Beans of the cacao plant, Theobroma cacao, are used in chocolates and other products, but they’re under threat, says the American Society for Microbiology (ASM). Increased drought associated with climate change has already begun to stress cacao-growing regions of Colombia and other countries, and models predict it will get worse. In recent research, scientists have found that fungal endophytes—microbes that live in a host plant without causing harm—may offer a novel strategy for boosting drought tolerance in cacao.
 
For a study published this week in ASM's mSphere journal, mycologists added fungal endophytes from a species of cactus to the soil of growing cacao plants. They found that the inoculated plants showed less negative levels of leaf water potential, possibly due to better control of the stomatal conductance, which is a key determinant of photosynthesis. These alterations could help the plant retain more water as it grows.

“We are losing a lot of species due to climate change,” says Silvia Restrepo, Ph.D., senior author on the study and a plant pathologist at the Boyce Thompson Institute in Ithaca, NY, and the Universidad de los Andes in Bogotá, Colombia. The new study, she said, shows how scientists can harness strategies by looking for solutions that have evolved in other organisms.

Restrepo has long studied the effects of fungal endophytes, isolated from plants growing in extreme conditions in Colombia, on threatened crops. In previous work, her lab found endophytes that could improve the growth of potatoes. More recently, she said, she’s been working with cacao growers on drought resistance.

For the new work, she and her collaborators collected root samples in two locations in Colombia from 12 Stenocereus cacti, a tree-like genus characterized by its ability to thrive in arid, hot conditions. They isolated more than 20 fungal endophytes from the samples and subjected the fungi to drought conditions. Five of the isolates lost less than 20% of their total biomass. The researchers added these isolates to soil of growing cacao plants and compared them to cacao plants growing in ordinary soil, then subjected both to drought conditions.

The endophytes did not affect the height of the plants, but treated cacao plants developed more and larger leaves. In addition, plants inoculated with endophytes were better able to recover from the drought conditions. Endophytes from the genera Fusarium and Phoma also promoted plant growth under non-drought conditions.

Restrepo says scientists don’t know exactly why the endophytes help cacao with drought resistance. “The fine details are an open question,” she notes. However, their analyses and observations suggest that the endophytes help the cacao plant manage the stomata, tiny pores that open and close to allow gas exchange, to avoid the rapid release of water vapor.

She suspects the endophytes may also confer similar benefits to other crops. “It’s easy to test in tomato, potato, and other crops,” she says. Her group is also developing an endophyte-based soil additive that farmers could use to help their crops better survive drought in Colombia and beyond. “We have to look at all possibilities to help the crops fighting against climate change," Restrepo finishes.

Related: Puratos to commercialize cultured cocoa