Plants have developed complex defense mechanisms against pests and pathogen attack. The diverse array of defense mechanisms includes the use of structural (cell wall) and biochemical defense (Hématy et al., 2009), production of antimicrobial phytoalexins (van Etten et al., 1989; Hammerschmidt and Dann 1999), pathogenesis-related proteins (Linthorst, 1991; Ponstein et al., 1994; van Loon et al., 2006), a large group of cysteine-rich proteins such as lipid transfer protein (Garcia-Olmedo et al., 1995), and plant defensins (Broekaert et al., 1995; Terras et al., 1995). Plant defensins are highly stable, small cationic peptides of 45–54 amino acid residues, these cysteine-rich peptides can form three to four disulfide bridges. They constitute a part of the innate defense system primarily directed against fungal pathogens. Broekaert et al. (1995) coined the term “plant defensin,” after comparison of a new class of plant antifungal peptides with known insect defensins. The first members of the family of plant defensins were isolated from wheat and barley grains (Colilla et al., 1990; Mendez et al., 1990). In Arabidopsis thaliana, at least 13 putative plant defensin genes (PDF) are present, encoding 11 different plant defensins (Thomma et al., 2002).
Defensins can be produced by plant, fungi, human, other mammals, birds, reptiles, fish, mollusks and arthropods (Thomma et al., 2002; Wong et al., 2007; Carvalho and Gomesa 2009 ). Plant defensins and defensin-like peptides are functionally diverse, regulating plant growth and development, disrupting microbial membranes, acting as ligands for cellular recognition and signaling (Stotz et al., 2009a ; Stotz et al., 2009b; Okuda et al., 2009). Besides these the variety of functions attributed to plant defensins include antifungal activity (Terras et al., 1995), antibacterial activity ( Zhang and Lewis 1997; Segura et al., 1998) and inhibition of α-amylases and proteases (Bloch and Richardson 1991: Wijaya et al., 2000).
Plant defensins are best characterized in seeds (Lay and Anderson 2005). During seed germination the seed loses its seed coat protection and becomes vulnerable to soil microorganism. Defensins being relatively abundant in seed tissue can protect seed from soil fungi resulting in enhanced seedling survival rate and apart from this they can also protect plant from microbial infection (Terras et al., 1995; Carvalho and Gomesa 2009). In addition, plant defensins are also localized in the xylem, stomata, stomata cells, parenchyma cells, and other peripheral areas (Kragh et al., 1995; Segura et al., 1998; Chen et al., 2002).
Plant defensin NaD1 (from the flowers of Nicotiana alata) targets filamentous fungi, the interaction begins with the fungal cell wall, followed by permeabilization of the plasma membrane and subsequent entry of the defensin into the cytoplasm (van der Weerden et al., 2008). Defensins interact with fungal-specific lipid components in the plasma membrane. The structural differences between membrane of fungi and plant cells probably account for the selective action of plant defensins against fungal pathogens and being nonphytotoxic (Thevissen et al., 2003).
Plant defensins show a constitutive pattern of expression in response to pathogen attack, injury and abiotic stresses (Bahramnejad et al., 2009; de Beer and Vivier, 2011). Antimicrobial microbial peptides (AMPs) display resistance against bacteria, fungi and viruses. In plant eight main classes of AMPs are cyclotides, lipid transfer proteins, defensins, thionins, snakins, hevein-like, vicilin-like, and knottins (Goyal and Mattoo, 2014). Variety of function attributed to plant defensin shows its possibility to be used as a commercial product in suppression of pathogens.
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