifolia is generally highly resistant to downy mildew, and almost no obvious lesions are observed (Feechan et al., 2013). Resistant varieties often initiate more rapid and effective defense responses when they encounter bacteria, including the large accumulation of related defense enzymes and antibacterial substances. For example, after the disease-resistant mountain grape varieties are infected by downy mildew, polyphenol antibacterial substances (such as resveratrol) and allergic necrosis reactions are stronger than disease-sensitive varieties. 4 Molecular Mechanisms and Signaling Pathways 4.1 Plant immune system: the role of PTI and ETI in downy mildew resistance The plant's immune system includes two main lines of defense: one is PAMP-triggered immunity (PTI) mediated by pattern recognition receptors, and the other is effector-triggered immunity (ETI) mediated by disease-resistant genes. Both immune response mechanisms play an important role in the grape-down mildew interaction. When the downy mildew sporangia or zoospores attach to grape leaves, pattern recognition receptors (such as LRR receptor-like kinases) on the host cell membrane can recognize conserved molecular patterns (PAMPs) on the surface of the pathogen, such as flagellin fragments, etc., thereby activate the PTI defense response (Fu et al., 2023).
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