Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

Document Type



The AVR4 effector, secreted by Cladosporium fulvum, has been demonstrated to be involved in pathogen virulence. Recent studies further demonstrated that Avr4 is highly conserved among several Cercospora species, indicating a potential important role of this gene in fungal virulence. Therefore, investigation to determine whether this fungal effector gene is present in Cercospora cf. flagellaris (previously known as C. kikuchii), the causal agent of soybean cercospora leaf blight (CLB) disease, and whether it plays any role in CLB disease development, is of great interest. In the present study, the Avr4 gene from C. cf. flagellaris was cloned and mutants lacking the expression of Avr4 were created by homologous recombination to investigate its role in fungal virulence, cercosporin production as well as CLB disease development. The ∆avr4 mutants produced little or no cercosporin in vitro and the mutants also had significantly reduced cercosporin toxin biosynthesis genes expression. Furthermore, ∆avr4 mutants grew faster and were more sensitive to chitinase in vitro than the wild type. Surprisingly, cercosporin could also directly suppress chitinolytic activity in vitro. When inoculated onto detached soybean leaves, these mutants exhibited reduced virulence compared to the wild type and no cercosporin was detected in mutant inoculated soybean leaves. Taken all together, the results suggest that AVR4 may contribute to the virulence of C. cf. flagellaris on soybean through protecting fungal hyphae and regulating cercosporin biosynthesis. Considering the importance of AVR4 in C. cf. flagellaris virulence, we selected a region of this gene for targeted gene suppression through host induced gene silencing (HIGS) to determine whether this can reduce CLB disease in soybean. It was found that HIGS plants carrying the BPMV-Avr4 construct showed less disease symptoms compared to control plants, and the reduction of symptoms was positively correlated with reduction in Avr4 transcript levels and fungal growth. To the best of our knowledge, this is the first study demonstrating the involvement of AVR4 in CLB disease development possibly through regulating cercosporin production as well as protecting fungal hyphae. In addition, this study also showed the potential of using HIGS as a tool to control this important disease of soybean



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Committee Chair

Chen, Zhiyuan