Plant cell walls are comprised of many complex polymers, such as cellulose and xylan. In order to be digested, these polymers must be broken down by enzymes such as cellulase or xylanase.
For decades scientists thought only microbes could produce cellulase, until cellulase genes were found in wood-feeding insects. Now, new research from the Max Planck Institute overturns another old theory. Scientists have discovered that stick insects (Phasmatodea) produce cellulases that can handle several types of cell wall polymers equally. The research is published in the journal Insect Biochemistry & Molecular Biology.
Cellulose, xylan, and xyloglucan are important components of plant cell walls, and walking sticks have multiple copies of cellulase genes, whose enzymes can attack the glucose backbone of cellulose. However, some of these enzymes can also break down the xylose-backbone of xylan, and others the xylose-glucose backbone of xyloglucan. This discovery marks the first known xyloglucanase of any kind to be found in multicellular animals. Such enzymes in animals were previously not thought to exist.
“If we hadn’t tested these enzymes on other substrates besides cellulose, there was no way we could have discovered these functions,” said Dr. Matan Shelomi, a postdoctoral fellow at the Max Planck Institute for Chemical Ecology and lead author of the study. “It was good that we did: nobody found these kind of powerful enzymes in an animal before.”
The researchers isolated the cellulase genes from seven species of stick insect, including the Australian Extatosoma tiaratum, the Vietnamese Ramulus artemis, and the Bornean Aretaon asperrimus. All express multiple different cellulase enzymes from the glycoside hydrolase family 9 (GH9). Maintaining redundant enzymes does not make sense if all have the same function, so the researchers hypothesized that some had lost their functions or evolved to do something new.
The ability to break down different polymers with the same enzymes means the Phasmatodea gut is unusually efficient. Along with other enzymes such as cellobiases and xylobiases, their guts can fully degrade nearly all of the plant cell wall into its component sugars, using them for nutrition as well as having more access to the easily digested cytoplasm within the cells. This means they can derive more nutrition from a leafy diet than other herbivores. Theoretically, they could even digest wood.
“There is a big community in Germany of people with stick insects as pets, and they report them nibbling on sticks, moss, bark, and even Styrofoam and electric cables,” said Shelomi. “But leaves are still their main food. Maybe their gut can break down wood, but their jaws are better suited for leaves, which probably taste better too.”
Next, the researchers will test other insects related to the stick insects to see if they have multifunctional cellulases too.
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