A Tn5 mutant of Agrobacterium vitis F2/5 (M1154) differs from the wild-type strain in that it has lost its abilities to cause necrosis on grape and a hypersensitive-like response (HR) on tobacco. The Tn5 insertion occurred in an open reading frame (ORF) aviR that is homologous to genes encoding the LuxR family of transcriptional regulators, thereby suggesting that the HR and necrosis are regulated by a quorum-sensing system. Fewer N-acyl-homoserine lactone autoinducers were detected in extracts from M1154 compared with extracts from F2/5 and from aviR-complemented M1154. The complemented mutant regained full ability to cause grape necrosis and HR. Eighteen ORFs located on a 36.6-kb insert in cosmid clone CPB221, which includes aviR, were sequenced and aligned with homologous genes from A. tumefaciens C58 and Sinorhizobium meliloti Rm1021. The order of several clustered genes is conserved among the bacteria; however, rearrangements are also apparent. Reverse transcriptase-polymerase chain reaction analysis indicated that ORF2 and ORF14 may be regulated by an aviR-encoded transcriptional regulator. Single site-directed mutations in each of the ORFs, however, had no effect on expression of HR or necrosis as compared with the wild-type parent.

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.

1. Agricultural crops can be grown to maturity with their root systems and associated soil sealed into 1680 1. respirometers. The crops appeared to grow normally and produced the expected agricultural yields.2. The mean daily oxygen consumption at maturity of the roots of the following crops grown at normal agricultural spacings were: potatoes 2·8 l./m2, marrowstem kale 5·6 l./m2 and tobacco 3·0 l./m2.

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