Mechanisms of crown gall formation: T-DNA transfer fromAgrobacterium tumefaciens to plant cells

Abstract

Agrobacterium tumefaciens harbouring the Ti plasmid incites crown gall tumor on dicotyledonous species. Upon infection of these plants, T-DNA in the Ti plasmid is transferred by unknown mechanisms to plant cells to be integrated into nuclear DNA. WhenAgrobacterium is incubated with protoplasts or seedlings of dicotyledonous plants, circulation of T-DNA and expression ofvir (virulence) genes on the Ti plasmid are induced. The circularization event is efficiently induced by mesophyll protoplasts of tobacco which are highly competent for transformation by the T-DNA, and is also induced by diffusible phenolic compounds excreted from the protoplasts. The circularization and formation of crown gall both require the expression of thevirD locus, one of the induciblevir genes. These results suggest that the circularization of T-DNA reflects one of steps of the T-DNA transfer during formation of crown gall. In contrast to dicotyledonous plants, monocotyledonous plants are thought to be unresponsive to infection byAgrobacterium. We showed that monocotyledonous plants do not excrete diffusible inducers for the expression ofvir genes, while they contain a novel type of a signal substance(s). This inducer is not detected in the exudates of seedlings of monocotyledonous plants, but is found in the extracts from the seedlings, and also those from the seeds, bran and germ of wheat and oats. This finding suggests that T-DNA processing, and possibly its transfer, should take place whenAgrobacterium invades seedlings and seeds of monocotyledonous plants.