(Plant-Pathogen Interaction Part I: Enzymes in Plant Pathology)
What are the enzymes involved in pathogenesis (plant pathology)?
Most of the fungal and bacterial parasites produce many enzymes that degrade the plant materials in vivo. Enzymes involved in pathogenesis or virulence (process of disease initiation) includes both constitutive and inducible enzymes.
(1). Constitutive enzymes are those enzymes which are present all the time in the cells.
(2). Inducible enzymes are those enzymes which are produced only when they are needed by the cells in response to certain internal or external stimuli.
Important classes of enzymes involved in pathogenesis are:
Plant surface are coated with thick or thin layer of cuticle for protection and preventions of water loss. Cuticle layer over the plant epidermis is usually impregnated and covered with wax. Cuticle is chiefly made up of cutin, a structural polymer composed of esters of hydroxy and hydroxyepoxy fatty acids. Cutinases are a group of enzymes which hydrolyse plant cuticle. Biochemically, cutinase enzyme belongs to serine hydrolase family. Many plant pathogenic fungi and few bacteria are known to produce cutinase enzyme during pathogenesis. Cutinase enzymes break the cuticle and release monomers and oligomers. These monomers enter into the cell and triggers further expression of cutinase genes. Cutinase enzyme is very essential for the direct penetration of host cuticle by pathogenic fungi. Highest concentrations of cutinase are present in the penetration point of hyphae and at the infection peg of appressorium.
Pectic substances are the components of middle lamella and they act as the intercellular cementing material between plant cells. Pectic substance also makes a large portion of the primary cell wall. In primary cell wall, pectin forms an amorphous gel filling the space between cellulose micro-fibrils. Plant pectin is a polysaccharide composed of large number of galacturonic acid residues. For the successful entry of pathogen into the host cells the degradation of the cell wall components is very essential. Many pathogenic fungi can produce pectinase enzyme during the process of infection. Pectin degradation by the enzyme pectinase results in liquefaction of the pectic substances and wakening of cell wall, leading to tissue maceration. Pectinases are the chief enzyme involved in soft rotting of tissues by fungi. The soft rotting and liquefaction of tissues facilitate the rapid invasion and establishment of pathogen.
Two classes of pectinases are known to be involved in plant pathogenesis.
(1). Pectin esterases (PE) or Pectin Methylesterases (PME): which removes small branches of the pectin chain
(2). Polygalacturonases (PG) or Pectic glycosidases: splits pectin chain to shorter chains of one or few molecules of galacturonic acids.
Galacturanan monomers produce by the pectin degradation enter into the cells and triggers more pectinase production in the pathogen. These monomer units which are absorbed by the pathogen act as the inducer for the mass production of pectinase in the cell.
Unlike animal cells, the plant cells are protected with a cellulosic cell wall. In order to establish the entry into host cell, the pathogen should break the thick cellulosic cell wall present around the cell. The ability to produce cellulase enzyme is a major characteristic of plant pathogens of bacterial and fungal origin. Cellulases are a class of enzyme which degrades plant cellulose, the chief component of cell wall. Different class cellulases are:
(1). Cellulase-C1: attack native cellulose and cleave cross linkage between chains
(2). Cellulase-C2: also attack native cellulose and breaks it into shorter chains
(3). Cellulase-Cx: enzyme attack shorter chains of celluloses and hydrolyzes to disaccharide units called cellobiose
(4). β-glucosidase: Cleave cellobiose and release glucose units.
Hemicellulose is complex mixtures of plysaccharide present along with cellulose and forms a major constituent of the primary cell wall, middle lamella and secondary cell wall of plants. Hemicellulases are the enzymes produced by many plant pathogens for the degradation of hemicellulose components in the cell wall. Several types of hemicellulases are produced by many plant pathogenic fungi. The variation in the hemicellulase produced by the pathogenic fungi is due to the variation of the monomer units released from the polymer. Major categories of hemicellulases involved in pathogenesis are (1). xylanase, (2). galactanase, (3). glucanase, (4). arabinase and (5). mannase.
Lignin is a complex organic polymer present in the secondary cell wall. Lignin is a complex polymer and the most important structural unit is a phenylpropanoid with one or more of its carbons having –OH, -O-CH3 or = O groups. Lignin is considered as the most resistant component in the plants. Ligninase are enzymes which degrade the lignin of the secondary cell wall. Only very small group micro-organisms are capable of degrading lignin. Mostly species of fungi belongs to the Basidiomycetes of the class Hymenomycetes are capable of producing ligninase enzyme and degrade lignin.
Liginase producing fungi cause a characteristic symptom called brown rot. The ligninase activity is the reason for wood decay by fungal infections. Even though fungi can degrade lignin, they cannot use the lignin as a food source. Apart from Basidiomycetes fungi, few white-rot fungi are also known to produce ligninase. This group of fungi can utilize lignin as a food source.
Lipases are the enzymes which hydrolyse the lipid bilayer of plasma membrane. Some bacterial and fungal pathogens are known to produce lipase enzyme.
In order to establish the pathogenisis, the pathogens are required to degrade many proteins of host origin. These proteins not only include the structural proteins of plasma membrane of the host; but also include many enzymes of host origin such as nucleases, host proteinases etc.. Almost all pathogenic fungi and bacteria can produce many proteinases.
These are the major enzymes involved in pathogenesis. Most of these enzyme productions in the parasite are triggered by small molecules of host origin. These small molecules, called inducers or elicitors, enter the parasite cell and they specifically interact with some target proteins such as transcription factors. The activated transcription factors then induce the mass production of concerned enzyme. The coordinated activities of all the above mentioned enzymes are necessary for the successful induction infection in plants.
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