Molecular Pathogens 2024, Vol.15, No.5, 237-245 http://microbescipublisher.com/index.php/mp 238 2 Biological Characteristics of Xanthomonas oryzae 2.1 Infection mechanism Xanthomonas oryzae pv. oryzae (Xoo) is a vascular pathogen that primarily infects rice through the xylem vessels. The infection process begins with the bacterium's chemotaxis response to components of rice xylem sap, such as glutamine, xylose, and methionine, which guide the pathogen to the entry points (Verma et al., 2018). Once inside the plant, Xoo utilizes its type III secretion system (T3SS) to inject effector proteins into the host cells, suppressing the plant's immune responses and facilitating colonization. Additionally, the pathogen employs a type II secretion system (T2SS) to secrete cell-wall-degrading enzymes, further aiding in the invasion and spread within the plant tissues (Jha et al., 2007). 2.2 Pathogenicity factors The virulence of Xanthomonas oryzae is largely attributed to its secretion systems and the effector proteins they deliver. The T3SS is crucial for injecting transcription activator-like (TAL) effectors into rice cells, which manipulate host gene expression to favor bacterial growth and suppress plant defenses (Verdier et al., 2020). Specific TAL effectors target rice susceptibility genes, such as the SWEET sucrose transporter genes, enhancing the pathogen's ability to extract nutrients from the host. Additionally, the T2SS secretes enzymes like cellulase and cellobiosidase, which degrade the plant cell wall and facilitate bacterial entry and spread. Other important virulence factors include the diffusible signal factor regulon, which modulates metabolic pathways critical for the pathogen's survival and virulence (Koduru et al., 2020). 2.3 Impact on rice physiology and yield Infection by Xanthomonas oryzae pv. oryzae leads to significant physiological changes in rice plants, ultimately resulting in severe yield losses. The pathogen's effector proteins disrupt normal cellular processes, leading to symptoms such as leaf blight, which is characterized by water-soaked lesions that turn yellow and necrotic (Sciallano et al., 2022). The infection impairs photosynthesis, signal transduction, and various metabolic pathways, including those involved in carbohydrate, lipid, and amino acid metabolism. These disruptions weaken the plant, reduce its growth, and significantly lower grain yield. In severe cases, bacterial blight can cause up to 50% yield loss, posing a major threat to rice production and food security (Jiang et al., 2020; Xu et al., 2020). 3 Current Status of Genetic Engineering Strategies for Disease Resistance 3.1 Discovery and application of rice resistance genes The identification and application of resistance (R) genes in rice have been pivotal in combating bacterial blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo). Extensive research has led to the discovery of numerous R genes, such as Xa10, Xa25, Xa33, and Xa7, which confer varying degrees of resistance to different Xoo strains. For instance, the Xa10 gene has been modified to create Xa10 (E5), which provides broad-spectrum and durable resistance by responding to multiple TAL effectors from Xoo (Zeng et al., 2015). Similarly, the Xa25 gene, a recessive gene, has been characterized to confer race-specific resistance by inhibiting Xoo growth (Liu et al., 2011). The Xa33 gene, identified from wild rice species, offers broad-spectrum resistance and has been fine-mapped for precise application in breeding programs (Kumar et al., 2012; Li and Jiong, 2024). The Xa7 gene, known for its durable resistance, has been shown to trigger a hypersensitive response and is effective even under high-temperature conditions, making it a valuable asset in the face of global warming (Chen et al., 2021; Wang et al., 2021). These discoveries underscore the importance of continuous identification and characterization of novel R genes to develop rice varieties with enhanced and durable resistance to BB (Jiang et al., 2020; Kumar et al., 2020). 3.2 Common genetic engineering techniques Genetic engineering techniques such as CRISPR-Cas9 and RNA interference (RNAi) have revolutionized the development of disease-resistant crops. CRISPR-Cas9, a precise genome-editing tool, has been employed to knock out susceptibility genes or introduce beneficial mutations in R genes, thereby enhancing resistance to pathogens like Xoo. For example, CRISPR-Cas9 has been used to modify the promoter regions of R genes to
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