Mechanism of suppression of host-plant defense PR1a gene expression by the plant tumor-inducing 6b gene of Agrobacterium tumefaciens

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Expression of plant defense genes, termed Pathogenesis Related (PR) genes, is turned on upon infection of pathogens to plants. A soil bacterium Agrobacterium tumefaciens induces crown gall tumors on a number of plants. We have shown that the tumor-inducing 6b (AK-6b) gene obtained from a strain AKE10 enhanced the accumulation of several phenylpropanoids, which also possess anti-pathogenic activity. Since regulatory factor(s) responsible for PR gene overlaps with the biosynthesis of phenylpropanoid, we analyzed the effect of AK-6b gene on PR1a gene expression. We generated tobacco plants transgenic for the AK-6b gene under the control of the promoters of its own, CaMV35S, and dexamethazone inducible ones. Tissues were grown in the presence or absence of plant hormones, mRNA was extracted, and PR1a gene expression was analyzed by Northern hybridization. There was no obvious relationship between PR1a transcript levels and AK-6b transcript levels. However, when timorous tissues were cultivated on hormone-free medium and transferred to auxin-containing medium and followed time course of PR1a transcript accumulation, there was a clear difference. It was found that PR1a mRNA levels of transgenic tissues were reduced compared to those of wild type tobacco tissues until 6 days post transfer. At day 6, PR1a transcript of both tissues was comparable. We concluded, therefore, that both the AK-6b gene expression and presence of auxin were responsible for reduction of PR1a mRNA accumulation, and that the reduction was transient. Exogenous application of salicylic acid induced PR1a transcripts of both transgenic and wild type tissues, demonstrating transgenic tissues possess a normal potential to induce PR1a gene expression. We also purified modified AK-6b protein tagged with T7 and His, to near homogeneity by using affinity chromatography.