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Plant-Bacteria Interactions

Research topic

Several human and animal endopathogenic bacteria were recently shown to grow also on plants. Many of them may infect crop plants like; tomatoes or lettuce causing subsequent infection of humans. Although diverse plant species support growth of endopathogenic bacteria, the underlying physiology and the molecular mechanisms of the Salmonella–plant interaction are largely unknown. Recent studies in our laboratory show that the human pathogen S. typhimurium triggers the activation of plant immune responses, including the enhanced transcription of PR genes. We also found that Salmonella can overcome plant defense and enter and proliferate inside various Arabidopsis cells, causing disease symptoms and eventually death of infected organs.

Endophytic growth of Salmonella in planta

Salmonella enterica serovar typhimurium (S. typhimurium) is a facultative endopathogen and the causative agent of various human diseases ranging from enteritis to typhoid fever. It is responsible for salmonellosis, which is the most frequent food-borne disease with around 1.5 billion yearly infections worldwide (WHO). In animals and humans, Salmonella actively enters epithelial and other cells in order to replicate and spread through the organism. We focused our attention on the question whether, similar to the situation in mammals, Salmonella can also invade plant cells. For this purpose, S. typhimurium was marked with green fluorescent protein (GFP). Three hours post-inoculation of liquid medium with immerged seedlings, GFP-marked Salmonella were localized inside root hairs and at 20 hours post-inoculation, inside rhizodermal. To our knowledge, this is the first report of an infection of the plant cytoplasm by a human enteropathogen. Salmonella was also found to form biofilm-like structures on the surface of roots and leaves, preferentially colonizing regions around emerging lateral roots and wounded tissues. These data demonstrate that Salmonella has the ability to enter and proliferate inside plant cells.

Arabidopsis-thaliana

GFP-marked Salmonella in the root cells of Arabidopsis thaliana


Plant defense mechanisms in response to Salmonella attack

The first event in plant-pathogen interaction is recognition of the pathogen by the plant. Although a number of PAMPs were already identified, only a few receptors have so far been identified. In the presence of the respective elicitors, such receptors trigger the activation of downstream kinases and defence responses. Salmonella attack of Arabidopsis plants results in the activation of MPK3 and MPK6. Since MPK3 and MPK6 are implicated in various pathways, the respective signalling complexes rather than the MAP kinases themselves, are thought to provide the necessary signalling specificity. MPK6 activation was totally compromised when mkk3 mutants were treated with Salmonella, revealing that Salmonella-induced activation is mediated by MKK3. In agreement with a role of MKK3 and MPK6 in the defence response against Salmonella, we recently found that MKK3 is also involved in defence against bacterial and fungal pathogens and becomes activated by reactive oxygen species. The role of MPK6 is also underscored by the fact that mpk6 mutant plants were significantly less resistant to Salmonella attack, allowing infection to occur significantly faster and the development of higher Salmonella levels inside Arabidopsis plant tissues.

MAP-kinase

MAP Kinase activation after 15 min of inoculation with Salmonella

Bacterial effectors and their function in plant cells

For an efficient establishment in their hosts, pathogens have evolved strategies to overcome defence responses. This is achieved through the inactivation of PAMP-induced signalling pathways, and often targets the MAPK cascade components.

The pathogen Pseudomonas syringae injects several effectors into plant cells. Among these, AvrPto and AvrPtoB interact with the FLS2 receptor and its co-receptor BAK1. In this way, both AvrPto and AvrPtoB interrupt signalling to the downstream MAPK module.

Pseudomonas syringae has yet another factor interacting directly with the MAPK cascade components: HopAI1 is a phosphothreonine lyase that dephosphorylates the threonine residue at which MAPKs are activated by their upstream MAPKKs. Strikingly, HopAI1 is also present in animal/human pathogens such as Shigella spp. (OspF) and Salmonella spp. (SpvC), where it interacts with the MAPKs ERK1/2 and p38.

In our research we focused on the known and unknown effectors and virulence factors from Salmonella. We test their function in plants cells and compare the mode of action between the animal and vegetal systems.

Reinforced of plant defense mechanisms by the 3-oxotetradecanoyl -homoserine lactone (oxo-C14-HSL)

N-acyl-L-homoserine lactones (AHLs) are small, diffusible molecules produced by Gram-negative bacteria. In a phenomenon called quorum sensing (QS), bacteria use these molecules for communication and to control their population density. AHLs control gene expression and permit a united behavior in the bacterial population; e.g expression of virulence genes required for effective infection of the host organism. On the other hand, host organisms (plants) can perceive AHLs. Recent reports show that different QS molecules from various bacteria influence the defense response in several plant species. However, the precise mechanism of this reinforcement is not known. We are interested in the impact of AHLs on the plant’s immune system, especially the involvement of signaling cascades in the AHL-induced resistance.

AHL-1-3

Different interaction of Hordeum plants with the biotrophic fungus Blumeria graminis