Trehalose Biosynthesis Promotes Pseudomonas aeruginosa Pathogenicity in Plants

Abstract

Pseudomonas aeruginosa strain PA14 is a multi-host pathogen that infects plants, nematodes, insects, and vertebrates. Many PA14 factors are required for virulence in more than one of these hosts. Noting that plants have a fundamentally different cellular architecture from animals, we sought to identify PA14 factors that are specifically required for plant pathogenesis. We show that synthesis by PA14 of the disaccharide trehalose is required for pathogenesis in Arabidopsis, but not in nematodes, insects, or mice. In-frame deletion of two closely-linked predicted trehalose biosynthetic operons, treYZ and treS, decreased growth in Arabidopsis leaves about 50 fold. Exogenously co-inoculated trehalose, ammonium, or nitrate, but not glucose, sulfate, or phosphate suppressed the phenotype of the double Δ treYZΔ treS mutant. Exogenous trehalose or ammonium nitrate does not suppress the growth defect of the double Δ treYZΔ treS mutant by suppressing the plant defense response. Trehalose also does not function intracellularly in P. aeruginosa to ameliorate a variety of stresses, but most likely functions extracellularly, because wild-type PA14 rescued the in vivo growth defect of the Δ treYZΔ treS in trans. Surprisingly, the growth defect of the double Δ treYZΔ treS double mutant was suppressed by various Arabidopsis cell wall mutants that affect xyloglucan synthesis, including an xxt1xxt2 double mutant that completely lacks xyloglucan, even though xyloglucan mutants are not more susceptible to pathogens and respond like wild-type plants to immune elicitors. An explanation of our data is that trehalose functions to promote the acquisition of nitrogen-containing nutrients in a process that involves the xyloglucan component of the plant cell wall, thereby allowing P. aeruginosa to replicate in the intercellular spaces in a leaf. This work shows how P. aeruginosa, a multi-host opportunistic pathogen, has repurposed a highly conserved “house-keeping” anabolic pathway (trehalose biosynthesis) as a potent virulence factor that allows it to replicate in the intercellular environment of a leaf.

Author Summary

Pseudomonas aeruginosa is an opportunistic human bacterial pathogen that infects a wide range of plants and animals, including the model laboratory plant Arabidopsis thaliana. P. aeruginosa utilizes many of the same virulence-related factors to infect both plants and animals. However, because plants have fundamentally different cellular architecture than animals, we hypothesized that P. aeruginosa synthesizes specific factors required for infecting plants but not animals. We found that synthesis of the sugar molecule trehalose, an unusual dimer of glucose, is required for plant but not animal pathogenesis. Although P. aeruginosa mutants defective in trehalose synthesis are non-pathogenic in Arabidopsis, Arabidopsis mutants that lack the polysaccharide xyloglucan in their cell walls can be infected by P. aeruginosa trehalose mutants. Moreover, application of ammonium nitrate overcomes the requirement for trehalose for infecting an Arabidopsis leaf. Our data suggest that trehalose promotes the acquisition of nitrogen-containing nutrients, thereby allowing P. aeruginosa to replicate in the nutrient-poor intercellular spaces in a leaf. This work shows how an opportunistic pathogen has repurposed a highly conserved “house-keeping” function (trehalose biosynthesis) as a potent virulence factor.