Molecular cloning and characterization of a novel bi-functional α-amylase/subtilisin inhibitor from Hevea brasiliensis

A novel cDNA encoding a bi-functional α-amylase/subtilisin inhibitor (HbASI) was isolated from rubber (Hevea brasiliensis) leaves cultivar RRIM600. The HbASI had strong homology with the soybean trypsin inhibitor (Kunitz) family of protease inhibitors. Its putative amino acid sequence was similar to that of the α-amylase/subtilisin inhibitor from Ricinus communis (72% identity). Genomic sequencing indicated that the HbASI gene contained no introns. The messenger RNA of HbASI was detected in leaf, hypocotyl and root. The recombinant HbASI expressed extracellularly in Pichia pastoris exhibited inhibitory activity against α-amylase from Aspergillus oryzae, trypsin and subtilisin A. The HbASI gene was induced in the rubber leaves infected with a rubber tree pathogen, Phytophthora palmivora. It was also enhanced by salicylic acid (SA) treatment and mechanical wounding. In addition, the biological activity of the HbASI protein involving in the plant defence responses was also investigated. The HbASI at a concentration of 0.16 mg mL could inhibit the mycelium growth of P. palmivora. These data suggested that the HbASI protein might play a crucial role in defence against pathogen of rubber trees