Plant immunity include both basal and inducible mechanisms. Antimicrobial peptides (AMPs) are oligopeptides with varying number of amino acids (from 5 to over a hundred) and provides resistance against bacteria, fungi or viruses (Bahar and Ren 2013). AMPs are diverse group of molecules produced in invertebrate, plant, animal and humans. AMPs regardless of their origin have similar properties such as positive charge, small in size and being amphipathic, which enables them to attach and insert into the membrane bilayer to form pores (Andreu and Rivas 1998; Brogden 2005). Translocated peptides may alter cytoplasmic membrane septum formation, inhibit synthesis of cell-wall, nucleic-acid and protein or inhibit enzymatic activity (Brogden 2005). Historically AMPs are referred to as follows (Bahar and Ren 2013):
- Cationic host defense (Brown and Hancock 2006)
- Anionic antimicrobial peptides/ proteins (Harris et al., 2009)
- Cationic amphipathic peptides (Groenink et al., 1999)
- Cationic AMPs (Bradshaw 2003)
- Host defense peptides (Riedl et al., 2011) and
- α-helical antimicrobial peptides (Huang et al., 2010).
Fungal cell wall contains chitin and some of antifungal peptides have the ability of binding to chitin (Yokoyama et al., 2009; Pushpanathan et al., 2012). Antifungal peptide bind to conidia wall constituent (De Lucca et al., 1998). Systemic activation of set of genes encode antimicrobial proteins in plant on being challenged by pathogen. Activation of gene depends on the signal produced by phytohormone ethylene and jasmonic acid (Penninckx et al., 1998).
Different AMPs are thionins, plant defensin, hevein-like peptides, knottin type peptides, α-hairpinin, lipid transfer protein, snakins and cyclotide family (Li et al., 2021). Plant defensin have antifungal activity. Cowpea-thionin II displays antifungal activity by permeabilizing membrane of fungal hyphae (Schmidt et al., 2019).
Thionins: Thionins are inducible plant protein involved in plant defense mechanism against pathogen infection (Bohlmann et al., 1988). Gamma-thionin or defensins are small proteins. They are able to inhibit digestive enzymes or act against bacteria or fungi (Pelegrini and Franco 2005). Leaf specific thionins of barley (Hordeum vulgare L.) is a cell wall protein, toxic to plant pathogenic fungi. Immunogold-labelling of thionins in several barley cultivar indicate resistance or susceptibility may be attributed to the presence or absence of thionins at the penetration site in walls and papillae of epidermal leaf cell (Ebrahim-Nesbat et al., 1989).
Hevein: Hevein-like peptides was identified in the latex of the rubber tree Hevea brasiliensis. The chitin-binding protein hevein demonstrate antifungal activity (Van Parijs et al., 1991).
Knottin: Cystine knot peptide from Cactaceae (cactus) family show bactericidal action, liposome leakage and membrane permeabilisation (Aboye et al., 2015). The typical structure of knottins involve conserved disulphide bonds between multiple cysteine pairs forming a cysteine knot (Li et al., 2021). Knottin are small sized and highly stable.
α-Hairpinin: α-hairpinin are component of plant innate immunity (Slavokhotova and Rogozhin 2020).
Lipid transfer protein (LTP): Elicitins and lipid-transfer proteins are small cysteine-rich lipid-binding proteins secreted by oomycetes and plant cells and have role in plant defense. Studies suggest that elicitins and lipid-transfer proteins share the same biological receptor indicating the role of lipid binding protein, in early recognition of the plant pathogen (Blein et al., 2002). The lipid-transfer proteins from Pisum sativum Ps-LTP1exhibits antifungal activity (Bogdanov et al., 2016).
Snakins: AMP snakin-1 (SN1) from potato tuber was active against bacterial and fungal pathogens. SN1 may be a component of constitutive defense barrier of storage and reproductive plant organs (Segura et al., 1999). Snakin-2 from potato (Solanum tuberosum cv Jaerla) (StSN2) is a component of both constitutive and inducible defense barriers and is found to be active against fungal and bacterial pathogens (Berrocal-Lobo et al., 2002).
Cyclotides are a novel family of backbone-cyclized cysteine-knot containing peptides from plants that possess insecticidal activity against Helicoverpa larvae (Colgrave et al., 2008). Cycloviolacin O2 (CyO2) was the most active cyclotide demonstrating the antibacterial activity (Pranting et al., 2010).
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