Lecture Glomeromycota. - Mycorrhizal Associations. Glomeromycota (Vesicular) Arbuscular endomycorrhizal fungi, or (V)AM fungi

Transcription

1 Lecture 18 - Glomeromycota - Mycorrhizal Associations Glomeromycota (Vesicular) Arbuscular endomycorrhizal fungi, or (V)AM fungi Mycorrhizal root system washed carefully from coarse sand to reveal the intact network with external hyphae (arrow) with spores (S) produced by Glomus mosseae. (bar = 100 um)

2 Glomeromycota (Vesicular) Arbuscular endomycorrhizal fungi, or (V)AM fungi - small group of about 130 described morphological species; --- associated with ca. 300,000 plant species (ca % of of land plants); - coenocytic hyphae ==> previously included in the Zygomycota, Order Glomales - multinucleate asexual spores can contain up to 5,000 nuclei. - sexual stage never observed in any member of the Glomales --- clonality? but extreme genetic variation of ribosomal DNA sequences is found within a single spore; still no data for other genes; these organisms are difficult to work with at the molecular level. - molecular data do not indicate monophyly between Zygomycota and Glomales ---- Glomeromycota are sister group to Asco-Basidio ==> the phylum Glomeromycota has been suggested by Schüssler et al., somatic hyphae are similar in all taxa. - little variation in the shape of arbuscules and vesicles (the latter not always present), - spores provide the most useful characteristics for differentiation of species. More info from: Glomeromycota (AM fungi) Arbuscules Structures in Soil - Hyphae - A network of hyphae forms in the soil with thicker hyphae which function as conduits and thin branched hyphae which are thought to absorb nutrients. - Spores - Large (for a fungus) asexual spherical structures ( um diameter) that form on hyphae in soil or roots. Structures in Roots - Hyphae - these are non-septate when young and ramify within the cortex. - Vesicles - storage structures formed by many fungi. - Arbuscules - intricately branched haustoria in cortex cells..

3 Glomeromycota (AM fungi) Glomeromycota (AM fungi) Soil hyphae produce appressoria between epidermal root cells (arrows) to penetrate plant tissue. (Bar = 100 um) Hyphae at an entry point (E) penetrating cortex cells (arrows). (Bar = 100 um)

4 Glomeromycota (AM fungi) Colony of a VAM fungus spreading from the entry point (E) by convoluted hyphae in the inner cortex of an Erythronium americanum root. (Bar = 100 um) Mature arbuscule of Glomus mosseae with numerous fine branch hyphae (Bar = 10 um). Glomeromycota (AM fungi) Vesicles (V) produced by a Glomus species in a leek root. This root also contains many intercellular hyphae. (Bar = 100 um) Lobed vesicles of an Acaulospora species in a clover root. (Bar = 100 um).

5 Glomeromycota (AM fungi) Lab techniques Spores can be separated from soil then sorted into morphospecies (size, colour, etc). The image on the right shows how spores on a piece of filter paper can be used to start a "pot culture" using pasteurised soil in which a host plant will be grown. ---> fungal/plant specificity? ---> effect on plant growth? ---> effect on disease resistance ---> etc. Glomeromycota (AM fungi) Research in John Klironomos lab at U. Guelph and others has shown that: - the effects of different species of arbuscular mycorrhizal fungi on plant growth varies with the plant species considered, ranging from positive to neutral and sometimes negative growth effects. - arbuscular mycorrhizal fungi can influence the reproductive success of individual plant species, for example through indirect positive effects on pollinator visitation rates. - AM fungi can have strong impacts on the composition and function of plant communities. Applications - ecosystem restoration - control of invasive species?

6 Glomeromycota (AM fungi) - three families and six genera;. Gigasporaceae : Gigaspora and Scutellospora - form only arbuscules (no vesicles) in roots. Acaulosporaceae: Acaulospora and Entrophospora - produce both vesicles and arbuscules in roots - produce "chlamydospores" in the soil - spores embedded in a swollen, sac-like structure -- Aculospora the spore forms laterally on the hyphae -- Entrophospora spore develops within the neck of the hyphae Glomaceae: Glomus and Sclerocystis - produce both vesicles and arbuscules in roots - chlamydospores are borne apically. Spore of Acaulospora sp. Pics from Spore of Glomus clarum Fossils from a Wisconsin roadcut show clearly that fungi and green plants moved from water onto land at about the same time, bolstering the theory that fungi helped plants successfully invade the land. This particular fossil shows no evidence of association with plants, however other Devonian fossils (400 million years ago) have been shown to contain structures indicative of arbuscular mycorrhizae. Glomus geosporum spore Light microscope picture of a fossil fungal spore with attached hypha from the Ordovician, 460 million years ago. (c)science Magazine CREDIT: Dirk Redecker/ UC Berkeley

7 Mycorrhizal symbiosis Fungal benefits - carbohydrates (= photosynthetic products) ==> converted to trehalose, mannitol, glycogen which are necessary cofactor for spore germination. Plant benefits - hyphae extend into the soil away from roots ---> increase the potential for water absorption ---> increase the potential for phosphorus uptake - currently debated to what extent, if any, fungi increase nitrogen uptake by host plant - endomycorrhizal associations may contribute to the resistance to certain root pathogens including various fungi and nematodes by production of antibiotic substances Two major types : AM vs. ECTO - AM penetrates cortex cells ; ECTO do not - ca.130 AM species for 300,000 plant species - > 5000 ECTO species for 2000 plant species - ECTO: mostly trees - AM: woody & herbaceous plants Mycorrhizal symbiosis From Kendrick

8 Mycorrhizal symbiosis The Hartig net and mantle are typical of ectomycorrhiza From Kendrick Mycorrhizal symbiosis : Ectomycorrhiza Thelephoroid ectomycorrhiza On Douglar fir Lactarius deliciosus on Pinus pinaster root tips double colonization by Tomentella sublilicina and Thelephoroid#2 on Bishop pine Pics from Tuber puberulum on Picea abies root tips unknown boletoid fungus on bishop pine

9 Mycorrhizal symbiosis : Ectomycorrhiza (A) penetration of hyphae between the cells of the root cortex to form a characteristic Hartig net (B) establishment of a mantle of hyphae around the outside of the root (the brown layer in the picture below); (C) extension of hyphae from the mantle into the surrounding soil From Kendrick Mycorrhizal symbiosis : Ectomycorrhiza Taxonomic Distribution of Ectomycorrhizal (EM) Fungi About 90 genera and 5,000 species Major taxa Basidiomycotina euagarics Amanita Hygrophorus Tricholoma Cortinarius Inocybe bolete clade about 15 traditional bolete genera, including Boletus, Suillus, Leccinum etc. Scleroderma Russuloid clade Russula, Lactarius Thelephoroid clade Thelephora, Tomentella Cantharelloid clade Cantharellus... Ascomycotina Pezizales Pezizaceae (1 genus) Balsamiaceae (3) Otideaceae (1) Helvellaceae (1) Pyronemataceae (3) Terfeziaceae (4) Tuberaceae (2) Elaphomyces Adapted from Kendrick

10 Mycorrhizal associations: Non AM or ECTO types Other morpho-types than AM and EM exist, but are less widespread. For instance: - orchid mycorrhizas; - ericaceous mycorrhizas (mostly Ericales species) --- ericoid mycorrhizes; --- arbutoid mycorrhizas --- monotropoid mycorrhizas Mycorrhizal associations Orchid mycorrhizas - Orchid seeds need a fungus to germinate --- plant depends on the fungus until chlorophyll-bearing leaf develop (can take 1-2 years) - commonly, these fungi are basidiomycetous anamorphs of the genus Rhizoctonia --- holomorphs mostly in genera Thanatephorus, Corticium, and Ceratobasidium --- Sebacina and Tulasnella species also commonly found. Note: We can now use molecular identification. From Kendrick The fungus enters cells of the root cortex ands develops coils or pelotons which eventually swell, degenerate and are absorbed by the plant cell.

11 Mycorrhizal associations Orchid mycorrhizas Different possible outcomes of seedling colonization by a fungus: (1) the seed becomes colonized by an appropriate fungus and thrives; (2) the fungal infection takes over, and kills the seedling; (3) the fungal invasion fails, the fungus is eliminated, and the seedling stops growing. ==> it does not appears to be a true mutualistic symbiosis Further specialization / dependence: achlorophyllous orchids, e.g., in genus Corallorizha -- other (ecto-!) fungi, e.g. Russula, Sebacina, etc --gain for the fungus? Is the plant cheating? Arbutoid / Ericoid mycorrhizas Mycorrhizal associations - intermediate between ecto- and endo-mycorrhizas: ---- sheathing fungal mantle and typical ecto- branching; ---- the fungus penetrates the cortical cells and fills them with densely coiled hyphae (not arbuscules as in VAM) - Arbutoid type: Hartignet present - Ericoid type: does not produce a typical Hartig net - characteristics ofericales (rhododendrons, blueberrys, etc.) --- but can also (more rarely) be formed in other plant families, e.g., Fagaceae. - predominant in low ph soils and high altitudes and latitudes (Heath soils, tundra) - tend to be present in pioneer species. - apparently transfer nitrogen, but not phosphorus, to the host. Some fungi can form normal ectomycorrhizas with some trees and ericoid mycorrhizas with members of the Ericaceae ===> plant signal Some fungi are restricted to Ericaceae and only form ericoid mycorrhizas. sheathing fungal mantle Ericoid type coiled hyphae (intracellular) See for more detailed info From Kendrick Ericoid type

12 Mycorrhizal associations Monotropoid mycorrhizas - known only with the achlorophyllous plant Monotropa ( Indian pipe ) ===> the plant (achlorophyllous) cannot provide carbohydrates to a fungal mycorrhizal partner so what? cheating! - the fungus produces a Hartig net and a haustorium-like peg into root cell. From Kendrick - Fungi involved: Tricholoma, Russula, Boletus; all typical ecto- - tri-ways relationship --- it has been shown that the fungus also has a normal mycorrhizal relationship with a neighboring plant. haustoriumlike peg Hartig net Evolution of ericaceous mycorrhizas in the Ericales monotropoid mycorrhizas arbutoid mycorrhizas ericoid mycorrhizes; Unknown status? References: Bidartondo, M. I.& Bruns, T. D Fine-level mycorrhizal specificity in the Monotropoideae (Ericaceae): specificity for fungal species groups. Molecular Ecology 11(3): M. I. Bidartondo and T. D. Bruns Extreme specificity in epiparasitic Monotropoideae (Ericaceae): widespread phylogenetic and geographical structure. Molecular Ecology 10(9):