2. What are Biotic stress?
It is a stress that occurs as a result of damage
done to plants by other living organisms,
such as bacteria, viruses, fungi, parasites,
beneficial and harmful insects, weeds and
cultivated or native plants.
Biotic stresses cause damage to plants via
living organisms, including fungi, bacteria,
insects and weeds. Viruses, although they are
not considered to be living organisms also
cause biotic stress to plants.
This Photo by Unknown author is licensed under CC BY-NC.
3. Fungi cause more diseases in plants than any other biotic stress factor. Over 8000 fungal
species are known to cause plant disease. On the other hand, only about 14 bacterial genera
cause diseases in plants.
Microbes can cause plant wilt, leaf spots, roots rot or seed damage. Insects can cause severe
physical damage to plants, including to the leaves, stem, bark and flower. Insects can also act as
a vector of viruses and bacteria from infected plants to healthy plants.
The method by which weeds, considered as unwanted and unprofitable plants inhibit the
growth of desirable plants such as crops or flowers is not by direct damage but
by competing with the desirable plants for space and nutrients. Because weeds
grow more quickly and produce an abundance of viable seeds and therefore, they are able to
dominate their presence in the surroundings in comparison to desirable plants.
4. Effects of insects on plants
1. Flowering
2. Fruit Production
3. Post-dispersal Seed Mortality
4. Seed Mortality
5. Growth And Reproduction
6. Defoliation
5. Effects of plant pathogen on plants
Interfere by chlorosis, necrosis and reduced growth and yield
Reduce amount of photosynthesis surface affect chloroplasts- degeneration
Produce toxins that inhibit enzymes involved
Stomata remain partially closed chlorophyll is reduced, and photosynthesis stops
Affect the integrity of function of root absorb less water
Growth in xylem vessels interfering with translocation, interfere with water economy
of plant by causing excessive transpiration
Physical presence in xylem Polysaccharides in the vessels
Increase in plant metabolism
6. Collapse of vessels
Development of tyloses
Reduced water tension in vessels due to pathogen induces alteration in foliar
transpiration
Pathogen attacks and destroys phloem elements interfering with downward
translocation of nutrients.
Starch accumulation in the leaves is a result of degeneration of the phloem
of infected plants
Increases when plant pathogens infect leaves because of an increase in leaf
cell permeability and stomata dysfunction.
Loss of water due to destruction of cuticle and epidermis which can further
lead to wilting of leaves
Rate of respiration increase when plants are infected.
7. Plant Disease And Resistance
Constitutive (continuous) defenses
Such as: Cell walls, waxy epidermal cuticles and bark.
They defend themselves from the pathogenic invaders by the methods of
producing toxic chemicals, pathogen degrading enzymes and deliberate cell
suicide.
8. Plan Pathogens: Sneak Attack Versus
Brute Force
SNEAK ATTACK BRUTE FORCE
Establish intimate connection Overwhelms plant defenses and releases
plant nutrients.
Pathogens that keep their host alive and feed on
living plant tissue are called biotrophs.
They are called necrotrophs.
• E.g. powdery mildew
fungus Bulmeria graminis and bacterial
rice pathogens Xanthomonas oryzae.
Gray mold fungus Botrytis cinerea and the
bacterial soft rot pathogen Erwinia
carotovora.
Some pathogens are biotrophic during the early stages of infection but become
necrotrophic during the latter stages of disease. These pathogens are called
hemibiotrophs and include the fungus Magnaporthe grisea, the causative agent of rice
blast.
9. Slightly different set of specialized genes and molecular mechanisms
required for each host-pathogen interaction.
Host range. E.g., BMV
Non host plant species
Non pathogens
Two outcomes are possible if a plant is with a living pathogen inside it and
that is: a. A Compatible response that leads to disease and
b. An incompatible response that leads to little or no
disease at all.
Disease resistance exists as a continuum of responses ranging from
immunity to highly resistance to highly susceptible disease symptoms.
10. Surveillance and Detection of Microbial
Pathogen
Multiple layers of surveillance mechanisms.
Basal resistance (innate immunity).
MAMPs
The hypersensitive response (HR).
More pathogen specific compared to basal resistance.
SAR
Plant activators such as 2,6-dichloroisonicotinic acid, β-aminobutyric
acid, probenazole, salicylic acid, riboflavin, etc.
RNA Silencing
12. Structural defenses
Cell wall is considered a major line of defense against fungal and bacterial
pathogens.
Deposition of papillae.
Presence Idioblasts
For e.g., Tannins in young red wines, Sclereid stone cells in pear fruit,
Hypodermic needles in stinging nettles, Crystalliferous cells in Dieffenbachia
sp.
14. Chemical Defenses
Secondary metabolites such as terpenoids, phenolics and alkaloids.
TERPENOIDS
Essential oils often work as insect toxins, and many protect against the bacterial and
fungal attack. Eg., menthol and menthone produced by Mentha sp.
Pyrethrins monoterpenoid in chrysanthemum
Pine tree resin produces turpentine
Many plants that produces essential oils that are harmful to the insects but harmless to
the humans such as: spearmint and peppermint, basil, oregano, rosemary, sage, savory,
thyme, black pepper, cinnamon and bay leaf.
17. Triterpenoids such as cardiac
glycosides are highly toxic to
vertebrate herbivore
including humans.
Foxglove is the principal
source of cardiac glycosides
digitoxin & digoxin.
Some organisms overcome
the toxic effect and use them
for their benfits such as
MONARCH BUTTERFLY.
Foxglove
Monarch butterfly
18.
19. Phenolics
Produced mainly in shikimic acid and malonic acid pathway in plants.
Consists of flavonoids, anthocyanins, phytoalexins, tannins, lignin and
furanocoumarins.
Flavonoids are one of the largest class.
Anthocyanins are colorful water-soluble flavonoids which protect foliage from
UV rays.
Phytoalexins = isoflavonids
Tanins are also water-soluble flavonoid which harm insects by the process of protein
inactivation and resulting in not gaining weight and die eventually.
20. Lignin is highly branched heterogenous
polymer found in secondary cel walls of
plants.
As it is insoluble, rigid and virtually
indigestible they provide excellent
defense against pathogen attacks.
Furanocoumarines are activated by UV light
and can be highly toxic as they integrate
into the DNA which contributes to cell
death.
21. Nitrogen compounds
Bitter in taste including caffeine, cocaine, morphine and nicotine. And are
derived from amino acids such as aspartate, lysine, tyrosine and
tryptophan.
Caffeine is an alkaloid found in plant such as coffee, tea and cocoa.
Caffeine produced by coffee can sometimes overwelm other plants in the
vicinity and make their seed inhibit to germinate, this process is called as
alleopathy.
Alleopathy is a phenomenon in which one plant species defend itself
against other plants that may compete for growing space and nutrient
resources.
23. Nightshade family I.e.,Solanaceae produces
many alkaloid compounds.
Nicotine is produced in roots of tobacco
plant and transported to the leaves where it
is stored in the vacuoles.
Atropine produced by bellandona is a
neurotoxin which is highly toxic.
Capsaicin produced by chilli
peppers give burning sensation in hot and
spicy food.
Cyauogenic glycosides are nitrogen
compounds that break down to produce
HCN e.g., mustard family.
Tobacco plant
25. Proteins and
enzymes
Many plant proteins are present that are
especially responsible fo inhibition of
pathogen. They are small and rich in amino acid
cystine.
They need great deal of plant resources and
energy for production.
They are produced in small quantities but once
they are active the inhibit the growth of fungi,
bacteria, nematodes and insects.
26. Defensins are small cysteine rich proteins that display broad anti-microbial activity.
They were first isolated from the endospeerm of barley and wheat.
These denfensins may inhibit pre-existing ion channels or form new
membrane pores that disrupt cellular ion balance.
Digestive enzyme inhibitors such as ricin in castor beans are highly potent toxin.
Hydrolytic enzymes degrade the cell wall of pathogenic fungi.
Chitinases are enzymes that catalyze the degradation of chitin (cell wall of fungi).
Glucanases degrade the glycosidic linkages in glucans (cell wall of oomycetes)
Lysozymes are hydrolytic enzymes that are capable of degrading bacterial cell walls.
27. Relevance
Biotic stress in plants is caused
by living organisms, especially
viruses, bacteria, fungi,
nematodes, insects, arachnids
and weeds.
The agents causing biotic
stress directly deprive their
host of its nutrients can lead
to death of plants.
Biotic stress can become major
because of pre- and postharvest
losses.
