|Fighting crown gall disease|
December 21, 2001
Here, GNN posts the abstracts of five papers about crown gall disease related to the feature The Genome of Agrobacterium tumefaciens
Agrobacterium tumefaciens causes crown gall disease in dicotyledonous plants by introducing a segment of DNA (T-DNA), derived from its tumour-inducing (Ti) plasmid, into plant cells at infection sites. Besides these natural hosts, Agrobacterium can deliver the T-DNA also to monocotyledonous plants, yeasts and fungi. The T-DNA integrates randomly into one of the chromosomes of the eukaryotic host by an unknown process. Here, we have used the yeast Saccharomyces cerevisiae as a T-DNA recipient to demonstrate that the non-homologous end-joining (NHEJ) proteins Yku70, Rad50, Mre11, Xrs2, Lig4 and Sir4 are required for the integration of T-DNA into the host genome. We discovered a minor pathway for T-DNA integration at the telomeric regions, which is still operational in the absence of Rad50, Mre11 or Xrs2, but not in the absence of Yku70. T-DNA integration at the telomeric regions in the rad50, mre11 and xrs2 mutants was accompanied by gross chromosomal rearrangements.
EMBO J 2001 Nov 15;20(22):6550-8.
Crown gall disease, caused by the soil bacterium Agrobacterium tumefaciens, results in significant economic losses in perennial crops worldwide. A. tumefaciens is one of the few organisms with a well characterized horizontal gene transfer system, possessing a suite of oncogenes that, when integrated into the plant genome, orchestrate de novo auxin and cytokinin biosynthesis to generate tumors. Specifically, the iaaM and ipt oncogenes, which show approximately 90% DNA sequence identity across studied A. tumefaciens strains, are required for tumor formation. By expressing two self-complementary RNA constructions designed to initiate RNA interference (RNAi) of iaaM and ipt, we generated transgenic Arabidopsis thaliana and Lycopersicon esculentum plants that are highly resistant to crown gall disease development. In in vitro root inoculation bioassays with two biovar I strains of A. tumefaciens, transgenic Arabidopsis lines averaged 0.0-1.5% tumorigenesis, whereas wild-type controls averaged 97.5% tumorigenesis. Similarly, several transformed tomato lines that were challenged by stem inoculation with three biovar I strains, one biovar II strain, and one biovar III strain of A. tumefaciens displayed between 0.0% and 24.2% tumorigenesis, whereas controls averaged 100% tumorigenesis. This mechanism of resistance, which is based on mRNA sequence homology rather than the highly specific receptor-ligand binding interactions characteristic of traditional plant resistance genes, should be highly durable. If successful and durable under field conditions, RNAi-mediated oncogene silencing may find broad applicability in the improvement of tree crop and ornamental rootstocks.
Proc Natl Acad Sci U S A 2001 Nov 6;98.
OccR is a LysR-type transcriptional regulator of Agrobacterium tumefaciens that positively regulates the octopine catabolism operon of the Ti plasmid and is also an autorepressor. Positive control of the occ genes occurs in response to octopine, a nutrient released from crown gall tumors. OccR binds to a site upstream of the occQ promoter in the presence and absence of octopine. Octopine causes prebound OccR to undergo a conformational change at the DNA binding site that causes changes in footprint length and DNA bending. In order to determine the roles of these conformational changes in transcriptional activation, we isolated 22 OccR mutants that were able to activate the occQ promoter in the absence of octopine. Thirteen of these mutants contained single amino acid substitutions while nine contained two base pair changes resulting in two amino acid substitutions, which in most cases acted synergistically. These mutations spanned the entire length of the protein. Most of these mutant proteins, in the absence of octopine, displayed DNA binding and bending properties characteristic of transcriptionally active OccR-octopine complexes.
J Biol Chem 2001 Nov 20.
A wild-type Agrobacterium tumefaciens strain CNI5 isolated from crown gall of chrysanthemum (Dendranthema grandiflora Tzvelev) was characterized. Strain CNI5 was classified into biovar 1, based on physiological and biochemical characteristics, and was resistant to ampicillin. Strain CNI5 induced tumors at a higher frequency and on a larger area of explants in most tested plant species, especially in chrysanthemum cultivars, than the octopine-type strain C58C1cmr (pTiB6S3). Agropine and mannopine were detected in tumors induced by strain CNI5 and were specifically catabolized by this strain. Strain CNI5 harbored five plasmids including one plasmid that shared sequence similarity to TL-DNA of the octopine-type Ti plasmid and four cryptic plasmids.
Arch Microbiol 2000 Apr;173(4):311-5.
Not until 1973 was it reported that strains of Agrobacterium that cause crown gall disease of grape form a specific group (later characterized as Agrobacterium vitis). Tumorigenic and nontumorigenic A. vitis have since been isolated from infected and symptomless grapes worldwide. Research on the genetic makeup of A. vitis has led to an improved understanding of pathogen biology and bacterial evolution. In addition, the identification of significant gene sequences has facilitated the development of PCR and RFLP-based identification procedures that continue to improve the detection of A. vitis in plants and soil. Current control practices rely on the use of disease-resistant cultivars, cultural practices that minimize plant injury, and the production of pathogen-free vines. Promising future controls include employment of biological control agents and development of crown gall-resistant transgenic grapevines.
Annu Rev Phytopathol 1999;37:53-80.
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