FUNGI AND OTHER ORGANISMS (PLANT) MYCOLOGY (MIC 206)

FUNGI AND PLANTS Fungi can be: 1) Facultative symbionts – can live freely or within plants 2) Obligate symbionts – can only grow in association with plants. Hyphae of symbiotic fungi can penetrate plant cells, some grow through tissues without entering or penetrating plants cells. Two way of parasitic relationships of fungi with plants: 1)Biotrophic 2)Necrotrophic

Biotrophic vs Necrotrophic Some fungi have phases where they became biotrophic or necrotrophic. Fungi can penetrate plant tissues through: a)aerial surfaces (stems, leaves, flowers, fruits). b)root surfaces (root epidermis). c)wounds (normal process where internal tissues are exposed due to breakage of plant’s surfaces (e.g. when fruits and leaves are formed).

MOA of Plant Pathogen Examples of fungi penetrating plant tissues: a)Aerial surfaces Examples are Cladosporium (leaf), Rhizopus stolonifer (fruit), Colleototrichum (fruit and leaf), Armillaria mellea (wood/ bark). b)Root surfaces Fusarium spp. (root), Sclerotinia sclerotonum (root). c)Wounds Sclerotinia fructigena (wasps or birds break epidermis of fruit and introduce conidia of fungus). d)Other internal tissues Armillaria mellea (tree pathogen and able to enter living trees but can live as saprotrops when trees die).

BIOTROPHIC ASSOCIATIONS

Biotrophic Associations From the Greek: bios = life, trophy = feeding. Live within the plant and obtain nourishment without causing cell death to plants. Long-term feeding relationship with the living cells of their hosts, rather than killing the host cells as part of the infection process.

Characteristics of Biotrophic Associations Characteristics: 1)Do not kill hosts so either parasitic or saprophytic. 2) Tissues that are invaded remain alive. 3) Symbiotic because two organisms live together. 4)A relationship between Fungus + Root = Mycorrhiza.

Construction of Haustaria Biotrophic fungi frequently produce haustaria. Haustoria: specialized hyphae that penetrate the cells of other organisms that obtains food from a host. Fungi grow between the host cells and invade only a few of the cells to produce nutrient-absorbing structures termed haustoria.

MOA of Haustaria Fungal haustoria will be produced from spores that germinate on the surface of plants, on leaves or stems. The germinating spores may produce a penetration peg known as an appressorium and penetrate the plant’s cell wall. After penetration, the hyphal tip forms an invagination within the cell that becomes the haustorium. Haustorium is a structure of an extended extracellular mycelium, or filaments, of the fungus.

appressorium Magnaporthe oryzae – Rice Blasts Diseases penetration of leaf penetration peg

CLASSIFICATION OF BIOTROPHICS ASSOCIATION Two importants groups: The Rust Fungi (Basidiomycota) - classic biotroph. The Powdery Mildew Fungi (Ascomycota)

BIOTROPHIC ASSOCIATIONS - RUST FUNGI

The Rust Fungi The infection behaviour of rust fungi is broadly similar to that of the powdery mildews. Involving nutrient absorption by haustoria to support abundant sporulation for epidemic spread. Close-up of lower surface of the leaf, showing cup-shaped pustules of aeciospores. Small lesions on the upper surface of a barberry leaf, with spermogonia in their centres.

Penetration and Colonization by Rust Fungi

Growth of barley rust germ tubes on a barley leaf replica

Bean Rust – Uromyces appendiculatus

Black Rot of Grape - Phyllosticta ampelicida

Phyllosticta ampelicida – germinated conidia and melanized appressoria

BIOTROPHIC ASSOCIATIONS – POWDERY MILDEW FUNGI

The Powdery Mildew Fungi (Ascomycota) Powdery mildews produce mycelium (fungal threads) that grow only on the surface of the plant. They never invade the tissues themselves. The fungi feed by sending haustoria, into the epidermal (top) cells of the plant. Typical forms: many powdery mildews, where the fungus forms a powdery coating of white spores on the leaf surface.

Powdery Mildew Fungi Other common examples in Britain are powdery mildew of hawthorn (Podosphaera oxyacanthae), gooseberry (Sphaerotheca morsuvae), and cereals and grasses (Erysiphe graminis). Powdery mildew of roses, caused by the fungus Sphaerotheca pannosa. Localised lesions of Erysiphe graminis on wheat leaves.

NECROTROPHIC ASSOCIATIONS

Necrotrophic Associations Term necro – relating to death. Necrotrophic: Kill cells and absorb nutrients from dead cells. How? → Frequently secrete enzymes that degrade plant components or toxins that kill the plant’s tissue. Subsequently live on nutrients from the tissue they have killed.

Characteristics of Necrotrophic Associations Characteristics: 1)Kills host cells. 2)Can be recognised by patches of dead, blackened tissue. 3)Produce toxins which kill plant cells and their enzymes to degrade plant constituents so that fungus can use them as food. 4)Can change to saprophytic or biotrophic in later stages. 5)Show low specificity to host plants. 6)They are easy to culture in the absence of their hosts. 7)They can survive in the absence of their hosts by production of spores or sclerotia in the dead tissue.

Necrotrophic Associations (con’t) Examples: 1)Botrytis cinerea cause grey moulds in young plants or grapes when conditions are cold and damp. Grey mould rot causes damage on stored carrot, parsnip, mangel, beet, endive, chicory, turnip, and many others.

Necrotrophic Associations (con’t) Examples: 2)Phytophthora spp. Cause black pod disease in cocoa plant and brown rot disease on orange fruit.