Enzymes

Role of Enzymes and Toxin in Disease Development  

                              Enzymes  

The activities of pathogen in plants are largely chemical in nature. The effects caused by the pathogens on plants are almost entirely the result of biochemical reactions taking place  
between substances secreted by the pathogens and those present in, or produced by the plants. 

 
Among the plant pathogens, all except viruses and viroids are not known to produce any  
substances themselves, but they induce the host cell to produce either excessive amount of certain substances already found in the healthy host cell or substances completely new to the host. Some of the substances are enzymes, growth regulators and toxins. In general,plant pathogenic enzymes disintegrate the structural components of the host cell, break down inert food substances in the cell, or affect the protoplast directly and interfere with its functioning system.

  

Enzymes are large protein molecules that catalyze all the interrelated reactions in the living  cells. For each kind of chemical reaction that occurs in cell, there is a different enzyme that catalyzes that reaction. Each enzyme being protein is coded for by specific gene. Some enzymes are present in cell at all times that is known constitutive enzyme. Many are produced only when they are needed by the cell in response to internal are external gene activators are called induced enzymes. Each type enzyme often exists in several forms known as isozymes.  

I. Enzymatic Degradation of Cell Wall Substances:  

1. Cuticular Wax: Plant waxes found as granular, blade, or rod like projections or as  continuous layers outside or within the cuticle. Puccinia hordei produce enzymes that  
degrade waxes. Another fungus Pestalotia malicda is also degrade the cuticular waxes 

 
2. Cutin: Cutin is the main component of the cuticular layer. The upper part of the cuticleis admixed with waxes, whereas its lower part, in the region where it merges into the outer walls of epidermal cells, is admixed with pectin and cellulose. Many fungi (Fusarium spp.) and at least one bacterium (Streptomyces scabies) have been shown to produce cutinases (enzymes that can degrade Cutin).

  
3. Pectic Substances: Pectic substances constitute the main component of the middle  
lamella i.e. the intercellular cement that holds in place the cells of plant tissues. Several  
enzymes known as pectinases or pectolytic enzymes degrade pectic substances. Pectic  
substances are degraded by the following three kinds of enzymes.

 
(i) Pectin methylesterases (PME): These enzymes remove small branches off the  
pectin chains and yield the linear chains of pectic acid. They remove the methyl ester from the branches of the pectic substances, and yield the polymers of pectinic acid of low methoxy content (which is later broken down into pectic acid) and methyl group.

  
(ii) Polygalacturonases (PG): These enzymes split the pectic acid chains and yield the monomers of galacturonic acid.

  
(iii) Pectin transeliminases (PT): These enzymes also act upon the pectic acid chain  
and the monomers of galacturonic acid

4. Cellulose: Cellulose is the major component and is the basic unit of structural  
framework of the plant cell walls. Cellulose degrading enzymes, cellulases have been  
shown to be produce by several phytopathogenic fungi, bacteria and nematodes. C1, Cx and β - glucosidase are the three enzymes that are involved in cellulose degradation.  
The cellulose content of tissues of grasses about 50%, in mature wood tissues to more  
than 90% in cotton fibers.

  
5. Hamicellulose: Hemicelluloses are the important part of mature and thickened cell  
walls of plants. They contain complex mixtures of such pentosans as xylans, mannans, galactans and arabans. Hamicellulases are the enzymes that degrade hemicelluloses in the plant cell walls. Certain components of hemicelluloses are degraded by cellulolytic enzymes also.

  
6. Lignin: Lignin is the most complex chemical compound in plant cell walls. Lignin is the found in the middle lamella as well as in the secondary cell wall of xylem vessels and fibrous that strengthen plants. Lignins are degraded by lignolytic or lignases enzymes that are mostly produced by white rot (wood rotting) fungi of Subdivision-Basidiomycotina. Among the phytopathogenic bacteria, the species that produce lignolytic enzymes are the species of Pseudomonas and Xanthomonas. 

 

II. Enzymatic degradation of Substances Contained in the Plant cell:  

Some of the nutrients e.g. sugars and amino acids are molecules sufficiently small to be absorbed by the pathogen directly. Some of the other plant constitutes, however, such as starch, proteins, and fats can be utilized only after degradation by enzymes secreted by the pathogen. 

 
1. Protein: Proteins are formed by the joining together of numerous molecules of about  
20 different kinds of amino acids. Degradation of proteins is through the proteases or  
proteinases or occasionally peptidase. It is also in group known proteolytic enzymes. 

 
2. Starch: Main reserved polysaccharides found in cells. It is synthesized in the  
chloroplast and, in non photosynthetic organs, in the amyloplast. Starch is a glucose  
polymer and exists in two forms: i) amylase - an essentially linear molecule, and ii)  
amylopectin- highly branched molecule of various chain lengths. The degradation of  
starch is brought about by the actions of enzymes called amylases.  

3. Lipids: In plant, most important being phospholipids and glycolipids. Oils and fats are found in many cells, especially in seeds where they function as energy storage  
compounds, wax lipids are found on most aerial epidermal cells. Several fungi bacteria  
and nematodes are known to be capable of degrading lipids by producing the enzymes  
called lipases or lipolytic enzymes.

           Toxins in Plant Diseases  

 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 can be defined as low molecular weight non-enzymatic microbial products toxic to the higher plants. In Plant Pathology, the term toxin is used for a product of the pathogen, its host or pathogen-host interaction which even at very low concentration directly acts on living host protoplasm to influence the course of disease development or symptom expression.
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.  
Ludwig (1960) has defined toxin “as a product of microorganism or microorganismhost
interaction which acts directly on host protoplasts to influence either the course of disease development or symptoms expression”.

Classification of Toxins:  
Toxins are classified into three broad classes (Older Classification):- 

1. Phytotoxins:
Any compound produced by a microorganism which is toxic to plants is a phytotoxin. These  
are toxic to both, host as well as non-host plants (non-specific in action), incite few or none of the symptoms that are incited by the pathogens. For example, Alternaric acid produced by Aliernaria solani. 

 
2. Vivotoxins: Defined as substance produced by the pathogen in the infected host which  
is involved in the production of disease but not initial incitant of the discase itself. Vivotoxins  
may also induce only a part of disease symptoms. For example, Fusaric acid and Pyricularin. 

 
3. Pathotoxins: They are defined as host specific toxins which induce all the typical  
symptoms of the disease that is caused by the producer pathogen at reasonable concentration. These toxins may be produced by the pathogen, host or interaction between them; for example Victorin produced by the fungus Cochliobolus victoriae.
In recent classification, toxins are divided into two categories:  
(I) Non-Host Specific or Non-Host Selective Toxins  
(II) Host-Specific or Host-Selective Toxins

 (I) Non-Host Specific or Non-Host Selective Toxins: 
They affect many unrelated plant species in addition to the main host of the pathogen  
producing toxin. It includes phytotoxins and vivotoxins. Several toxic substances produced by phytopathogenic microorganisms have been shown to produce all or part of the disease syndrome not only on the host plant, but also on other species of plants that are not normally attacked by the pathogen in nature. Such toxins called non-host-specific or non-hostselective toxins.  
Some examples of non-host selective toxins are:  
Tabtoxin  
 Tabtoxin is produced by the bacterium Pseudomonas syringae pv. tabaci, which causes the wildfire disease of tobacco.  
Phaseolotoxin  
Phaseolotoxin is produced by the bacterium Pseudomonas syringae pv. phaseolicola,  
the cause of halo blight of bean and some other legumes.  
Tentoxin  
Tentoxin is produced by the fungus Alternaria alternata (previously called A. tenuis),  
which causes spots and chlorosis in plants of many species.  
Cercosporin  
Cercosporin is produced by the fungus Cercospora and by several other fungi. It causes damaging leaf spot and blight diseases of many crop plants, such as Cercospora leaf spot of zinnia and grey leaf spot of corn. 

 
Other non-host-specific toxins are  
• Fumaric acid- produced by Rhizopus spp. in almond hull rot disease  
• Oxalic acid -Sclerotium and Sclerotinia spp. in various plants they infect and by Cryphonectria parasitica, the cause of chestnut blight  
• Alternaric acid, alternariol, and zinniol -Alternaria spp. in leaf spot diseases of various plants  
• Ceratoulmin- Ophiostoma ulmi in Dutch elm disease  
• Fusicoccin- Fusicoccum amygdali in the twig blight disease of almond and peach trees  
• Ophiobolin -several Cochliobolus spp. In diseases of grain crops  
• Pyricularin- Pyricularia grisea in rice blast disease
• Fusaric acid and lycomarasmin -Fusarium oxysporum in tomato wilt  

(II) Host-specific or host-selective toxins  
They affect only the specific host of the pathogen. It includes pathotoxins. A host-
specific or host-selective toxin is a substance produced by a pathogenic microorganism that,  at physiological concentrations, is toxic only to the hosts of that pathogen and shows little or no toxicity against non-susceptible plants.  
Most host-specific toxins must be present for the producing microorganism to be able  
to cause disease. Host-specific toxins have been shown to be produced only by certain fungi (Cochliobolus, Alternaria, Periconia, Phyllosticta, Corynespora, and Hypoxylon), although certain bacterial polysaccharides from Pseudomonas and Xanthomonas have been reported to be host specific. Some examples of host selective toxins are:  
Victorin, or HV Toxin- Victorin, or HV-toxin is produced by the fungus Cochliobolus  
(Helminthosporium) victoriae, which causes Victoria blight of oats.

T Toxin [HMT Toxin]- T toxin is produced by race T of Cochliobolus heterostrophus  
(anamorph: Bipolaris maydis, earlier called Helminthosporium maydis), the cause of southern corn leaf blight.

 
HC Toxin- Race 1 of Cochliobolus carbonum (anamorph:Bipolaris(Helminthosporium)  
zeicola) causes northern leaf spot and ear rot disease in maize. It also produces the hostspecific HC toxin, which is toxic only on specific maize lines.

 
Alternaria alternata Toxins- Several pathotypes of Alternaria alternata attack different host plants and on each they produce one of several multiple forms of related compounds that are toxic only on the particular host plant of each pathotype.
• Some of the toxins and the hosts on which they are produced and affect  
• AK toxin causing black spot on Japanese peat fruit,  
• AAL toxin causing stem canker on tomato,  
• AF toxin on strawberry,  
• AM toxin on apple,  
• ACT toxin on tangerine,  
• ACL toxin on rough lemon, the HS toxin on sugar cane.  
• As an example of Alternaria alternata toxins, the AM toxin is produced by the apple  
pathotype of A. alternata, known previously as Alternaria mali, the cause of Alternaria leaf  
blotch of apple.