International Journal of Horticulture, 2024, Vol.14, No.4, 263-274 http://hortherbpublisher.com/index.php/ijh 268 al., 2021). Gene pyramiding, which involves combining multiple resistance genes into a single genotype, enhances the durability and spectrum of disease resistance. For example, in Chinese cabbage, multiple genes for self-compatibility, multilocular ovaries, and clubroot resistance were pyramided using molecular markers (Zheng et al., 2022). These strategies can be adapted for carrot breeding to develop varieties with robust and broad-spectrum disease resistance. Disease-resistant carrot breeding involves integrating identified resistance markers into superior varieties through Marker-Assisted Selection (MAS). This approach significantly enhances the efficiency and accuracy of traditional breeding methods by selecting for disease resistance traits at the seedling stage, without the need to wait for the plants to be exposed to pathogens for screening. A common strategy is to pyramid multiple resistance genes to create broad-spectrum resistance. By combining several resistance genes, breeders can develop carrot varieties that are less susceptible to disease pressures under varying environmental conditions, which is particularly important in regions where multiple pathogens coexist (Rogozina et al., 2021). Another strategy involves using backcrossing, where a disease-resistant donor parent is crossed with a high-yielding but susceptible parent. The offspring are then backcrossed with the high-yielding parent, and resistance markers are selected at each generation. This process continues until an ideal combination of high yield and disease resistance traits is achieved. In some cases, genetic engineering techniques, such as CRISPR-Cas9, are used to introduce or enhance resistance genes within the carrot genome. However, these methods require regulatory approval and public acceptance before they can be widely adopted (Hu et al., 2023). 4.3 Case studies of disease-resistant carrot varieties Several successful case studies demonstrate the effectiveness of MAS in developing disease-resistant carrot varieties. A notable example is the research on developing a variety resistant to ALB disease. Koutouan et al. (2023) identified seven terpenes potentially associated with resistance through QTL co-localization analysis, including α-pinene, camphene, α-bisabolene, α-humulene, β-cubebene, caryophyllene, and dauca-4,8-diene. Further in vitro experiments showed that α-humulene and caryophyllene exhibited significant antifungal activity, capable of inhibiting the growth of Alternaria dauci mycelium. The results suggest that terpenes may play a crucial role in the carrot resistance mechanism, particularly α-humulene and caryophyllene, which may be directly involved in the disease resistance process, while other terpenes like α-pinene and camphene may function in other stages of resistance (Figure 2). Figure 2 Co-localization between rQTL and mQTL of terpenes presenting significant negative correlation with ALB disease score (Adopted from Koutouan et al., 2023) Image caption: In the figure, bars of different colors represent the positions of terpene mQTLs and rQTLs on the chromosomes, with blue representing sesquiterpenes and green representing monoterpenes. The results show that multiple terpene mQTLs co-localize with rQTLs on chromosomes 1, 4, 6, and 8, with significant hotspot regions particularly on chromosomes 4 and 8. These co-localization results suggest that these terpenes may directly participate in the carrot's resistance mechanism against Alternaria dauci, confirming the important role of terpenes in disease resistance (Adapted from Koutouan et al., 2023)
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