Plant pathogens can interfere with key plant physiological functions such as photosynthesis, respiration, transpiration, nutrient transport, and cellular processes. This document discusses how pathogens disrupt these functions through tissue damage, toxin production, and cellular changes. Specifically, it notes that pathogens reduce photosynthesis by destroying chlorophyll or inhibiting related enzymes. They also increase plant respiration and interfere with nutrient transport through the xylem and phloem.

2. Plant physiology is the science which deals with the life processes
of plants, or it deals with the functions of cells, tissues, organs or
the plant as a whole.
Plant disease is an outcome of interaction between host and
pathogen.
Interaction begins with physical contact and effects remain
localised until host tissues have been penetrated.
Depending on pathogen and the plant organ and tissue they infect
it interfere with the different physiological functions of the plant.

3.  Harmful effects of infection on physiology of the host are the main cause of
symptom expression, damage and loss.
Some harmful effects are-
i) Disintegration of tissues
ii) Effect on growth due to hormonal imbalance
iii) Reduction in photosynthesis
iv) Abnormal respiration
Pathogens infecting flowers, interfere with the seed production of plants.
Root pathogens affect translocation resulting in its wilting and death of
plant.

4.  Foliar pathogen-
• Infect the part of the leaves ,stem ,petiole and other aerial parts.
• Destroy chlorophyll formation leading to reduced photosynthesis
• Effects growth and yield of the plants
Changes in the cell biology-
• Striking increase in protoplasm.
• Movement of nucleus at site of penetration
• Granulation of cytoplasmic particles.
• Cell contents becomes yellow and finally dark brown leading to death of cell.

5.  Photosynthesis
 Respiration
 Translocation of water and nutrients
 Transpiration
 Permeability of the cell membrane
 Transcription and Translation

6. Photosynthesis is the basic function of green plants.
It transform light energy into chemical energy, which is utilized in
all cell activities.
6CO2+6H20 C6H12O6+ 6O2
Any interference of pathogen with photosynthesis result in plant
diseased condition.
 Once the pathogen is established in the host colony it redirects
the host nutrients for their own use. This condition is known as
metabolic sink.
Light
Chlorophyll

7. Effect on photosynthesis
In leaf spots, blight, and other diseases in which there is destruction of leaf tissue
e.g., in cereal rusts and fungal leaf spots, bacterial leaf spots, viral mosaics,
yellowing and stunting diseases, or in defoliations, photosynthesis is reduced.
Chlorosis, necrosis and reduced growth and yield.
Reduce amount of photosynthetic surface
Chlorophyll degeneration
Produce toxins that inhibit enzyme Eg; tentoxin, tabtoxin
Vascular pathogens reduced the chlorophyll and photosynthesis rate.
Stomata remain partially closed

8. Leaf spot of barley by
Rhynchosporium sp.
Leaf spot of egg plant by
Cercospora melongenae
Leaf spot caused by Alternaria
sp
Wheat leaf rust caused by
Puccinia recondita
Angular leaf spot of cucurbits
by Pseudomonas syringae
Mosaic of mung bean
by mung bean mosaic
virus
Yellowing and stunting of
rice by Tungro virus

9. Respiration is the process by which the cells, through enzymatic
oxidation of organic materials(energy rich carbohydrates and fatty
acids), produces energy for various activities and carbon skeletons.
 In plants the process of respiration takes place in two major steps:
1.Glycolysis or EMP pathway
2.The Terminal phase
GLUCOSE
PYRUVIC ACID
C2H5OH+CO2+ENERGY CO2+H2O+ENERGY
COMMON TO BOTH AEROBIC AND
ANAEROBIC RESPIRATION
GLYCOLYSIS OR
EMP PATHWAY
ANAEROBIC RESIRATION AEROBBIC RESPIRATION
Acetyl CoA

10. Infected plant shows increased respiration rate and slight rise in temperature.
Affected tissue use up their reserve carbohydrates faster than the healthy tissue.
Increase in respiration starts soon after inoculation and rises to a maximum rate
coincident with the sporulation of a fungal pathogen and then declines to normal or
subnormal level.
In resistant plants the rate of increase in respiration is very rapid but declines soon
while in susceptible plants respiration rises slowly but lasts for longer time.
Increase in plant metabolism and protoplasmic streaming.
Increased respiration has been noticed in cereal rusts, powdery mildews, blast of
rice, late blight of potato and many other diseases.

11. Levels of many enzymes associated with respiratory processes is increased in the
diseased plants.
 Accumulation and oxidation of phenols also increases with increase in respiration.
Diseased plants carry out more fermentation than the healthy plants.

12. Blast of rice by Pyricularia grisea Late blight of potato by
Phytopthora infestens
Powdery mildew of soybean by
Microsphaera diffusa

13. XYLEM
• Water and minerals absorbed by Roots
• Translocated through the stem xylem vessel to vascular bundle
of petioles and leaf veins then enter into the leaf cells
• Most of the water evaporates and diffuses into the atmosphere
via the stomata
• Results in water tension in the xylem
• Pulls the water column upward
• Stimulates additional uptake
EFFECT ON UPTAKE AND TRANSLOCATION OF
WATER AND NUTRIENTS

14. Pathogens Interfere with Normal Translocation Process
• Flow of nutrients or water blocked
• Disease in affected cells/tissues
• Inability to carry out functions
• Shortage of their products
• Disease in entire plant
Pathogens Impair Translocation of Water and Inorganic Nutrients
1. Root damage – Damping-off fungi –ex-Pythium sp., Rhizoctonia sp.
root-rot fungi, bacteria, nematodes and some viruses.
Mechanisms of damage-
i) Cause direct injury to roots
ii) Inhibit root hair production
iii) Alter permeability of root cells

15. 2. Xylem gets destroyed –i) Rot or canker pathogens – E.g-Sclerotium rolfsii
ii) Gall formation – E.g-Agrobacterium tumefaciens, Plasmodiophora
brassicae, root knot nematode(Meloidogyne sp.)
3. Xylem gets clogged –i) Growth of vascular wilt pathogens
E.g- Ceratocystis, Ophiostoma, Fusarium, Verticillium, Ralstonia
ii) Secretions of tylose by plant in response to the pathogen.

16. Seedling damping off by Pythium sp. Wilt of tomato by Fusarium sp.
Wilt to tomato by Ralstonia solanacearumBlight of pepper by Phytopthora sp.

17. Crown gall of apple by
Agrobacterium tumefacience
Root galls by Meloidogyne sp.

18.  Carbohydrates produced in leaf cells move
through plasmodesmata into adjoining
phloem elements
 Move down phloem sieve tubes
 Move into non-photosynthetic cells or into
storage organs
 Nutrients are removed from ‘Circulation’.
PHLOEM

19.  Pathogen attacks and destroys phloem elements interferring with downward
translocation of nutrients.
 Starch accumulation in the leaves is a result of degeneration of the phloem of
infected plants (leaf curling viruses).
 Obligate fungal parasites (eg: Rust and Mildew) – Respiration increases –
photosynthesis decreases.
 Viruses – Viruses that cause leaf-curling e.g. tomato leaf curl virus and some yellows
diseases cause death of phloem
 Some viruses inhibit enzymes that break down starch into smaller, trans-locatable
molecules.
How Can Pathogens Impair Translocation in the Phloem?

20. Transpiration is the process of water movement through the plant and its
evaporation from aerial parts, such as leaves, stems and flowers.
Effects of pathogens on transpiration
Disintegration of cuticle
Increased transpiration
Disfunctioning in permeability of leaf cells
Collapse of vessels or formation of tyloses
Powdery mildew- epidermal cells invaded by the fungus
Rust- leaf surface get exposed due to rupture of epidermis cause unrestricted
loss of water
Blight- number of active and healthy cells reduced, transport of water to leaves
by xylem is reduced

21. Wheat leaf rust by
Puccinia recontida
Powdery mildew of grapes by
Uncinula necator
Tomato leaf blight by Septoria sp. Scab of apple by Venturia inequalis

22.  Cell Membrane
• Double layer of lipid molecules
• Protein molecules embedded
– Parts usually protrude on one or both sides of lipid bilayer
• Membranes are selective barriers
– Allow entry of substances the cell needs
– Inhibit passage out of the cell
• Cell wall keeps large molecules away from membrane
• Small water-soluble molecules such as ions, sugars and
amino acids flow through or are pumped through membrane
channels (proteins)

23.  Disruption of the cell membrane alters permeability – uncontrollable loss of useful
substances and entry of harmful substances.
 Membrane permeability is often the first detectable response of cells to infection
 Leakage of electrolytes
o Unknown if membrane leakage is primary or secondary effect.
o If direct attack, pathogens probably use one of the following strategies
• Stimulate membrane-bound enzymes that pump H+ in and K+ out
• Interfere with maintenance and repair of membrane’s fluid film
• Degrade the lipid or protein components of membrane
Effect on cell membrane

24. E.g.-
i. Fusicoccin produced by Fusicoccum amygdali-activates membrane ATPase and
disrupts solute fluxes.
ii. Sclerotium rolfsii and Botrytis cinerea-produces lipid degrading enzymes.
iii. Rhizoctonia solani-degrade protein

25. • Transcription – Copying coding on DNA onto messenger RNA
• Translation – Use of messenger RNA as a template to produce proteins
 Disturbances of any of the processes may affect expression of genes and cause
drastic, unfavourable changes in structure and function of affected cells.

26. Some pathogens change composition, structure or function of chromatin associated with
cell DNA
Virus uses host cell’s nucleotides and machinery to make its own RNA
Sometimes increase in activity of enzymes that break down RNA
 Pathogen may be forcing plant to produce new kinds of enzymes not produced in
healthy plants
Higher levels of RNA in infected plants, especially resistant ones
• probably due to increased synthesis of substances involved in defence mechanisms.
EFFECT ON TRANSCRIPTION

27. In biotrophic infections(e.g. Plasmodiophora brassicae,cabbage; Fusarium
solani,pea; powdery mildew, barley; rust disease,wheat) host nuclei and nucleoli
increases in size.
EFFECT ON TRANSLATION
Higher energy needs
– Increased activity in enzymes associated with respiration
 Increased production of phenols or oxidation of phenolic compounds
– Used in defence reactions
 Resistant plants have increased protein synthesis in first few minutes of
infection
– Levels remain high up to 2 to 20 hours after inoculation

28. CONCLUSION
Human and animal survival depend on plant growth.
Agriculture, the world’s biggest business, is based on ability of plants to grow
and convert simple raw material to complex substances in order to suit human
needs.
The biochemical pathways in a plant are very highly integrated an inter-
dependent and any disruption represents a loss of control by the host.
Even small and localised changes may have drastic consequences to the plant
as a whole.
Pathogen effects plants by inducing changes in its physiological functions that
leads to different types of symptoms and diseases.
This leads to the reduction in quality and quantity of plant products.
So, it directly or indirectly effects our crop economy.