Since many peptides of group IV have a high hemolytic activity, we selected peptides of group I and group III for further studies. The most promising candidates are derivatives of SP1 and SP10. These peptides were highly active against a broad spectrum of bacteria, but CP-690550 showed low hemolytic activity. First, we analysed the toxicity of these peptides for plant cells by incubation with plant protoplast. All three peptides showed a very low phytotoxicity to plant protoplasts and therefore seem to be well suited as a plant protecting agent. Hints to cell death are loss of spherical shape, chloroplast release and agglomeration of protoplasts. Bacteria, fungi and viruses can dramatically affect yield and quality of crop plants, which can have enormous economic consequences. But the damage is not only due to loss of money. Moreover, some of these phytopathogenic Niraparib PARP inhibitor microorganisms can produce toxins, which can affect health of the consumers. For instance aflatoxins, contaminants produced by the fungi Aspergillus flavus and Aspergillus parasiticus in a variety of food crops, are known to cause human liver cancer, to affect growth of human and animals and to be immunosuppressive. Therefore, the control of the plant pathogens is not only important from the economic point of view, but also very important for human and animal health. The control of the pathogenic microorganisms relies mainly on chemical pesticides. However, many countries have undertaken regulatory changes in pesticide registration requirements with the aim of retaining only compounds being 
more selective, with lower toxicity and reduced negative environmental impact. The use of AMPs in agriculture was already proposed along with their discovery in the early 1980s. To date, nearly thousand natural antimicrobial peptides have been isolated from different organisms, such as mammals, insects, amphibians, and plants and can be found in AMP databases. The use of AMPs in animal or human medicine and plant protection may offer new possibilities to control microbial diseases that are still challenging to combat. However, most natural occurring AMPs exhibit a narrow activity spectrum, low activity against important pathogens or high toxicity against human and plant cells. To overcome these problems different types of antimicrobial peptides were designed. We developed helical antimicrobial peptides harbouring positively charged and hydrophobic clusters. In a first approach 16 different peptides with a length of 12 and 20 amino acids were developed. All designed peptides have a positive net charge of at least 3.76 at physiological pH, as it is the case for most of the natural occurring AMPs. SP1 and SP2 differ mainly in their N-terminal half and showed a broad antibacterial activity in a concentration range of 1�C2 mg/ml. Interestingly, SP3 and SP4 displayed a significant lower antibacterial activity, although the arrangement of hydrophobic and charged amino acid residues is the same as in SP1 and SP2. Notably, SP1 contains no alanine, whereas SP2, SP3 and SP4 contain two, three and four alanine residues, respectively. Wang et al. analyzed the amino acid composition of AMPs from different kingdoms of life listed in The Antimicrobial Peptide Database and determined frequently occurring residues. The most abundant residues in bacterial peptides are glycine and alanine. Possibly, with increasing alanine content the designed peptides reflect more analogy to bacterial peptides resulting in lower antimicrobial activity. Another difference among the peptides of group I is the slightly lower hydrophobicity of SP3 and SP4. This might affect the possibility of SP3 and SP4 to interact with bacterial membranes or to adopt membrane-induced amphipathicity at the expense of inhibiting bacterial growth.