The type III (Hrp) secretion pathway of plant pathogenic bacteria: trafficking harpins, Avr proteins, and death.

The ability of plant pathogenic bacteria to deliver deathtriggering proteins to the interior of plant cells was revealed in a rapid succession of papers in 1996 that transformed our concepts of bacterial plant pathogenicity. The breakthrough came with the convergence of work on Hrp systems and Avr proteins, an understanding of which requires an introduction to the most prevalent bacterial pathogens of plants, the cardinal importance of the Hrp pathway, and the paradoxical phenotype associated with avr genes. Plant pathogenic bacteria in the genera Erwinia, Pseudomonas, Xanthomonas, and Ralstonia cause diverse, and sometimes devastating, diseases in many different plants, but they all share three characteristics: they colonize the intercellular spaces of plants, they are capable of killing plant cells, and they possess hrp genes. Many of these pathogens are host specific. In host plants, they produce various symptoms after several days of multiplication, whereas in nonhost plants, they trigger the hypersensitive response (HR), a rapid, defense-associated, programmed death of plant cells at the site of invasion (21, 43). With inoculum levels typically encountered in natural environments, the HR produces individual dead plant cells that are scattered within successfully defended healthy tissue (71). However, experimental infiltration of high inoculum levels (.10 bacterial cells/ml) results in macroscopically observable death of the entire infiltrated tissue, usually within 24 h (42). Pioneer screens for random transposon mutants with impaired plant interactions yielded a prevalent class that was designated Hrp, that is, deficient in both HR elicitation in nonhost plant species and pathogenicity (and parasitic growth) in host species (49, 56). This complete loss of pathogenic behavior results from mutation of any one of several hrp genes, which largely encode components of a type III protein secretion system (73). Because the capacity to elicit the HR is a convenient marker for the capacity to be pathogenic and these two abilities have a common genetic basis, the “simple” problem of HR elicitation is being studied as an entry to the larger problem of pathogenesis. A key part of the HR puzzle is that HR elicitation and the resulting limitation in host range can occur if the pathogen possesses any one of many possible avr (avirulence) genes that interact with corresponding R (resistance) genes in the host plant. Such “gene-for-gene” interactions result in recognition of the bacterium and the triggering of plant defenses. For example, Pseudomonas syringae pv. glycinea is one of over 40 P. syringae pathovars differing largely in host range among plant species and is subdivided into races on the basis of their interactions with genetically distinct cultivars of its host, soybean. Those race-cultivar interactions involving matching bacterial avr and plant R genes result in the HR and avirulence, i.e.; failure of the bacterium to produce disease. The R genes encode components of a parasite surveillance system and are crossed into crops from wild relatives by plant breeders for disease control. avr genes are identified and cloned on the basis of the avirulence they confer on virulent races in appropriate test plants (39, 69). In most cases, it is not clear why plant pathogens carry avr genes that betray them to host defenses but new insights into this question are discussed below. Both hrp and avr genes were originally defined on the basis of the phenotypes they confer on bacteria interacting with plants. Molecular studies have revealed a functional relationship between the products of these two classes of genes and an underlying similarity with a key virulence system of several animal pathogens. Yersinia, Salmonella, and Shigella spp. transfer virulence effector proteins directly into animal cells via the type III pathway (16, 17, 62, 67, 84). Similarly, plant pathogens use the Hrp type III pathway to transfer Avr effector proteins to the interior of plant cells. The genetic dissection of type III secretion systems is just beginning, and little is known of the mechanisms of protein translocation. In this review, we will describe (i) the recently completed inventory of genes directing type III secretion in plant pathogens and new insights into type III secretion mechanisms gained from research with Hrp systems, (ii) two classes of proteins (harpins and pilins) that are secreted by the Hrp type III pathway when plant pathogens are grown in media that mimic plant intercellular fluids, (iii) evidence that Avr proteins are delivered by the Hrp pathway directly to the interior of plant cells, and (iv) a resulting new paradigm for bacterial plant pathogenicity. The focus will be on quite recent work, and readers are referred to other reviews for a classic introduction to the HR phenomenon (43), earlier investigations of the Hrp system (11), avr genes (20, 46), and a wider perspective on bacterial virulence systems and plant responses (2).