Ti plasmids from Agrobacterium characterize rootstock clones that initiated a spread of crown gall disease in Mediterranean countries

Crown gall caused by Agrobacterium is one of the predominant diseases encou ntered in rose cultures. However, our current knowledge of the bacterial st rains that invade rose plants and the way in which they spread is limited. Here, we describe the integrated physiological and molecular analyses of 30 Agrobacterium isolates obtained from crown gall tumors and of several refe rence strains. Characterization was based on the determination of the biova r, analysis of 16S ribosomal DNA restriction fragment length polymorphisms by PCR (PCR-RFLP), elucidation of the opine type, and PCR-RFLP analysis of genes involved in virulence and oncogenesis. This study led to the classifi cation of rose isolates into seven groups with common chromosome characteri stics and seven groups with common Ti plasmid characteristics. Altogether, the rose isolates formed 14 independent groups, with no specific associatio n of plasmid- and chromosome-encoded traits. The predominant Ti plasmid cha racteristic was that 16 of the isolates induced the production of the uncom mon opine succinamopine, while the other 14 were nopaline-producing isolate s. With the exception of one, all succinamopine Ti plasmids belonged to the same plasmid group. Conversely, the nopaline Ti plasmids belonged to five groups, one of these containing seven isolates. We showed that outbreaks of disease provoked by the succinamopine-producing isolates in different coun tries and nurseries concurred with a common origin of specific rootstock cl ones. Similarly, groups of nopaline-producing isolates were associated with particular rootstock clones. These results strongly suggest that the causa l agent of crown gall disease in rose planta; is transmitted via rootstock material.