Comparative analysis of interactions between five strains of Pseudomonas syringae pv. tabaci and Nicotiana benthamiana

Pseudomonas syringae pv. tabaci 6605 (Pta 6605), the agent of wildfire disease in tobacco, has been used as a model strain for elucidating the virulence mechanisms of Pta. However, the host genes involved in resistance or susceptibility to Pta remain largely unknown. Nicotiana benthamiana is a model plant species in the Solanaceae family and is useful in functional analyses of genes. We herein compared five Pta strains (6605, 6823, 7372, 7375, and 7380) in terms of their phenotypes on medium and interactions with N. benthamiana. Pta 6605 and Pta 6823 showed more active proliferation than the other strains in a high cell density culture. Moreover, Pta 6605 exhibited markedly higher swarming motility than the other strains. In inoculated leaves of N. benthamiana, Pta 6605 and Pta 6823 caused more severe disease symptoms and proliferated to a higher cell density than the other strains. However, Pta 6823 as well as Pta 7372 and Pta 7380 induced the high accumulation of salicylic acid (SA). Moreover, the inoculations of Pta 6823 and Pta 7372 resulted in the upregulation of ethylene biosynthesis genes. On the other hand, Pta 6605 induced neither SA accumulation nor the expression of ethylene biosynthesis genes, and suppressed the expression of jasmonate biosynthesis genes. Moreover, chlorosis was clearly induced in the upper uninoculated leaves of Pta 6605-infected plants. These results suggest that Pta 6605 escapes from or suppresses plant immune systems and, thus, is the most virulent on N. benthamiana among the five strains tested.