Molecular Pathogens 2024, Vol.15, No.5, 246-254 http://microbescipublisher.com/index.php/mp 248 for the activation of the bHLH132 transcription factor in tomato, a gene involved in both growth and defense (Kim and Mudgett, 2019). Similarly, XopC2 from Xanthomonas axonopodis pv. punicae suppresses immune responses such as callose deposition and reactive oxygen species (ROS) production, facilitating bacterial growth and disease development in pomegranate. TAL effectors can also inhibit the expression of plant defense genes. For example, Tal7 fromXanthomonas oryzae pv. oryzicola activates the rice gene Os09g29100, which suppresses the defense response mediated by the Xa7 resistance gene, thereby enhancing susceptibility to bacterial infection (Cai et al., 2017). This suppression of effector-triggered immunity (ETI) is a key strategy employed by Xanthomonas to overcome plant defenses. 3.3 Impact on host metabolism and growth TAL effectors can significantly alter host metabolism, including photosynthesis and nutrient transport. For instance, the activation of specific host genes by TAL effectors can lead to changes in the expression of genes involved in these processes, thereby affecting the overall health and growth of the plant. This alteration in host metabolism not only facilitates pathogen growth but also contributes to the visible symptoms of disease (Wu et al., 2021). TAL effectors also influence hormonal pathways in the host plant. The activation of OsF3H04g by Tal2c in rice leads to reduced salicylic acid production, a hormone crucial for plant defense, thereby increasing susceptibility to infection. The interaction of XopC2 with plant proteins involved in hormone signaling suggests that non-TAL effectors can also modulate hormonal pathways to suppress plant immunity and promote bacterial growth (Mondal et al., 2020). 4 Host Resistance to TAL Effectors 4.1 Resistant gene (Rgene) mechanisms Resistance genes (Rgenes) play a crucial role in plant defense against pathogens by recognizing specific pathogen effectors and triggering immune responses. In the context of Xanthomonas, some isolates of the rice pathogen Xanthomonas oryzae use truncated versions of TAL effectors, known as interfering TAL effectors (iTALEs), to overcome R-gene-mediated resistance. These iTALEs lack a transcription activation domain but retain nuclear localization motifs, allowing them to interfere with the resistance conferred by the Rgene Xa1, which recognizes multiple TAL effectors (Zhang et al., 2015; Ji et al., 2016). This illustrates the dynamic co-evolution between plant Rgenes and pathogen effectors. 4.2 Engineering disease-resistant legumes Engineering disease-resistant legumes involves modifying the promoter regions of susceptibility (S) genes to prevent TAL effector binding. For instance, targeted promoter editing of the OsSWEET14 gene in rice has been shown to confer resistance to Xanthomonas oryzae pv. oryzae strains that rely on specific TAL effectors like AvrXa7 and Tal5. By creating mutations in the effector binding elements (EBEs) of these genes, researchers have successfully generated rice lines resistant to bacterial strains dependent on these TAL effectors (Blanvillain-Baufumé et al., 2016; Zhou and Chen, 2024). This strategy can be applied to legumes by identifying and editing the EBEs of key S genes targeted by Xanthomonas TAL effectors. 4.3 Role of loss-of-susceptibility (LOS) mutations Loss-of-susceptibility (LOS) mutations are another effective strategy for developing disease-resistant crops. These mutations involve altering the EBEs of S genes so that TAL effectors can no longer bind and activate them. For example, mutations in the EBEs of the CsLOB1 gene in citrus have been shown to prevent the TAL effector PthA4 from inducing gene expression, thereby conferring resistance to Xanthomonas citri subsp. citri (Figure 1) (Teper and Wang, 2021). Similarly, the identification of a single nucleotide polymorphism (SNP) in the promoter of the OsSWEET13 gene in rice, which prevents the TAL effector PthXo2 from binding, has revealed cryptic resistance to bacterial blight (Zhou et al., 2015). These findings highlight the potential of LOS mutations in breeding disease-resistant legumes by disrupting the interaction between TAL effectors and their target Sgenes.
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