Zoospore

Abstract: The variation in two aspects of pathogenicity, aggressiveness and virulence (physiologic race), among single zoospore cultures from three wild isolates has been examined. Two components of aggressiveness were measured, rate of growth on tubers and generation time on detached leaflets. Virulence was assessed from the pattern of compatible and incompatible reactions to members of the R gene differential series. Extensive variation in aggressiveness was detected in samples of zoospore cultures from all three isolates. The level of aggressiveness ranged from a high, equivalent to that of the parental wild isolates, to a complete lack of pathogenicity; as much as 45% was of the latter type. No instance of a change in virulence was found among 104 cultures tested.The significance of the observed pathogenic variation is discussed. It is concluded that zoospore variation is not important as a source of new physiologic races or strains adapted to particular, horizontally resistant varieties. Comparison of populati...

Abstract: Bacteria emit volatile organic compounds with a wide range of effects on bacteria, fungi, plants, and animals. The antifungal potential of bacterial volatiles has been investigated with a broad span of phytopathogenic organisms, yet the reaction of oomycetes to these volatile signals is largely unknown. For instance, the response of the late blight-causing agent and most devastating oomycete pathogen worldwide, Phytophthora infestans, to bacterial volatiles has not been assessed so far. In this work, we analyzed this response and compared it to that of selected fungal and bacterial potato pathogens, using newly isolated, potato-associated bacterial strains as volatile emitters. P. infestans was highly susceptible to bacterial volatiles, while fungal and bacterial pathogens were less sensitive. Cyanogenic Pseudomonas strains were the most active, leading to complete growth inhibition, yet noncyanogenic ones also produced antioomycete volatiles. Headspace analysis of the emitted volatiles revealed 1-undecene as a compound produced by strains inducing volatile-mediated P. infestans growth inhibition. Supplying pure 1-undecene to P. infestans significantly reduced mycelial growth, sporangium formation, germination, and zoospore release in a dose-dependent manner. This work demonstrates the high sensitivity of P. infestans to bacterial volatiles and opens new perspectives for sustainable control of this devastating pathogen.

Abstract: The green marine macroalga, Ulva linza, adopts an “atypical” form when grown in the absence of bacteria. Twenty unique strains of periphytic bacteria, isolated from three species of Ulva, were identified by 16S rDNA sequencing. These isolates were assessed for their effect on the growth and morphological development of axenic plantlets of U. linza. Results showed that the effect of bacterial strains was strain- but not taxon-specific. Thirteen isolates returned the aberrant morphology to normal and of these, five also significantly increased growth rate. One isolate increased growth, but had no effect on morphology. Biofilms of some of these isolates stimulated the settlement of Ulva zoospores but there was no correlation between bacterial isolates that stimulated zoospore settlement and those that initiated changes in morphology and/or growth of the cultured alga.

Abstract: Summary Previous studies have shown that the rate of settle- ment of zoospores of the green alga Enteromorpha is stimulated by mixed microbial biofilms and that the number of zoospores settling is positively correlated with the number of bacteria in the biofilm In the present study the specificity of this relationship has been investigated Ninety-nine strains of marine bac- teria were isolated from natural biofilms on rocks and the surface of Enteromorpha plants Isolates were screened by denaturing gradient gel electrophoresis (DGGE) to eliminate replicates and 16S rDNA sequencing identified a total of 37 unique strains Phylogenetic analysis revealed that the isolated bac- terial strains belonged to three groups g - Proteobacte- ria (28 strains), Cytophaga-Flavobacteria-Bacteroid (CFB) group (six strains) and a - Proteobacteria (one strain) Two strains were unassigned, showing < < < 93% sequence similarity with the CFB group The main genera of g - Proteobacteria were Pseudoalteromonas (14 strains), Vibrio (five strains), Shewanella (five strains), Halomonas (three strains) and Pseudomo- nas (one strain) Spore settlement experiments were conducted on single-species biofilms, developed for different times on glass slides The effect of correct- ing spore settlement values for biofilm density was evaluated Results showed that the effect of bacterial strains on spore settlement was strain- but not taxon- specific and activity varied with the age of the biofilm However, most of the strains belonging to genera Vibrio and Shewanella showed stimulation Pseudoalteromonas strains showed a range of effects including settlement-inhibiting, paralysing and lysing activities Spatial analysis of bacterial density in the presence and absence of spores revealed a range of different types of association between spores and bacteria Overall, the spatial association between spores and bacteria appears to be independent of the overall quantitative influence of bacterial cells on spore settlement