Abstract
Prior to infection, phytopathogenic bacteria face a challenging environment on the plant surface, where they are exposed to nutrient starvation and abiotic stresses. Pathways enabling surface adhesion, stress tolerance and epiphytic survival are important for successful plant pathogenesis. Understanding the roles and regulation of these pathways is therefore crucial to fully understand bacterial plant infections. The phytopathogen Pseudomonas syringae pv. tomato (Pst) encodes multiple polysaccharides that are implicated in biofilm formation, stress survival and virulence in other microbes. To examine how these polysaccharides impact Pst epiphytic survival and pathogenesis, we analysed mutants in multiple polysaccharide loci to determine their intersecting contributions to epiphytic survival and infection. In parallel, we used qRT-PCR to analyse the regulation of each pathway. Pst polysaccharides are tightly coordinated by multiple environmental signals. Nutrient availability, temperature and surface association strongly affect the expression of different polysaccharides under the control of the signalling proteins ladS and cbrB and the second messenger cyclic-di-GMP. Furthermore, functionally redundant, combinatorial phenotypes were observed for several polysaccharides. Exopolysaccharides and WapQ-mediated lipopolysaccharide production are important for leaf adhesion, while α-glucan and alginate together confer desiccation tolerance. Our results suggest that polysaccharides play important roles in overcoming environmental challenges to Pst during plant infection.
Highlight Pseudomonas syringae uses the coordinated deployment of polysaccharides to address environmental challenges during plant colonization. Functional redundancy renders individual polysaccharides dispensable during plant infection, but their combined loss impedes pathogenicity.
Competing Interest Statement
The authors have declared no competing interest.
Abbreviations
- Pst
- Pseudomonas syringae pv. tomato DC3000
- LPS
- lipopolysaccharide
- EPS
- exopolysaccharide
- PBS
- phosphate buffered saline
- KB
- King’s B medium
- L
- Lennox medium with 0.1% glucose
- SFM
- soy flour mannitol medium
- M9
- M9 minimal nutrient medium
- cdG
- cyclic di GMP
- WapQ
- orthologue of LPS kinase WapG/WaaQ/InaA
- Waap
- Kdo/Waap family LPS kinase
- Waal
- LPS biosynthesis protein
- WbpL
- glycosyltransferase/LPS biosynthesis protein
- RPM
- revolutions per minute
- RH
- relative humidity
- CFU
- colony-forming units
- cDNA
- complementary DNA
- CR
- Congo Red dye
- X-gal
- 5-bromo-4-chloro-3-indolyl β-D-galactopyranoside
- ddH2O
- double distilled H2O
- Δ
- deletion of specified gene(s) or operon
- G1P
- glucose 1-phosphate
- G6P
- glucose 6-phosphate
- M1P
- maltose 1-phosphate
- OD600
- optical density at λ 600 nm
- NMR
- nuclear magnetic resonance
- TMSP
- trimethylsilyl propanoic acid
- SD
- standard deviation
- CbrB
- member of the CbrAB two component regulatory system.