Exploring Pseudomonas syringae pv. tomato biofilm‐like aggregate formation in susceptible and PTI‐responding Arabidopsis thaliana

Bacterial biofilm‐like aggregates have been observed in plants, but their role in pathogenicity is underinvestigated. In the present study, we observed that extracellular DNA and polysaccharides colocalized with green fluorescent protein (GFP)‐expressing Pseudomonas syringae pv. tomato (Pst) aggregates in Arabidopsis leaves, suggesting that Pst aggregates are biofilms. GFP‐expressing Pst, Pst ΔalgU ΔmucAB (Pst algU mutant), and Pst ΔalgD ΔalgU ΔmucAB (Pst algU algD mutant) were examined to explore the roles of (1) alginate, a potential biofilm component; (2) Pst AlgU, thought to regulate alginate biosynthesis and some type III secretion system effector genes; and (3) intercellular salicylic acid (SA) accumulation during pathogen‐associated molecular pattern‐triggered immunity (PTI). Pst formed extensive aggregates in susceptible plants, whereas aggregate numbers and size were reduced in Pst algU and Pst algD algU mutants, and both multiplied poorly in planta, suggesting that aggregate formation contributes to Pst success in planta. However, in SA‐deficient sid2‐2 plants, Pst algD algU mutant multiplication and aggregate formation were partially restored, suggesting plant‐produced SA contributes to suppression of Pst aggregate formation. Pst algD algU mutants formed fewer and smaller aggregates than Pst algU mutants, suggesting both AlgU and AlgD contribute to Pst aggregate formation. Col‐0 plants accumulated low levels of SA in response to Pst and both mutants (Pst algU and Pst algD algU), suggesting the regulatory functions of AlgU are not involved in suppressing SA‐mediated plant defence. Plant PTI was associated with highly reduced Pst aggregate formation and accumulation of intercellular SA in flg22‐induced PTI‐responding wild‐type Col‐0, but not in PTI‐incompetent fls2, suggesting intercellular SA accumulation by Arabidopsis contributes to suppression of Pst biofilm‐like aggregate formation during PTI. PTI‐responding leaves accumulate intercellular SA and few GFP‐expressing Pseudomonas aggregates, whereas susceptible leaves do not accumulate intercellular SA and have many large GFP‐expressing Pseudomonas aggregates.