Use of Ti Plasmid DNA Probes for Determining Tumorigenicity of Agrobacterium Strains

Resistance to crown gall disease in transgenic grapevine rootstocks containing truncated virE2 of Agrobacterium

A truncated form of the Ti-plasmid virE2 gene from Agrobacterium tumefaciens strains C58 and A6, and A. vitis strain CG450 was transferred and expressed in somatic embryos of grapevine rootstocks 110 Richter (Vitis rupestris × V. berlandieri), 3309 Couderc (V. rupestris × V. riparia) and Teleki 5C (V. berlandieri × V. riparia) via Agrobacterium-mediated transformation to confer resistance to crown gall disease. Transformation was confirmed in 98% of the 322 lines by enzyme-linked immunosorbent assay for the neomycin phosphotransferase II protein and 97% of 295 lines by polymerase chain reaction for the truncated virE2 transgene. Southern blot analysis revealed the insertion of truncated virE2 at one to three loci in a subset of seven transgenic 110 Richter lines. In vitro resistance screening assays based on inoculations of shoot internode sections showed reduced tumorigenicity and very small galls in 23 of 154 transgenic lines. Non-transformed controls had a 100% tumorigenicity rate with very large galls. Disease resistance assay at the whole plant level in the greenhouse revealed seven transgenic lines (3 lines of 110 Richter, 2 lines of 3309 Couderc and 2 lines of Teleki 5C) were resistant to A. tumefaciens strain C58 and A. vitis strains TM4 and CG450 with a substantially reduced percentage of inoculation sites showing gall as compared to controls. No association was found between the level of resistance to crown gall disease and the source Agrobacterium strain of virE2. Taken together, our data showed that resistance to crown gall disease can be achieved by expressing a truncated form of virE2 in grapevines.

Characterization of Agrobacterium vitis Strains Isolated from Feral Vitis riparia

Agrobacterium vitis was isolated from roots of 41 of 66 feral Vitis riparia vines collected in three different regions of New York State. Two of the regions were more than 150 km from commercial vineyards. The strains were highly diverse as determined by DNA fingerprinting of the chromosomal region lying between the 16S and 23S rRNA genes. Of 24 strains examined, 15 different fingerprints were generated, and none was identical to fingerprints generated by previously identified groups of tumorigenic A. vitis strains. Results of physiological tests that were done to characterize strains from V. riparia conformed closely to those expected for A. vitis, except that 23 of 26 strains did not utilize tartrate. All strains were nontumorigenic, did not hybridize with a probe consisting of T-DNA genes, did not utilize octopine or nopaline, and carried zero to three plasmids. Of 26 strains, 7 inhibited A. vitis strain K306 from causing galls at wound sites on grape as well as or better than a previously studied nontumorigenic A. vitis strain, F2/5, that is known to have biological control activity.

CROWN GALL OF GRAPE: Biology and Disease Management

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.

Biological control of grape crown gall by strain f2/5 is not associated with agrocin production or competition for attachment sites on grape cells

ABSTRACT Agrocin-minus mutants of nontumorigenic Agrobacterium vitis strain F2/5 controlled grape crown gall as well as the wild-type strain, indicating that agrocin is not a major factor in the mechanism of biological control. Relative levels of attachment to grape cells by tumorigenic and biocontrol strains were also measured. Attachment of tumorigenic strains (CG49 and K306) and biological control strains (F2/5 and agrocin-minus mutant 1077) was often reduced when mixtures of the strains were applied. However, high populations (10(3) to 10(5) CFU/ml) of all strains attached following mixed inoculations, suggesting that competition for attachment sites is also not a factor in the mechanism of biological control. Transfer of T-DNA to grape by CG49 was prevented or greatly inhibited in the presence of F2/5 or 1077 as measured by expression of the GUS reporter gene. The Ti plasmid virulence genes, however, were induced by exudates from grape shoots that had been inoculated with F2/5. Sonicated and autoclaved preparations of F2/5 and 1077 did not control crown gall or inhibit T-DNA transfer. Control by F2/5 is specific to grape, since gall formation on tomato, sunflower, and Kalanchoe daigremontiana were not inhibited.

Attachment, Chemotaxis, and Multiplication of Agrobacterium tumefaciens Biovar 1 and Biovar 3 on Grapevine and Pea

Tumorigenic (CG49) and nontumorigenic (CG484) strains of Agrobacterium tumefaciens bv. 3 attached to grape roots at a higher level than did a nonpectinolytic mutant of CG49 (CG50) or a tumorigenic strain of A. tumefaciens bv. 1 (CG628). Strains attached equally well to wounded and unwounded grape roots. Strains responded differently to pea plants in that biovar 3 strains consistently attached to unwounded roots at a lower level than they did to wounded roots, whereas CG628 attached equally well regardless of wounding. The lowest levels of attachment to pea roots were consistently observed for CG50. Population curves were calculated for the strains inoculated into wound sites on grape and pea roots. A. tumefaciens bv. 3 wild-type strains developed greater populations at wound sites on grape roots after 100 h (resulting in root decay) than did CG50 or CG628. Population curves for strains at wound sites on pea roots were different from those on grape roots. There were no significant differences in populations after 100 h, and no strains caused root decay. No differences in the chemotaxis of wild-type and mutant A. tumefaciens bv. 3 strains towards grape roots, crown pieces, or root extracts were observed, but the biovar 1 strain, CG628, always migrated the greatest distance towards all substrates. Polygalacturonase production may affect attachment to grape roots and multiplication of A. tumefaciens bv. 3 at wound sites and thus be associated with the specificity of the bacterium for grape.

Polygalacturonase Production by Agrobacterium tumefaciens Biovar 3

Agrobacterium tumefaciens biovar 3 causes both crown gall and root decay of grape. Twenty-two Agrobacterium strains representing biovars 1, 2, and 3 were analyzed for tumorigenicity, presence of a Ti plasmid, ability to cause grape seedling root decay, and pectolytic activity. All of the biovar 3 strains, regardless of their tumorigenicity or presence of a Ti plasmid, caused root decay and were pectolytic, whereas none of the biovar 1 and 2 strains had these capacities. Isoelectrically focused gels that were activity stained with differentially buffered polygalacturonate-agarose overlays revealed that all of the biovar 3 strains produced a single polygalacturonase with a pH optimum of 4.5 and pIs ranging from 4.8 to 5.2. The enzyme was largely extracellular and was produced constitutively in basal medium supplemented with a variety of carbon sources including polygalacturonic acid. Lesions on grape seedling roots inoculated with A. tumefaciens biovar 3 strain CG49 yielded polygalacturonase activity with a pI similar to that of the enzyme produced by the bacterium in culture. These observations support the hypothesis that the polygalacturonase produced by A. tumefaciens biovar 3 has a role in grape root decay.