Toxins for microbial attack and plant defence
Millions of years of coevolution of plants and microbial pathogens have shaped both the abilities of microbial pathogens to overcome plant disease resistance and the abilities of plants to cope with microbial invasion. Phytopathogens from different taxonomic origins secrete structurally unrelated effectors into plants to establish infection and to suppress host defences. In addition, phytopathogenic micro-organisms produce a wide range of cytolytic toxins that function as virulence determinants.
Microbial pattern recognition is a prerequisite for the initiation of antimicrobial defenses in all multicellular organisms, including plants. The bipartite plant immune system is based upon recognition of pathogen-associated molecular patterns by pattern-recognition receptors as well as upon the activities of resistance proteins that have evolved to recognize the presence or activities of microbial effectors. In addition to the recognition of microbial patterns and effectors, plants also possess capacities to sense host-derived damage patterns that originate, for example, from the degradation of the plant cell wall by microbial hydrolytic enzymes.
Paradoxically, some phytopathogenic microbe-derived cytolytic toxins have also been reported to elicit plant defences. However, for virtually all microbial toxins with plant defence-stimulating potential, it is unknown whether activation of plant defences results from toxin-induced cellular distress or, independently of toxin action, from recognition of toxins as microbial patterns by plant pattern-recognition receptors.
Subscribe to podcasts (free):
[iTunes] Enhanced podcasts & videos
[RSS] mp3 podcasts (audio only)
Play this episode: Enhanced version | Audio only
NLPs are a superfamily of proteins that are produced by various phytopathogenic micro-organisms, both prokaryotes and eukaryotes. Necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLPs) trigger leaf necrosis that is genetically distinct from immunity-associated programmed cell death and stimulate immunity associated defences in all dicotyledonous plants tested, but not in monocotyledons such as grasses. Hence, NLPs were proposed to have dual functions in plant pathogen interactions, acting both as triggers of immune responses and as toxin-like virulence factors. The broad taxonomic distribution of NLPs, in particular their occurrence in both prokaryotic and eukaryotic species, is unusual for known microbial phytotoxins, the production of which is restricted to a narrow range of microbial species.
Recent work has determined the crystal structure of an NLP from a phytopathogenic fungus (A common toxin fold mediates microbial attack and plant defense. PNAS USA June 11 2009, doi: 10.1073/pnas.0902362106). Computational modeling of the three-dimensional structure of NLPs from another fungus and from a phytopathogenic bacterium reveals a high degree of conservation. Expression of the fungus NLPs in an NLP-deficient phytopathogenic bacteria restored bacterial virulence.
Mutation analysis revealed that identical structural properties were required to cause plasma membrane permeabilization and cytolysis in plant cells, as well as to restore bacterial virulence. The conclusion is that NLPs are conserved virulence factors whose wide taxonomic distribution is exceptional for microbial phytotoxins, and that contribute to host infection by plasma membrane destruction and cytolysis. Phytotoxin-induced cellular damage-associated activation of plant defenses is reminiscent of microbial toxin-induced inflammatory activation in vertebrates and may constitute another conserved element in animal and plant innate immunity.
Related:
Tags: Agriculture, Biology, Immunology, infection, Microbiology, plants, Podcast, Science, Toxins
