Carnivorous or insectivorous plants fascinate people, including scientists such as Charles Darwin. He was the first to write a book on this topic (Darwin, 1875). Although Darwin described plants equipped with pitfall traps, he never saw species of the genus Nepenthes which occur in south-east Asia.
Like other carnivorous plants, Nepenthes species grow on poor soil. Therefore, they need to complement their mineral nutrients – primarily with nitrogen and phosphorus – from caught and digested prey. When visiting the pitfall traps, the attracted prey, mainly arthropods, falls into the trap, drowns and is digested by the enzyme cocktail of the pitcher fluid. Because the digestive liquid can be easily harvested from the pitcher, Nepenthes plants are ideal objects to study enzymes and other compounds involved in this plant carnivory. As a consequence, compared with other carnivorous taxa, the protein composition of the digestive fluid of Nepenthes is fairly well analysed and documented.
Due to the fact that closed Nepenthes pitchers have no direct contact with the environment, it has been widely claimed that their pitcher fluid is sterile and that all proteins and compounds identified in this pitcher fluid are solely plant-derived. Interestingly, only two experiments have been conducted to demonstrate the sterility of pitcher liquid: fluid taken from a closed pitcher was plated either on plain nutrient agar (Hepburn, 1918) or on meat agar plates (Lüttge, 1964) and incubated for several days. In no case was any bacterial growth detected and the authors concluded that the pitcher fluid is sterile. However, the presence of microbes cannot be excluded by such simple experiments because most micro-organisms cannot be grown in culture.
Researchers have now analysed the composition of Nepenthes digestive fluid from closed pitchers to reveal whether or not pitchers are really sterile inside and how these plants manage to keep microbial growth under control. The content of proteins, inorganic ion compositions and secondary metabolites were studied. In addition, the effect of pitcher fluid on microbial growth was investigated. The results reveal that the fluid of closed Nepenthes pitchers is composed provides anti-microbial conditions. Thus these plants can avoid, at least to some extent, the growth of microbes that compete with the plant for the prey-derived nutrients available in the pitcher.
Franziska Buch, Matthias Rott, Sandy Rottloff, Christian Paetz, Ines Hilke, Michael Raessler and Axel Mithöfer. Secreted pitfall-trap fluid of carnivorous Nepenthes plants is unsuitable for microbial growth. (2013) Annals of Botany 111 (3): 375–383
Carnivorous plants of the genus Nepenthes possess modified leaves that form pitfall traps in order to capture prey, mainly arthropods, to make additional nutrients available for the plant. These pitchers contain a digestive fluid due to the presence of hydrolytic enzymes. In this study, the composition of the digestive fluid was further analysed with regard to mineral nutrients and low molecular-weight compounds. A potential contribution of microbes to the composition of pitcher fluid was investigated. Fluids from closed pitchers were harvested and analysed for mineral nutrients using analytical techniques based on ion-chromatography and inductively coupled plasma–optical emission spectroscopy. Secondary metabolites were identified by a combination of LC-MS and NMR. The presence of bacteria in the pitcher fluid was investigated by PCR of 16S-rRNA genes. Growth analyses of bacteria and yeast were performed in vitro with harvested pitcher fluid and in vivo within pitchers with injected microbes. The pitcher fluid from closed pitchers was found to be primarily an approx. 25-mM KCl solution, which is free of bacteria and unsuitable for microbial growth probably due to the lack of essential mineral nutrients such as phosphate and inorganic nitrogen. The fluid also contained antimicrobial naphthoquinones, plumbagin and 7-methyl-juglone, and defensive proteins such as the thaumatin-like protein. Challenging with bacteria or yeast caused bactericide as well as fungistatic properties in the fluid. Our results reveal that Nepenthes pitcher fluids represent a dynamic system that is able to react to the presence of microbes. The secreted liquid of closed and freshly opened Nepenthes pitchers is exclusively plant-derived. It is unsuitable to serve as an environment for microbial growth. Thus, Nepenthes plants can avoid and control, at least to some extent, the microbial colonization of their pitfall traps and, thereby, reduce the need to vie with microbes for the prey-derived nutrients.
- The role of multiple partners in a digestive mutualism with a protocarnivorous plant. (2013) Annals of Botany 111 (1): 143–150
- Distribution of naphthoquinones, plumbagin, droserone, and 5-O-methyl droserone in chitin-induced and uninduced Nepenthes khasiana: molecular events in prey capture. (2011) Annals of Botany 62 (15): 5429–5436