Surface motility and associated surfactant production in Agrobacterium vitis

Aims: Agrobacterium vitis is the causal agent of crown gall of grapevine. Surface motility (swarming), an important mechanism for bacterial colonization of new environments and a previously unknown behaviour of Ag. vitis, was demonstrated. Methods: Surface motility assays were performed on half-strength potato dextrose agar (Difco) containing 0 center dot 75% agar. To test for surfactant production, a drop-collapse test was used. Quorum-sensing (QS) negative and complemented mutants were tested for swarming activity. Results: Ninety-one Agrobacterium strains representing -Agrobacterium tumefaciens (17 strains), Agrobacterium rhizogenes (14 strains) and Ag. vitis (60 strains) were tested for swarming and production of surfactant. All Ag. vitis strains expressed a surface-related motility. In contrast, none of 17 strains of Ag. tumefaciens or 14 strains of Ag. rhizogenes exhibited this behaviour. Surface motility in Ag. vitis was associated with surfactant secretion; both of which are regulated by a QS system previously associated with induction of a hypersensitive response on tobacco and necrosis on grape. An aviR (belongs to luxR family) mutant was surface motility negative and did not produce surfactant. An avsI mutant (autoinducer synthase) was also surface motility negative and was complemented with an Ag. tumefaciens clone expressing avsI. Conclusions: Agrobacterium vitis is able to produce a characteristic swarming phenotype that is regulated by a complex QS system. Significance and Impact of the Study: Swarming activity is unique to Ag. vitis among Agrobacterium sp. and may be associated with the ability of the pathogen to colonize grapevines.