Role of Extracellular Polysaccharide (EPS) Slime of Plant Pathogenic Bacteria in Protecting Cells to Reactive Oxygen Species

Abstract

Pseudomonas syringae pv. phaseolicola, a phytopathogenic bacterium, seemed very sensitive in planta to the adverse action of reactive oxygen species (ROS) produced by two chemical systems. The disease symptoms in host plants were also suppressed by ROS. Several other plant pathogenic bacteria (P. syringae pv. pisi, Erwinia amylovora. Xanthomonas campestris pv. pelargonii) as well as P. fluorescens were also sensitive in vitro to the inhibiting or killing action of ROS. It was shown that O2" and H2O2 were produced in our two chemical systems and were involved in the killing action. OH' however was not involved in the adverse action on bacteria of the ROS. Superoxide dismutase and catalase were able to reverse the killing action of ROS. When the EPS slime around bacteria was removed by washing and centrifuging the cells, bacteria were more sensitive to ROS. However, when the cells of EPS" mutants were washed and centrifuged, their sensitivity to the killing action of ROS did not change because the lack of slime around the mutant cells.The EPS" Tn5 mutants of P. syringae pv. phaseolicola and the natural EPS" mutant of E. amylovora were more sensitive to ROS than the wild type strains. These results support the idea that the EPS slime protects bacteria from ROS (O^ and H2O2). Zusammenfassung Die Bedeutung des aus extrazdlularen Polysacchariden (EPS) bestehenden ScUeims phytopathogener Bakterien fiir den Schutz der Zeilen vor reaktiven SauerstofTspeziesDas phytopathogene Bakterium Pseudomonas syringae pv. phaseolicola wurde in planta sehr leicht von reaktiven Sauerstoffspezies (ROS) geschadigt, welche durch zwei chemische Systeme gebildet wurden. Die ROS unterdruckten aucb die Krankheitssymptome von Wirtspflanzen. Verschiedene andere pbytopathogene Bakterien (P. syringae pv. pisi, Erwinia amylovora, Xanthomonas campestris pv. pelargonii) und P. fluorescens wurden auch in vitro von ROS gehemmt oder getotet. Es wurde gezeigt, daB in unseren beiden chemischen Systemen Oi" und H2O2 entstanden und ftir die abtotende Wirkung bedeutsam waren. OH' Radikale waren an der bakterienschadigenden Wirkung der ROS dagegen nicht beteiligt. Superoxid-Dismutase und Katalase konnten die abtotende Wirkung der ROS aufheben, Wurde der die Bakterien umgebende EPS-Schleim durch Waschen und Zentrifugieren der Zellen entfernt., so reagierten die Bakterien sensitiver auf die ROS. Wurden jedoch die Zellen von EPS~-Mutanten gewaschen und zentrifugiert, anderte sich ihre Sensitivitat fur die abtotende Wirkung der ROS aufgrund ihrer fehlenden Schleimhtllle nicbt.Die EPS~-Tn5-Mutanten von P. syringae pv. phaseolicola und die naturlich vorkommende EPS"-Mutante von E. amylovora reagierten im Vergleich zu den WildtypStammen sensitiver auf die ROS. Diese Ergebnisse untermauern die Ansicht, daB der EPS-Schleim die Bakterien vor ROS (O\~ und H2O2) schutzt.

Microbiology of the Phyllosphere

“…Given that water availability is likely one of the most highly fluctuating factors on leaf surfaces, the heavy EPS slime within aggregates may shield the bacteria from desiccation stress by buffering the matric and osmotic potentials of their surroundings. Additionally, EPS has a role in protecting plant-associated bacteria from reactive oxygen species (59), which are often encountered on plants. It has been demonstrated that aggregated bacteria resist oxidative stress better than planktonic bacteria (101).…”

Exopolysaccharides from Sinorhizobium meliloti Can Protect against H2O2-Dependent Damage

“…Pseudomonas syringae cells devoid of exopolysaccharides are sensitive to ROS (12). In a study of S. meliloti, Davies and Walker (13) carried out a two-part screen for mutants sensitive to H 2 O 2 that were also defective in forming productive nitrogen-fixing nodules on alfalfa (Medicago sativa).…”

Diguanylate Cyclases AdrA and STM1987 Regulate Salmonella enterica Exopolysaccharide Production during Plant Colonization in an Environment-Dependent Manner

“…In contrast, the rhizosphere, such as that of the roots of germinating seeds used in this study, is bathed in the nutrients from root exudates, and S. enterica grows exponentially on or around the roots (36). Aggregation or biofilm formation can protect bacteria from environmental stresses (37)(38)(39). Accordingly, S. enterica uses attachment factors and biofilm formation to utilize plants as vectors to mammalian hosts (7,8).…”

“…Patterns of water and nutrient availability on a leaf surface are comparable to oases in a desert (49); thus, the arrival site on the leaf predisposes a subset of bacterial cells for survival or death (50). In addition, bacterial aggregation or formation of biofilms can provide a protective layer, trap nutrients, and generally aid in stress tolerance (37)(38)(39). S. enterica biofilm mutants are significantly impaired in persistence on produce leaves (51,52).…”