Mechanisms of pathogen attacks on plants

1. Mechanisms of pathogen
attacks on plants
• By
• H.M.N.M.Watagodakumbura
• Senior Lecturer in Agriculture

2. Mechanisms of pathogen attacks
on plants
• Mainly two kinds
1. Mechanical forces on host tissue
2. Chemical weapons of pathogens

3. 1.Mechanical forces on host tissue
Eg. Some fungi and nematodes
1. Only some fungi {Producing appresorium and
penetration peg}
a) After germination, germination tube make a
bulb like structure is called appresorium.
b) Appresorium increases area of attach to the
plant. at the end of the appresorium, penetration
peg is produced.
c) Water is absorbed into the appresorium.

4. d) Then due to turgor pressure in the
appresorium physical penetration by
infection peg is happened.
e) And also penetration is easy due to pre-
softening by enzymatic secretions.
2. Nematodes- By using stylet make
mechanical forces to penetrate into cells.

5. 2. Chemical weapons of
pathogens
• Allmost all pathogens except virus and
viroids produce-
1. Enzymes
2. Toxins
3. Growth regulators
4. Polysaccharides

6. • Viruses and viroids are not producing any
substances themselves.
But,
They induce the host cell to produce needed
substances like enzymes etc.

7. Production of Enzymes
– Disintegrate the structural components of
the4 host cells
– Interfere functional system of protoplast
– Breakdown inert food substances in the cell
Enzymetic degradation of cell wall substances

9. • Aerial plant part or cell wall surface consist mainly cuticle and
cellulose layer.
• Root cell wall consist only cellulose layer.
• Cell wall consist of
- cutin
-wax
-protein and lignin
-cellulose
-pectin
• If pathogen attack the plants, they should digest above
substances.
• They are using various kinds of enzymes for this.

10. 1. Chitinases
Eg. Many fungi and bacteria
• It can digest cutin.
• Enzyme reaches its highest concentration in the
germ tube and infection peg in appresorium
forming fungi.
• Fungi that enter into host by passively, there are
no any need of chitinases.
• Virulent pathogen produce more chitinases

11. 2.Pectinases
• Break down pectin chain at random sites.
• Occur soft rotting of tissues. Because the
middle lamella contains pectin.

12. 3. Cellulases
Softening and disintegration of cell wall material
4. Ligninases
• Breakdown lignin of cell wall, mostly those in the xylem.
• Some microbes have ability to breakdown wood causing
wood decay.
• Brown- rot fungi degrade lignin but, they cannot utilize it
• How ever, most of lignin is degraded and utilize by
white- rot fungi
• Some bacteria degrade lignin by causing soft rot cavities.

13. 5.Other enzymes
• By proteases produced by pathogens,can
digest proteins.
• Most pathogens depends on starch with
the use of amylases.
• Some pathogens degrade lipids by using
lipases.

14. 2. Production of Growth regulators
• Most importantgrowth regulators are
• Auxins
• Gibberalines
• Cytokinines
• They make abnormal growth such as-
Stunting Overgrowth
Rossetting
Excessive root branching
Stem malformation
Defoliation
Suppression of bud growth.

15. 1. Auxins
• IAA- cell elongation and differentiation
• It increases respiration of plant tissues
• It promotes the synthesis of enzymes of structural
proteins.
• Eg. Fungi causing corn smut
Root knot nematode
Bacterial crown gall
Club root

16. 2.Gibberalines
• Eg foolish seedling disease of rice
• It increases plant height
3. Cytokinine
• Occur chorosis, leaf abscission
• Stimulation of adventious roots
• Premature fruit ripening

17. 4. Polysaccharides
• Fungi, bacteria, nematodes and some others
release mucilaginous substances
• Large polysaccharides molecules released by
the pathogen in the xylem cause mechanical
blockage of vascular bundles initiating wilting in
plants.