Molecular Plant Breeding 2024, Vol.15, No.2, 70-80 http://genbreedpublisher.com/index.php/mpb 74 One notable example is the editing of the MdDIPM gene in apple, which is known to be involved in the susceptibility to fire blight (Figure 2). By knocking out this gene, researchers have successfully developed apple lines that exhibit enhanced resistance to this devastating disease (Yin and Qiu, 2019). Similarly, targeting the MdMLO gene, which is associated with powdery mildew susceptibility, has resulted in apple varieties with improved resistance to this fungal pathogen (Zhou et al., 2020; Keul et al., 2022). Figure 2 Fire blight severity in MdDIPM4-edited plants of ‘Gala’ and ‘Golden Delicious’ cultivars (Adopted from Pompili et al., 2019) Image caption: (a) Boxplot summarizing the percentage of necrosis (calculated as length of the necrosis/total length of the shoot × 100) of candidate MdDIPM4 CRISPR/Cas9-edited plants inoculated by the method of scissor with Erwinia amylovora strain Ea273. The number of inoculated biological replicates for each line is indicated (n). Boxes comprise values between 25% and 75% of the group. Horizontal central lines represent medians. Mean is shown as+. Whiskers (Tukey) determine values within ±1.5 interquartile ranges from the median. Circles indicate outliers. Lettering indicates statistically significant differences between plant lines (for ‘Gala’ lower case, for ‘Golden Delicious’ upper case) according to Kruskal–Wallis test followed by multiple comparison of mean rank (α = 0.05). (b) Pictures, taken 1 month after inoculation, showing the fire blight-induced necrotic phenotype in wild-type and some transgenic lines. Red and blue arrows indicate the interruption of necrosis and new regenerated shoots, respectively. Gala (G); Golden Delicious (GD) (Adopted from Pompili et al., 2019) Pompili et al. (2019) demonstrates that CRISPR/Cas9-mediated editing of the MdDIPM4 gene in apple cultivars 'Gala' and 'Golden Delicious' significantly reduces susceptibility to fire blight, caused by Erwinia amylovora. The graph shows a substantial decrease in necrosis percentages in edited lines compared to non-edited controls, indicating successful gene editing. Edited lines exhibit a reduced rate of disease progression and show new, healthy shoot growth post-infection, unlike the continuous disease progression observed in control plants. This highlights the potential of CRISPR/Cas9 technology in enhancing disease resistance in apple cultivars, paving the way for improved crop resilience and productivity. The visual comparison of plants also underscores the practical benefits of genetic editing in real-world agricultural applications. The precision and efficiency of CRISPR/Cas9 have also enabled the development of apple varieties with multiple disease resistances. For example, simultaneous editing of multiple susceptibility genes has been demonstrated, leading to apple trees that are resistant to both fire blight and apple scab (Yin and Qiu, 2019; Zhou et al., 2020; Keul et al., 2022). This multi-target approach not only enhances disease resistance but also reduces the need for chemical treatments, promoting more sustainable apple production. 4.1.2 Citrus gene editing for HLB resistance Huanglongbing (HLB), also known as citrus greening disease, is one of the most devastating diseases affecting citrus crops worldwide. The disease is caused by the bacterium Candidatus liberibacter asiaticus and leads to significant yield losses and tree mortality. Traditional breeding methods have struggled to develop HLB-resistant citrus varieties due to the complex genetics and long generation times of citrus trees. However, recent advancements in genome editing technologies have opened new avenues for developing HLB-resistant citrus.
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