Thallus organization and nutrition In Fungi

THALLUS ORGANIZATION Ø

 Except some unicellular forms (e.g. yeasts, Synchytrium). Ø The fungal body is a thallus called mycelium. Ø The mycelium is an interwoven mass of thread-like hyphae (Sing., hypha). Ø Hyphae may be septate (with cross wall) and aseptate (without cross wall). Ø Some fungi are dimorphic that found as both unicellular and mycelial forms e.g. Candida albicans.

DIFFERENT FORMS OF MYCELIUM

 (a) Plectenchyma (fungal tissue): In a fungal mycelium, hyphae organized loosely or compactly woven to form a tissue called plectenchyma. It is two types:

 i. Prosenchyma or Prosoplectenchyma: In these fungal tissue hyphae are loosely interwoven lying more or less parallel to each other.

ii. Pseudoparenchyma or paraplectenchyma: In these fungal tissue hyphae are compactly interwoven looking like a parenchyma in cross-section.

Sclerotia (Gr. Skleros=haid): These are hard dormant bodies consist of compact hyphae protected by external thickened hyphae. Each Sclerotium germinates into a mycelium, on return of favourable condition, e.g., Penicillium.

Rhizomorphs: They are root-like compactly interwoven hyphae with distinct growing tip. They help in absorption and perennation (to tide over the unfavourable periods), e.g., Armillaria mellea.

NUTRITION

 The fungi lack chlorophyll. Therefore, they cannot synthesize their own food. Depending on from where and how they get nutrition, fungi are of following types: (a) Saprotrophs (= saprobes): They obtain food from dead and decaying organic matter. They secrete digesting enzymes to outside which digest the substratum and then absorb nutrients, e.g., Mucor, Rhizopus (bread mould) etc. (b) Parasitic: Ø They obtain food from living. Ø They may be facultative or obligate. Ø Facultative parasites grow on a variety of tissues and often cause ‘soft rot’ of the tissue, e.g., Ustilago. (In facultative mutualism, each organism can survive independently, but it benefits both to remain together.)

Ø The obligate- parasites absorb through specialized haustoria. (In obligate mutualism, one organism cannot survive without the other. This term is easy to remember because both organisms are obligated, or forced to, rely on one another).

The parasitic fungi that grow on surface of host cells and absorb food through haustoria are called ectoparasites or ectophytic parasites (e.g., Mucor, Erisphae). Ø When parasitic fungi grow inside the host tissue arc called endoparasites or endophytic parasites (e.g., Pythium, Puccinia). (b) Predacious: Some soil fungi develop ring-like noses to trap annelids, nematodes etc. e.g., Arthrobotrys, Zoophagus, Dactylella etc. (d) Symbiotic: Ø They live in mutualistic relationship with another organism by which both are benefited. Ø The two common examples are lichens and mycorrhiza.

Lichens are symbiotic associations between fungi and algae. The fungal partner is a member of ascomycetes or basidiomycetes that provides water and nutrients, while the algal partner is a green alga or cyanobacteria that prepares food by photosynthesis. Ø Mycorrhizas or mycorrhizae (fungus roots in Greek) are the mutualistic symbiotic associations between soil fungi and the roots of most plant species (95% of all plant families are predominantly mycorrhizal). According to the carbohydrate theory (Bjorkman, 1949), the plants that grow in soils deficient in P and N, and high intensity light develop mycorrhizas. Ø The two most common types of mycorrhizas are the ectomycorrhizas (ECM) and the endomycorrhizas (also known as arbuscular mycorrhiza, AM or VAM).

The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate the cell wall of the plant’s root cells, Ø While the hyphae of arbuscular mycorrhizal fungi penetrate the cell.

HETEROTHALLISM AND HOMOTHALLISM Ø

F. Blakeslee (1904) discovered mating types or genetically distinct strains in Mucor.

 He called fungi with different mating types are called heterothallic and Ø Fungi without mating types are called homothallic.

 Nowadays we call some fungi and algae homothallic if both male and female gametes produce in the same individual can fertilize each other and

 Heterothallic if the gametes can only be fertilized by gametes from another individual of the same species.