Plants normally grow on substrates that are extremely rich in microorganisms, but infection remains a rare event. During evolution, plants have developed a variety of defense systems to protect themselves from potential pathogens. Proteins including thionins, plant defensins and chitinases have been shown to play active roles against pathogen infections. Plants produce a wide array of antimicrobial compounds for escaping from microorganisms by producing some proteins, which are having antibacterial, antifungal activity, etc. Antimicrobial peptides extracted from plants inhibit the growth of a variety of fungi, oomycetes, Gram-positive bacterial phytopathogenes and Saccharomyces cerevisiae. Many antimicrobial peptides (AMPs) which contain cysteine residues abundantly have been isolated from plants, and these are classified into the plant defensin family. Plant defensins can be classified broadly into three types (hevein type, C6 type, and C8 type) according to the number and the position of cysteine residues in the molecules. The C6 type of plant defensins are highly basic proteins that contains 6 cysteine residues and a continuous sequence of cysteine (-CC-). All the 6 cysteines are involved in disulfide bond formation for stabilizing protein tertiary structure [1,2,3,4].

                           +----------+
                           |          |
                           CxxxxCxxxxCCxxxCxxxxxxC
                                |    |    |      |
                                +----|----+      |
                                     +-----------+

'C': conserved cysteine involved in a disulfide bond.

The overall tertiary structure of Ac-AMP2 consists of a centrally located, twisted anti-parallel β-sheet, which acts as the core of the protein and is more or less covered by the rest of the protein (see <PDB:1MMC>) [4].

Some C6 type AMPs are listed below:

• Mirabilis jalapa AMPs Mj-AMP1 and Mj-AMP2. They exhibit a broad spectrum of antifungal activity [1].
• Phytolacca americana (pokeweed) seeds AMP (Pa-AMP-1). It inhibits the growth of several filamentous fungi and gram-positive bacteria [2].
• Amaranthus caudatus AMPs Ac-AMP1 and Ac-AMP2, which are probably involved in the protection of seeds or seedlings against fungi and microorganisms. Although presently uncertain, the mode of action is most probably related to the fact that they can bind to chitin, a polymer of β-1,4-N-acetyl-D- glucosamine [4].
• Impatiens balsamina (balsam) AMPs Ib-AMP1, Ib-AMP2, Ib-AMP3, and Ib-AMP4, which play a role in the defense of the germinating seed against micro- organisms, by inhibiting the growth of a range of filamentous fungi and bacteria, especially Gram-positive bacteria [5].
• Mesembryanthemum crystallinum (common ice plant) AMP Mc-AMP-1. It possesses antifungal and antibacterial activity.

Our signature pattern for plant C6 type antimicrobial peptides contains the six conserved cysteines involved in disulfide bonds with knottin scaffold [E1].