Vol. 9, Issue 4, Part I (2025)

This article is a summary of the thirty-fourth New Phytologist Symposium Plant Pathogen Interactions held at the John Innes Centre, Norwich, UK on 16 July, 2009. Plant pathogen interactions have provided fundamental insights into plant biology. Furthermore, they are of cross-disciplinary interest to many scientists including plant, microbial and fungal scientists. In the molecular dialogue between plant and bacterial pathogens, plants have evolved an effective immune system that recognizes conserved microbial structures. This system has been termed PAMP triggered immunity (PTI) since it is initiated upon recognition of microbial elicitors termed pathogen associated molecular patterns (PAMPs). Successful pathogens suppress or evade PTI using an array of effectors. In turn, plants have evolved resistance (R) proteins that recognize effector activity, either directly or indirectly, resulting in effector triggered immunity (ETI). In Gram-negative bacteria, the type III secretion system (T3SS) is central to pathogenicity, and ∼29 avirulence proteins (AVRs) are secreted by the Pseudomonas syringae pv. tomato T3SS. Recognition of these AVRs by cognate plant R proteins regulates many aspects of gene expression in the plant, often via the action of the defence hormone salicylic acid. Combining microarray analysis of the dynamics of the plant Arabidopsis thaliana during the establishment of primordia at infection sites with complementary analysis of the P. syringae transcriptome offers an insight into the molecular dialogue between host and pathogen. The P. syringae T3SS can suppress the entry of P. fluorescens, P. putida, and A. tumefaciens Agropine Ti plasmids into plants independent of the presence of the phytohormone auxin.