Agrobacterium tumefaciens: chemotaxis and crop protection

Abstract

Chemotaxis in Agrobacierium tumefaciens was studied. Several plant derived monocyclic phenolic compounds were analysed for their ability to act as chemoattractants for A. tumefaciens C58C (^1) and as inducers of the Ti-plasmid virulence operons. The results divided the phenolics into 4 groups. A strong correlation between vir- inducing ability and Ti-plasmid requirement for chemo taxis was established and chemical structure rules for vir induction and chemo taxis are outlined. Furthermore, virA and virG were found to be the Ti-plasmid virulence genes required for chemo taxis towards the monocyclic phenolic compound acetosyringone. Chemotaxis towards both monocotyledonous and dicotyledonous plant extracts was analysed. Undiluted shoot and root extracts from both sources elicited a response from both Ti-plasmid harbouring and cured A. tumefaciens C58C(^1) However, when diluted extracts of Wheat and Kalanchoe shoot homogenate were analysed, a distinct enhancement of chemotaxis was conferred by the Ti-plasmid, suggesting that recognition of, and attraction towards, susceptible plants is not the step blocked in monocot transformation. Analysis of cell wall material revealed that native cell wall components are not required for chemotaxis of A. tumefaciens C58C (^1) towards plant extracts. Results obtained on chemotaxis along with current knowledge of vir- induction allowed the development of a novel idea involving Agrobacterium as a biocontrol agent. A chitinase gene from Serratia marcescens was manipulated such that its promotor was removed. The promotorless cassette was linked to the virB pro-motor from an octopine Ti-plasmid and the construct introduced into Agrobacterium harbouring virA and virG. The potential benefit of this biocontrol system with respect to other existing biocontrol systems is that expression of the pesticidal gene is regulated by components of wound exudate and therefore is a conservative process, pesticide being produced only when a plant is wounded, at a time when it is most susceptible to attack by plant pathogens, and then exclusively in the microrhizosphere around the wound site.