Ralstonia solanacearum and Phytophthora parasitica promote the infection of each other by disturbing host defense response together

Multiple infections (multi-infection), either sequential or simultaneous, affecting a single plant or crop are now considered common in plant disease epidemics. The consequences of multi-infection have been studied from the aspects of pathogen virulence, accumulation, transmission, and epidemics, as well as genetic diversity, population structure, and evolutionary trajectory. However, the dynamic changes of host defense response during multi-infection are much unclear. In this study, <i>Rs</i> (<i>Ralstonia solanacearum</i>) and <i>Pp</i> (<i>Phytophthora parasitica</i>) were used to simulate the multi-infection and uncover the defense response changes of tobacco. Results showed that the lesion diameter of <i>Rs</i>+<i>Pp</i> was 350% higher than that of <i>Pp</i> and 54.2% higher than that of <i>Rs</i>, indicating that co-infection with <i>Rs</i> and <i>Pp</i> makes tobacco more susceptible to disease. Further analysis showed that co-infection could increase the contents of Aseorbate peroxidase (APX) and Peroxidase (POD), thus lead to the excessive accumulation of Reactive oxygen species (ROS). Meanwhile, most <i>pathogenesis</i>-<i>related</i> (<i>PR</i>) genes were down-regulated, revealing that the immune defense response was disturbed by co-infection and resulted in susceptibility. Our study preliminarily reveals the underlying ways that <i>Rs</i> and <i>Pp</i> co-infection suppress the host defense response, which will provide a theoretical basis for scientific, reasonable and effective tobacco disease management.