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infections is only rudimentary. Much remains to be learned about individual virulence factors essential for pathogenicity and the mechanism by which these factors work together during pathogenesis.

P. aeruginosa virulence factors facilitate tissue invasion and systemic spread and include pili and flagella, endotoxins, exotoxins, vascular permeability factors, and a variety of excreted enzymes (16). P. aeruginosa proteases degrade a variety of host proteins and have a direct destructive effect on skin tissue (17, 18). Elastase (LasB), a potent metalloprotease with broad substrate specificity, degrades host proteins, such as elastin, collagen, transferrin, Ig, and some of the components of complement (19). The LasB elastase acts in concert with both LasA protease and alkaline protease to cause efficient elastolysis, which is required for the tissue damage associated with P. aeruginosa pathogenesis (20). The P. aeruginosa alginate capsule plays an essential role in chronic pulmonary infection in cystic fibrosis patients (21, 22). Finally, the P. aeruginosa lipopolysaccharide also has been shown to be an important virulence factor (23).

The synthesis of several P. aeruginosa-secreted virulence factors is transcriptionally regulated by environmental stimuli (24, 25 and 26). For example, phospholipase C is regulated by the availability of phosphate (27, 28), whereas exotoxin A and elastase are regulated by the concentration of iron in the growth medium (29, 30, 31 and 32). Moreover, the production of a large number of exoproducts, including elastase, alkaline protease, the LasA protease, hemolysin, pyocyanin, and rhamnolipids is cell densitydependent and is regulated by the so-called quorum-sensing cascade (33). The P. aeruginosa quorum-sensing cascade is mediated by low molecular mass homoserine lactones that are synthesized by the products of the lasI and rhlI genes. The concentration of these homoserine lactones is monitored by the products of the lasR and rhlR genes, which serve as global transcriptional activators of a variety of exoproducts relevant for P. aeruginosa pathogenesis. In addition to homoserine lactones, it recently has been shown that 2-heptyl-3-hydroxy-4-quinolone also serves as an intracellular signal molecule involved in the activation of pathogeneicity-related factors (34). This molecule belongs to the 4-quinolone chemical family, best known for the antibiotic activity of many of its members.

Fig. 1. Macroscopic and microscopic symptoms elicited by P. aeruginosa strain UCBPP-PA14 infiltrated into Arabidopsis leaves. (A) The upper part of an Arabidopsis leaf (ecotype LL-O) was infiltrated with bacterial suspensions at a titer of 103 cfu/cm2 and photographed 2 days postinfection. A chlorotic zone (white arrow) surrounds the soft-rot symptoms. (B and C) Arabidopsis leaves infiltrated with bacterial suspension and stained with trypan blue 2 days postinfection. Whole leaves were examined by using a Zeiss Axioskop and photographed. Bacterial movement was observed along leaf veins (arrows indicate vessel parenchyma filled with bacteria). Bacteria are absent in xylem vessels (arrowheads).

In addition to regulating the production of a variety of specific virulence-related factors, the quorum-sensing cascade as well as other cell-to-cell signaling mechanisms are involved in the differentiation of P. aeruginosa biofilms (35, 36), matrix-enclosed bacterial populations that attach to each other and to biotic, or abiotic surfaces (37). P. aeruginosa is protected from adverse environmental conditions and from antibacterial agents while growing in this matrix-enclosed structure that has been implicated as a contributing factor in the persistence and severity of P. aeruginosa infections. It is thought that when nutrient conditions become limiting, bacteria are shed from the biofilm and enter the planktonic (free-living) phase (37). In this manner, cells are able to colonize new habitats and form new biofilms.

Another quorum-sensing regulated virulence factor is pyocyanin, a blue-green-colored phenazine (33, 38). This secondary metabolite has antimicrobial activity against several species of bacteria, fungi, and protozoa, a quality attributed to its redoxactive potential (39). Although little is known about the nature of the enzymes that catalyze the formation of pyocyanin in P. aeruginosa, the conversion of chorismate to anthranilate is thought to be a key step in the pathway that is most likely catalyzed by the anthranilate synthetase encoded by the phnA and phnB genes (40). Even though pyocyanin-induced free radical production appears to be responsible for much of its antimicrobial activity (41), the role of pyocyanin in P. aeruginosa-associated tissue injury is less clear.

Development of the Arabidopsis-P. aeruginosa Model

As described briefly above, we developed the Arabidopsis-P. aeruginosa pathogenesis model based on the P. aeruginosa strain PA14. This was accomplished as follows. A collection of 75 P. aeruginosa strains, of which 30 were human isolates, was screened for its ability to elicit disease on leaves of at least four different Arabidopsis ecotypes (4). Several strains elicited disease characterized by varying degrees of soft-rot symptoms in all four ecotypes (wild varieties) tested. Importantly, two strains, PA14 (a human isolate) and UCBPP-PA29 (a plant isolate), caused severe soft-rot symptom in some, but not all of the ecotypes tested. The finding that these two P. aeruginosa strains exhibit ecotype specificity is typical of plant pathogen interac-

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