Proximity of Agrobacterium to living plant tissues induces conversion to a filamentous bacterial form

Changes in Agrobacterium colony and cell morphology were observed following co-culture of the bacterium with a variety of different plant tissues. Bacterial colonies grown in the presence of plant tissue became opaque and appeared to grow as a thick mat of cells. A single bacterial colony would often grow to fill an entire 100-mm-diameter petri dish. Ultrastructural observations of the bacteria in these colonies revealed the formation of a predominantly filamentous form of the bacterium. The bacteria ranged from 5 to 100 mum in length as compared to 2 mum in the non-filamentous form. The filamentous form was observed 2-3 days after coculture and only if the bacteria were either in direct contact with or in close proximity (<5 mm) to living plant tissues. The filamentous form was observed with both wild-type and engineered Agrobacterium strains. As proximity of the bacteria to living tissue was necessary for induction, plant tissues apparently produce inductive compounds that are either very labile or have their effect only at high concentrations.