Bacterial Pathogens in Plants: Life up against the Wall.

Abstract

of transposon mutagenesis, broad-host-range cosmid vectors, and marker-exchange mutagenesis to identify and manipulate bacterial genes that have a readily scored phenotype when mutated, conjugated into a related strain, or expressed in Escherichia coli. These approaches have yielded a large inventory of hrp (hypersensitive [esponse and pathogenicity) and avr (eirulence) genes that directly relate to the HR puzzle as we!l as numerous other genes associated with pectic enzyme, toxin, and extracellular polysaccharide (EPS) production. Rather than detail this inventory (which may be fundamentally incomplete; see below), we use representative components to develop a model for bacterial plant pathogenesis that is based on the very recent third development-the discovery that the hrp genes encode a protein secretion system, shared in plant and animal pathogens, that has the potential to transfer virulence proteins into eukaryotic host cells. The necrogenic bacteria have diverse pathogenic personalities with a bewildering array of symptoms and host specificities. The growing evidence that the hrp genes are ubiquitous in these pathogens, controlling early (and generally essential) interactions with plants, provides a unifying entry point for exploring bacterial phytopathogenicity. Hence, after introducing the representative pathogens, we explore the dynamic operation of the Hrp system and then turn briefly to factors such as toxins, EPS, and pectic enzymes that affect the full development of plant disease.