Shi En Kim

Reporter

Nicole Hynson normally gets roped in to help when all else fails. The conservation biologist from the University of Hawaii is involved in bringing back all kinds of critically endangered plants from the brink of extinction. Unfortunately, she’s kept busy in her home state, Hawaii, which is also known as the extinction capital of the world. Her latest conservation target is a flowering tree that’s fighting a losing battle in the wild: the Gardenia brighamii, or, as it’s known among some local communities, the na’u.

The na’u is one of three gardenia species endemic to the archipelago. Historical records showed that the Native Hawaiian population fashioned tools out of the na’u wood and harvested its fruit for dye. The na’u’s crowning glory is its fragrant flower, a pearly blossom that was once frequently woven into traditional floral wreaths called leis. But now the tree’s wood, fruit and flowers are too rare for commercial or casual use. Due to drought, agriculture, competition with invasive plants and wildfires, the species that was once present throughout Hawaii is now confined to a single island, Lanai. Only ten or so known individuals remain in the wild there, making the na’u one of the rarest plants in the world.

“These gardenia are absolutely beautiful, wonderful plants with deep cultural history and value,” says Mike Opgenorth, a director at the National Tropical Botanical Garden and Hynson’s former graduate student who worked on G. brighamii conservation. Like other local flora, they’re central to traditional Native knowledge. “You lose some of Hawaiian culture when you lose some of these plants.”

Gardenia Brighamii Tree
The G. brighamii plant is a slow-growing woody tree that can reach 20 feet in height. The last wild individuals are all found on the island of Lanai. Nicole Hynson

In order to grow robust saplings that can bolster the species in the wild, Hynson’s team is wielding an emerging tool in the arsenal of plant conservation: mycorrhizal fungi. The community of beneficial microorganisms that dwell among the roots of plants are the botanical equivalent of a gut microbiome. In exchange for food, mycorrhizal fungi perform all kinds of functions for their plant host—they supply plants with mineral nutrients from the soil, act as root extensions to help the plant suck up more water and increase the host’s resistance against pathogens. In return, they gain energy-rich sugars and lipids from photosynthetic plants. Plant-fungi interactions are rich symbiotic pacts that go back to when plants first colonized land around 500 million years ago.

For all their myriad and longstanding services to plants throughout their evolutionary history, mycorrhizal fungi have been largely neglected by agronomists and botanists until recently. To raise healthy plants, conventional horticulture banked on the intensive use of chemicals such as fungicides to ward off all kinds of diseases. But this practice meant that fortifying fungi, and the nutrition-accessing benefits they bring, were excluded from shaping plant health. Mycorrhizal exposure in na’u restoration is a step away from that extreme approach, and it fosters growth conditions that more resemble a plant’s natural habitat.

Harnessing nature’s agents

For more than 20 years, conservationists have “babied” G. brighamii seedlings in greenhouses, growing them in sterile environments and plying them with fertilizers and pesticides. In this resource-intensive approach, the survival rate of rewilded G. brighamii is less than 10 percent, according to a report authored by Hynson. Out in the open, the coddled seedlings falter when they encounter the full brunt of the elements and other natural threats. Often, these individuals have to depend on synthetic chemicals for the rest of their lives, Hynson says.

Hynson’s strategy is to replace the artificial chemicals entirely with mycorrhizal fungi probiotics that the plants would normally curate themselves over time in the wild. In theory, these root-based living defenses should help the plant live more independently.

Lena Neuenkamp, a plant and mycorrhizal ecologist at Germany’s University of Münster who didn’t participate in the project, says it’s a little like what you would do before rewilding captive animals from a zoo. She says you want to give them the best start in life, such as a healthy inner microbiome, so they have the sunniest chances of making it on their own once back in their native habitat.

“Native mycorrhizal contact can be really important to high-conservation-value plants and their establishment,” says Jim Bever, a plant ecologist from the University of Kansas who wasn’t involved in the endeavor. He has used mycorrhizal inoculation for prairie grass restoration in the American heartland. “Our experience in the Midwest is very consistent with Nicole’s goal.”

Hynson’s group gathers soil samples from wild G. brighamii individuals to collect friendly fungi. “It’s quite an expedition to bring back some of these samples,” she says, given that many of the last G. brighamii trees live in remote locations. Back in the greenhouse, Hynson cultivates the microbes present in a dummy grass host. After a few months of amplifying the fungal numbers, the researchers collect the spores and spritz them onto G. brighamii seedlings in the lab.

Gardenia Brighamii Seedling
G. brighamii seedlings are first grown in test tubes before they are exposed to mycorrhizal fungi. Nicole Hynson

So far, the results are promising. The team observed that inoculated seedlings grew three times as fast as the uninitiated ones. It will take another half a year at least before the seedlings are ready for the ultimate test: transplantation in outdoor restoration sites.

Of course, rapid growth in the greenhouse doesn’t necessarily guarantee survival in the wild. But given the challenges of breeding G. brighamii in captivity in the first place, Hynson counts the early success with the boosted seedlings as a win. “My expectation is that they’re going to continue on this positive trajectory,” Hyson says. “It’s not always clear whether they’re going to survive; all we can do is try to make them as strong as possible.”

Given that the mycorrhizal method doesn’t involve any fertilizers and pesticides, the practice may be more sustainable in the long term. “If we can generate plants that do well, without all that input, then that’s the savings on all fronts,” Hynson says.

A race against time

The challenge in working with mycorrhizal fungi is that plants and fungi are exquisitely choosy with whom they strike up partnerships. Store-bought generic mycorrhizal mix for houseplants often doesn’t confer the expected benefits for this reason. The mixes usually come loaded with fertilizer to coax out some growth effects, which might mislead plant parents to think that inoculation is working, when really the fertilizer is doing the work.

Which fungal helpers a plant admits into its circle is complex—a healthy mycorrhizal pit crew for a single plant can tally as many as 80 members strong, Hynson says.

Neuenkamp says mycorrhizal fungal species are not hard to find because they’re everywhere. But the trick to building the dream team is to find which ones make a positive difference.

Gardenia Brighamii
A G. brighamii blossom flowers on one of the few remaining trees in the wild. Nicole Hynson

For now, Hynson’s team is sourcing for entire microbial communities from the few wild gardenia relics. The researchers are testing soil samples from all over the state—gathered from healthy plants, flailing ones and even the deceased—and growing hundreds of seedlings to identify the best mycorrhizal mixture. “The idea is, if we spread out our collecting among a bunch of different trees, hopefully we can kind of hit a pot of gold,” Hynson says.

Researchers can’t say which soil sample will yield the best outcome. But once the results roll in, they plan to conduct DNA fingerprinting in the most successful soil sample to identify which fungi are present and what services they provide their host. This will help the researchers reproducibly nail down the optimal mycorrhizal inoculation formula for all future na’u cultivation.

The entire conservation process is a slow one, and the researchers are keenly aware that they’re racing against time. In the three years since Hynson has embarked on this quest to save the species, she has seen wild individuals die out faster than she has managed to raise greenhouse cultivars into adulthood.

Last year, she and a team of other botanists ventured to Oahu to check on its last surviving na’u tree. But their grueling three-hour journey of uphill bushwhacking in the rain ended in disappointment. Where Oahu’s last tree once stood, only a skeleton of a toppled tree remained, its trunk split open and its boughs bare. It had succumbed to the combined threats of competition from exotic fauna and climate change-induced habitat shifts. “We all just kind of took in the moment of realizing, that’s it for the wild individuals of this tree [on Oahu],” Hynson recalls. Down the mountain in the Nanakuli Valley, schoolchildren still chant about their “flower of the valley” in cultural songs, more of a requiem than an ode to a beloved na’u tree that’s no more on their island.

And the same overarching pattern of biodiversity decline is also playing out well beyond the archipelago’s shores. “Hawaii is a microcosm of the greater world,” Opgenorth says. Nearly half of endangered and threatened American plants are housed in Hawaii. Globally, an estimated 40 percent of terrestrial plant species are at risk of extinction.

But if the mycorrhizal inoculation of the na’u works, it might serve as a template strategy for saving other imperiled flora. In this way, Hawaii would position itself as a shining example of ecological restoration for the rest of the world. “If we can figure things out in Hawaii and find a way to live more sustainably,” Opgenorth adds, “that will reverberate—and be lessons that others can appreciate outside the islands as well.”

Get the latest Science stories in your inbox.

Shi En Kim

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.