Identification of Collimonas gene loci involved in the biosynthesis of a diffusible secondary metabolite with broad-spectrum antifungal activity and plant-protective properties.

In greenhouse and field trials, a bacterial mixture of Collimonas arenae Cal35 and Bacillus velezensis FZB42, but not Cal35 alone or FZB42 alone, was able to protect tomato plants from challenge with the soilborne fungal pathogen Fusarium oxysporum f.sp. lycopersici (Fol). To identify genes and mechanisms underlying this property in Cal35, we screened a random transposon insertion library for loss of function and identified two mutants that were impaired completely or partially in their ability to halt the growth of a wide range of fungal species. In mutant 46A06, the transposon insertion was located in a biosynthetic gene cluster that was predicted to code for a hybrid polyketide synthase-non-ribosomal peptide synthetase, while mutant 60C09 was impacted in a gene cluster for the synthesis and secretion of sugar repeat units. Our data are consistent with a model in which both gene clusters are necessary for the production of an antifungal compound we refer to as carenaemins. We also show that the ability to produce carenaemin contributed significantly to the observed synergy between Cal35 and FZB42 in protecting tomato plants from Fol. We discuss the potential for supplementing Bacillus-based biocontrol products with Collimonas bacteria to boost efficacy of such products.

The Piriformospora indica effector PIIN_08944 promotes the mutualistic Sebacinalean symbiosis.

Pathogenic and mutualistic microbes actively suppress plant defense by secreting effector proteins to manipulate the host responses for their own benefit. Current knowledge about fungal effectors has been mainly derived from biotrophic and hemibiotrophic plant pathogenic fungi and oomycetes with restricted host range. We studied colonization strategies of the root endophytic basidiomycete Piriformospora indica that colonizes a wide range of plant species thereby establishing long-term mutualistic relationships. The release of P. indica's genome helped to identify hundreds of genes coding for candidate effectors and provides an opportunity to investigate the role of those proteins in a mutualistic symbiosis. We demonstrate that the candidate effector PIIN_08944 plays a crucial role during fungal colonization of Arabidopsis thaliana roots. PIIN_08944 expression was detected during chlamydospore germination, and fungal deletion mutants (PiΔ08944) showed delayed root colonization. Constitutive over-expression of PIIN_08944 in Arabidopsis rescued the delayed colonization phenotype of the deletion mutant. PIIN_08944-expressing Arabidopsis showed a reduced expression of flg22-induced marker genes of pattern-triggered immunity (PTI) and the salicylic acid (SA) defense pathway, and expression of PIIN_08944 in barley reduced the burst of reactive oxygen species (ROS) triggered by flg22 and chitin. These data suggest that PIIN_08944 contributes to root colonization by P. indica by interfering with SA-mediated basal immune responses of the host plant. Consistent with this, PIIN_08944-expressing Arabidopsis also supported the growth of the biotrophic oomycete Hyaloperonospora arabidopsidis while growth of the necrotrophic fungi Botrytis cinerea on Arabidopsis and Fusarium graminearum on barley was not affected.

The Salmonella effector protein SpvC, a phosphothreonine lyase is functional in plant cells.

Salmonella is one of the most prominent causes of food poisoning and growing evidence indicates that contaminated fruits and vegetables are an increasing concern for human health. Successful infection demands the suppression of the host immune system, which is often achieved via injection of bacterial effector proteins into host cells. In this report we present the function of Salmonella effector protein in plant cell, supporting the new concept of trans-kingdom competence of this bacterium. We screened a range of Salmonella Typhimurium effector proteins for interference with plant immunity. Among these, the phosphothreonine lyase SpvC attenuated the induction of immunity-related genes when present in plant cells. Using in vitro and in vivo systems we show that this effector protein interacts with and dephosphorylates activated Arabidopsis Mitogen-activated Protein Kinase 6 (MPK6), thereby inhibiting defense signaling. Moreover, the requirement of Salmonella SpvC was shown by the decreased proliferation of the ΔspvC mutant in Arabidopsis plants. These results suggest that some Salmonella effector proteins could have a conserved function during proliferation in different hosts. The fact that Salmonella and other Enterobacteriaceae use plants as hosts strongly suggests that plants represent a much larger reservoir for animal pathogens than so far estimated.