fungi: heterothallism, heterokaryosis, parasexuality,fungi sex hormones, Mycorrhizae, Types of mycorrhizae, Defence mechanism in plants- structural and biochemical.
1. Fungi: Heterothallism,
Heterokaryosis and Parasexuality
o Heterokaryosis was proposed by Hansen and Smith in 1932,
o Heterokaryosis occurs naturally in certain fungi, in which it results from
the fusion of the cytoplasm of cells from different strains withoutthe
fusion of their nuclei.
Heterokaryosisis the mainsourceof variationintheanamorphic
(imperfect) fungi, which lacksexualreproduction.
The presenceof genetically-different nuclei inan individualis
called heterokaryosis, and the organism heterokaryon.
o The cell, and the hypha or mycelium containing it, is known as a
heterokaryon; the mostcommon type of heterokaryon is a dikaryon.
o Heterokaryosis is the main sourceof variation in the anamorphic
(imperfect) fungi, which lack sexual reproduction.
o a heterokaryon possessestwo sets of chromosomes, justlikea diploid
organism, butinstead of being contained in a single nucleus, the two
sets of chromosomes lie in separate nuclei, sharing the same cytoplasm.
Heterokaryons show dominanceand, thus, resemblediploids in many
respects.
Heterokaryosis is a major factor in natural variability and sexuality.
The heterokaryotic condition can arise in a fungus by three methods, viz.,
(1) Mutation,
2. (2) Anastomosis i.e., fusion between genetically-different hyphae, and
(3) Diplodization-fusion between haploid nuclei to form diploid nuclei.
Mutations occur frequently in fungi, and a homokaryotic mycelium is
frequently converted into a heterokaryotic one.
Anastomosis between spores and hyphae is a universalfeature of higher
fungi and certainly must be a potential sourceof heterokaryosis and,
thus, of variability.
Whether nuclei migrate from one thallus to another is a debated point
but the hyphaehaving nuclei of both parents ariseat the point of fusion.
Heterokaryosis is often accompanied by parasexualcycle.
Heterothallism in Fungi:
o A.F. Blakeslee,an American Geneticist,in 1904 observe
Mucor, and discover Heterothallism.
o Blakeslee observed, that while some isolates of Mucor
formed sporangia as well as zygospores (e.g., M. tenuis),
some others failed to form the zygospores and
reproduced only by sporangiospores.
o When he grew these non-sexually reproducing isolate
with other similar isolates,zygospores appeared in the
region where the hyphae of the different isolates came in
contact with each other.
Blakeslee coined the terms homothallism and heterothallism to
explain this phenomenon.
o The homothallic species were those that produced zygospores
independently,
o while heterothallic species required the presence of the
opposite mating type. M. hiemalis,M. mucedo, Rhizopus
nigricans are examples of heterothallic species.
o Since the two mating types were morphologically
indistinguishable, Blakeslee designated them as the (+) and (-)
mating types or strains (not male or female).
3. Pattern of Distribution of Sex Organs in Fungi:
On the basis of the distribution of sex organs, fungi can be put
in the following categories:
1. Hermaphrodite, in which both male and female sex organs occur
on the same thallus.
2. Dioecious (sexually dimorphic)- The two sex organs are present
on different thalli.
3. Sexually undifferentiated-The male and female sex organs are
morphologically similar and, therefore,indistinguishable.
Physiological heterothallism
When the two sex organs, present on the same mycelium,are
unable to mate, this is because of self-sterility and is called
physiological heterothallism.
Such fungi need genetically-different nuclei, which does not occur
when the same thallus forms both the sex organs.
Morphological heterothallism
The dioecious fungi, in which the male and the female sex organs
are borne on different thalli are, heterothallic.This is called
morphological heterothallism.
4. In this case, heterothallism is made obligatory because the opposite
and morphologically distinct sex organs are formed only on
different thalli.
So, heterothallism,according to Whitehouse
(1949) can be caused by the absence of the morphological sex
organs of the opposite type (morphological heterothallism) orby
the absence of genetically-different nuclei (physiological
heterothallism).
Parasexuality
o Called genetic recombinationwithout meiosis.
o In the absence of meiosis during the life cycle of imperfectfungi,
recombinationof hereditary properties and genetic variation still
occur by a mechanism called parasexuality.
o first discovered by Pontecarvoand Roper of Universityof Glasgow
in 1952 in Aspergillusnidulans
o The ParasexualCycle is defined as a cycle in which plasmogamy,
karyogamyand meiosis(haploidisation) takeplacebut not at a
specified timeor at specified pointsin the life-cycle of an organism.
o occursin those fungi in which true sexualcycle does not takeplace.
The membersof class Deuteromycetes(Deuteromycotina) inwhich
sexual cycle does not occur, exhibit parasexualcyclegenerally.
It includes
(i) Formationof heterokaryotic mycelium
(ii) Fusion betweentwo nuclei (Karyogamy)
(a) Fusion betweenlike nuclei
(b) Fusion betweenunlike nuclei
(iii) Multiplicationofdiploid nuclei
(iv) OccasionalMitoticcrossing over.
(v) Sorting out of diploid nuclei
(v) Occasionalhaploidisationof diploid nuclei, and
6. Presence of Sex Hormones in Achlya | Fungi
Hormones play an essential role in this communication as chemical
signaling molecules.
Hormones are substances produced in one portion of an organism and
transported by any means, including diffusion, to other portions of the
same individual or other individuals of the same species where they induce
specific responses.
Although a large number of sex hormones have been reported, only
a few have been chemically characterised and extensively studied.
These are – Sirenin, antheridiol and oogoniol, trisporic acid and
yeast D factor.
These are briefly described here:
7. 1. Sirenin:
It is a sperm attracting hormone produced by water mould
Allomyces, A. macrogynous and A. arbuscula. It is the female
gametes which release sirenin to attract the male gametes.
The synthesis of sirenin by female gametes and their function to
attract male gametes was demonstrated by Machlis
2. Antheridiol and Oogoniol:
Antheridiol
It is reported to stimulate four types of reactions:
(a) initiation of antheridial hyphae on male plant,
(b) chemotropic stimulation of antheridial hyphae,
(c) stimulation of male hyphae for production of oogoniol and
(d) delimiation of antheridia.
Oogoniol:
The hormone is synthesizedby male hyphae of Achlya ambisexualis
only in the presence of antheridiol.
However, Barksdali et al. (1974) reported that oogoniol is
synthesisedby some hermaphrodite strains without the stimulus of
antheridiol.
3. Trisporic Acid:
It has been found to play active role in sexual reproduction of
several members of the order Mucorales.
It is an unsaturatedand oxygenated form of trimethyl cyclo-hexane.
Three kinds of trisporic acid have been identified, trisporic acid A, B
and C.
Trisporic acid C plays the major role (80%) as a sexhormone,
followedwith trisporic acid B with 15% activity and trisporic acid A
is least active with 1-2% activity.
8. The sex hormone trisporic acid, present in this species is
synthesizedfrom B-carotene.
In the heterothallicmycelia, trisporic acid B and C stimulate the
development of zygophores.
4. Yeast α Factor:
the haploid cells are of two mating types a and α which conjugate to
form diploid cells.
In 1956, Levi showed that the α haploid cells produce a diffusible
chemical which induces the formation of copulatory process by
compatible a cells.
These a cells, due to influence of the chemical substance produced
by α cells, stop their growth and reproduction by budding.
Mycorrhiza
A mycorrhiza (from Greek mýkēs, "fungus", and rhiza, "root"; is a
mutual symbiotic association between a fungus and a plant.
The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere,
its root system. Mycorrhizae play important roles in plant nutrition, soil
biology and soil chemistry.
A mycorrhiza is a symbiotic association between a green plant and a fungus. The
plant makes organic molecules such as sugars by photosynthesis and supplies them
to the fungus, and the fungus supplies to the plant water and mineral nutrients, such
as phosphorus, taken from the soil.
Mycorrhizas are commonly divide -
Ectomycorrhizas - Endomycorrhizas.
The two types are differentiated by the fact that the hyphae of ectomycorrhizal fungi
do not penetrate individual cells within the root,
while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate
the cell membrane.
9. Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza, while arbutoid
mycorrhizas can be classified as ectoendomycorrhizas. Monotropoid mycorrhizas form
a special category.
Features of Mycorrhiza:
Scannerini (1988) briefly pointed out the common features of
mutualisticsymbionts.
These include:
(i) Absence of any phytopathological symptoms in the partners
during the active phase of mutualism,
(ii) Presence of complex interfaces between cells of the partners
with a predominant type of perisymbiotic membrane, surrounding
intracellularsymbionts,
(iii) Presence of various types of phagocyte-like structures during
establishment of symbionts and during harvestingphase to control
the symbiotic population by the host.
Types of Mycorrhiza:
Peterson and Farquhar (1994) classified the mycorrhizae
into seven (7) distinct types.
These are :
(1) Ectomycorrhizae,
(2) Vesicular-arbuscular mycorrhizae,
(3) Ectendomycorrhizae (Arbutoid),
(4) Ericoid mycorrhizae,
(5) Centianoid mycorrhizae,
(6) Orchidoid mycorrhizae, and
(7) Monotropoid mycorrhizae.
10. (1) Ectomycorrhizae:
Ectomycorrhiza is commonly called “sheathingmycorrhiza”. They
occur in 3% of all seed plants in forests of temperate regions,
especially on pine, beech, spruce, birch etc.
They appear in various colours like white, brown, yellow, black etc.,
depending on the colour of the fungus. Members of
Gastercmycetes under Basidiomycotina like Rhizopogon and
Scleroderma are involved in this process.
(2) Vesicular-arbuscular mycorrhizae(VAM):
It is a type of endomycorrhizal association, where both vesicles and
arbuscles are developed together. VAM is common of all
mycorrhizae and has been reported in more than 90% of land
plants.
They are found in bryophytes, pteridophytes, gymnosperm (except
Pinaceae) and most of angiosperms, commonly in Leguminosae
(Fabaceae), Rosaceae, Gramineae (Poaceae)
The VAM is so named because of the presence of two
characteristic structures i.e., vesicles and arbuscles:
(i) The vesicles are thin or thick walled vesicular structures
produced intra-cellularly and stored materials like polyphosphate
and other minerals .
11. (ii) The arbuscles are repeated dichotomously branched haustoria
which grow intracellularly .The arbuscles live for four days and then
get lysed releasing the stored food as oil droplets, mostly
polyphosphate.
(3) Ericoid mycorrhizae
This is actually a type of endomycorrhiza. Ericoid mycorrhizae are
found in the different members of Ericaceae like Rhododendron etc.
The fungi are slow-growing, septate and mostly sterile.
Most of the members of Ericaceae grow in acid soil with less
amount of P and N nutrition. The fungus gets the photosynthate
from the host and improves the mineral uptake and nutrition of the
host, especially P and N.
(4) Ectendomycorrhizae (Arbutoid):
Some members of the family Ericaceae and members of other
families of Ericales have mycorrhizae intermediate in form between
12. ecto- and endomycorrhizae types, called ectendomycorrhizae.
Arbustus and Arctostaphylos of Ericaceae show this type of
mycorrhizal association.
(5) Gentianoid mycorrhizae:
Seedlings of some members of Gentianaceae (G. amarella, etc.) get
infected within 2 weeks of germination. In root, the cortical cells
become full of irregular coils of aseptate hyphae. With time the
hyphae become lysed. Vesicles are occasionally seen attached to
these coils.
(6) Orchidoid mycorrhizae:
Orchids produce millions of tiny seeds per capsule, majority of
seeds are unable to germinate without exogenous supply of
carbohydrates. Therefore, mycorrhizal association is obligatory for
the seeds to germinate. The fungus provides C-nutrition to the
seeds. Initially the fungus enters the embryo and colonises,
being restricted to the cortical cells and provides the nutrition.
Role of Mycorrhizae in Agriculture and Foresty:
Role in Agriculture:
1. The mycorrhizal association helps in the formation of
dichotomous branching and profuse root growth, thus enhances
plant growth.
13. 2. Ectotrophic mycorrhiza helps in uptake of mineral ions and also
acts as reservoir.
3. They also help in absorption of nutrients.
4. In nutrient deficient soil, the mycelial association helps in the
absorption of N, Ca, P, Zn, Fe, Na and others.
5. Mycorrhizal association is obligatory for the germination of
orchid seeds.
Mycorrhizal growth in orchids (Rhizoctoniarepens with Orchis
militaris tuber tissues) causes the synthesis of phytoalexins —
orchinol and hirsinol. Both the compounds act as a barrier to pro-
tect infection by other pathogens.
6. Inoculation of VAM as biofertiliserprovides a distinct possibility
for the uptake of P in phosphorus-deficient soil.
Role in Foresty:
1. Mycorrhiza plays an important role to establish forest in
unfavourable location, barren land, waste lands etc.
2. Trees with facultative endomycorrhiza act as first invader in
waste lands as pioneer in plant succession.
3. The application of mycorrhizal fungi in forest bed enhances the
formation of mycorrhizal association that prevents the entry of
fungal root pathogens. This method is very much effective in the
root of Pinus clausa against Phytophthora cinnamoni infection.
4. Mycorrhizamixed nitrogenous compounds such as nitrate;
ammonia etc. is available to the plants. Thus it helps in plant
growth, especially in acid soil.
Defense Mechanism in Plants
structural and biochemical defense mechanisms
Structural
14. (A) Preexisting defense structures
o Cuticular Wax - Deposition of wax on the cuticular surface is
thought to play a defensive role by forming a hydrophobic
surface where water is repelled. As a result, the pathogen does
not get sufficient water to germinate or multiply.
o Cuticle Thickness - The thickness of cuticle is most
important for those which try to enter the host through
the leaf surface. The cuticle thickness obstructs the path
of pathogen.
o Structure of Epidermal Cell Wall - Tough and thick outer
walls of epidermal cells may directly prevent the entry of
the pathogen completely . The presence or absence of
lignin and silicic acid in the cell walls may show variation
in resistance to penetration of the pathogen.
o Structure of Natural openings - Structure of natural openings
like stomata lenticels etc. also decide the fate of the entry of
the pathogen. Resistant varieties of apple, presence of
abundant hairs in the nectaries acts as a defense mechanism
while susceptible varieties are devoid of abundant hairs.
(B) Defense structures developed after the attack of the pathogen.
After the pathogen has successfully managedto overcome the pre-
existing defence mechanisms of the host, it invades the cells and
tissues of the host. In order to check the furtherinvasion by the
pathogen, the host plants develop some structures.
(i) Defense Reactions in the Cytoplasm - cytoplasm of the invaded
cell surrounds the hyphae of the pathogen and the nucleus of the
host cell gets stretched to break into two. cytoplasm becomes
granular and dense result in the disintegration of the pathogen
mycelium seen in weak pathogens like Annillaria and some
mycorrhizal fungi.
ii) Cell Wall Defense Structures - Cell walls thicken in response to
the pathogen by producing a cellulose material, thus preventing the
entry of the pathogen.
15. (iii) Defense Structures Developed by the Tissues
(a) Gum Deposition - Plants produce a variety of gummy
substances around lesions or spots as a result of infection.
These gummy substances inhibit the progress of the
pathogen.
(b) Abcission Layers -
(c) Tyloses - Tyloses are out growths of protoplasts of adjacent
live parenchyma cells protruding into xylem vessels through
pits under stress or in response to attack by the vascular
pathogens.
(d) Formation of Layer - These layers inhibit the further
invasion by the pathogen and also block the flow of toxic
substances secreted by the pathogen. Eg. necrotic lesions on
tobacco caused by tobacco mosaic virus.
IV. Necrosis or Hypersensitive Type of Defense:
II. Biochemical Defense:
It play more important role than the structural defense
mechanisms.This has been supplementedby the fact that many
pathogens entering non host plants naturally or artificially
inoculated fail to cause infections in absence of any structural
barriers.
(A) Preexisting Biochemical Defense:
(i) Inhibitors Released in the Prepenetration Stage - Plant generally
exudes organic substance through above ground parts
(phyllosphere) and roots (rhizosphere). For example fungistatic
chemicals released by tomato and sugar beet prevent the
germination of Botrytis and Cercospora. Presence of several
phenolics, tannins and some fatty acid like compounds such as
dienes in cells of young fruits, leaves or seeds afford them resistance
to Botrytis.
ii) Lack of nutrients essential for the pathogen is another
preexisting biochemical defense mechanism.Plant varieties or
16. species which do not produce any of the chemicals essential for the
growth of pathogen may act as resistant variety.
(B) Post-Infection-Biochemical Defense Mechanism
(i) Phenolic Compounds - These are the most common compounds
produced by plants in response to injury or infection. The synthesis
of phenolic compounds takes place eitherthrough “acetic acid
pathway” or “Shikimic acid pathway”.
(ii) Phytoalexins - Phytoalexins are toxic antimicrobial substances
synthesized‘de novo’ in the plants in response to injury, infectious
agents or their products and physiological stimuli. They stop the
growth of pathogens by altering the plasma membrane and
inhibiting the oxidative phosphorylation.
iii)Detoxification of Pathogen Toxins and Enzymes - Some hosts
produce chemicals which neutralise the enzymes produced by
pathogen, thus defending the host. Therefore these substances help
plants to defend themselves from the attack of the pathogen.
