Xanthomonas axonopodis pv. manihotis gumD gene is essential for EPS production and pathogenicity and enhances epiphytic survival on cassava (Manihot esculenta)

Abstract: Extracellular polysaccharide-defective (EPS−) mutants of the cassava blight pathogen, Xanthomonas axonopodis pv. manihotis, were created by targeted disruption of the biosynthetic gene gumD. Pathogenicity of mutants was severely attenuated. Two EPS− mutants infiltrated into the leaf apoplast, caused initial water-soaking and chlorosis but symptoms were confined to inoculated zones, whereas those of wild type I56 rapidly extended and developed into flaccidity. This pattern reflected failure of EPS− mutants to spread. In entire lobes the wild-type population exceeded by 200-fold that of EPS− mutants. Petioles injected with EPS− mutants remained symptomless, in contrast to severe wilting with I56. EPS conferred significant resistance to H2O2, known to be produced by cassava cells during the oxidative burst. At 1h exposure to 1mM peroxide, only 3% EPS− compared with 48% wild type cells survived. Also, EPS− mutants were more vulnerable to UV irradiation. These phenomena correlated with reduced epiphytic survival of EPS− following spray application. The requirement for EPS in pathogenicity and epiphytic fitness is considered. [Copyright &y& Elsevier]