Plant growth-promoting rhizobacterium Bacillus cereus AR156 - induced systemic resistance against multiple pathogens by Priming of phytoalexin synthesis and secretion

Phytoalexins play an important role in plant immunity. However, whether they are involved in the induction of ISR by plant growth-promoting rhizobacteria against different pathogens is poorly studied. This study showed that B. cereus AR156 could trigger ISR against broad-spectrum disease. RNA-seq and camalexin content assays showed that AR156-triggered ISR can induce the accumulation of phytoalexins such as camalexin synthesis and secretion-related genes. Moreover, it showed that AR156-triggered ISR elevates camalexin accumulation by increasing the expression of camalexin synthesis genes upon pathogen infection. Further studies revealed that WRKY33 was required for the induction of camalexin accumulation by AR156 during the pathogen infection. Compared to the control inoculated with P. capsici and B. cinerea only, the biomass of P. capsici and B. cinerea in AR156 pretreated wrky33 mutant plants were quite similar. AR156-induced ISR resistance to Pst DC3000 was significantly attenuated in the wrky33 mutant. Moreover, the study reveals that AR156 could up-regulate the expression level of PEN3 and PDR12, which act as camalexin transporter. In addition, we found that PEN3 and PDR12 served as positive regulators involved in AR156-triggered ISR against pathogens. Specifically, PEN3 and PDR12 participated in AR156-triggered ISR against fungi and oomycetes, while PEN3 was involved in AR156-triggered ISR against Pst DC3000. In summary, Bacillus cereus AR156 triggered induced systemic resistance against Botrytis cinerea, Pseudomonas syringae pv. tomato DC3000 and Phytophthora capsici by priming of phytoalexin synthesis and secretion.