Differential Virulence Contributions of the Efflux Transporter MexAB-OprM in Pseudomonas syringae Infecting a Variety of Host Plants

Abstract

Efflux transporters such as MexAB-OprM contribute to bacterial resistance to diverse antimicrobial compounds. Here, we show that MexB contributes to epiphytic and late-stage apoplastic growth of Pseudomonas syringae strain B728a, as well as lesion formation in common bean (Phaseolus vulgaris). Although a ∆mexB mutant formed fewer lesions after topical application to common bean, these lesions contain the same number of cells (105 to 107 cells) as those caused by the wild-type strain. The internalized population size of both the wild-type and the ∆mexB mutant within small samples of surface-sterilized asymptomatic portions of leaves varied from undetectably low to as high as 105 cells/cm2. Localized bacterial populations within individual lesions consistently exceeded 105 cells/cm2. Strain B728a was capable of moderate to extensive apoplastic growth in diverse host plants, including lima bean (P. lunatus), fava bean (Vicia faba), pepper (Capsicum annuum), Nicotiana benthamiana, sunflower (Helianthus annuus), and tomato (Solanum lycopersicum), but MexB was not required for growth in a subset of these plant species. A model is proposed that MexB provides resistance to as-yet-unidentified antimicrobials that differ between plant species. [Graphic: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Keywords

• apoplast
• epiphytes
• host range
• lesions
• nonhost resistance
• phyllosphere