Plants genetic makeup ensures their survival. Despite having no immune system plants can resist infection by constitutive or induced defense mechanism. The synthesis of plant protein induced due to pathological or related situation are referred to as pathogenesis-related (PR) protein (Antoniw et al., 1981). PR protein play an important role in induced resistance and are also accumulated on mechanical wounding (Chang et al., 1995) and abiotic stresses (Ernst et al., 1992; Green and Fluhr 1995). The physicochemical properties of PR proteins enables them to resist the acidic pH and proteolytic cleavage thus ensuring their survival in vacuolar compartment or cell wall or intercellular spaces (Stintzi et al., 1993). Plant pathogen and insect elicit PR protein biosynthesis and affect their development (Conrads-Strauch et al., 1990; Ryan 1990). Most PR proteins are induced through the action of the signaling compounds salicylic acid, jasmonic acid, or ethylene and possess antimicrobial activities (van Loon et al., 2006). The rhizosphere microorganisms or some plant growth promoting rhizobacteria induce PR proteins (Park and Kloepper 2000). Though 17 families of pathogenesis-related proteins have been classified. The 14 families of PR protein (van Loon and van Strien 1999) mentioned are as follows:
PR-1 found to display differential fungicidal activity (Niderman et al., 1995).
The PR-2 family consists of endo- β-1,3-glucanases and PR-3, PR-4, PR-8 and PR-11 are all classified as endochitinases. Type of chitinases are distinguished by class i.e. based on their activities over a wide range of substrate. Class III (PR-8) are basic isoform possessing substantial lysozyme activity. Chitinases and β-1,3-glucanases degrade fungal cell wall.
PR-5 belongs to thaumatin-like proteins (homology to permatin), permeabilize fungal membrane (Vigers et al., 1991).
PR-6 are proteinase inhibitors. They are involved in defense against insect, other herbivores, microorganisms and nematodes (Koiwa et al., 1997; Ryan 1990).
PR-7 acts as endoproteinase. Lysis of cell wall i.e. degradation of cell wall protein in addition to hydrolysis of chitin and glucan (Haran et al., 1996).
PR-9 strengthens plant cell wall by catalyzing lignin deposition in response to microbial attack. Peroxidases in plant cell is associated with lignin biosynthesis in response to pathogen attack (Ostergaard et al., 2000).
PR-10 is structurally related to ribonucleases.
PR-12 are plant defensins (antimicrobial activities).
PR-13 protein are thionins (antimicrobial activities). The electrostatic interaction of the positively charged thionin with the negatively charged phospholipids making up the membrane forms pore which leads to disruption of the membrane (Florack and Stiekema 1994).
PR-14 are lipid transfer proteins (LTPs). They exhibit antifungal and antibacterial activity. LTPs are active plant-defense protein (García-Olmedo et al., 1995).
In some cases PR-4 and PR-10 exhibit ribonuclease activity and this activity is related to their fungicidal properties. These proteins inhibit the growth of fungi (Filipenko et al., 2013). The antifungal activity of PR-ribonucleases have cytotoxic impact by degrading the mRNA pool of fungi (penetration of nucleases into the cell of the pathogen) as well as the PR- ribonucleases participate in induction of plant cell apoptosis leading to hypersensitive responses restricting the spread of the pathogen (Filipenko et al., 2013).
Compost made up of agricultural residue provide protection against soil –borne pathogens (Hoitink et al., 1996; Hoitink and Boehm 1999). The compost mediated induced resistance may be due to the activation of plant defense related genes and accumulation of peroxidases, β-1,3-glucanases and pathogenesis–related protein PR-1 (Zhang et al., 1998; Kavroulakis et al., 2005). Colonization of plant with rhizobacteria leads to induced systemic resistance (ISR) providing protection against pathogen. The rhizosphere fungi and bacteria exhibit antagonistic property. Kavroulakis et al., (2006) reported that expression of PR genes may be triggered by the microflora of the compost or could be associated with abiotic factor of the compost. Though the PR protein role in plant defense has been studied but there are few PR proteins whose biological functions are unknown.
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