Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/3/2011
Publication Date: 2/2/2012
Citation: Maketon, C., Fortuna, A., Okubara, P.A. 2012. Cultivar-Dependent Transcript Accumulation in Wheat Roots Colonized by Pseudomonas fluorescens Q8r1-96 Wild Type and Mutant Strains. Biological Control. 60(2):216-224. Interpretive Summary: Specific biological control bacteria induce the expression of defense genes in the roots of their host plant, wheat. Biocontrol bacterium of the genus Pseudomonas produce antifungal metabolites, such as DAPG, that might trigger host defense responses. They also harbor structural proteins and biochemical components on their surface that are known to act as endogenous triggers of immunity, and inject bacterial proteins into the plant that cause changes in host cellular processes. To better understand the mechanism(s) by which biocontrol Pseudomonas induce wheat root defense responses, we tested the effects on defense gene expression of a mutant bacterium unable to produce the key antifungal metabolite, DAPG, and a second mutant unable to inject its proteins (also called effectors). To expand the study, we used two different wheat cultivars that varied in accumulation of root DAPG. Our findings indicated that the absence of DAPG or effectors had no global impact on the expression of the 15 genes we monitored in the 24-hour time period of the study. However, we noted that the expression of specific genes was strain-dependent. We also observed a marked cultivar dependence on root defense gene expression.
Technical Abstract: In Triticum aestivum L. (wheat), the root-colonizing bacterium Pseudomonas fluorescens strain Q8r1-96 produces the antifungal metabolite 2,4-diacetylphloroglucinol (DAPG), suppresses damage caused by soilborne root pathogens, and modulates multiple stress or defense pathways in wheat roots. To test the hypothesis that DAPG is an inducer of root defense responses, we compared the impact of wild type and a DAPG null mutant on the abundance of fifteen wheat root defense gene homologues in two wheat cultivars, Buchanan and Tara, using real-time PCR. We also sought to determine whether the type III secretion system (T3SS) of Q8r1-96 is important for root defense gene induction by using a T3SS deletion mutant of Q8r1-96. Transcripts encoding dihydroxyacetone reductase, monodehydroascorbate reductase, a hypersensitive response-induced protein, pathogenesis-related protein10a (PR-10a) and two glutathione-S-transferases were induced in a strain–dependent manner relative to no-bacterium controls within the 24-hour interval of the study. Our findings indicated that neither DAPG nor the T3SS were key single factors in the expression of all the root defense homologues. However, gene expression was induced to a greater degree in cv. Buchanan compared to cv. Tara, and cultivar-by-strain interactions were observed for transcripts encoding ascorbate peroxidase, PR-10a and the herbicide conjugation/detoxification enzyme In2.1-18. Clusters of co-induced transcripts also differed with the host cultivar-bacterial strain combination. Cultivar dependence of molecular root defense responses to biocontrol P. fluorescens has implications for the utility of biocontrol pseudomonads in greenhouse and field crop production systems.