Deep inside the roots of a columnar cactus baking in Colombia’s Tatacoa Desert, a fungus is doing something quietly remarkable. It is surviving conditions that would kill most organisms, hunkered down in tissue where soil temperatures routinely crack 40 degrees Celsius and rain might not fall for months. That kind of resilience, it turns out, is transferable. Scientists have now shown that fungal endophytes pulled from these cacti can help young cacao plants, the source of the world’s chocolate, withstand drought they would otherwise not survive.
Key Takeaways
- Fungal endophytes from desert cacti can help young cacao plants withstand drought, showing great promise for combating climate change.
- Cacao plants require significant water to thrive, making them vulnerable to increasing drought conditions predicted for Colombia.
- The Phoma endophyte improved water retention in cacao seedlings and potentially altered their stress response during drought.
- While the research shows potential, safety testing is necessary due to risks from some fungal pathogens isolated during the study.
- The approach could be applicable to other crops, like tomatoes and potatoes, expanding the benefits of utilizing stress-adapted microbes.
The finding matters because cacao is in trouble. Climate models predict worsening drought across Colombia’s key growing regions, including Santander and parts of Tolima, and the crop is notoriously fussy about water; it needs somewhere between 1,500 and 2,500 millimeters of rainfall a year to thrive. Seedlings and juveniles are especially vulnerable, with shallow roots that cannot chase retreating moisture deeper into the soil. “We are losing a lot of species due to climate change,” said Silvia Restrepo, a plant pathologist at the Boyce Thompson Institute in Ithaca, New York, and the Universidad de los Andes in Bogota, who is the study’s senior author.
Endophytes are microorganisms (fungi, in this case) that live inside a plant’s tissues without causing harm. Some actively help their hosts, boosting growth or defending against pathogens. Researchers have known for years that certain endophytes confer stress tolerance, but the idea of borrowing them from one plant species and transplanting them into another is relatively new, and a bit audacious. You are essentially asking a fungus that evolved inside a desert cactus to cooperate with a tropical tree that likes its feet wet.
Restrepo has been working on exactly this sort of microbial matchmaking for years. Her lab previously found endophytes that could improve potato growth. More recently she turned her attention to cacao.
For the new study, published in mSphere, Restrepo and her collaborators collected root samples from 12 Stenocereus cacti at two sites in Colombia: the Tatacoa Desert and the coastal town of Taganga. They isolated more than 20 fungal endophytes, then subjected them to simulated drought in the lab using polyethylene glycol to lower osmotic pressure. Five isolates lost less than 20% of their biomass under those conditions. A pretty impressive showing.
Those five hardy fungi were then introduced into the soil of five-month-old cacao seedlings grown in a greenhouse in Pitalito, in Colombia’s Huila department. Half the plants were subjected to 14 days without water; the other half kept to a normal watering schedule. The results were mixed but intriguing. Inoculated plants did not grow any taller than controls, but they developed more leaves with greater surface area. And one fungus in particular, a species of Phoma, stood out.
Cacao seedlings treated with the Phoma endophyte maintained significantly less negative leaf water potential under drought, which is a rough proxy for how much water stress the plant is under. Their stomata, the tiny pores on leaf surfaces that open to let carbon dioxide in and close to keep water from escaping, seemed better regulated. Even more curious, the Phoma-treated plants accumulated less proline, an amino acid that normally spikes when plants are water-stressed. Typically, high proline signals a plant in crisis mode, so lower levels could suggest the fungus is somehow dampening the stress response before it ramps up. Two Phoma-inoculated plants even survived the full drought and recovery cycle, while all the non-inoculated controls subjected to drought died.
“The fine details are an open question,” Restrepo said, when asked exactly how the endophytes achieve this effect. The team’s analyses suggest stomatal regulation is part of the answer, but the proline result complicates the picture. In other plant species, Phoma fungi have been shown to increase proline production under drought, not decrease it.
There is also the question of safety. Several of the fungal genera isolated in the study, including Fusarium, have members that are known plant pathogens. The researchers note that any eventual deployment of these strains in farmers’ fields would need to be preceded by pathogenic host range studies, essentially checking that the cure does not become a disease.
Restrepo reckons the approach could extend well beyond cacao. “It’s easy to test in tomato, potato and other crops,” she said. Her group is developing an endophyte-based soil additive that farmers could use directly, something practical and low-cost enough for smallholders in drought-vulnerable regions. Colombia alone has roughly 52,000 families who depend on cacao for their livelihoods, and the crop has particular social significance there; it has been promoted as an alternative to illicit crops in post-conflict territories.
“We have to look at all possibilities to help the crops fighting against climate change,” Restrepo said. If a fungus that has spent millions of years figuring out how to keep a cactus alive in a desert can teach a chocolate tree the same trick, that would be quite the partnership. Whether it scales from greenhouse to field remains the next, harder chapter.
Possibly, yes. Researchers in Colombia have shown that fungal endophytes isolated from drought-adapted cacti can improve the water stress tolerance of young cacao plants in greenhouse conditions. The approach is still at an early stage, but the principle of borrowing stress-adapted microbes from hardy plants and applying them to vulnerable crops is gaining traction across agricultural research.
The exact mechanisms are not fully understood, but the evidence points to better regulation of stomata, the pores that control gas exchange and water loss on leaf surfaces. In this study, cacao plants treated with a Phoma fungus maintained better water status and, surprisingly, accumulated less of the stress-signaling amino acid proline than untreated plants. That suggests the fungus may be intervening in the plant’s stress response at a fundamental metabolic level.
The researchers believe so. Fungal endophytes generally are not locked to a single host species, and the lead scientist on this study has noted that testing on tomato, potato, and other crops would be straightforward. Previous work from the same lab found endophytes that improved potato growth, which supports the idea that these fungi can cooperate with a range of agricultural plants.
Several things. The research has only been tested on young cacao plants in a greenhouse, not mature trees in real field conditions. Some of the fungal genera involved, such as Fusarium, include species that are known plant pathogens, so safety testing is essential before any commercial deployment. The research team is working on developing a practical soil additive, but scaling from lab to field is a significant hurdle that has yet to be cleared.
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