ARS | Publication request: Dissection of Wheat Genes Expressed During Rhizosphere Interactions with Biocontrol Pseudomonas Fluorescens

Submitted to: Annual Missouri Symposium
Publication Type: Abstract
Publication Acceptance Date: April 25, 2006
Publication Date: May 25, 2006
Citation: Okubara, P.A., Kwak, Y., Walter, N., Call, D., Skinner, D.Z. Dissection of wheat genes expressed during rhizosphere interactions with biocontrol pseudomonas fluorescens. Annual Missouri Symposium. Abstract #17, p. 43-44

Technical Abstract: Root-colonizing Pseudomonas fluorescens can suppress root diseases in host plants by the production of compounds inhibitory to soilborne pathogens, or by stimulation of growth and defense responses in the plant. The aim of our research is to characterize the molecular responses of Triticum aestivum L. (hexaploid wheat) to such biocontrol bacteria, and to identify wheat genes that govern interactions with P. fluorescens. We have demonstrated that cultivars of wheat vary in ability to sustain root populations of Q8r1-96 and Q2-87V1, two strains that produce the disease-suppressive compound 2,4-diacetylphloroglucinol (DAPG) but vary in rhizosphere persistence. The cultivars also vary in ability to accumulate DAPG early during root colonization in a soil-free system. Roots of cultivar Tara displayed a striking differential in metabolite accumulation by Q8r1-96 compared to Q2-87V1. In contrast, Buchanan roots showed no differential when colonized by the strains. In greenhouse assays, suppression of the root disease take-all by Q8r1-96 was most significant on cv. Tara. Our findings indicate that DAPG accumulation on wheat roots depends on an interaction between cultivar and bacterial strain, and that a threshold level of the compound is needed for disease suppression on roots. To examine the impact of root colonization by P. fluorescens on defense gene expression in Triticum aestivum L. (hexaploid wheat), we constructed 60-mer oligonucleotide microarrays representing 142 wheat root homologs of defense- and stress-related genes from Arabidopsis, tomato, rice, and barley. The genes were selected from an in silico analysis of root-expressed sequences generated by the NSF Wheat EST Genomics Project and the International Triticeae EST Cooperative, or retrieved from the TIGR Wheat Gene Index. The microarrays were interrogated using whole-root or coleoptile transcript populations expressed 6, 12, 24 and 48 hr, or 4 da after inoculation with P. fluorescens Q8r1-96. In three to four independent interrogations, transcripts encoding Ca2+-dependent protein kinases, an HR-induced polypeptide, PR-10a, and ethylene and cold stress signal components were induced two-fold or more in roots during P. fluorescens interactions. Expression of specific genes in coleoptiles was also modulated by Q8r1-96 treatment and distinct from that in roots. Our findings suggest that molecular responses to biocontrol pseudomonads in wheat seedlings involve modulation of defense and stress gene expression. Progress on experiments using applied DAPG and DAPG knockout mutants of P. fluorescens will be presented.