Biotic factors affecting expression of the 2,4-diacetylphloroglucinol biosynthesis gene phlA in Pseudomonas fluorescens biocontrol strain CHA0 in therhizosphere

Production of the polyketide antimicrobial metabolite 2,4-diacetyl-phlorogl ucinol (DAPG) is a key factor in the biocontrol activity of Pseudomonas flu orescens CHA0. Strain CHA0 carrying a translational phlA '-' lacZ fusion wa s used to monitor expression of the phl biosynthetic genes in vitro and in the rhizosphere. Expression of the reporter gene accurately reflected actua l production of DAPG in vitro and in planta as determined by direct extract ion of the antimicrobial compound. In a gnotobiotic system containing a cla y and sand-based artificial soil, reporter gene expression was significantl y greater in the rhizospheres of two monocots (maize and wheat) compared wi th gene expression in the rhizospheres of two dicots (bean and cucumber). W e observed this host genotype effect on bacterial gene expression also at t he level of cultivars. Significant differences were found among six additio nal maize cultivars tested under gnotobiotic conditions. There was no diffe rence between transgenic maize expressing the Bacillus thuringiensis insect icidal gene cry1Ab and the near-isogeaic parent line. Plant age had a signi ficant impact on gene expression. Using maize as a model, expression of the phlA '-' lacZ reporter gene peaked at 24 h after planting of pregerminated seedlings, and dropped to a fourth of that value within 48 h, remaining at that level throughout 22 days of plant growth. Root infection by Pythium u ltimum stimulated bacterial gene expression on both cucumber and maize, and this was independent of differences in rhizosphere colonization on these h ost plants. To our knowledge, this is the first comprehensive evaluation of how biotic factors that commonly confront bacterial inoculants in agricult ural systems (host genotype, host age, and pathogen infection) modulate the expression of key biocontrol genes for disease suppression.