The Conidium

Daniel J. Ebbole

doi:10.1128/9781555816636.ch36

Chapter 36 : The Conidium

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Affiliations: 1: Program for the Biology of Filamentous Fungi, Department of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843

Content Type: Monograph

Publication Year: 2010

Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis

Book DOI: 10.1128/9781555816636

Chapter DOI: 10.1128/9781555816636.ch36

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Abstract:

A discussion of hyphal growth and polarity determination, cell cycle and signal transduction, is integral to understanding the cell biology of development. In Saccharomyces cerevisiae Tup1p can alter chromatin structure as a mechanism to regulate gene expression. The rco-1 ortholog in Aspergillus nidulans, rcoA, has been shown to affect chromatin structure at some promoters. Heterotrimeric G proteins positively regulate adenylate cyclase in Neurospora crassa, and mutation of gna-3 has the most dramatic effect of the three G-alpha subunit mutations in derepressing conidiation, suggesting that GNA-3 plays the greatest role in stimulating cAMP levels under vegetative growth conditions. Thus, signals that downregulate cAMP levels are likely to stimulate conidiophore morphogenesis. A dominant activated G-alpha subunit stimulates conidiation. Although G-protein and cAMP signal transduction is used as part of the overall pathway controlling conidiation in all A. nidulans, N. crassa, and Magnaporthe grisea, the wiring of the circuit differs among them, just as the effect of light on conidiation can differ (stimulatory in A. nidulans and N. crassa and inhibitory in M. grisea). The central regulatory pathway controlling conidiation in A. nidulans involves BrlA, AbaA (for “abacus”), and WetA (for “wet-white”) to regulate production of vesicles, sterigmata (metulae and phialide cells), and conidia. Principles gained from defining the evolution of conidiation pathways are likely to be informative for understanding the origins of other novel developmental pathways, such as pathogenesis.

Citation: Ebbole D. 2010. The Conidium, p 577-590. In Borkovich K, Ebbole D (ed), Cellular and Molecular Biology of Filamentous Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555816636.ch36

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The Conidium

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FIGURE 1

Cytoplasmic cleavage in zoosporogenesis. The cytoplasm of the sporangium is divided into membrane-bound compartments that give rise to uniflagellate zoospores. Control treatments of zoospore formation in living sporangia of Allomyces macrogynus were observed with video-enhanced differential interference contrast optics (A through D) and confocal microscopy after FM4–64 staining (E through H). Minutes and seconds in the upper left corners indicate postinduction times. Scale bars = 10 mm. (A through D) Nuclei were located in the cortical cytoplasm during early stages of zoospore formation (arrow, A) and by 20 to 30 min postinduction were positioned throughout the cytoplasm (arrows, B). Cytoplasmic domains became distinct (C), and eventually the papillum (asterisk, A) deliquesced and mature zoospores (arrowhead, D) maneuvered out of the sporangium into the surrounding medium. (E through H) By 12 to 20 min postinduction, areas of increased fluorescence (arrows, E) were observed along regions of the plasma membrane. Cleavage elements initially extended from these regions within the sporangial cortex (arrows, F) followed by a rapid elongation inward toward the center of the sporangium (G). By 40 to 50 min postinduction, zoospore initials were delimited into polyhedral cells (asterisk, H). Reprinted from Lowry et al. (2004) with permission from the Mycological Society of America.

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FIGURE 1

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FIGURE 2

Developmental stages of Rhizopus sporangiophores and sporangia as seen under low-temperature scanning electron microscopy. (A) Immature sporangiophore; (B) young sporangiophore with developing spores in sporangia; (C) mature sporangium with sporangiospores; (D) sporangiospores starting to be released from an old sporangium; (E) columella with only a few spores left. Arrows indicate examples of anomalies in sporangiospore size and shape. (A) R. rhizopodiformis CBS 102277; 3 days, MEA; (B) R. oligosporus CBS 339.62; 3 days, malt extract agar (MEA); (C) R. homothallicus CBS 111232; 4 days, oatmeal agar; (D through E) R. rhizopodiformis CBS 536.80, 12 days, MEA. Bars, 10 μm. Reprinted from Jennessen et al. (2008) with permission from Elsevier.

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FIGURE 2

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FIGURE 3

Conidial development in Trinacrium subtile, Reiss (1852). (A) The conidia are produced in a sympodial fashion, and the sites of conidial production are apparent after detachment from the conidiogenous cell. (B) Note the Y-shaped conidia of this Ingoldian aquatic hyphomycete. (C) Illustration of conidia and conidiogenous cells. Images provided by S. S. Tzean, National Taiwan University.

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FIGURE 3

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FIGURE 4

Coiled conidia of helicosporous fungi. (A and E) Conidia from the Tubeufiaceae; (B through D) conidia from other lineages. The conidia in panels A through C are three-dimensional and doliiform (barrel-shaped); those in panels D and E are two-dimensional and planate. Species represented are Helicoon gigantisporum (A), Helicodendron tubulosum (B and C), Helicoma olivaceum(D), and Helicomyces roseus (E), with a conidium developing from a conspicuous, erect conidiophore. Images are differential interference contrast micrographs; scale bar for panels A, B, and D, 30 μm; for panel C, 25 μm; for panel E, 15 μm. Reprinted from Tsui and Berbee (2006) with permission from Elsevier.

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FIGURE 4

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FIGURE 5

Ontogeny of blastic and thallic conidia. (A) Conidiophore of N. crassa showing holoblastic conidiation. Conidia are produced by repeated apical budding with an intact outer cell wall during macroconidium formation. (B) N. crassa holoblastic macroconidiation showing arthric separation of individual conidia. (C) Enteroblastic conidia development observed in the conidiophore of A. nidulans. The phialide, P, cell wall differentiates to form an inner wall layer (arrows) that forms a blastic conidium initial, CI, which pushes through the phialide cell wall to form the conidial cell wall (arrow labeled CW), distinct from the phialide cell wall layer. Photograph from Sewall et al. (1990) with permission from Elsevier. (D) Microconidia of N. crassa pushing through the phialide cell wall. Photographs for panels A, B, and D provided courtesy of Matthew Springer, University of California—San Francisco, and Oxford University Press ( Davis, 2000 ).

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FIGURE 5

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FIGURE 6

Effect of light on conidiophore development and spore release of M. oryzae. (A) Aerial development and conidium formation are repressed by light. M. oryzae growing (left to right) across a strip of medium and exposed to light (white boxes) and dark (black boxes). (B) Spore release activated by light. Cultures grown in constant darkness fail to release spores (line with boxes); 12-h exposure to light induces spore release (line with diamonds). Reprinted from Lee et al. (2006) with permission from Elsevier.

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FIGURE 6

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FIGURE 7

Deep etch of an N. crassa conidium showing hydrophobin rodlet layer. The initial fracture passed through the cell wall (CWF), exposing the plasma membrane face (PF). Etching then exposed the outside of the cell wall proper (CWE) and shows that in some regions the cell wall is covered by rodlets. Bar, 1 μm. Reproduced from Dempsey and Beever (1979) with permission from the American Society for Microbiology.

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FIGURE 7

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FIGURE 8

Analogous features in conidiation of A. nidulans and N. crassa. (A) Time coursean A. nidulans conidiation. (C) Regulatory genes controlling development in A. nidulans. (D) Timing of expression of conidiation-specific genes. (E) Regulatory genes controlling development in N. crassa. (F) Morphological landmarks in N. crassa conidiation. The flbC, flbD, acon-2, and acon-4 play roles in activating the nonhomologous key regulatory genes brlA and fl. The role of abaA in N. crassa appears to be played by acon-3, which is required for major constriction chain formation. The wetA, csp-1, and csp-2 genes are required for late stages of conidial morphogenesis and maturation. The medA and stuA (asm-1) genes play roles in conidiation across fungal species in cell patterning and conidiophore structure.aa

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FIGURE 8

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FIGURE 9

Conservation of developmental regulator function in evolution. The different fungal species display analogous effects on morphogenesis. The top row illustrates medA. In A. nidulans, medA mutants reiterate sterigmata and sometimes form new conidiophores from the vesicle. In F. oxysporum, the ren1 mutants are unable to make either micro- or macroconidia and instead produce a reiteration of a new cell type. In M. oryzae, acr mutants fail to produce a sympodial arrangement of conidia and instead reiterate conidia, one on top of the preceding cell. This is called an acropetal mode of spore ontogeny. The bottom row illustrates phenotypic similarity in the stuA ortholog mutants across the fungi. In A. nidulans, the stalk cell is stunted and the sterigma is reduced (stunted). In F. oxysporum, the conidiophore is reduced so that conidiophores are not produced and macro-conidia form directly from the substrate hyphae (intercalary phialides). Microconidium formation is not affected. In N. crassa, aerial hyphae are stunted, but conidiation is otherwise normal. Figure modified from Ohara et al., 2004 , and Ohara and Tsuge, 2004 .

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FIGURE 9

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FIGURE 10

Patterns of gene evolution for regulators of conidiation. (A) Genes that are conserved across fungi and play a recognizably analogous role in conidiation. This pattern is represented by medA and stuA orthologs across a broad group of fungi. These genes also appear to be involved in sexual development in all fungi examined to date. (B) Lineage-specific regulators are genes that are unique to a particular fungal clade either through loss in most other fungal groups or gain in a particular clade. BrlA and FL exemplify this group. (C) Genes that are conserved across fungi may retain a common function, but in some lineages the gene has been adapted to also regulate a conidiation pathway. Alternatively, the ancestral gene was involved in development and this role was lost in some lineages. FlbD is an example of a protein that has retained its biochemical function across a broad evolutionary distance but plays different roles in the different fungal lineages.

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FIGURE 10

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