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Sclerotinia sclerotiorum is unusual among necrotrophic pathogens in its requirement for senescent tissues to establish an infection and to complete the life cycle. A model for the infection process has emerged whereby the pathogenic phase is bounded by saprophytic phases; the distinction being that the dead tissues in the latter are generated by the actions of the pathogen. Initial colonization of dead tissue provides nutrients for pathogen establishment and resources to infect healthy plant tissue. The early pathogenicity stage involves production of oxalic acid and the expression of cell wall degrading enzymes, such as specific isoforms of polygalacturonase (SSPG1) and protease (ASPS), at the expanding edge of the lesion. Such activities release small molecules (oligo-galacturonides and peptides) that serve to induce the expression of a second wave of degradative enzymes that collectively bring about the total dissolution of the plant tissue. Oxalic acid and other metabolites and enzymes suppress host defences during the pathogenic phase, while other components initiate host cell death responses leading to the formation of necrotic tissue. The pathogenic phase is followed by a second saprophytic phase, the transition to which is effected by declining cAMP levels as glucose becomes available and further hydrolytic enzyme synthesis is repressed. Low cAMP levels and an acidic environment generated by the secretion of oxalic acid promote sclerotial development and completion of the life cycle. This review brings together histological, biochemical and molecular information gathered over the past several decades to develop this tri-phasic model for infection. In several instances, studies with Botrytis species are drawn upon for supplemental and supportive evidence for this model. In this process, we attempt to outline how the interplay between glucose levels, cAMP and ambient pH serves to coordinate the transition between these phases and dictate the biochemical and developmental events that define them. 相似文献
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《Fungal Ecology》2014
The wheat grain mycobiome is only scarcely investigated and focus has been on seed-transmitted wheat pathogens of agricultural importance. In this study, we used next generation sequencing to study the mycobiome of Danish wheat grain samples at harvest. In total 228,421 sequences were obtained from 90 samples that were taken from locations across Denmark during three years. These sequences could be grouped into 173 non-singleton operational taxonomic units (OTUs) of which 21 OTUs, identified as belonging to genera such as Fusarium, Alternaria, Cladosporium. Phaeosphaeria and Microdochium, were identified as ‘core’ OTUs as they were found in all or almost all samples and accounted for almost 99 % of all sequences. The remaining OTUs were only sporadically found and only in small amounts. Cluster and factor analyses showed patterns of co-existence among the core species. Cluster analysis grouped the 21 core OTUs into three clusters: cluster 1 consisting of saprotrophs, cluster 2 consisting mainly of yeasts and saprotrophs and cluster 3 consisting of wheat pathogens. Principal component extraction showed that the Fusarium graminearum group was inversely related to OTUs of clusters 1 and 2. 相似文献
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《Fungal Ecology》2015
Lichens produce various oxidoreductases including heme-containing peroxidases and the copper-containing phenol oxidases tyrosinase and laccase. Our earlier findings suggested that significant oxidoreductase activity occurs mainly in lichens from the order Peltigerales. Here we show that the non-Peltigeralean lichen Usnea can display significant activities of peroxidases and laccases. Strong evidence for the involvement of peroxidases and laccases in saprotrophic activities comes from the observation that their activities are induced by “starvation” due to prolonged dark storage, and also by treatment with soluble cellulose and lignin breakdown products. We also show that, given a quinone and chelated Fe, Usnea can produce hydroxyl radicals; these radicals contribute to the break down of carbohydrates or lignin. However, hydroxyl radical production is independent of laccase and peroxidase activity. Laccases and peroxidases are involved in other aspects of lichen biology; here we show that peroxidases, but not laccases, can break down lichen substances. Reduction in the amounts of lichen substances will reduce photoprotection, which will increase the photosynthetic capacity of thalli during winter when light intensities are low. 相似文献
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Pollination biology of the saprophytic speciesPetrosavia sakuraii was investigated in Central Japan. The flower opens at any time of the day, and the anthers of outer stamens dehisce after
about one day and those of the inner stamens after two days. The stigmata on the semicarpous (nearly apocarpous) pistil, which
are already receptive when the flower opens, are situated in the center of the flower and move gradually outwards to touch
the anthers of outer stamens about five days after anthesis. The breeding experiments show that the stigmata are receptive
at least for five days, and many seeds are produced through autonomous self-pollination. That means the nearly apocarpous
pistil, which is in a primitive condition, is adapted to the autonomous self-pollination. The selfing rate including insect-mediated
self-pollination is very high, and this sexual-reproductive system seems favorable to the saprophytic plant which is probably
severely limited in its resources. However, cross-pollination also may be performed by the small bees of Lasioglossum and
some other insects, even though they do not so frequently visit the flowers and the out-crossing rate is low. 相似文献
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