The weedy vine Mikania micrantha, originally from Central and South America, has become a worldwide menace. Thus far, it’s spread to tropical climes in China and Southeast Asia, Australia, and islands of the Pacific and Indian Oceans, as well as Hawaii and Florida in the United States. Now, with the global invader’s genome recently reported in Nature Communications, researchers are getting clues as to how it has spread so far and wide: The plant is able to collect carbon from the air during both day and night, and it seems to recruit soil microbes that help to gather nitrogen from the atmosphere as well.
M. micrantha’s tiny seeds ride the winds or travel wedged into agricultural equipment. Chunks of stem or root can also regenerate a new plant, and these segments flow with waterways such as Asia’s Mekong River. By climbing into the canopy and blocking sunlight, it smothers forests, orchards, and rice paddies. Known as “mile-a-minute weed” for its remarkable growth rate, M. micrantha actually elongates up to 20 centimeters per day.
“Through the study of the M. micrantha genome, [we found] the reason the species becomes invasive can be explained at the molecular level,” co-first author Bo Liu, a genomicist at the Agricultural Genomics Institute in Shenzhen, says via email. “This will help us identify genes related to growth and reproduction, and develop targeted molecular control agents.”
Leslie Weston, a plant biologist at Charles Sturt University in Wagga Wagga, Australia, who was not involved in the project, was struck by the number of duplications evident in the M. micrantha genome, including a whole-genome copying event and duplication of several genomic segments. “That is unusual,” says Weston. She suspects these genetic features, many acquired during the last one million years, could somehow account for the plant’s swift growth and ability to adapt to a variety of environments. Transposons, known as jumping genes, also pervade much of the genome.
The plant’s speedy growth seems to stem from a remarkable ability to assimilate carbon. Examining the plants and their carbon uptake, the study authors found a peculiar hybrid metabolism that helps the plant thrive. Most plants open pores on the undersurface of leaves, called stomata, during the day to collect carbon dioxide. Some others, such as cacti, open their stomata at night to avoid losing water through them. M. micrantha does both, sucking in as much carbon dioxide as it can to fuel its extraordinary growth. “It kind of gets the best of both worlds,” says David Clements, a plant ecologist at Trinity Western University in Langley, Canada, who was not involved in the study.
The authors also found that M. micrantha stems can photosynthesize, further boosting its growth rate. The plant kept photosynthesizing and growing normally, even when the authors stripped the leaves from the plant.
The researchers were particularly surprised by the weed’s actions on nutrient uptake in the soil. M. micrantha makes compounds called sesquiterpene lactones (STLs), which are known, in other plants, to influence neighboring organisms via the soil. The team grew the mile-a-minute weed in pots and analyzed the nitrogen in its soil. Compared to pots containing native Chinese plants, mile-a-minute pots contained more of the nitrogen forms a plant can use, such as ammonium nitrogen. M. micrantha plants also collected more total nitrogen from the soil.
Soil microbes convert atmospheric nitrogen into those plant-friendly compounds. When the researchers sequenced microbial DNA from those soils, they discovered more genes related to nitrogen metabolism in the M. micrantha pots, suggesting the plant promoted a highly active microbiome. M. micrantha soils also contained more microorganisms that make potassium and phosphorus available to plants.
Furthermore, when the team directly supplemented soils with five mile-a-minute weed STLs, the availability of carbon, nitrogen, and other nutrients burgeoned. They speculate that these compounds might be used as a new type of fertilizer.
The exceptional photosynthesis rate of M. micrantha points to potential avenues for improving crop plants, says Clements. In fact, geneticists are already looking to develop crops with a similar hybrid metabolism. Most plants already possess the genes to work at night, but don’t activate them. It might be possible to turn them on.
At the same time, accelerated photosynthesis suggests M. micrantha will continue to be troublesome to eliminate, says Weston. The best bet, she says, might be preventive measures, such as biosecurity inspections to catch seeds or fragments before they reach waterways or new regions.
Looking for other means of control, researchers collaborating with the Chinese team are investigating the ability of a Central and South American fungus, Puccinia spegazzinii, to curb the mile-a-minute weed, as it doesn’t cause disease in related plants. However, the fungus doesn’t spread readily in dry seasons, and researchers must match the right variant of fungus to the genotype of weed it infects. Given the stem photosynthesis, Clements suggests the ideal biocontrol agent would spread beyond the leaves. P. spegazzinii mainly infects leaves, but also invades stems to some extent.
Sweet potato vines can sometimes outcompete the mile-a-minute, Clements notes, so there is hope. “It’s not totally indomitable,” he says.