Fungi – Taxonomic status, Body, Nutrition, Growth of Fungi

Taxonomic status of fungi

The taxonomic status of fungi has actually altered from that of ‘a group of Plant kingdom’ to a different kingdom “Fungi”. They resemble plants in some aspects – have a cell wall, lack centrioles, and are non-motile. But fungi resemble more animals than plants.

Unlike plants and like animals, fungi are heterotrophs, lack cellulose in their cell wall and include chitin – the chemical present in the external skeleton of arthropods. For this reason, some mycologists (scientists who study fungi) think that fungi and animals most likely emerged from a common ancestor. However, fungi are different from animals in having a cell wall, being absorptive heterotrophs, and non-motile. So, fungi are neither plants nor animals.

Their DNA studies also confirm that they are different from all other organisms. They show a characteristic kind of mitosis, called ‘nuclear mitosis’. During nuclear mitosis, the nuclear envelope does not break; rather the mitotic spindle kinds within the nucleus and the nuclear membrane constricts between the two clusters of daughter chromosomes. (In some fungi nuclear envelope dismantles late). Because fungi stand out from plants, animals, and protists in lots of methods, they are appointed to a separate kingdom ‘Fungi’.

Body of Fungi

The body of fungi, called mycelium, includes long, slender, branched tubular thread-like filaments called the hyphae (singular hypha). Hyphae spread extensively over the surface area of the substratum. Chitin in their wall is more resistant to decay than are cellulose and lignin which make up the plant cell walls.

Hyphae might be septate or non-septate. Septate hyphae are divided by cross-walls called septa (particular septum) into individual cells consisting of one or more nuclei. Non-septate hyphae lack septa and are not divided into individual cells; instead, these are in the type of an extended multinucleated large cell. Such hyphae are called coenocytic hyphae, in which the cytoplasm moves effectively, distributing the products throughout.

Septa of lots of septate fungi have a pore through which cytoplasm flows from cell to cell, carrying the products to growing ideas and making it possible for the hyphae to proliferate when food and water are plentiful and the temperature level is favorable. All parts of fungus growing through the substrate are metabolically active. An extensive spreading system of hyphae supplies a huge surface area for absorption.

Hyphae may be packed together and organized to form complex reproductive structures such as mushrooms, puffballs, morels and so on which can expand rapidly. Yeast is non-hyphal unicellular fungi. All fungal nuclei are haploid except for short-term diploid zygote that forms during sexual reproduction.

Might it be the world’s largest organism???

A circular clone of Armillaria, a pathogenic fungus afflicting conifers, growing out from a central focus, has been measured up to 15 hectares (1hectare = 10000 m²).

Nutrition in Fungi

All fungi lack chlorophyll and are heterotrophs (getting carbon and energy from raw material). They get their food by direct absorption from the immediate environment and are thus absorptive heterotrophs. Most fungi are saprotrophs (or saprobes), decomposers that acquire their food (energy, carbon, and nitrogen) direct from dead raw material.

They produce out digestive enzymes that digest dead organic matter, and the organic molecules hence produced are absorbed back into the fungi. Saprobic fungi anchor to the substrate by modified hyphae, the rhizoids.

Fungi are the primary decomposers of cellulose and lignin, the primary parts of plant cell walls (most bacteria can not break them). An extensive system of fast-growing hyphae supplies massive surface for an absorptive mode of nutrition. Saprobic fungi, along with bacteria, are the significant decomposers of the biosphere, adding to the recycling of the elements (C, N, P, 0, H, etc) used by living things.

 

Some fungi are parasites, some are even predators, and still, others are mutualists. Parasitic fungi absorb nutrients straight from the living host cytoplasm with the help of unique hyphal tips called haustoria.

They may be obligate or facultative. Obligate parasites can grow only on their living host and can not be grown on available defined growth culture medium. Different mildews and most rust species are obligate parasites. Facultative parasites can grow parasitically on their host as well as by themselves on artificial growth media.

Some fungi are active predators. The oyster mushroom (Pleurotus ostreatus) is an omnivorous (predatory) fungus. It paralyzes the nematodes, penetrates them, and absorb their dietary contents, primarily to fulfill its nitrogen requirements. It fulfills its glucose requirements by breaking the wood. Some types of Arthrobotrys trap soil nematodes by forming a restricting ring, their hyphae invade and absorb the unlucky victim. Other predators have other adaptations, such as secretion of sticky compounds.

and desiccation and itself gets food through the courtesy of alga

Mutualistic Symbiotic Associations in Fungi

Fungi form two major mutualistic symbiotic associations (associations of benefit to both partners). These are lichens and mycorrhizae.

Lichens

Lichens are mutualistic symbiotic associations between particular fungi (mainly Ascomycetes and imperfect fungi, and few Basidiomycetes – about 20 out of 15000 species of lichens) and certain photoautotrophs-either green algae or a cyanobacterium, or some times both.

The majority of the visible part of a lichen consists of fungus, and algal components are present within the hyphae. Fungus protects the algal partner from strong light and desiccation and itself gets food through the courtesy of alga.

Lichens can grow at such locations where neither of the components alone can, even in harsh areas such as bare rocks and so on. Lichens differ in color, shape, total look, growth form. They are environmentally extremely important as bioindicators of air pollution.

 

Mycorrhizae

Mycorrhizae are mutualistic association between particular fungi and roots of vascular plants (about 95% of all kinds of vascular plants). The fungal hyphae significantly increase the amount of soil contact and overall area for absorption and aid in the direct absorption of phosphorus, zinc, copper, and other nutrients from the soil into the roots. Such plants reveal better development than those without this association. The plant, on the other hand, supplies natural carbon to fungal hyphae.

There are two primary types of mycorrhizae: endomycorrhizae, in which the fungal hyphae permeate the external cells of the plant root, forming coils, swellings, and minute branches, and also extend out into surrounding soil; and ectomycorrhizae, in which the hyphae surround and extend between the cells however do not permeate the cell walls of the roots. These are primarily formed with pines, firs, etc. However, the mycelium extends far out into the soil in both sorts of mycorrhizae.

Growth of Fungi

Fungus grows best in damp environments, however, are discovered anywhere raw material is present. They endure dry conditions in some resting stage or by producing resistant spores. They can also endure a large range of pH from 2 – 9, a broad temperature range, and high osmotic pressure such as in concentrated salt/sugar solutions as in jelly, jam, etc. These features also help them in their survival on land. Fungi store surplus food generally as lipid droplets or glycogen in the mycelium.

Multiple-Choice Questions (MCQs) with Answers

 

FAQs (Frequently Asked Questions) – Fungi Tutorial