Production of pectinases and cellulase by six Pythium species isolated from necrotic strawberry roots
Polygalacturonase and polymethylgalacturonase activity was detected in filtrates ofPythium irregulare, P. perniciosum, P. sylvaticum, andP. dissotocum but not in filtrates ofP. rostratum andP. acanthicum. OnlyP. irregulare, P. perniciosum andP. dissotocum elaborated pectin methyl esterase and in trace amounts. None of the species produced detectable pectintrans-eliminase and “macerating enzyme” activity.P. sylvaticum was the only species of four tested which produced cellulase.
Carbohydrate-Active Enzymes in Pythium and Their Role in Plant Cell Wall and Storage Polysaccharide Degradation
Carbohydrate-active enzymes (CAZymes) are involved in the metabolism of glycoconjugates, oligosaccharides, and polysaccharides and, in the case of plant pathogens, in the degradation of the host cell wall and storage compounds. We performed an in silico analysis of CAZymes predicted from the genomes of seven Pythium species (Py. aphanidermatum, Py. arrhenomanes, Py. irregulare, Py. iwayamai, Py. ultimum var. ultimum, Py. ultimum var. sporangiiferum and Py. vexans) using the “CAZymes Analysis Toolkit” and “Database for Automated Carbohydrate-active Enzyme Annotation” and compared them to previously published oomycete genomes. Growth of Pythium spp. was assessed in a minimal medium containing selected carbon sources that are usually present in plants. The in silico analyses, coupled with our in vitro growth assays, suggest that most of the predicted CAZymes are involved in the metabolism of the oomycete cell wall with starch and sucrose serving as the main carbohydrate sources for growth of these plant pathogens. The genomes of Pythium spp. also encode pectinases and cellulases that facilitate degradation of the plant cell wall and are important in hyphal penetration; however, the species examined in this study lack the requisite genes for the complete saccharification of these carbohydrates for use as a carbon source. Genes encoding for xylan, xyloglucan, (galacto)(gluco)mannan and cutin degradation were absent or infrequent in Pythium spp.. Comparative analyses of predicted CAZymes in oomycetes indicated distinct evolutionary histories. Furthermore, CAZyme gene families among Pythium spp. were not uniformly distributed in the genomes, suggesting independent gene loss events, reflective of the polyphyletic relationships among some of the species.
The Erwinia-Coliform Relationship
Erwinia, a genus of bacterial plant pathogens, has long been recognized as closely related to the Escherichia-Aerobacter group. This view, although readily accepted, has been the object of little comparative study; the relationship is usually taken for granted, the result being a relatively poor understanding of the similarities of the two groups. On the basis of its disease-producing ability in plants, Bergey et al. (1939) gives the group tribal ranking, Erwineae, in the family Enterobacteriaceae. Other than the obvious facts that the species of Erwinia are peritrichously flagellated, gram negative, do not form spores and ferment many carbohydrates, including lactose, our knowledge of the relationship is meager. The possibility that the Erwinial might be coliform organisms has been given comparatively little thought. Stuart et al. (1938, 1940) recognized the possibility of Erwinia cultures being confused with coliform organisms, or vice versa, and included some plant cultures in their studies. Stanley (1939) likewise declared the soft-rot group to belong to the colon-typhoid-dysentery group, and suggested that organisms identified as Erwinia might be Escherichia coli, the latter interpreted in a broad sense. According to Parr (1939), F. D. Chester stated that the genus Erwinia was established on a purely utilitarian basis and had no genetic standing. Dowson (1939), following the usual British practice, advocated the inclusion of the soft-rot pathogens in the genus Bacterium Lehmann and Neumann. By this procedure he recognized the apparent close relationship of the coliform and Erwinia groups, as the former are likewise placed in this genus. Unfortunately, the biochemical characteristics of the soft-rot organisms are as variable as those of coliform isolates (Stanley 1939, Elrod 1941). It has been shown, especially by Stanley, that fermentative shifts are common, tending to throw a culture from one species to another. These changes usually concern a change from an aerogenic to an anaerogenic state, although some variation has been noted in the methyl-red, Voges-Proskauer and citrate tests. Elrod (1941) found that the soft-rot group is as antigenicaily heterogeneous as the coliform group. In the light of our abundant knowledge of the coliform group, it was decided first to employ "coliform" methods in an attempt to arrive at a better understanding of the Erwinia-Escherichia-Aerobacter relationship. The recent work of Stuart, Mickle and Borman (1940) on aberrant coliforms offers a way of