To be or not to be: How Pseudomonas solanacearum decides whether or not to express virulence genes

Pseudomonas solanacearum is a soil-borne phytopathogen that causes a lethal wilting disease of many plants, due in part to production of the unusual exopolysaccharide EPS I and numerous extracellular proteins (EXPs). Levels of EPS I and many EXPs are differentially controlled by a complex sensory array whose size, organization, and other properties set it apart from others found in prokaryotes. This network not only controls reversible switching between two morphotypes, each probably specialized for survival in different ecological niches (plant vs. soil), but also fine tunes transcription of virulence genes in response to multiple environmental signals. The interacting and cascading nature of the network is reminiscent of a primitive neural network, apparently designed to guide virulence gene expression during the dynamic interaction of the pathogen with its environment. This minireview focuses on the unique aspects of the network and its regulated targets.