Peptides and proteins with antimicrobial activity are produced in bacteria, fungi, plants, invertebrates and vertebrates. These proteins contribute to constitutive or induced defense mechanism against microorganisms (Marx 2004). Many antimicrobial peptides (AMPs) have the ability to kill pathogens while others have the capability of modulating the host defense system (Mahlapuu et al., 2016). Prediction of function rely on many proteins of known sequence and structures (Whisstock and Lesk 2003). Extracellular plant peptides perform variety of functions including signaling and defense. Plant peptides, protein molecules smaller than 10 kDa are divided into two categories:
- Bioactive peptides produced by selective action of peptidases on larger precursor proteins. This group has a role in plant growth regulation through cell to cell signaling, endurance against pest and pathogens by acting as toxins or elicitors and detoxification of heavy metals by sequestration
- Degraded peptides that results from the activity of proteolytic enzymes during protein turnover.
Both groups are the product of proteolysis and they differ on the basis of how they act within the cell. The second group may play a role in nitrogen mobilization across cellular membranes (Farrokhi et al., 2007).
Cystine-rich plant AMPs are defensins (PR-12 family), thionin (PR-13 family), lipid transfer proteins (PR-14 family), hevein- and knottin-like peptides, α-hairpinin, snakins and cyclotides (Egorov and Odintsova 2012; Tam et al., 2015). Plant AMPs are rich in cysteine residues which forms multiple disulfides. The presence of disulphide bonds stabilizes the structure (Nawrot et al., 2014). Antimicrobial peptides are defense peptides which are short and positively charged. They are generally fewer than 50 amino acid residues (Montesinos 2007). The positive charge and amphipathic nature of AMPs are related to their defensive roles allowing AMPs to interact with membrane lipids (Nawrot et al., 2014). Based on their cationic character defensin interact with negatively charged plasma membrane components of sensitive microorganisms (Hegedus and Marx 2013).
Thionins are present in seeds to protect the seed and germinating seedlings from attack by phytopathogens (Hughes et al., 2000). Synthesis of thionin can be triggered by pathogens indicating thionin are inducible plant protein which are involved in plant defense against plant pathogens. The leaf-specific thionins of barley are toxic to plant pathogenic fungi and suggest that they are naturally occurring as well as inducible plant protein (Bohlmann et al., 1988). Membrane lysis is the first effect exerted by plant toxin thionin that initiates cascade of cytoplasmic events leading to cell death (Stec et al., 2004). Purothionins are basic polypeptides with antimicrobial properties and are found in the endosperm of wheat and other cereal species. Susceptibility of phytopathogenic bacteria to wheat purothionins has been observed (de Caleya et al., 1972). Richard et al.(2005) suggest that toxicity of beta-purothionin is associated with the formation of functional channels in cell membrane rather than with lytic phenomenon.
Plant defensins denotes defense-related proteins. Small cysteine-rich proteins Raphanus sativus-antifungal protein 1 (Rs-AFP1) and Rs-AFP2 from radish exhibits antifungal activity in vitro (Terras et al., 1995). Novel antimicrobial peptides Fa-AMP1 and Fa-AMP2 were purified from seeds of buckwheat (Fagopyrum esculentum Moench.) (Fujimura et al., 2003). Antifungal protein PAF from Penicillium chrysogenum inhibits growth of opportunistic zoo-pathogens including Aspergillus nidulans and many plant pathogenic fungi. PAF elicits hyperpolarization of the plasma membrane and the activation of ion channel followed by an increase in reactive oxygen species in the cell (Marx et al., 2008). Changes in the membrane potential may influence the activity of ion channels/pumps/transporters or be a result of their activation (Hegedus and Marx 2013). Observation suggest that translocated peptides may alter cytoplasmic membrane septum formation, inhibit cell wall, nucleic acid and protein synthesis (Brogden 2005).
α-Hairpinins are short cysteine-rich peptides with antifungal and antibacterial activity, whereas, there are some members with trypsin inhibitory and ribosome inactivation activities (Slavokhotova and Rogozhin 2020). Hevein-like peptides binds chitins while knottins-type peptides inhibit proteases enzyme and lipid transfer proteins binds lipids to disrupt microbial penetration into cell membranes (Tam et al., 2015). Hevein-like AMPs has antifungal activity and the chitin-binding site interacts with chitin of fungal cell walls (Odintsova et al., 2020). The radish seed nonspecific lipid transfer protein-like protein inhibits the growth of fungi in vitro (Terras et al., 1992).
The nucleopolypeptide derivatives polyoxins, nikkomycins, blasticidin and mildiomycin are pseudopeptides with antifungal activity (Copping and Menn 2000; Montesinos 2007). Polyoxins are commercially used to control fungal disease in plant and are pyrimidinyl dipeptides that inhibit chitin synthesis in Alternaria spp., Botrytis cinerea and Rhizoctonia solani. Nikkomycins are pyridinyl derivatives similar to polyoxins. Blasticidin inhibits protein biosynthesis in prokaryotes and also has activity against Pyricularia oryzae (Montesinos 2007). Wide range of material and organisms can be classified as biopesticide (Copping and Menn 2000). AMPs secreted by several microorganisms can be used as biopesticides.
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