Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2010
Publication Date: 9/6/2010
Citation: Okubara, P.A., Call, D.R., Kwak, Y., Skinner, D.Z. 2010. Induction of defense gene homologues in wheat roots during interactions with pseudomonas fluorescens. Biological Control. 55:118-125. Interpretive Summary: Plant growth-promoting rhizobacteria (PGPR) can suppress soilborne fungal pathogens of wheat but their effect on the host plant can be deleterious in the absence of the pathogen. Furthermore, molecular changes induced by PGPR in the roots of host plants is not known. To explore the how an aggressive PGPR (biocontrol) strain of Pseudomonas fluorescens changes defense gene expression in wheat roots, we mined and annotated over 17,000 root-expressed gene sequences generated by the NSF Wheat EST Genomics Project. A subset of ESTs (150) were used to construct the first wheat defense/stress root microarrays. Microarray and real-time PCR analyses showed that three defense pathways are induced in wheat roots within 6 hours of treatment with biocontrol bacteria. A few genes also are induced or repressed in shoots of wheat. Our findings provide leads for understanding the impact of rhizobacteria on the host, and indicate wheat genes that might be modulated for better biological control of soilborne pathogens.
Technical Abstract: Specific strains of Pseudomonas fluorescens directly inhibit soilborne fungal pathogens of Triticum aestivum (wheat) during colonization of the wheat rhizosphere, but until now the impact of these beneficial bacteria on wheat gene expression was unknown. To test the hypothesis that P. fluorescens induces defense genes in wheat roots, we constructed a custom microarray of 192 oligonucleotides representing 84 wheat root expressed sequence tags (ESTs) homologous to defense/stress genes from Arabidopsis, tomato, rice, and barley, and 11 candidate root developmental genes. The ESTs were selected from existing wheat root EST libraries. Arrays were interrogated with Alexa Fluor 546-labeled transcript (cDNA) populations from roots or coleoptiles of cultivar Finley or lines 442 or 443, near-isogenic for the cold temperature-dependent vrn1A flowering locus, four days after seed inoculation with the take-all-suppressive strain P. fluorescens Q8r1-96. Twenty-two transcripts encoding Ca2+-dependent protein kinases, components of the oxidative stress, cold stress and jasmonic acid pathways, and proteins associated with the hypersensitive response were induced or repressed in wheat roots during P. fluorescens interactions. Transcripts encoding pathogenesis-related protein Pr-10a and hypersensitive response protein HRin1 also were induced in coleoptiles. Real-time PCR demonstrated that eleven transcripts were induced in root tissue between 2 and 6 hours and remained elevated at 24 hours post-inoculation. Our findings suggest that biocontrol P. fluorescens modulates defense/stress gene expression in wheat roots.