Full text loading...
Chapter 40 : Necrotrophic Fungi: Live and Let Die
Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
Ebook: Choose a downloadable PDF or ePub file. Chapter is a downloadable PDF file. File must be downloaded within 48 hours of purchase
Necrotrophic fungi are pathogens that obtain nutrients from dead cells. In this chapter three major fungal necrotrophs, Sclerotinia sclerotiorum, Botrytis cinerea, and Alternaria brassicicola, are compared and contrasted. All three fungi discussed in the chapter have recently completed genome sequences. Effective pathogenesis by S. sclerotiorum requires the secretion of oxalic acid (OA). The role of OA in fungal pathogenicity was originally demonstrated using a genetic approach. In all eukaryotes examined, reactive oxygen species (ROS) are produced during normal cellular metabolism. It is now evident that low, nonlethal concentrations of ROS can function beneficially as regulatory molecules in cell-signaling pathways. Programmed cell death (PCD) is an intentional cellular suicide that is genetically based. The result of PCD is the orderly removal of unwanted, unneeded, used, or pathological cells and under normal homeostatic conditions, is of benefit to the organism. Diseases caused by B. cinerea occur in important crop plants in all temperate climate zones, both during plant cultivation and on harvested commodities, often during storage. The SNF1 kinase plays a central role in carbon catabolite repression in Saccharomyces cerevisiae. Importantly, the addition of tryptone to spores of both Δabste12 and Δabnik1 during plant inoculation resulted in a complete restoration of pathogenicity. These results might suggest the presence of a previously undescribed nutrient- or polypeptide-sensing pathway downstream of Amk1/AbSte12 signaling pathways and a putative AbNIK1 osmoregulation pathway.
Key Concept Ranking
- Fungal Signal Transduction
Typical symptoms caused by S. sclerotiorum and structures of the fungus. (a) Apothecia (arrow) below a soybean canopy. (b) White mold on dry bean stalks with sclerotia (arrow). (c) Symptoms of white mold on dry bean: water-soaked lesions with fluffy mycelium. (d) Oilseed rape infected with S. sclerotiorum showing bleached stem tissue and necrotic side branch. (e) Carpogenic germination of sclerotia resulting in apothecia with ascospores. (f) GFP-tagged S. sclerotiorum under fluorescence microscopy ( De Silva et al., 2005 ). (g) Sclerotia in a soybean harvest sample (arrow).
Life stages of B. cinerea (Botryotinia fuckeliana). (a) B. cinerea conidiophore with mature conidia in situ (low-temperature scanning electron microscopy). (b) Apothecia of B. fuckeliana, ~10 weeks after spermatization. (c) Two B. fuckeliana asci each containing eight ascospores, surrounded by ascospores released from damaged asci. (d) Scanning electron micrograph of calcium oxalate crystals (indicated by arrows) in tomato leaves infected by B. cinerea. The white dashed line in the center of the picture indicates the border of the lesion between the external mycelium surrounded by crystals (on the left-hand side) and a concentric zone of collapsed epidermal cells (on the right-hand side). Both zones are colonized by mycelium, growing below the epidermis. The white dashed line at the right-hand side of the picture represents the border between the colonized area and noninvaded leaf tissue. Panels a through c reproduced from Williamson et al., 2007 , with permission from Blackwell Publishers. Panel d reproduced from Prins et al., 2000b , with permission.
A. brassicicola . (A) Germinating spores of A. brassicicola in vitro. (B) Black spot symptoms on cultivated mustard, B. juncea. (C) Trypan blue staining of infected cabbage tissue. The arrow points to an invasive hypha. Note dark blue staining of tissue surrounding hypha, suggesting damaged walls and membranes of host cells due to the action of toxic secreted proteins and metabolites. (D) GFP-tagged A. brassicicola germinating on a cabbage leaf. Arrows depict entry through stomata and appressorium-like structures. Reprinted from Lawrence et al. (2008) .
Features of the genome sequences of S. sclerotiorum, B. cinérea, and A. brassicicola