Overexpression of BAS1 in rice blast fungus can promote blast fungus growth, sporulation and virulence in planta

Background: BAS1 is biotrophy-associated secreted protein of rice blast strain (Magnaporthe oryzae). In order to study the effect of BAS1 on virulence of rice blast strain, we characterized function of BAS1 using a purified prokaryotic expression product of BAS1 and its overexpression strain. Results: Our results showed in vitro the purified expression product caused rapid callose deposition and ROS production in rice leaves and calli, indicated it triggered transient basal defense. When the purified expression product of BAS1 was sprayed onto rice leaves, and 24 h later the leaves were inoculated with blast strain, the results showed the size and number of lesions, on purified BAS1 product-pretreated leaves of the Lijiangxintuanheigu (LTH) challenged with blast strain, was higher than those in BAS1-untreated leaves directly challenged with the same strain, which suggested the defense response trigged by BAS1 can be overcome by other effectors of the fungus. More severe symptoms, higher sporulation, higher relative fungal growth and more lower expression level of defense-related genes appeared in LTH leaves challenged with overexpression strain 35S:BAS1/Mo-2 than those in LTH inoculated with wild-type strain. Conclusions: These data suggest both in vitro pretreatment with BAS1 prokaryotic expression products and overexpression in blast strains can increase virulence of blast fungus.