Expression of a single, host-specific, bacterial pathogenicity gene in plant cells elicits division, enlargement, and cell death

A fundamental question about microbial pathogens is how they elicit host-specific symptoms. We report here that expression of a single gene from a plant-pathogenic bacterium in plant cells elicits host-specific symptoms diagnostic of the disease caused by the pathogen, Expression of pthA from Xanthomonas citri in citrus cells is sufficient to cause division, enlargement, and death of host cells. Since elicitation of these symptoms depends on a functional type III protein secretion system in X, citri, we deduce that the PthA protein is a specific plant signal, its site of action is inside the plant cell, and it is a major determinant of host range.