Lux gene reporting of the interaction between Gaeumannomyces graminis and an antagonistic pseudomonad in the wheat rhizosphere
In a series of short term, laboratory-based experiments, the interaction between Gaeumannomyces graminis var. tritici (Ggt) and the antagonist Pseudomonas corrugata lux in the wheat rhizosphere was studied. The pseudomonad had been chromosomally huxAB marked, providing luminescence as the basis of measurement. During initial in vitro characterisation of the interaction between the Ggt and P.corrugata hix, the antagonist was observed to luminesce more in areas of colonies nearest to the fungus, possibly suggesting that signalling had occurred. The phenomenon was shown to be dependent on spatial and temporal factors and luminescence was enhanced at lower pH, but was peculiar to only Ggt. The signalling mechanism was studied by a series of investigations, using disciplines ranging from molecular biology to analytical chemistry and study of mitogenetic radiation. P.corrugata lux was shown to control gene expression by autoinduction mediated by HSL type molecules. This compound was produced by Ggt in batch culture and the mechanism of signalling was established; however, there was secondary partial evidence of signalling between the antagonist and the pathogen, caused by the phenomenon of mitogenetic radiation. Characterisation of volatile organic compounds in the wheat rhizosphere in the presence of Ggt indicated a wide range of VOCs were produced. In a series of subsequent experiments, the spatial dynamics of the pathogen-antagonist interaction were investigated in soil microcosms supporting wheat seedlings. These studied determined that position in the rhizosphere and cell density of the antagonist were associated with antagonistic success in terms of control of Ggt. The experiments further showed the usefulness of the lux-marker system in characterising pathogen-antagonist interactions in the rhizosphere and in providing experimental data with which to test mathematical models of these systems.