Mycorrhiza

Mycorrhiza (Gk. mykes-fungus, rhiza - root; Frank, 1985) is a symbiotic association between fungus and the root of a plant.

Ectomycorrhiza and Endomycorrhiza : Depending upon residence of the fungus in the root, mycorrhiza is of two types, ectomycorrhia and endomycorrhiza. In ectomycorrhiza the fungus forms a mantle of hyphae over the surface of root as well as penetrates the root cortex but remains in its intercellular spaces, producing Hartig’s net. The cortical cells of the root secrete nutrients into the intercellular spaces for absorption of the fungal hyphae. Example of ectomycorrhizal fungi include Boletus, Amanita, Rhizopogon, Scleroderma, etc. In endomycorrhiza the  external mantle is absent. Root hairs are present along with hyphae.  Fungal hyphae pass into cortex where they are both intercellular and intracellular.

Inside the cortical cells the fungus either swollen vesicles or finely branched masses called arbuscules. Because of them, endomycorrhiza is also called VAM or vesicular arbuscular mycorrhiza. Example of some endomycorrhizal fungi Rhizoctonia (a basidiomycete), Endogone, Glomus and Sclerocystis.

Advantages of Mycorrhizal Association :

1. Absorption of Minerals : Fungal hyphae absorb minerals from a large area of soil and hand over the same to the root.

2. Growth Promotion : Fungus produces growth promoting hormones. In the absence of fungus, Pinus and Betula show stunted growth despite providing all types of nutrients.

3. Antimicrobial Substances : Fungus protects the root from parasitic fungi and harmful bacteria by secreting antimicrobial substances.

Points to Remember

—Smallest fungus – Yeast (3-15 mm × 2-10 mm)

—Longest fungus – Ganoderma applanatus (fruiting body 60 cm across)

—Largest fungus – Giant puffball (Clavatia, 90-120 cm across)

—Most complex fungi – Mushroom

—Chytridiomycetes are called chytrids.

—Oomycetes are called egg fungi.

—Fungi are plants because of presence of cell wall, immobile nature, spread out appearance and apical growth.

—Most of the fungi grow well in acidic pH (5-6.5). Light is generally not essential for the growth of fungi.

—Basidiomycetes are most commonly seen fungi.

—Aspergillus flavus is called Guinea Pig of Plant Kingdom.

—Neurospora crassa is called Drosophila of plant kingdom.

—Some fungi are dimorphic, existing both as single celled and multicellular hyphal state, e.g., Candida albicans.

—Pilobolus is commonly called squirting fungus or fungus gun because its mature sporangia are shot to a distance of more than six feet in the air.

—Ascomycetes are our worst fungus enemies.

—Hormonal control of sexual mechanism in fungi was demostrated by Burgeff (1924). In Rhizopus, it is under the control of trisporic acid.

—Conjugation between parent and bud is called pedogamy, e.g., Zygosaccharomyces chevallieri.

—Saprophytic fungi have been called vegetative vultures by Rolfe and Rolfe (1926) as they function as natural scavengers.

—Emperor Claudius Caesar was murdered by his wife by giving extract of toad stool fungus Amanita phalloides which stops m-RNA synthesis.

—Dactylella and Arthrobotrys (Deuteromycetes) are predaceous fungi.

—Yeast belongs to both ascomycetes and basidiomycetes. True yeasts are ascomycetous and produce ascospores. False yeasts don’t produce ascospores, e.g., Candida, Cryptococcus, Mycoderma.

—Armillaria is the largest organism, which can spread in an area of 1480 acres.

—Millardet discovered fungicide Bordeaux mixture (mixture of quick lime and CuSO4).

Micro-organisms like bacteria sometimes can exist without cell wall. The cell membrane and its intact contents are then called as protoplast. (Osmotically fragile). Young actively growing gram(+) bacteria are sensitive to penicillin. So these bacteria can be made protoplasts.

Mycoplasma are also called Jokers of “Plant kingdom” because of showing pleomorphism.

The archaebacteria and eubacteria possibly arose from a more ancient form of life called progenote.

The non-symbiotic free living nitrogen fixing bacterium is Clostridium pasteurianum.

Chromatium, Rhodospirillum, are photosynthetic nitrogen synthesising bacteria.

Nostoc. sp occurs within the thalli of Blasia and Anthoceros (bryophytes), in the cells of Geosiphon pyreforme (a fungus), in the petioles of Gunnera (an angiosperm)

Trifolium alexandrium (clover) contains Nostoc in its roots.

Reddish colour of Red sea is due to a cyanobacterium Trichodesmium erythraeum.

Death factors VFDF (very fast), FDF (fast), SDF (slow) are toxins produced by cyanobacteria.

Biological nitrogen fixation has been discovered by Winogradsky.

Some cyanobacteria may occur at very unusual places eg. Phormidium-hot water springs. Some have been reported from arctic and antarctic regions.

The cells of certain bacteria like Aquaspirillum magnetotacticum contains structures composed of iron in the form of magnetite (Fe3O4) which are called magnetosomes. They help in orientation among themselves along geomagnetic lines.

Mycobacterium and Xanthomonas form nodules in leaves of Ardisia and Pavettla, while Frankia forms nodules in roots of Alnus and Casuarina.

When gram positive bacteria are treated with lysozyme (found in egg white, secretion of skin and mucous membranes and tears). They are rapidly denuded of their cell walls and become naked protoplasts. While peptidoglycan of cell wall of Gram negative bacteria is protected by outer layer of lipo complex (it can be removed by ethylene diamine tetra acetate or EDTA). So the cell wall of gram (–ve) bacteria is completely removed. Such only partially denuded cells are called ‘sphaeroplasts’.

Term protista was given by Ernst Haeckel.

Laveran (1880) discovered Plasmodium in erythrocytes.

Ronald Ross (1897) found oocysts in the stomach of mosquito and transfer of parasite to second host.

Flagellated cells are absent in red algae, higher seed plants and higher fungi.

Proterosporangia is a colonial protozoan protist having choanocyte like flagellated and collared cells. It is a connecting link between protista and porifera.

Auxospores are rejuvescent spores formed in diatoms to correct the size which decreases with each binary fission.

Some dinoflagellates such as Noctiluca are phosphorescent (bioluminescent). They make the sea surface glow in dark.

E.coli is found as an endocommensal in the colon of about 50% of population.