Enzyme action of plant disease

1. Lecture 6.
Role of Enzymes in Plant Disease Development

2. Chemical weapons of pathogens in pathogenesis
1. Enzymes –disintegrates and degrades the plant cell wall and
weakens the plants structural integrity
2. Toxins – low molecular weight extra- cellular secondary
metabolites of the pathogen and they directly interferes
mainly the permeability of the cell membrane
3. Growth regulators- exert a hormonal effect on the plant cell
and either increase and decrease their ability to divide and
enlarge.
4. Polysaccharides – plugs the vascular tissues and interferes
with the translocation of waters

3. Enzymes are the large protein molecules that catalyze all the
interrelated reaction in a living cell.
• Some enzymes are present in cells at all times (Constitutive) .
• Many of them are produced only when they are needed by the
cell is response to external or internal stimuli (induced).
• The hydrolytic enzymes are extremely important in
pathogenesis because
they provide to the pathogen a chemical means of
entrance into the host and a process where by nutrients can be
digested.

4. Anatomy of leaf

5. Plant cell wall composition
Aerial plant part surfaces consist primarily of cuticle and/or cellulose,
whereas root cell wall surfaces consist only of cellulose
Cuticle consists primarily of cutin, more or less impregnated with wax.
Below the cuticle the epidermal cells followed by mesophyll cells
These cells are made up of two walls. 1. primary cell wall and secondary cell
wall. Cell walls are made cellulose and hemicellulose and proteins.
Primary cell wall – cellulose and pectins
Secondary cell wall- cellulose
Middle lamella is cement like structure that act as a glue to hold the cells
together. It is made up of pectins.

6. Pathogens come in contact with their host plant at their
surface.
• Which consist primarily of cuticle and cellulose.
• The cuticle comprises of cutin and wax.
• The penetration of pathogen into parenchymatous tissue
brought by the break down of cell wall consist of
cellulose, pectin's, hemicelluloses, lignin, proteins and
middle lamella and
each of these substances are degraded by the action of
one or set of enzymes.

7. Cuticular wax
Plant waxes are formed as granular or rod like projections or as a continuous
layer
outside or within the cuticle of many aerial plant parts
Cutin
It is the main component of the cuticular layer. The upper part of the layer is
admixed with waxes while in its lower part its admixed with cellulose and
pectin.
Pectic Substances
Pectic substances constitute the main components of the middle lamella, i.e.,
the intercellular cement that holds in place the cells of plant tissues.
Pectic substances also make up a large portion of the primary cell wall in which
they form an amorphous gel filling the spaces between the cellulose
microfibrils
Pectic substances are polysaccharides consisting mostly of chains of
galacturonan molecules
Several enzymes degrade pectic substances and are known as pectinases or
pectolytic enzymes

8. Hemicellulose
They include several polymers. Xyloglucan is important
hemicelluloses in the
primary wall region
It is made of a β1, 4 linked glucopyranose chain with terminal
branches of 1,6 linked D- Xylopyranose.
Other polymers are glucomannar and galactogluco-mannan
Cellulose
It is an aggregation of β-D-Glucopyranos in which C1 is linked to
C4
It is the major constituent of plant cell wall.
Lignin
It has high molecular weight.
Formed in the middle lamella

9. Cell wall proteins
5 classes of structural proteins have been foundain cell walls
1. Extensious – glycoprotein-0.5% in healthy tissues, 5-10% in
infected tissues.
2. Proline rich proteins
3. Glucine rich proteins
4. Solanaceous lectius
5. Other structural proteins
Lipids
1. Phosphelipids and glycolipids along with proteins are the
main constituents of all
2. plant cell membranes.

10. Cuticle degrading enzymes
• Degradation of waxes layer in or on cuticle
Puccinia hordei produce enzyme degrade the wax layer.
Fungi and bacteria that enter through wounds don’t
produce cutinase, when other produced cutinase
enzyme to penetrate the cuticle.
• Ex. Penicillium spinulosm produced 2 cutinolytic enzymes viz.,
cutin esterase and cutin peroxidase to degrade cuticle.
• The other fungi reported to produce cutinase during pathogenesis
are Colletotrichum gloeosporioides on orange leaves & papaya
fruits;

11. Cell wall degrading enzymes
• The concept of cell wall degrading enzymes in induced in
plant cell by biotic agent was introduced for first time by de
Bary (1886)
• The celluloses, pectins, hemicellulose, lignin and protein
present in the cell wall are degraded respectively by
cellulolytic, pectin, hemicellulolytic, lignolytic and proteolytic
enzymes.

12. Cellulolytic enzymes (Cellullase)
• The pathogen cause root rot (Rhizoctonia solani, Macrophomina
phaseolina);
• Wilt (Fusarium, Verticillum sp, Sclerotium sp.,
Ralstonia solanacearum etc)
• Soft rot –(Pectobacterium carotovorum , Botrytis
cinerea, Penicillium spp.,)
• Wood rotting (Polyporus versicolor) symptoms in plants
enzymes break down cellulose in the wood and discoloration
inside wood .
• Soft –rot fungi need fixed nitrogen in order to synthesize
enzymes- Chaetomium , Ceratocystis

13. The pectic enzymes are classified as
1. pectin methylesterase (PME), which converts pectin into
polygalacturonic acid by removing small methyl branches off the pectin
chains.
2. Polygalacturonase (PG)- split the pectic chain by adding a molecule of
water and breaking (hydrolyzing) the linkage between two galacturonan
molecules
a) exo-polygalacturonase (exo-PG), which cleaves polygalacturonic acids
in a terminal manner releasing monomeric products (galacturonic acids), and
b) endo-polygalacturonase (endo-PG), which cleaves polygalacturonic
acids in a random manner releasing oligogalacturonic acid.
3. pectin lyases (PL)- split the chain by removing a molecule of water from
the linkage, thereby breaking it and releasing products with an unsaturated
double bond
a) endopectinases - break the pectin chain at random sites and release
shorter chains,
b) exopectinases or can break only the terminal linkage of the chain and
release single units of galacturonan.

14. Pectinolytic or pectic enzymes
• Produced by fungi and bacteria
• Divide into two major groups viz., Pectinesterases (PE)
and Polygalacturonases (PG)
• Pectic enzymes are great importance in soft rot diseases in
which parenchymatous tissue of host plant is rapidly invaded and
the diseased area contains mass of soft rot, disorganized, water
soaked tissue due to maceration and dissolution of middle
lamella.
• Some organism that cause soft rot in this manner are
Pectobacterium carotovorum sub. sp. carotovorum ,
Penicillum expansum, Botrytis cinerea , Sclerotinia fructigena ,
Phythium spp., Phytophthora spp., Rhizopus spp., and
Rhizoctonia spp.
• Vascular pathogens like Fusarium, Verticillium and Ralstonia
solanacearum also produced pectolytic enzymes.

16. Leaf
anatomy
Cell wall parts Made up of Degrading enzymes
Cuticle cuticle Cutin, wax Cutinases
Cell wall Primary cell
wall
Cellulose, pectates For cellulose degradation
Cellulolytic enzymes or cellulases and
hemicellulases
Secondary cell
wall
Cellulose
Middle
lamella
Pectate Pectinases or pectiolytic enzymes such
a) Pectin methyl esterases,
b) poly galacturonases and
c) pectin lyases
Plamame
mbrane
Cell
membrane
Phospholipids Lipases and phospholipases A and
phospholipases B

17. THANK YOU