Identification of genes involved in rhizobacteria-mediated induced systemic resistance in Arabidopsis

Different forms of biologically induced disease resistance have been identified in plants. Following attack by a necrotizing pathogen systemic acquired resistance (SAR) is induced, leading to a broad-spectrum disease resistance that is associated with an increase in salicylic acid (SA) levels and the accumulation of pathogenesis-related (PR) proteins. Selected strains of non-pathogenic, rootcolonizing fluorescent Pseudomonas spp. can induce systemic resistance as well, without provoking any symptoms themselves. This rhizobacteria-mediated induced systemic resistance (ISR) is phenotypically similar to pathogen-induced SAR in that it is effective against various pathogens. In Arabidopsis thaliana, the ISR signaling pathway triggered by Pseudomonas fluorescens WCS417r requires responsiveness to both jasmonate (JA) and ethylene (ET) and is independent of SA and the accumulation of PR proteins. The state of pathogen-induced SAR is characterised by the concomitant activation of a set of PR genes. Of many defence-related genes tested in Arabidopsis, none were upregulated prior to challenge in plants expressing WCS417r-mediated ISR. In an attempt to isolate ISRrelated genes, we screened a large collection of Arabidopsis lines containing enhancer-trap Ds transposons and the β-glucuronidase (GUS) reporter gene with minimal promoter. One enhancer-trap line showed local GUS activity in the roots upon colonization with WCS417r. This local GUS expression was not observed after treatment of the roots with Escherichia coli, indicating that the induction was Pseudomonas specific. Interestingly, a similar expression pattern was observed after treatment of the roots with the ET precursor ACC, indicating that this line contains a transposon insertion in the vicinity of an ET-inducible gene that is up-regulated upon colonization with WCS417r. There are several candidate genes in the vicinity of the enhancer-trap Ds transposon, one of which encodes a thaumatin-like protein. Gene expression analyses confirmed that this thaumatin-like gene, designated THL1, is up-regulated in response to treatment of the roots with WCS417r or ACC. Analysis of the role of THLI in ISR might provide more insight into the molecular mechanisms involved in rhizobacteria-mediated ISR