The pretty ferns that adorn windowsills and gardens have some surprising powers. Biologists have long known that this ancient group of plants wards off hungry insects better than other flora, and now they’re homing in on why. They’ve discovered fern proteins that kill and deter pests, including, most recently, one that shows promise against bugs resistant to widely used natural pesticides.
The new protein, described last month in the Proceedings of the National Academy of Sciences (PNAS), adds to a growing arsenal that could one day provide a fresh alternative to chemical insecticides. “These proteins have great potential and may represent a new mode of pesticide action,” says Juan Luis Jurat-Fuentes, an entomologist at the University of Tennessee, Knoxville. They are exciting, says Kristina Sepčić, a biochemist at the University of Ljubljana, because they “have proven to be active against insect [populations] resistant to certain bacterial toxins.”
Since the late 1930s, proteins isolated from a soil bacterium called Bacillus thuringensis (Bt) have become a mainstay of natural pest control. They were first used as an insecticidal spray, but more recently scientists engineered genes for these proteins into crops. Farmers around the world planted more than 100 million hectares of these transgenic plants in 2019.
Transgenic corn and cotton alone saved growers more than $50 billion in lost crops in the first 2 decades of their use, according to Corteva Agriscience. Bt pest control also brought environmental benefits, reducing the use of organophosphate insecticides and other toxic chemicals.
But it may not be working as well as it used to. When Bruce Tabashnik, an entomologist at the University of Arizona, reviewed 25 years of data on corn, sugarcane, cotton, soybeans, and other Bt crops from seven countries, he found signs that populations of 11 pest species have evolved substantial resistance to the proteins. Cases of resistance jumped from three in 2005 to 26 in 2020, he and his colleagues reported in April in the Journal of Economic Entomology. That trend is continuing, Jurat-Fuentes says. Tabashnik has found 17 additional instances where pests were becoming resistant.
“This is of great concern,” says Marilyn Anderson, a biochemist at La Trobe University. “We do not want to return to heavy use of chemical insecticides.” She is among a small group of scientists eyeing fern proteins as an alternative. In the wild, these ancient plants, which evolved long before the plants now used as crops, often seem unaffected by insects.
In the 1990s, researchers sprayed crops with fern extracts, with mixed results. Otherwise, ferns and other non–seed producing plants got little attention as possible insect killers. Then, in 2016, researchers from India inserted a gene from a halberd fern (genus Tectaria) into cotton, hoping to fight sap-sucking whiteflies. Because no other natural insecticides had ever worked against this pest, says P.K. Singh, a plant biotechnologist at the CSIR-National Botanical Research Institute, “We thought to explore nonobvious and unrelated sources for insecticidal activity.”
The halberd fern gene protected the cotton from whiteflies and other sucking pests, and Singh has now isolated other fern compounds that deter chewing insects, such as caterpillars. He says his team has engineered the corresponding genes into cotton and seen very “interesting” and “promising” results in field studies.
Evidence that ferns might harbor useful insecticides also emerged from a collaboration between Corteva and Anderson’s company, Hexima. Starting 8 years ago, Anderson’s team examined 10,000 Australian plants, testing extracts against pest insects in the lab and exposing them to digestive enzymes to determine whether they’d likely break down in the human gut and therefore be safe to use on crops. Corteva, meanwhile, screened plants from North America and elsewhere. Both teams looked for proteins with a novel mechanism that could replace Bt, Anderson says.
In 2019, Corteva reported that genes for proteins found in maidenhair ferns could protect soybeans from soybean looper and velvetbean caterpillars, and since then both groups have sharpened their focus on ferns. “We have since discovered several families of insecticidal proteins from these plants,” Corteva said in a statement. They don’t yet know exactly how these proteins work.
In the recent PNAS paper, a team including Anderson and the Corteva scientists report the latest potential weapon against pests: a protein from Pteris cretica cv. Albolineata, sometimes called Cretan brake fern, ribbon fern, or table fern, which is a common houseplant native to Europe, Asia, and Africa. In the lab, extracts of the fern stunted the growth of soybean looper and corn earworm. Distant relatives of the fern have variants of this protein, the researchers discovered, indicating it arose early in fern evolution, about 300 million years ago. They dubbed this group of proteins IPD113.
Co-author Megan Maher, a structural biologist at the University of Melbourne, and colleagues solved the structure of one variant. They found that it resembles the Bt proteins used as insecticides, except it has just two major active parts, whereas Bt proteins have three. Bt proteins work by puncturing the insect gut. The researchers think the fern proteins do, too, but because the active part missing in fern proteins is the one Bt proteins use to bind receptors on the cell membranes, the fern proteins may bind different receptors. “The hope is the new fern proteins can be Goldilocks insecticides—similar enough to Bt to be safe and effective yet different enough to kill insects that evolved resistance to Bt,” Tabashnik says.
When the Corteva team transferred the genes for the most effective IPD113 versions into maize, leaf damage from key pests such as fall armyworm and corn earworm fell to at most 30% compared with more than 50% in unmodified maize. The fern proteins also worked against insect strains resistant to Bt proteins. The paper “is an excellent advance and establishes ferns as a repertoire of new molecules,” Singh says.Click here to see more...