Cyclic lipopeptides produced by Pseudomonas spp.: an untapped reservoir to control plant diseases by direct antagonism and induced resistance
Cyclic lipopeptides (CLPs) are metabolites with biosurfactant activity that are produced and secreted by a range of bacterial genera including rhizosphere bacteria belonging to the genus Pseudomonas. CLPs are composed of a hydrophobic fatty acid tail linked to an amphipathic oligopeptide that is partly or completely organized in a cyclic structure. CLPs of Pseudomonas show an enormous functional diversity and are currently classified in at least 14 different groups based on the length and composition of the oligopeptide and number of amino acids in the cyclic peptide moiety. They are implicated in bacterial motility and attachment to surfaces, including the formation of biofilms and they show growth-inhibitory activities against a broad range of micro-organisms. Moreover, they play an important role in the interaction of their producers with plants, either as inducers of systemic resistance or as phytotoxins contributing to plant pathogenesis. We have a very diverse collection of well-characterized CLP-producing Pseudomonas bacteria obtained from the roots of the tropical tuber crop cocoyam in Cameroon and Nigeria. We have tested these bacteria and their CLPs for biocontrol in various pathosystems including rice-Magnaporthe oryzae, grapevine-Plasmopara viticola, bean-Rhizoctonia solani and others. Some CLPs have direct antagonistic activity against pathogens and suppress germination, zoospore release, or appressorium formation and cause hyphal branching or hyphal leakage. Other CLPs induce local and systemic resistance in plants. Our hypothesis is that these compounds cause their effects because they interact with the lipids in membranes of pathogens and plants. We are currently studying structure-function relationships and mode of actions by looking at early defense responses in plant cell suspension cultures and on plant roots.