Little lab of horrors
Enzymes from insect-eating plants could give us new antibacterial products.
Carnivorous plants are not the first organisms to come to mind when searching for biomedical compounds. Yet, like something from science fiction, researchers are discovering enzymes in the digestive fluids of carnivorous pitcher plants that could prove useful in controlling infections.
Most plants support their growth by absorbing nutrients such as nitrogen, phosphorus and potassium from the soil. But for those unlucky enough to live in regions where soils lack these nutrients, alternative arrangements have evolved — such as organs that can catch, kill and digest insects.
Some of these organs develop as spiky mouths that close on unsuspecting insects when they land on them; some develop as seemingly normal leaves that are covered with goo, rather like flypaper; others, such as the structures sported by the plant Nepenthes alata, are slippery pitchers that function like pit traps. Nepenthes alata uses a combination of bright colours and sweet scent to attract insects to the pitcher, where slippery side walls and a deep pit filled with acidic fluid trap and kill the victims.
The fluid at the base of the trap had long been thought to contain digestive enzymes. Previous research had confirmed this, but exactly which enzymes were present was anyone’s guess. “Digestion in pitcher plants has been actively studied for more than 150 years and we still don’t know how it works [because] it is such a complex process,” says Chris Frazier at the University of New Mexico in Albuquerque.
Now, Naoya Hatano from the Harima Institute in Riken and Tatsuro Hamada from Ishikawa Prefectural University in Japan have identified seven proteins in the carnivorous plant's fluid. They grew the carnivorous plants in their lab, and collected the fluid from newly opened pitchers to prevent contamination from recently captured insects. Then they used polyacrylamide gel electrophoresis to separate out the proteins, and mass spectrometry to identify what type of enzymes the proteins were likely to be. Because some of the enzymes they found were unfamiliar, they searched protein databases to find enzymes with similar structures and noted that some of them were probably not digestive at all.
Hatano and Hamada found that although three of the enzymes are certainly capable of digesting insects, the others probably play a role in preservation of the prey because they are closely related to enzymes that prevent fungal and bacterial infections in other plants. They report their results in the Journal of Proteome Research1.
The concept of preservational enzymes in digestive juice may not at first make sense, but these plants consume insects very slowly, so they compete with bacteria that grow on the insect, stealing the nutrients from the plant, explains Hamada. Covering the prey with antibacterial enzymes leaves more insect for the plant to drain of nutrients later.
“These enzymes could potentially be useful in preventing bacterial and fungal infections,” says Hamada. However, further research is needed for their full potential to be realized in agriculture and medicine, he adds.
“Whether bacteria and fungi are helpful or detrimental to digestion is still not at all clear,” notes Frazier. Like bacteria in the human gut, they could be assisting the digestive process. The new proteins may serve to limit microbial activity as Hamada suggests, but it is just as possible that they have some other function that is not yet understood, he explains. “Knowing they are there, however, is a big step ... we need to learn what they are doing” Frazier adds.
- Hatano, N. & Hamada, T. J. Prot. Res. doi:10.1021/pr700566d (2008).
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