Isolation and characterization of chitinase genes from pitchers of the carnivorous plant Nepenthes khasiana

The genus Nepenthes represents carnivorous plants with pitcher traps capable of efficient prey capture and digestion. The possible involvement of plant chitinases in this process was studied in Nepenthes khasiana. Two different types of endochitinases were identified in the liquid of closed traps exhibiting substrate specificity for either long chitin polymers or N-acetylglucosamine (GlcNAc) oligomers. Injection of chitin into such closed sterile pitchers induced the appearance of additional endochitinase isoenzymes, with substrate specificity only for long chitin polymers. No significant exochitinase (N-acetyl-beta-glucosaminidase) or chitobiosidase activity could be detected in the non-induced or induced trap liquid. Four genes representing two subgroups of basic chitinases, denoted as Nkchit1b and Nkchit2b, were isolated from the secretory region of N. khasiana pitchers. The main differences between the two subgroups are the presence of a proline-rich hinge region only in NkCHIT1b and a C-terminal putative vacuole targeting extension only in NkCHIT2b, indicating different compartmentalization of the two enzymes. Reverse transcription-polymerase chain reaction (RT-PCR) evaluation of mRNA levels showed that the Nkchit2b genes are constitutively expressed in the secretory cells while transcription of Nkchit1b genes is induced by chitin injection. These results show for the first time the involvement of genes encoding chitinases in prey-trap interaction and their differential expression and activity during prey trapping.