Abstract
Plant defensins are small peptides of 45 to 54 amino acids undertaking the antimicrobial role in various plants. Part of the family members is responsible for the constitutive guards against pathogen attack. In addition, some members are activated by exogenous agents to further plant protection. This activation is related to methyl jasmonic acid (MJ) and ethylene (ET). From a MJ subtraction library of Bupleurum kaoi, two defensin genes, BkDF1 and BkDF2, were cloned. According to the sequence comparison with other related defensins and protease digestion result of the purified BkDF1 protein, these two genes belong to group II of plant defensins and a considerable antimicrobial activity was implied. To characterize the defensins, BkDF1 and its mature region were expressed in Escherichia coli and Pichia pastoris separately. Expression of BkDF1 is toxic to E. coli. The antimicrobial activity of mature BkDF1 protein expressed in P. pastoris was tested against several fungi and bacteria. In addition, the transcription level of BkDF1 was studied. Our results suggest that BkDF1 may protect roots and leaves against microorganisms and is a downstream gene of the jasmonic acid (JA) signal pathway. Its expression is inhibited by salicylic acid (SA) and demonstrates the antagonistic relationship between JA and SA signal pathways, which are the two major defense response systems in plants. The promoter regions of these two genes will be further investigated to understand more about the regulation of defensin genes.