EPWS 310 - Plant Pathology

Lectures - Fall 2002

 


Lecture 6

Readings Chapter 5

How pathogens attack plants - in more detail...

A. It is a war out there!!!

 

 PATHOGEN

HOST PLANT

1. Penetration with pressure

Intricate cell wall and cuticle

2. Penetration with enzymes

Suberization, antifungal enzymes (Chitinases, B-glucanases). Deactivation of pathogen enzymes

3. Toxins

Detoxification by enzymes

4. Growth regulators and general virulence factors

Biochemical defense (Phytoalexins, hypersensitive response, phenols)

 B. Mechanical forces exerted by pathogens on host tissues

For a pathogen to exist it must enter the plant, obtain nutrients from the plant and neutralize the plant defenses.

1. What plant parasites enter by mechanical pressure??

2. Show overhead of cell wall diagram-

3. The parasite first adheres to the plant surface. The fungus forms an appressorium-a flattened bulb like structure. This increases the area of adherence between the two organisms. From the appressorium a penetration peg forms.

C. Chemical weapons of pathogens

The effects caused by pathogens in plants are largely chemical in nature.

Main groups:

Enzymes (most common)

Toxins (2nd most common)

Growth regulators (3rd most common)

Polysaccharides (least common)

All plant pathogens can produce these compounds except for the viruses and viroids. But viruses and viroids can stimulate the plant to produce these compounds.

1. Enzymes-large protein molecules that catalyze all the interrelated chemical reactions in a living cell.

(Show cell wall overhead)

Enzymatic degradation of cell wall substances

a. cuticular wax- no pathogens are known that produce enzymes that can degrade waxes. Fungi and parasitic higher plants apparently penetrate wax layers by means of mechanical force alone.

b. Cutin- an insoluble polyester of mostly branched derivatives of C16 and C18 hydroxy fatty acids.

Cutinases - esterases - they break the ester linkages between cutin molecules and release monomers as well as oligomers.

Many fungi and at least one bacterium have been shown to produce cutinases. Fungi produce small amts. of cutinase all the time. When this small amt. acts on the cutin, the cutin monmers cause a 1000x increase in the amt. of cutinase.

c. Pectic substances - constitute the main component of the middle lamella. They are polysaccharides consisting mostly of galacturonan molecules interspersed with rhamnose molecules and side chans of galacturonan and other sugars.

Pectinases-pectolytic enzymes

*Pectin methyl esterases-removes small branches of pectin chain that does not effect the pectin chain length, but this does affect the solubility and open it up to attack by other enzymes.

*Polygalacturonase-chain splitting pectinase. Adds a water molecule hydrolyzing the linkage between 2 galacturonan molecules.

*Pectic lyases or transeliminases- Split the chain by removing a molecule of water.

* Endo-pectinases vs. exo-pectinases.

Autocatalytic induction-the galacturonan monomers serve as inducers for enhanced syntheses and release of pectolytic enzymes which further increase the amount of galacturonan monomers.

When the monomer concentrations become too high, catabolite repression occurs and the production of enzymes is stopped.

*damage done by pectic enzymes-tissue maceration-separation of cell walls. Occlusion of vessels. Weaken the primary cell wall and upset the osmotic balance of the cell causing the cell to burst.

d. Cellulose-consist of chains of glucose molecules. Several Cellulases- C1-attacks native cellulose by cleaving cross linkages between chains.

C2-breaks the chains down farther.

CX-breaks the chains down into disaccharide cellobiose.

-glucosidase- breaks disaccharide into glucose.

e. Hemicelluloses-complex mixtures of polysaccharide polymers, the composition and frequency of which seem to vary among plant tissues, plant species, and with the developmental stage of the plant. They are a primary component of the cell wall and may also be in the middle lamella and the secondary wall.

*many different kinds of hemicellulases-xylanase, galactanase, glucanase, arabinase, mannase...................................

f. Lignin - Found in the middle lamella, in the cell wall of xylem vessels, and in the fibers that strengthen plants.

Structure of lignin-phenylpropanoid.

Some basidiomycetes produce ligninases.

 Enzymatic degradation of substances contained in plant cells

Proteinases-

Amylases- starch

Lipases, phospholipases-oils and fats, membranes.

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