Basal resistance against bacteria in Nicotiana benthamiana leaves is accompanied by reduced vascular staining and suppressed by multiple Pseudomonas syringae type III secretion system effector proteins

Summary

Basal resistance in plants is induced by flagellin and several other common bacterial molecules and is implicated in the immunity of plants to most bacteria and other microbes. However, basal resistance can be suppressed by effector proteins that are injected by the type III secretion system (TTSS) of pathogens such as Pseudomonas syringae. This study demonstrates that basal resistance in the leaves of Nicotiana benthamiana is accompanied by reduced vascular flow into minor veins. Reduced vascular flow was assayed by feeding leaves, via freshly excised petioles, with 1% (weight in volume, w/v) neutral red (NR) and then observing differential staining of minor veins or altered levels of extractable dye in excised leaf samples. The reduced vascular staining was localized to tissues expressing basal resistance and was observable when resistance was induced by either the non-pathogen Pseudomonas fluorescens, a TTSS-deficient mutant of P. syringae pv. tabaci, or flg22 (a flagellin-derived peptide elicitor of basal resistance). Nicotiana benthamiana leaf areas expressing basal resistance no longer elicited the hypersensitive response when challenge inoculated with P. syringae pv. tomato DC3000. The reduced vascular staining effect was suppressed by wild-type P. syringae pv. tabaci and P. fluorescens heterologously expressing a P. syringae TTSS and AvrPto1_PtoJL1065. TTSS-proficient P. fluorescens was used to test the ability of several P. syringae pv. tomato DC3000 effectors for their ability to suppress the basal resistance-associated reduced vascular staining effect. AvrE_PtoDC3000, HopM1_PtoDC3000 (formerly known as HopPtoM), HopF2_PtoDC3000 (HopPtoF) and HopG1_PtoDC3000 (HopPtoG) suppressed basal resistance by this test, whereas HopC1_PtoDC3000 (HopPtoC) did not. In summary, basal resistance locally alters vascular function and the vascular dye uptake assay should be a useful tool for characterizing effectors that suppress basal resistance.