Biological and Molecular Characteristics of Microorganism-Stimulated Defence Response in Lycopersicon esculentum –L

Abstract: Microorganisms, including two fungi, Phytophthora cryptogea and Fusarium oxysporum strain Fo-(IMI 386351), and one bacterium, Pesudomonas sp. strain MF30, were tested for their abilities to stimulate plant defence responses in tomato (Lycopersicon esculentum –L.) and to serve as effective biocontrol agents (Bs). The study included in vivo and in vitro characterization of biological attributes of the microorganisms, pertaining to their abilities to stimulate plant immunity against a fungal pathogen, Fusarium oxysporum f. sp. lycopersici (Fol), the causal agent of tomato wilt disease. Using Lycopersicon esculentum –L. as a model plant for examining some fundamental elements of the plant-microorganism interaction, the study reveals and clarifies some aspects of the close association and the complexity of such systems.For each B, the results revealed a B-distinct plant-microorganism interaction, which included systemic induced resistance (SIR). A phylogenetic analyses of the partial sequences of two Fo-(IMI 386351) genes, a mitochondrial small subunit ribosomal DNA (mtSSU rDNA) and the nuclear translation elongation factor 1α (EF-1α), provided phylogenetic trees confirming that Fo-(IMI 386351) might be a member of Fol or of F. oxysporum f. sp. melonis, which have polyphyletic evolutionary origins. RFLP analysis (mtDNA), suggested that Fo-(IMI 386351) probably belongs to Fol. For routine and accurate differentiation between two morphologically indistinguishable F. oxysporum formae speciales strains, F. oxysporum f. sp. lycopersici and F. oxysporum f. sp. radicis-lycopersici, a molecular method (mtDNA RFLP analysis) was developed, and its usefulness for such differentiation was compared with that of two other methods: isozyme analysis and an osmotic method, revealed with high performance liquid chromatography (HPLC). The HPLC-spectra of Fo-(IMI 386351) had an extra peak for the two tested fractions, indicating that activation of the observed plant defence mechanism could have been at least partially the result of one of the products of the eliciting microbe. Preliminary results obtained by nuclear magnetic resonance spectrometry of those fractions suggest that the extra peak probably represents an oligosaccharide, which may have acted as a mobile signal and triggered the plant defence mechanisms.We concluded that (1) our three tested microorganisms are able to stimulate plant defence mechanisms by triggering SIR (plant immunity), (2) the complexity and elaborateness of evolved plant-microbe interactions involving plant defence can, at least in some cases, be observed and studied in the laboratory, and (3) molecular tools can be a powerful means for identifying fungal strains and for clarifying their taxonomical relationships.