Fundamentals of Plant Pathology

ENZYMES

• Enzymes are generally large protein molecules that catalyze organic reactions in living cells and in solutions.
• Because most kinds of chemical reactions that occur in a cell are enzymatic, there are almost as many kinds of enzymes as there are chemical reactions.
• Each enzyme, being a protein, is coded for by a specific gene.
• Some enzymes are present in cells at all times (constitutive).
• Many are produced only when they are needed by the cell in response to internal or external gene activators (induced).
• Each type of enzyme often exists in several forms known as isozymes that carry out the same function but may vary from one another in several properties, requirements, and mechanism of action.

Plant Substances and Their Enzymatic Degradation

• Aerial plant part surfaces consist primarily of cuticle and/or cellulose, whereas root cell wall surfaces consist only of cellulose.
• Cuticle consists of cutin, wax and covering of layer of wax
• The lower part of cutin contains pectin, cellulose lamellae and a thin layer of pectic substances; below that there is a layer of cellulose.

Cuticular wax

• Plant waxes are found as granular, blade, or rod-like projections or as continuous layers outside or within the cuticle of many aerial plant parts.
• The presence and condition of waxes at the leaf surface affect the degree of colonization of leaves and the effect varies with the plant species.
• Electron microscope studies suggest that several pathogens, e.g., Puccinia hordei, produce enzymes that can degrade waxes.

Cutin

• Main component of the cuticle.
• The upper part is admixed with waxes.
• The lower part is admixed with pectin and cellulose.
• Cutin is insoluble polyester of 16- and 18-carbon hydroxy fatty acids.
• Many fungi and a few bacteria produce cutinases and/or nonspecific esterases, i.e., enzymes that can degrade cutin.
• Cutinases break cutin molecules and release monomers (single molecules) as well as oligomers (small groups of molecules) of the component fatty acid derivatives from the insoluble cutin polymer.

Pectic substances

• Main components of the middle lamella, i.e., the intercellular cement that holds in place the cells of plant tissues.
• 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.
• Are polysaccharides consisting mostly of chains of galacturonan molecules interspersed with a much smaller number of rhamnose molecules and small side chains of galacturonan, xylan and some other five carbon sugars.

Degradation of pectic substances

• Several enzymes degrade pectic substances and are known as pectinases or pectolytic enzymes.
• The pectin methyl esterases remove small branches off the pectin chains.
• Pectin methyl esterases have no effect on the overall chain length, but they alter the solubility of the pectins and affect the rate at which they can be attacked by the chain-splitting pectinases.
• Pectinases cleave the pectic chain and release shorter chain portions containing one or a few molecules of galacturonan.
• Some chain splitting pectinases, called polygalacturonases, split the pectic chain by adding a molecule of water and breaking (hydrolyzing) the linkage between two galacturonan molecules.
• Others, known as pectin lyases, split the chain by removing a molecule of water from the linkage, thereby breaking it and releasing products with an unsaturated double bond.

Cellulose

• Cellulose - a polysaccharide, but consists of chains of glucose (1–4) ß-d-glucan molecules.
• The glucose is produced by a series of enzymatic reactions carried out by several cellulases and other enzymes.
• One cellulase (C1) attacks native cellulose by cleaving cross-linkages between chains.
• A second cellulase (C2) also attacks native cellulose and breaks it into shorter chains.
• These are then attacked by a third group of cellulases (Cx), which degrade them to the disaccharide cellobiose.
• Finally, cellobiose is degraded by the enzyme ß -glucosidase into glucose.

Cross-linking glycans (hemicelluloses)

• The enzymatic breakdown of hemicelluloses requires the activity of many enzymes.
• Several hemicellulases seem to be produced by many plant pathogenic fungi.
• Depending on the monomer released from the polymer on which they act, the particular enzymes are called xylanase, galactanase, glucanase, arabinase, mannase, and so on.
• The non-enzymatic breakdown of hemicelluloses by activated oxygen, hydroxyl, and other radicals produced by attacking fungi also occurs.

Suberin

• Suberin is found in certain tissues of various underground organs, such as roots, tubers, and stolons, and in periderm layers, such as cork and bark tissues.
• Suberins are also formed in response to wounding and to pathogen-induced defenses of certain organs and cell types.
• Although plants obviously produce enzymes that synthesize suberin, it is not known whether or how pathogens break it down during infection.

Lignin

• Found in the middle lamella, as well as in the secondary cell wall of xylem vessels and the fibres that strengthen plants.
• Also found in epidermal and occasionally hypodermal cell walls of some plants.
• The lignin content of mature woody plants varies from 15 to 38% and is second only to cellulose in abundance.
• Lignin is an amorphous, three-dimensional polymer that is different from both carbohydrates and proteins in composition and properties.
• It is obvious that enormous amounts of lignin are degraded by microorganisms in nature, as is evidenced by the yearly decomposition of all annual plants and a large portion of perennial plants.

Cell wall flavonoids

• Flavonoids are a large class of phenolic compounds that occur in most plant tissues and, especially, in the vacuoles.
• Also occur as mixtures of single and polymeric components in various barks and heartwoods.
• Among the various functions of flavonoids, some act as signaling molecules for certain functions in specific plant-microbe combinations.

Cell wall structural proteins

• Cell walls consist primarily of polysaccharides, i.e., cellulose fibres embedded in a matrix of hemicelluloses and pectin, but structural proteins, in the form of glycoproteins, may also form networks in the cell wall.
• Four classes of structural proteins have been found in cell walls.
• hydroxyproline-rich glycoproteins (HRGPs)
• proline-rich proteins (PRPs)
• glycine-rich proteins (GRPs)
• arabino-galactan proteins (AGPs)

Enzymatic degradation of substances contained in plant cells

• Most kinds of pathogens spend all or part of their lives in association with or inside the living protoplast.
• These pathogens obviously derive nutrients from the protoplast.
• The great majority of fungi and bacteria -obtain nutrients from protoplasts after the latter have been killed.
• Some of the nutrients, e.g., sugars and amino acids are sufficiently small molecules to be absorbed by the pathogen directly.
• Some of the other plant cell constituents, however, such as starch, proteins and fats can be utilized only after degradation by enzymes secreted by the pathogens.

Proteins

• Plant cells contain innumerable different proteins, which play diverse roles as catalysts of cellular reactions (enzymes) or as structural material (in membranes and cell walls).
• Proteins are formed by the joining together of numerous molecules of about 20 different kinds of amino acids.
• All pathogens seem to be capable of degrading many kinds of protein molecules.
• The plant pathogenic enzymes involved in protein degradation are similar to those present in higher plants and animals and are called proteases or proteinases or, occasionally, peptidases.

Starch

• Starch is a glucose polymer and exists in two forms:
• amylose, an essentially linear molecule
• amylopectin, a highly branched molecule of various chain lengths
• Most pathogens utilize starch and other reserve polysaccharides in their metabolic activities.
• The degradation of starch is brought about by the action of enzymes called amylases.
• The end product of starch breakdown is glucose and is used by the pathogens directly.

Lipids

• Various types of lipids occur in all plant cells, with the most important being phospholipids and glycolipids, both of which, along with proteins, are the main constituents of all plant cell membranes.
• The latter form a hydrophobic barrier that is critical to life by separating cells from their surroundings and keeping organelles such as chloroplasts and mitochondria intact and separate from the cytoplasm.
• Oils and fats are found in many cells, especially in seeds where they function as energy storage compound.
• The common characteristic of all lipids is that they contain fatty acids, which may be saturated or unsaturated.
• Several fungi, bacteria, and nematodes are known to be capable of degrading lipids. Lipolytic enzymes, called lipases, phospholipases, and so on, hydrolyze the lipid molecules with liberation of the fatty acids.

Microbial Toxins in Plant Disease

• Toxins are extremely poisonous substances and are effective in very low concentrations.
• Fungi and bacteria may produce toxins in infected plants as well as in culture medium.
• Toxins injure host cells either by affecting the permeability of the cell membrane or by inactivating or inhibiting enzymes and subsequently interrupting the corresponding enzymatic reactions.
• Certain toxins act as antimetabolites and induce a deficiency for an essential growth factor.