Aspartic proteinases are expressed in pitchers of the carnivorous plant Nepenthes alata Blanco
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Carnivorous plants acquire significant amounts of nitrogen from insects. The tropical carnivorous plant Nepenthes accumulates acidic fluid containing aspartic proteinase (AP) in its trapping organs (pitchers), suggesting that the plant utilizes insect protein as a nitrogen source. Aspartic proteinases have been purified and characterized from sterile pitcher fluid of several species of Nepenthes; however, there is, as of yet, no information about sequence and expression of Nepenthes AP genes. To identify the pitcher AP, we cloned plant AP homologs from N. alata and examined their expressions. Five AP homologs (NaAP1–NaAP5) were obtained by reverse transcription-polymerase chain reaction with degenerate primers designed for the conserved sequences of plant APs. Alignment of deduced amino acid sequences with other plant APs demonstrated that NaAP1–NaAP4 contained a plant-specific insert (PSI), a unique sequence of plant AP. However, NaAP5 did not possess the insert, and had a shorter sequence (by >100 amino acids) than the other APs. Northern analysis using a part of the coding region of NaAP1 as a probe showed that bands of approx. 1.8 kb corresponding to the sizes of NaAP1–NaAP4 mRNA were present in roots, stems, leaves, tendrils, and lower part of the pitchers, but a band of approx. 1.3 kb corresponding to the size of NaAP5 mRNA was not observed in any organs. In pitchers, highest expressions of NaAP1–NaAP4 were seen in the lower part of open pitchers containing natural prey, suggesting that the expressions of NaAP1–NaAP4 are coupled with prey capture. Transcripts of NaAP2 and NaAP4 were detected in the digestive glands, where AP secretion may occur. This result suggests that NaAP2 and NaAP4 are the possible APs secreted into the pitcher of N. alata.
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