Fungi and the Origins of Land Plants

What is the definition of Fungi?

Fungi constitute a diverse kingdom, with 'mycetes' meaning fungus, including an estimated 2.2 to 3.8 million species, ranging from microscopic yeasts to large mushrooms, playing crucial roles as decomposers, mutualists, and pathogens.

What are Cryptomycetes and Microsporidians?

Cryptomycetes are basal fungal groups, often aquatic endosymbionts. Microsporidians are obligate intracellular parasites that can cause serious human health issues, particularly in immunocompromised individuals.

What is the ecological impact of Chytrids?

Chytrids are aquatic fungi, many parasitic, with species like Batrachochytrium dendrobatidis (Bd) responsible for amphibian extinctions worldwide.

What is the primary composition of fungal cell walls?

Fungal cell walls are uniquely made of chitin, a tough, nitrogenous polysaccharide that provides structural support and protection against osmotic lysis.

How do fungal cell walls differ from plant cell walls?

Fungal cell walls contain chitin, whereas plant cell walls are primarily composed of cellulose.

Why are fungi considered phylogenetically closer to animals than plants?

Molecular evidence, including genetic sequencing and ultrastructural similarities, places fungi closer to animals, forming a clade called Opisthokonta, partly due to the presence of chitin in both fungal cell walls and animal exoskeletons.

What are unicellular fungi called?

Yeasts are prominent unicellular fungi. While most fungi are multicellular, they lack the complex organ and tissue organization of true plants.

Hypha/Hyphae

Branched, thread-like filamentous strands that collectively make up the main body of a fungus, responsible for nutrient absorption. They can be septate (divided by walls) or coenocytic (lacking septa).

Mycelium/Mycelia

The entire mass or extensive network of interconnected hyphae that constitutes the vegetative part of a fungus, typically hidden within a substrate. Its large surface area-to-volume ratio is highly efficient for nutrient absorption.

What is a fungal fruiting structure?

Structures like a toadstool or mushroom are fruiting bodies (also called sporocarps or basidiocarps/ascocarps). These are specialized, transient reproductive structures primarily associated with sexual reproduction, designed for spore dispersal.

Mycorrhizae

A crucial, close, and mutually beneficial symbiotic relationship between fungi ('myco') and the roots ('rhizal') of approximately 80\% of all vascular plants, critical for their colonization of land.

How do mycorrhizal fungi benefit the plant?

The fungal hyphae extensively increase the plant's root surface area (by 100 to 1000 times), providing enhanced access to water and essential mineral nutrients (e.g., phosphates, nitrates, zinc, copper) from a larger soil volume.

How do plants benefit mycorrhizal fungi?

The plant provides the fungus with readily available organic carbon compounds (sugars) produced through photosynthesis, as fungi are heterotrophic.

Arbuscule

Specialized, highly branched structures formed by fungal hyphae that extend into plant root cells, characteristic of arbuscular mycorrhizal fungi (a type of endomycorrhizae), facilitating nutrient exchange between fungus and plant.

Describe the predatory mechanism of Arthrobotrys.

Arthrobotrys is a predatory fungus that forms constricting loop-like structures or sticky networks of hyphae. When a nematode passes through a loop, it rapidly constricts, trapping the worm. The fungus then produces haustoria to grow into the nematode and absorb nutrients.

Haustorium/Haustoria

Specialized penetration structures produced by parasitic fungi that grow into a host's body to extract nutrients. While similar in appearance to arbuscules, haustoria serve a parasitic function.

What are the three main nuclear stages in the generalized fungal life cycle (Ascomycota and Basidiomycota)?

Haploid (n), diploid (2n), and an intermediate dikaryotic (n+n) stage.

Spore Germination (fungal life cycle)

Haploid spores (from meiosis or mitosis) germinate under suitable conditions to produce new haploid hyphae or a haploid mycelium, often two compatible mating types ('+' and '-').

Plasmogamy

The fusion of cytoplasm from two compatible haploid mycelia, the initial step in fungal sexual reproduction, analogous to the fusion of egg and sperm cytoplasm in animals.

Dikaryotic (n+n)

A unique stage in fungal sexual life cycles (characteristic of Ascomycetes and Basidiomycetes) where fused cells contain two distinct haploid nuclei, one from each parent mycelium, that coexist without fusing. This stage can be short-lived or dominate the life cycle.

Karyogamy

The fusion of the two haploid nuclei within a dikaryotic cell, occurring after plasmogamy, resulting in the formation of a transient diploid (2n) nucleus.

Meiosis (fungal life cycle)

The newly formed diploid nucleus immediately undergoes meiosis (reduction division), producing four genetically distinct haploid nuclei, which are then incorporated into spores.

What is the largest phylum of fungi?

Ascomycetes (Sac Fungi).

Ascomycota

The largest phylum of fungi, known as "sac fungi" due to the ascus, a microscopic, sac-like, sexual spore-bearing structure. Includes yeasts, molds, cup fungi, morels, and truffles.

What is an ascus?

A microscopic, sac-like, sexual spore-bearing structure characteristic of Ascomycetes. Within each ascus, usually eight ascospores are formed after meiosis and subsequent mitosis.

What are ascocarps?

Visible, macroscopic fruiting structures (e.g., cup-shaped in cup fungi) formed by the dikaryotic phase of many Ascomycetes.

Give examples of edible Ascomycetes and mention a caution.

Morels (Morchella esculenta) and Truffles (Tuber melanosporum). Caution: 'False morels' (Gyromitra esculenta) are poisonous.

Basidiomycota

The phylum of fungi known as "club fungi," including many familiar mushrooms, toadstools, puffballs, and shelf fungi. Their name is derived from the basidium.

Basidium

An elongated, microscopic, club-shaped cell in Basidiomycetes where karyogamy and meiosis occur, typically producing four external basidiospores at its tip.

Basidiospore

Haploid spores produced externally from a basidium during sexual reproduction in Basidiomycetes, typically four per basidium.

Basidiocarp

The visible fruiting body of a Basidiomycete, such as a mushroom, entirely composed of dikaryotic hyphae, designed for spore dispersal.

What are some examples of Basidiomycetes?

Mushrooms, toadstools, puffballs, shelf or bracket fungi, rusts, smuts, and stinkhorns.

Describe 'Fairy Rings'.

A natural phenomenon where basidiocarps (mushrooms) grow in a distinctive circular or arc-like pattern in open grassy areas. This occurs as a single dikaryotic mycelium grows outwards radially within the soil, with new fruiting bodies emerging at the expanding outer margin.

Lichen

A remarkable mutualistic symbiotic association involving a fungus (mycobiont, typically an Ascomycete) and a photosynthetic partner (photobiont, either a green alga or a cyanobacterium, or both).

What are the common growth forms of lichens?

Fruticose (shrub-like), foliose (leaf-like), and crustose (crust-like).

Why are lichens considered bioindicators?

Lichens are renowned for their extreme sensitivity to airborne pollutants, particularly sulfur dioxide (SO2), making their presence or absence and species composition useful for assessing air quality.

How do lichens reproduce asexually?

Asexual reproduction is often achieved via specialized propagules like soredia (powdery clusters of fungal hyphae and algal cells) or isidia (small outgrowths of the thallus), which contain both fungal and photosynthetic material.

What are Charophytes?

A specific lineage of freshwater green algae (e.g., Chara, Zygnema) considered the closest living relatives to land plants (embryophytes), supported by molecular and morphological evidence.

Compare Chara and Zygnema.

Chara is a macroscopic charophyte with a complex body plan resembling a higher plant, living submerged. Zygnema is a simpler, unbranched filamentous charophyte, identified by molecular evidence as the single closest living relative to land plants.

Archaeplastida

A supergroup of eukaryotes that includes all organisms that acquired plastids (chloroplasts) through a single primary endosymbiosis event, encompassing red algae, green algae (chlorophytes and charophytes), and land plants.

Why is 'Green Algae' considered a paraphyletic group?

It is a traditional grouping that includes Chlorophytes and Charophytes, but it excludes the land plants (embryophytes), even though land plants evolved directly from within the charophyte lineage.

Streptophytes

A true monophyletic clade comprising the Charophytes and the Embryophytes (land plants), uniting the closest algal relatives with the land plants.

Viridiplantae ('Green Plants')

A larger, comprehensively monophyletic group encompassing all green algae (both Chlorophytes and Charophytes) and all land plants, representing all descendants of the common ancestor that first acquired green plastids.

Phragmoplast

A complex system of microtubules and endoplasmic reticulum fragments that forms in the equatorial plane of the dividing cell during late anaphase and telophase of mitosis in charophytes and land plants, critical for cell plate formation.

Multicellular Gametangia

Specialized, complex, multicellular structures (e.g., antheridium for sperm, archegonium for eggs) found in some charophytes and all land plants, responsible for the production and protection of gametes.

Apical Meristem

Regions of perpetually embryonic, undifferentiated 'stem cells' located at the tips of shoots and roots (in land plants) or growing points of charophytes, allowing for indeterminate growth and tissue differentiation.

Dependent Embryo

A defining feature of embryophytes (land plants), referring to a multicellular diploid sporophyte that is uniquely retained, protected, and nourished within the tissues of the female gametophyte.

Cuticle

A waxy, non-living, extracellular layer composed of cutin and waxes that covers the exposed epidermal surfaces of stems and leaves in land plants, forming a hydrophobic barrier to minimize water loss.

Alternation of Generation

A life cycle pattern in land plants (and some algae) involving two distinct, multicellular stages: a haploid gametophyte (which produces gametes by mitosis) and a diploid sporophyte (which produces spores by meiosis).

Mating Type (+) or (-)

Designations for genetically distinct haploid mycelia that are compatible for sexual reproduction.