Antisense-mediated resistance to Agrobacterium vitis
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Agrobacterium vitis is the causal organism of crown gall in grapevine. Infection is particularly severe in areas that experience winter damage to the vine. Improving resistance to A. vitis will require a detailed knowledge about this organism. In this study, 18 grapevine isolates of A. vitis were obtained from different vineyards around Lubbock, Texas. Isolates were subjected to a phenotypic characterization using several biochemical tests. Isolates were also evaluated for host range, tumor morphology and opine utilization and the oncogenic regions of the T-DNA were compared at the molecular level by Southern hybridzation. Although all isolates were able to metabolize tartarte and grow on Roy-sasser media, there was high variability based on other biochemical tests. This variability was also observed in terms of tumor morphology and opine utilization. Southern hybridization analysis, however, showed that the oncogenic regions of A. vitis isolates have strong homology with previously characterized Agrobacterium T-DNA genes. This study suggests that the local isolates were probably of clonal origin. Tobacco was used as a model to test whether transgenic plants with TDNA auxin biosynthetic genes in an antisense orientation would block tumor morphology. The expression of iaaM antisense RNA resulted in an approximately 45% reduction in the number of tumors. The iaaM antisense also affected tumor morphology by reducing gall size. Plants expressing iaaH antisense had fewer tumors than the control, but more than 60% of samples formed roots when infected with Agrobacterium. The incomplete suppression of tumor formation may be due to the presence of other loci on the T-DNA that are also involved in auxin biosynthesis. Although a complete inhibition of tumorgenesis was not achieved, the fact that both tumor frequency and morphology changed, indicated that further manipulation of phytohormone genes using antisense technology may result in complete resistance to Agrobacterium tumefaciens species in transgenic plants.