Introduction to botany
Fungi structure and reproduction
The fungi are a group of eukaryotic,
non-vascular organism. Which are of diverse forms, sizes, physiology and reproduces both by sexual (meiotic)
and asexual (mitotic) spores. Examples of fungi :-Mushrooms, yeasts, molds, Penicillium- the first of the wonder
drugs, penicillin, was isolated from this fungus and crop parasites.Fungi are most often associated with the roots of
plant species, and this type of symbiotic associations is known as mycorrhizae.
The study of fungi is known as mycology (Mykes = mushroom + logos = discourse) or mycetology and who study about
fungus is known as mycologist.
General characteristics of Fungi
- Most fungi are eukaryotic,multinucleate, multicellular organisms,
except yeasts which are unicellular.
- Nutrition Fungi are heterotrophic because these lack chlorophyll (green pigment) and thus cannot create their own
food through photosynthesis.Fungi acquire their nutrients from dead organic matter by secretion of
extracellular enzymes followed by absorption. These are either :-
- Saprophytes or Saprobes :-
Most fungi are saprophytes ,obtaining nutrients by absorbing dead organic matter.
- Parasitic :-Some fungi are
parasites, living in or on another organism (called a host) from which they obtain their nutrients. This relationship usually
harms the host. Such parasitic fungi usually have specialized tissues called haustoria that penetrate the host's body for food
- Symbionts :- Some fungi live in a mutually beneficial symbiotic relationship with another organism.
- Lichen (association of fungi + algae)Some fungi are associated with either cynobacteria or green algae and this type of
symbiotic associations is known as lichen.
- Mycorrhizae (fungi + plants) :- Some fungi are most often associated with the roots of
plant species, and this type of symbiotic associations is known as mycorrhizae.Mutualistic association of plant roots and fungi
increase the absorptive surface area of plant roots.
(Fungus gets organic nutrients from plant;
Plant gets minerals from the soil via the fungus).
- Thallus organization (structure):-
- Most of the fungi, are filamentous and the vegetative
(=assimilative) stage is a tubular
branching thread-like filament called a hypha (plural = hyphae).
- The hypha extends by tip growth, and multiplies by
branching, creating a fine network called a mycelium.
- Two of the phyla Zygomycotina and the Chytridiomycotina (also known as lower fungi) vegetative mycelium is without cross
-walls known as non-septate (aseptate) or coenocytes. Complete septa are only found in reproductive structures. In aseptate
mycelium numerous nuclei lie in a common mass of protoplasm.This condition is called coenocytes.
- The other two phyla Ascomycotina and the Basidiomycotina( higher fungi)the hyphae have cross walls that break them up
into cells. A cross wall in fungi is called a septum ( plural septa).The septum has a central pore which enable exchange of
nuclei and most organelles within the cytoplasm between adjacent compartments.
- Septum formation is a simple process.
Wall ingrowth towards the centre of the compartment results in a complete or incomplete blockage of the hypha. Inward growth
may be followed by modification of the outer wall. The septum also increases rigidity of the hypha as it can function as a
structural support to the turgor pressure within the compartment.
- Ultrastructure of fungal thallus:-
- Rigid cell walls are strengthened with chitin, a polymer of
(except oomycetes where cellulose present).
- Food reserves are in the form of glycogen
- Hyphae contain nuclei, mitochondria, ribosomes, golgi and membrane-bound vesicles within a plasma-membrane bound cytoplasm.
The sub-cellular structures are supported and organized by micro-tubules and endoplasmic reticulum. The cytoplasm and most
organelles and inclusions of fungal cytoplasm are typical of eukaryotic organisms. However, chloroplasts or plastids are
- Reproduction :-
- At the time of reproduction when entire cell asin case of unicellular fungus may be converted into
Such a condition is known as holocarpic. But if only a portion of thallus is converted into reproductive structure , it is
called eucarpic.Some fungi are unicellular and are termed as yeasts. These grow by binary fission or budding, creating
new individuals from the parent cell.
- Members of the Ascomycotina produce asexual conidiospores and sexual ascospores
in sac-shaped cells called asci.
- Fungi from the Basidiomycotina rarely produce asexual spores, and produce their
sexual spores from club-shaped basidia in complex fruit bodies
Classification of Fungi
Over 60,000 species of fungi are known. Fungi are classified primarily by their method of reproduction (both sexual and
asexual) and fruiting bodies (asexual or sexual spores surrounded by highly organised protective structures).
In the earliest classification, there were only two recognized kingdoms:- Plants and Animals.This two kingdom system was used
until Whitaker (1969) proposed that organisms be classified into five kingdoms :- Monera (=Bacteria), Protista (=Mostly Algae
and Protozoans), Plantae (=Plants), Mycetae (=Fungi) and Animalia (=Animals).
Fungi belong to Domain-Eukarya and Kingdom -Mycetae (=Fungi) The classification of fungi, as proposed by Ainsworth (1973), is
commonly followed :-
The more recent systems of classification that is based, in part, on molecular research are :-
Division :- Myxomycota (plasmodial slime molds)
The organisms included in this division are commonly known as the plasmodial or acellular slime molds. The exact systemic
position of the Myxomycota is not clear. Olive (1975) classified them in the Kingdom
Protista. Ainsworth (1973) classification is most accepted in which Division Myxomycota belong to Kingdom
Within the Myxomycota, the class Myxomycetes,
the acellular slime molds.
- Vegetative phase is unicellular, without a cell wall, a multinucleate mass of protoplasm called a
- They engulf bacteria, protozoa, and other food particles by phagocytosis.During this mode of ingestion,
particles, usually bacteria, beceome surrounded by the pseudopodia of the myxamoeba. Once the food has been engulfed in this
matter, it is surrounded by a membrane or food vacuole where hydrolytic enzymes are secreted that will digest the food.
vegetative stage in slime molds is morphologically similar to that of an amoeba, because of that known as myxamoeba.
Plasmodium produces one or more sporangia where meiosis takes place.
Class: Myxomycetes There are approximately 71 genera and 500 species of Myxomycetes.
Habitats (Occurrence) :-There are commonly occur in damp places especially on decaying wood, all of which are
terrestrial (found on land).
- Asexual reproduction occurs by binary fission of myxamoeba or fragmentation of the
- Sexual reproduction takes place by the plasmogamy (fusion of compatible gametes) occurs between myxamoebae or swarm cells
(some species are heterothallic).Karyogamy (fusion of nucleus) occurs shortly after plasmogamy to form zygote (2n). Zygote
forms plasmodium – longest lived vegetative stage.
Life Cycle of Myxomycetes:- Bi-phasic life cycle with a haploid and a diploid phase.
The diploid stage is a naked coenocytic protoplast (that is, it consists of a multinucleate mass of
cytoplasm that is enclosed only by a plasma membrane and does not have a cell wall).When the sporangia matured and turned
grey. Normally globose, with a definite, rather
cell wall,unicellular, uninucelate and haploid spores are released by rupture of the sporangial coat.The spore surface may
range from almost smooth to reticulate. Spores of myxomycetes are small (4-20 µm) and are easily picked up by air currents,
arthropods and other animals
On germination a spore produces one or four myxamoebae or flagellate cells known as swarm cells- that have one or two anterior
flagella, all of which are of the whiplash type. In most cases swarm cells possess one long prominent flagellum and a second
shorter, inconspicuous flagellum that is directed backward and appressed to the cell surface.
Myxamoebae and swarm cells can be interconverted. When food is abundant and environmental conditions are favorable, myxamoebae
divide repeatedly, giving rise to a large population of cells. The nuclear divisions are centric and open (the nuclear membrane
breaks down during prophase and is reconstituted after telophase.) Under unfavorable conditions myxamoebae round up and secrete
a galactosamine wall to form microcysts (sclerotia). When favorable conditions return, the microcysts germinate and either a
myxamoeba or swarm cell emerges from each wall.
When free water is available myxamoeba can differentiate into flagellated swarm cells
Swarm cells and myxamoeba may function as gametes (both in homothallic or heterothallic strains).The compatible gametes fuse in
pairs (two swarm cells, two myxamoebae) to form a diploid zygote.As the zygote grows, its nucleus undergoes successive
synchronous mitotic divisions without cytokinesis. And the cell becomes transformed into a multinucleate, amoeboid structure,
Plasmodium:-The plasmodium is a diploid structure.Plasmodium is a naked, multinucleate, motile mass of
protoplasm; no cell wall around it.
However, in most species, the plasmodium is enveloped by a gelatinous slime sheath that contains microfibrils. Just inside the
slime sheath is the plasma membrane that surrounds and confines the cytoplasm
Plasmodia are of various colors. Examples- Physarum polycephalum it is a bright yellow, slimy structure and Didymium iridis
the plasmodium is colorless
In nature plasmodia probably feed on bacteria, spores of fungi and plants, and possibly on protozoa and even on bits of
nonliving organic matter
Growth is accompanied by successive mitotic divisions of the nuclei embedded in the cytoplasm. In growing plasmodia of P.
polycephalum, nuclear division occurs almost simultaneously every 8-10 hours throughout the plasmodium and requires 20 to 40
minutes for completion. There are three types of plasmodia :-
protoplasmodium - microscopic throughout its existence; gives rise to only a single sporangium when it fruits.
aphanoplasmodium - resembles a protoplasmodium in its initial stages, but soon elongates, branches, and becomes a network of
very fine, transparent strands; Stemonitales.
phaneroplasmodium - characteristic of Physarales, also resembles a protoplasmodium at first; it grows larger and becomes more
massive. Its protoplasm is very granular, and the plasmodium is visible even at an early stage of development. The gelified and
fluid portions of the veins are easily distinguishable and the rhythmic, reversible streaming is very conspicuous
Sporulation and Sporophores :-
- Under favorable conditions, the plasmodium will migrate and feed for a period of time
before being converted to one or more sporophores.Sporophores posses brightly coloured sporangia.
- Entire plasmodium of
a myxomycete usually is converted into one or more sporophores so
that the somatic and reproductive
phases seldom coexist in the same individual.
- Sporangium consists of the following parts: -
- Peridium (persistent or evanescent):- The fragile, outer layer of the sporangium is the peridium (pl.=peridia), which may
be persistent or degenerate by the time the sporangium is ready to disperse its spores.
- Hypothallus - The hypothallus is a plasmodial remnant forming the base for one or more fruiting bodies. The hypothallus
connects the stalk or stipe to the substrate. It may be dull or brightly colored, thin and delicate or coarse.The hypothallus
may be composed of calcium carbonate
- Stalk :- Basal portion of sporangium, may or may not be present, may be hollow or filled with material
Stalks formed from secretions of plasmodium and are acellular (in contrast to cellular slime molds
- Columella and Pseudocolumella :-The columella appears as an extension of the stalk into the spore mass, although it may not
resemble the stalk. In a sessile fruiting body, the columella may be an area on the inside of the peridium where it contacts
the substrate or appears as a dome-shaped structure. A pseudocolumella (pseudo=false) is a columella that does not attach to
the stalk. The pseudocolumella is found only in the order Physarales, existing as a lime mass within the spore mass.
Capillitial elements may be attached to the columella or pseudocolumella.
- Capillitium and Pseudocapillitium :-The capillitium consists of threadlike elements inside the sporophores, intermingled
with the spores.Some elements of capillitium may be elastic, allowing for expansion when the peridium opens, while other types
are hygroscopic and capable of dispersing spores by a twisting motion. A pseudocapillitium is present in some aethalia and
pseudoaethalia producing species. Pseudocapillitial elements are highly variable in size and shape, and may appear as bristles,
threads or perforated plates.
General characteristics of Mastigomycotina :-
- One large group of the Mastigomycotina is aquatic. While
another group of the Mastigomycotina are primarily terrestrial, although the organisms still form motile zoospores when open
water is available
- The members of Mastigomycotina produce
flagellated zoospores in their life cycle.
- Most of them are filamentous and have coenocytic mycelium. However, unicellular form are present, and some genera show
the pseudosepta (false cross wall) formation.
- Rhizoids are present in some of unicellular forms.
- They show centric nuclear division. Their centrioles remain functional during nuclear division.
- Live either as saprophytes or parasites.Due to presence of haustoria in a majority of Mastigomycotina , the mode of
nutrition is typically absorptive.
- The sexual reproduction takes place by different methods , oospores formation are common in almost all Mastigomycotina
Three classes are
included in this sub-division, on the basis of zoospore and oospore and comprise 204
genera and 1160 species:-
- Chytridiomycetes produces posteriorly uniflagellate zoospores Chytridiomycetous fungi occur as saprobes on plants and
animal remains in water while other members occur as parasites on algae and aquatic animals.
- Hyphochytridiomycetes :-
Zoospores are anteriorly uniflagellate.The Hyphochytridiomycetes are those aquatic fungi whose thallus is holocarpic or
eucarpic, monocentric or polycentric and their vegetative system is rhizoidal or hypha-like with intercalary swellings.
- Oomycetes :- The Oomycetes contain 74
genera and 580 species, which are mostly aquatic, though some are terrestrial and live as parasites or saprophytes.Includes
classic “water molds” in the Order
Saprolegniales and the “downy mildews” in the
General characteristics of Class :- Oomycetes :-
- Vegetative body is filamentous and coenocytic except
the unicellular Lagenidiales.
- Holocarpic or Eucarpic
- Entire thallus converted into reproductive structures
- Reproductive organs arise from only a portion of the thallus and remainder continues as somatic
Majority of species are eucarpic.
- Cell wall contains cellulose and glucans.Chitin is absent.
- Asexual reproduction is by biflagellate heterokont (different) and anisokont (unequal) zoospores that are produced in
- Zoosporangia- Modified hyphae that are usually terminal and delimited by a septum
- Zoospores are diploid formed by mitosis
- Anteriorly directed flagellum is tinsel type and posteriorly directed is whiplash type .Depending on genera single
type-monomorphic or two types of zoospores are formed-dimorphic . Two types of zoospores are
formed in the life cycle are :-
- Primary zoospores :- First formed, pip-shaped, and the flagella are located anteriorly. Primary zoospore is
released from the zoosporangium, encyst and germinates to form the secondary zoospore.
- Secondary zoospores :- The
secondary zoospore which is reniform or bean-shaped and laterally flagellated.
- Zooporangium and zoospores are the
major dispersal agents for most species.
- Sexual reproduction:- Sexual reproduction is heterogamous (oogamous) by oogonia (female) and antheridia (male). Female
produced by an oogonium.
Depending on taxon, there may be one to many oospheres per oogonium
Male gamete is produced by antheridium and transferred to the oogonium by gametangial contact and migration of male nuclei into
oogonia and fertilize oospheres
Homothallic– self-fertile or Heterothallic– opposite mating types required for sexual reproduction. A swimming sperm is absent
in the Oomycetes. This type of sexual reproduction is referred to as gametangial copulation. In antheridia and oogonia meiosis
The eggs and sperms are products of meiosis and the only parts of the life cycle that are haploid.
Diploid zygote develops into thick-walled resistant oospore that germinates and give rise to vegatative diploid hyphae that
reproduce asexually by production of zoospores.
The vegetative body is diploid and the life cycle is diplontic.
Class Oomycetes is divided into four orders.
The most economically important group of Oomycetes is the Peronosporales that contain the late blight of potato fungus
Phytophthora infestans and relatives such as Peronospora, Bremia, Plasmopara and others that cause “downy mildews”, the
“damping off” fungi, Pythium spp., and the white rust fungi, Albugo spp.
- Lagenidiales (Salilagenidiales)
- Peronosporales :- This order has
some of the most well known pathogens (fungi cause diseases) cause
diseases to many a crop plants. Peronosporales:- divided into three families :-
- Pythiaceae - Pithium,Phytophthora
- Albuginaceae -Albugo.
- Peronosporales differs from the Saprolegniales in producing
only secondary zoospores in a zoosporangium
- That is differentiated from hyphae (eucarpic) and one oosphere
(egg) per oogonium.
- Zoosporangia often deciduous
- Zoospores often formed in vesicle
- They are aquatic, amphibious, terrestrial and some of the most destructive plant pathogens .
However, although members of Mastigomycotina are
they share no close phylogenetic
relationships to fungi. The combination of cellulose cell wall,
biflatellated zoospores, one flagellum of the tinsel type and the other of the whiplash type, and gametangial copulation are
characteristics that are shared with some members of the algal divisions Phaeophyta and Chrysophyta. This has recently led to
recognition of yet another kingdom, Stramenopila, which includes the divisions Bacillariophyta, Chrysophyta, Phaeophyta,
Hyphochytridiomycota and Oomycota. These divisions are now thought to be derived from a common ancestor .
Sub-division : -Zygomycotina
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