Wound entry of plants by bacterial pathogens

Wounding of plants by wind, hail, rain, sand storms, and frost is common in nature. Many epiphytic leaf pathogens take advantage of this opportunity to infect plants, causing outbreak of diseases after e.g. frost and sand storms. Although plants have evolved effective immune responses to protect wound sites, molecular mechanisms underpinning defence at wound sites are still poorly understood. Also mechanisms that pathogens use to overcome these barriers at wounding sites are not all understood.

We discovered that various strains of the bacterial model plant pathogen Pseudomonas syringae can enter wound sites and spread into the leaf tissue through the xylem (Misas-Villamil et al., 2011). The P. syringae pv. syringae B728a strain achieves this by producing syringolin A (SylA), a phytotoxin that targets and inhibits the host proteasome and thereby blocks signalling through the stress hormone salicylic acid (Misas-Villamil et al., 2013). The diffusion of SylA from the primary infection site creates a zone of SA-insensitive tissue that is made ready for subsequent colonization. Although this mechanism is evident for SylA-producing strains, several other strains that cannot produce SylA can still efficiently enter wound sites (Misas-Villamil et al., 2011). The underlying mechanism is currently unknown.

The aims of this project are: i) to identify novel mechanisms that P. syringae strains use to facilitate wound entry, and ii) to determine the genetic network that host plants use to prevent wound entry. Wound entry assays with GFP-expressing P. syringae bacteria on the model plant Nicotiana benthamiana using fluorescence microscopy will be combined with genetics on both host and pathogen site. Candidate bacterial genes will be targeted by reverse genetics and metabolite extracts from bacteria will be investigated for wound-entry promoting molecules. Roles of quorum sensing, different motility mechanisms and putative phytotoxin gene clusters will be investigated. Vice versa, candidate genes preventing wound entry (e.g. involved in jasmonate signalling and oxidative burst) will be inactivated in N. benthamiana using virus-induced gene silencing (VIGS) and CRISPR/Cas9 genome editing.

Student Profile

This is an exciting, challenging project for a devoted, inquisitive minded student with an appetite for plant pathology and genetics. Experience with molecular cloning is essential, and practice working with bacterial genetics is advantageous. The student will be part of a strong, dynamic interdisciplinary research team that closely collaborates with Prof. Gail Preston, a specialist in P. syringae genetics, who will act as co-supervisor in this project.