Suvi Taira and Martin Romantschuk - General Microbiology

Plant pathogenic bacteria

Docent Suvi Taira and Professor Martin Romantschuk

Plant pathogen group focuses on type III secretion system of  plant-infecting bacteria .

Many pathogenic Gram negative bacteria are able to inject virulence proteins into the host cell by a specialized secretion apparatus, termed Type III Secretion System (TTSS). The proteins secreted by the TTSS are believed to interfere with the host cells defense responses, presumably by silencing the eukaryotic signaling cascades. Among the TTSS harbouring bacteria are several important human pathogens, such as Yersinia, Salmonella and Shigella. Interestingly also several bacteria infecting plants are using a homologous secretion apparatus for the same purpose. Our model bacterium is Pseudomonas syringae, and our model plant is Arabidopsis thaliana. Both organisms are classical models for studying plant-microbe interactions and full genome sequence is available for both organisms.

Our main focus has been on the TTSS dependent secretion process i) the selection of virulence proteins to the secretion route and ii) the translocation of proteins through bacterial cell wall, plant cell wall and membranes present in bacteria and plants. We have shown that one structural component of the secretion apparatus is a needle-like extension on the bacterial surface. This extension, termed Hrp pilus, forms a physical bridge between the bacterium and the plant and functions as a conduit for the secreted proteins (EMBO J. 2002, 21:1909-1915).

Our more recent focus has been on the secreted proteins, their function and their interaction with plant proteins. Plants defense signaling is a complicated network involving dozens of proteins. The TTSS-secreted virulence proteins are also numerous. One single strain of P. syringae has been estimated to carry over 30 genes encoding proteins secreted by TTSS. Revealing the complicated interplay between the plant and the bacterial proteins is a future challenge that the scientific community is now focusing on (Mol. Plant-Microb. Interact. 2005, 18:60-66).