Molecular biology of the Agrobacterium-plant interaction

The Farrand laboratory conducted research in the biology and molecular biology of the plant pathogen, Agrobacterium tumefaciens. This organism causes cancerous tumors, called crown galls, on susceptible plants. Tumors result from the transfer of a small piece of DNA, called the T-DNA, from the bacterium to the plant cell. The T-DNA becomes integrated into plant cell nuclear DNA and expression of genes on this segment causes the normal plant cell to differentiate into a tumor cell. Expression of additional T-DNA genes causes the tumor cells to produce and secrete novel small carbon compounds called opines. In turn, Agrobacterium cells can utilize opines as sole carbon and energy sources. In the bacterium, the T-DNA and the genes for opine catabolism reside on a large, extrachromosomal virulence element called the Ti plasmid. This plasmid itself is transmissible from the bacterium to recipient bacteria by a mating process called conjugation. Plant signaling and Ti plasmid conjugation. T-DNA transfer is initiated when the bacterium senses a proper chemical signal produced by a wounded plant. Similarly, Ti plasmid conjugal transfer requires a specific plant signal, in this case an opine secreted by the crown gall tumor. These signalling events are crucial to the interaction between Agrobacterium and its host plant, and also between this bacterium and other related bacteria in the plant rhizosphere.

Farrand and collaborators investigated the nature and the mechanism of the signaling events that lead to Ti plasmid conjugal transfer. Using approaches ranging from physiology through genetics to molecular biology, we dissected the communication system operative in signalling bacterial donor cells to initiate Ti plasmid transfer. We identified, mapped and sequenced the loci of the Ti plasmid required for conjugal transfer and opine utilization. We identified the mechanism by which Agrobacterium recognizes and processes the opine signal, and we have cloned and sequenced the genes involved in this regulatory process. Additionally, we discovered that bacterium-to-bacterium signaling between members of the donor population is required for induction of conjugal transfer. We have identified the components of this quorum-sensing system and determined the hierarchical link between opine regulation and the second messenger system. We also identified another regulatory component, an antiactivator called TraM, that is essential for the quorum-dependent nature of the regulatory system.

The lab also investigated the mechanism by which the second messenger, called AAI, activates TraR, the quorum-sensing transcription factor.