No abstract

Understanding the molecular mechanisms underpinning the ecological success of plant pathogens is critical to develop strategies for controlling diseases and protecting crops. Recent observations have shown that plant pathogenic bacteria, particularly Pseudomonas, exist in a range of natural environments away from their natural plant host e.g., water courses, soil, non-host plants. This exposes them to a variety of eukaryotic predators such as nematodes, insects and amoebae present in the environment. Nematodes and amoeba in particular are bacterial predators while insect herbivores may act as indirect predators, ingesting bacteria on plant tissue. We therefore postulated that bacteria are probably under selective pressure to avoid or survive predation and have therefore developed appropriate coping mechanisms. We tested the hypothesis that plant pathogenic Pseudomonas syringae are able to cope with predation pressure and found that three pathovars show weak, but significant resistance or toxicity. To identify the gene systems that contribute to resistance or toxicity we applied a heterologous screening technique, called Rapid Virulence Annotation (RVA), for anti-predation and toxicity mechanisms. Three cosmid libraries for P. syringae pv. aesculi, pv. tomato and pv. phaseolicola, of approximately 2000 cosmids each, were screened in the susceptible/non-toxic bacterium Escherichia coli against nematode, amoebae and an insect. A number of potential conserved and unique genes were identified which included genes encoding haemolysins, biofilm formation, motility and adhesion. These data provide the first multi-pathovar comparative insight to how plant pathogens cope with different predation pressures and infection of an insect gut and provide a foundation for further study into the function of selected genes and their role in ecological success.

Regional variations in the population structure of Pseudomonas syringae pathovar phaseolicola from Spain are revealed by typing with PmeI pulsed-field gel electrophoresis, plasmid profiling and virulence gene complement One hundred and twenty pathogenic isolates of Pseudomonas syringae pv. phaseolicola recovered in Spain were subjected to biochemical and genomic typing, and investigated for virulence gene complement. Fifty-six were recovered from common beans (Phaseolus vulgaris) of the type Granja Asturiana, grown in a northern Spanish region (Asturias), and 64 from other common beans cultured in the neighbouring region of Castilla y Leó n. Typing by PmeI digestion followed by pulsed-field gel electrophoresis revealed 27 profiles, with only three being common to both regions. Relationships between profiles distributed the isolates into two clusters: A (subdivided into subclusters A1 and A2) and B. Cluster A included all isolates from Granja Asturiana and about a quarter of the isolates from Castilla y Leó n. Isolates from cluster A were negative for mannitol utilization and hybridized to probes for the argK-tox region responsible for phaseolotoxin production. Isolates that grouped in cluster B, which were only found in Castilla y Leó n, were able to utilize mannitol but did not hybridize to probes for the argK-tox region. Separation of the isolates into three genomic groups, subsequently termed PphA1, PphA2 and PphB, was also supported by effector gene complement and location. In PphB, all effector genes tested (hopX1, hopF1, avrB2 and avrD1) mapped on chromosomal fragments, but faint hybridization of avrB2 with plasmids of about 40 kb was also observed. In PphA hopX1 mapped on the chromosome; in PphA1 avrB2 and avrD1 were carried on virulence plasmids (most of approx. 125 kb) and hopF1 was not detected, while in PphA2 the three genes were located on plasmids (approx. 75-160 kb). These results can be used as a framework to investigate the basis of regional variation in population structure, and for further epidemiological surveillance of P. syringae pv. phaseolicola. INTRODUCTIONHalo blight disease of common beans (Phaseolus vulgaris L.) is caused by Pseudomonas syringae pathovar phaseolicola (hereafter P. syringae pv. phaseolicola), a seed-borne bacterial pathogen with worldwide distribution (Saettler, 1991;Taylor et al., 1996). The ability of this and other phytopathogenic pseudomonads to cause disease on susceptible cultivars, and to elicit the hypersensitivity response in non-susceptible cultivars, requires a type III secretion system encoded by the hrp/hrc genes of chromosomal location (Alfano et al., 2000;Joardar et al., 2005). This system is used to translocate effector proteins, termed Hop (Hrp outer protein) or Avr (avirulence) into plant host cells (Lindeberg et al., 2006). In P. syringae pv. phaseolicola, effectors can be encoded either on the bacterial chromosome or on plasmids, including pAV511 (a 154 kb plasmid found in strain 1449B; Jackson et al., 1999) and p1448A-A (132 kb, carried by strain 14...

ABSTRACTA virulentPseudomonas viridiflava-related bacterium has been identified as a new pathogen of soybean, one of the most important crops worldwide. The bacterium was recovered from forage soybean leaves with dark-reddish spots, and damage on petioles and pods was also observed. In contrast, common bean was not affected.