1. EFFECT OF PATHOGENS ON PLANT PHYSIOLOGY
By Irda Safni

2. Effect of pathogens on plant physiology:
Photosyntesis
Respiration
Permeability of cell membrane
Translocation of water and nutrients
Transcription and translation
Plant Growth Hormones

3. Physiological function Example of disease/pathogen
Table 1. Common plant disease symptoms, the physiological process affected, and examples of a disease
Symptoms
Physiological function
Example of disease/pathogen
Chlorosis
Photosynthesis
TMV
Wilting
Xylem transport
Bacterial wilt of tobacco, tomato
Hyperplasia – cell division
Growth hormone regulation
Crown gall ; Black knot of plum (Dibotryon marbosum)
Necrosis
Many different functions
Fire blight of apple
Hyperthrophy – cell enlargement
Root knots
Leaf abscission
Coffea rust (Hemileia vastatrix)
Etiolation
Foolish seedling” of rice (Gibberella fujikuroi)
Stunting
Man different viral diseases
Abnormal leaf formation
Growth hormone regulation, respiration
CMV

4. Photosynthesis
How do pathogens affect plant physiology?
Interfere by chlorosis, necrosis, and reduced growth and yield.
Reduce amount of photosynthetic surface  affect chloroplast - degenaration
Produce toxins that inhibit enzymes involved
Stomata remains unpartially closed  chlorophyl is reduced & photosynthesis stops

5. Chlorosis of Tomato Splottle Leaf Virus
Necrosis of Bacterial leaf spot on lettuce (Xanthomonas campestris pv. vitians)
Chlorosis of Tomato Splottle Leaf Virus
Affected leaves can carry on little or no photosynthesis.

6. Photosynthesis The influence of virus infection :
Reduction of chloroplast numbers
Reduction in chlorophyll content
Chloroplast abnormalities
Reduction in photochemical activity
Stimulating CO2 incorporation at early stage of infection, but declined after virus infection for several days.
Reduction in sucrose content

7. Photosynthesis The influence of bacterial infection :
Decrease in chloroplast stroma (chloroplast content)
Disorientation of chloroplasts
Destroy of chloroplast integrity - HR
Suppression of CO2 fixation

8. Hypersensitive Reaction (HR)
E. C. Stakman (1915) is generally credited with the use of the term, hypersensitive reaction (HR).
HR involves the extremely rapid death of only a few host cells which limits the progression of the infection.

9. Characteristics of Hypersensitive Reaction
Cessation of cytoplasmic streaming
Membrane damage
Burst of reactive oxygen species
Protoplast (vacuoles) collapse
Release of second metabolites – fluorescent compounds
Browning of cells

11. Appearance of HR
42 hpi
120 hpi
Oat Rodney (Pg-2) infected with incompatible isolates of Puccinia graminis f.sp. avenae Pga-1H

12. Photosynthesis The influence of fungal infection :
Reduce chloroplast RNA content
Loss of chlorophyll
Inhibit photophosphorylation coupling mechanism
Inhibit electron transport
Suppress CO2 fixation

13. Photosynthesis
Stimulation of CO2 fixation in uncolonized leaves
A stimulation of CO2 fixation in the light is characteristic of the noninfected leaves of heavily rusted bean plants.
Altered translocation of organic compounds

14. Green island on wheat infected with wheat powdery mildew

15. Green island
Occurred in plant infected by obligate parasite, such as powdery mildew or rust fungi.
It usually occurred in later stage of disease.
Active starch accumulation and chlorophyll synthesis.

16. Green island
Green island has been supposed to be due to juvenility effect of cytokinins and their action on nutrient metabolism.
Cytokinin produced at the infection sites exerts a juvenility effect on tissues and directly govern short- and long-distance movement of nutrients.
The cytokinins increase in rust-infected bean leaves is of host origin.

21. Respiration
Respiration increases when plant pathogens infect leaves because of an increase in leaf cell permaeability and stomata dysfunction.
Destruction of a considerable portion of the cuticle and epidermis  an uncontrolled loss of water from the affected areas.
If water absorption and translocation cannot keep up with excessive loss of water  loss of turgor and wilting of leaves.

22. Respiration Pathological respiration induced by viruses:
Respiration rate :
- Nonhypersensitive hosts (systemic hosts) Slightly increase in respiration rate of inoculated leaves
- Hypersensitive hosts
A much more comsiderable increase in respiration activity than systemic hosts.

23. Respiration Pathological respiration induced by bacteria:
In pepper – Xanthomonas vesicatoria interaction, an immediate increase in O2 consumption was detected in resistant tissue, whereas susceptible tissue did not reflect an increase in respiration until about 30h after inoculation.
In incompatible interaction, the bacteria usually caused great increase in respiratory rate.

24. Respiration Pathological respiration induced by fungi :
Respiration rate is usually increased in diseased plants.
In the early stage of disease, synthetic processes induce high rates of respiration, whereas in the late stages injury and decomposition of tissues lead to increase respiration.

25. Respiration Pathological respiration in resistant hosts
O2 consumption increases more rapidly in resistant plants infected either by obligate or facultative parasites at the early stage of disease; later, the respiration rate gradually decreases.

26. Permeability of Host Cell membranes
Cell membranes consist of a double layer of lipid molecules, which are embeded many kinds of protein molecules. Membranes functions as permeability barriers that allow passage into a cell only a substances the cell needs and inhibit passage out of the cell of substances needed by the cell. The lipid bilayer is impremeable to most biological molecules. In plant cells, because of the cell wall, only small molecules reach the cell membranes.

27. Permeability of Host Cell membranes
Pathogens can change the permeability of host cell membranes by mechanical injury, enzymatic degradation, or toxins.
Changes in cell permeability are often the first detectable responses of cells to infection by pathogens.

28. Permeability of Host Cell membranes
The most commonly observed effect of changes in cell membrane permeability is the lost of electrolytes.
Electrolyte leakage occurs much sooner when the host-pathogen interaction is incompatible, and the host remains resistant, but when the host is susceptible and the host develops extensive symptoms.

29. Translocation of Water and Nutrients
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
Example: Fusarium crown rot

30. Translocation of Water and Nutrients
Plants infected by viruses:
Although there are exceptions, virus infection generally leads to a reduction in transpiration rate, which is often correlated with a reduced leaf stomatal aperture.
Accumulation of carbohydrates in leaf tissue is a characteristic of severe virus diseases. It is usually accompanied by phloem necrosis and/or gummosis, particularly in the later stage of disease.

31. Gummosis on cherry plant

32. Translocation of Water and Nutrients
Plants infected by bacteria :
Bacteria can enter the vascular systems, both xylem and phloem, through wounds.
Production of EPS might clot the vascular system and cause wilting.

33. Translocation of Water and Nutrients
Infection of Ralstonia solanacearum on tomato
Infection of Xanthomonas spp. on banana

34. Translocation of Water and Nutrients
Plants infected by fungi :
Absorption of water by diseased roots is usually inhibited.
In vascular wilt diseases, water flow through the vessels of diseased stems is reduced.

35. Translocation of Water and Nutrients
In plants with vascular disease, the transpiration is significantly less than in healthy plants. This lower transpiration is closely related to the plugging of vessels and the resultant shortage of water in the leaves.
Polysaccharides (ex., tyloses) produced by Fusarium may also be involved in the obstruction of normal water translocation.

37. Translocation of Water and Nutrients
Powdery mildew strongly inhibits stomatal opening, thereby reducing transpiration rate.
However, when epidermis is ruptured, enhanced transpiration is observed.

38. Translocation of Water and Nutrients
How do plant pathogens interfere with translocation in xylem vessels?
Physical presence (mycelium, conidia, bacterial cells) in xylem
Polysaccharides in the vessels
Collapse of vessels
Development of tyloses
Reduced water tension in vessels due to pathogen-induces alteration in foliar transpiration

39. Translocation of Water and Nutrients
How do plant pathogens interfere with translocation of nutrients through phloem?
Pathogen attacks and destroys phloem elements interferring with downward translocation of nutrient
Starch accumulation in the leaves is a result of degeneration of the phloem of infected plants (leaf curling viruses)

40. Transcription and Translation

41. Transcription and Translation
Transcription of cellular DNA into messenger RNA & translation of messenger RNA to produce proteins are two most basic, general, and precisely controlled process in the biology of any normal cell.
Disturbance of any of these process may cause drastic changes in the structure and function of the affected cells.

42. Transcription and Translation
Transcription and translation of host cells usually increase in response to pathogen infection, but with a higher level for resistant plants, since the resistant plants need to proceed the defense reactions.

43. Transcription and Translation Effect on Transcription
Several pahogens, particularly viruses & fungal obligate parasites such as the rusts and powdery mildews, affect the transcription process in infected cells.
Pathogens affect transcription by changing the composition, structure, or function of the chromatin associated with the cell DNA.

45. Transcription and Translation Effect on Transcription
For viruses, through its own enzyme or by modifying the host enzyme that makes RNA, utilizes the host cell nucleotides to make its own RNA.
The activity of ribonucleases is increased.
Infected plants (particularly resistant ones) contain higher levels of RNA than healthy plants, especially in the early stages of infection.
Greater DNA levels and increased transcription in cell indicate increased synthesis of substances involved in the defence mechanisms of plant cells.
Host enzyme = RNA polymerase.
Ribonucleases = enzymes that break down RNA

46. Transcription and Translation
Effect on Translation
Infected plant tissues often have increased activity in several enzymes.
Protein synthesis is increased in infected tissues of primarily resistant plants.
The increased protein synthesis attacked by pathogens reflects the increased production of enzymes and another protein involved in the defence reactions of plants.
Several enzymes  particularly those associated with generation of energy (respiration) or with production or oxidation of various phenolic compounds, some of which may be involved in (defense) reactions to infection.
Protein synthesis is increased in the early stages of infection (the first few minutes and up to 2-20 hours after inoculation)

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