in the wild, like the mint on the mountain, or improve output
of crops ranging from breadbasket wheat to tropical cacao.
Beyond the garden store
Certain microbial plant partners are well-known, and there are
scores of microbial products already on the market. Gardeners,
for instance, can spike their watering pails with microbes to
encourage flowering and boost plant immunity. But “we know
very little about how the products out there actually do work,”
says Jeff Dangl, a geneticist at the University of North Carolina
at Chapel Hill. “None of those garden supply store products
have proven useful at large scale.”
Big farms can use microbial treatments. The main one
applied broadly in large-scale agriculture helps roots collect
nitrogen, Dangl says, which plants use to produce chlorophyll
Farmers may soon have many more microbial helpers to
choose from. Scientists studying plant microbiomes have
described numerous unfamiliar plant partners in recent
decades. Those researchers say they’ve only scratched the surface of possibilities. Many start-up companies are researching
and releasing novel microbial treatments. “The last five years
have seen an explosion in this,” says Dangl, who cofounded
AgBiome, which soon plans to market a bacterial treatment
that combats fungal diseases. Agricultural giants like Bayer AG,
which recently bought Monsanto, are also investing hundreds of
millions of dollars in potential microbial treatments for plants.
The hope is that microbes can provide the next great revolution in agriculture — a revolution that’s sorely needed. With
the human population predicted to skyrocket from today’s
7. 6 billion to nearly 10 billion by 2050, our need for plant-based
food, fibers and animal feed is expected to double.
“We’re going to need to increase yield,” says Posy Busby, an
ecologist at Oregon State University in Corvallis. “If we can
manage and manipulate microbiomes … this could potentially
represent an untapped area for increasing plant yield in agricultural settings.” Meanwhile, scientists like Zahn are eyeing
the microbiome to save endangered plants.
But before microbiome-based farming and conservation can
truly take off, many questions need answers. Several revolve
around the complex interactions between plants, their diverse
microbial denizens and the environments they live in. One concern is that the microbes that help some plants might, under
certain conditions, harm others elsewhere, warns microbiologist Luis Mejía of the Institute of Scientific Research and
High Technology Services in Panama City.
Save the chocolate
Cacao crops — and thus humankind’s precious M&M’s supply —
are under constant threat from undesirable fungi, such as
Phytophthora palmivora, which causes black pod rot. But there
are good guys in cacao’s microbiome too, particularly the fungus Colletotrichum tropicale, which seems to protect the trees.
Natalie Christian, as a graduate student at Indiana University
Bloomington, traveled to the Smithsonian Tropical Research
Institute on Panama’s Barro Colorado Island in 2014 to study
how entire communities of microbes colonize and influence
cacao plants ( Theobroma cacao). Christian suspected that the
prime source of a young cacao tree’s microbiome would be the
dead and decaying leaves on the rainforest or orchard floor.
To test this hunch and see what kind of protection microbes
picked up from leaf litter might offer, Christian raised fungus-free cacao seedlings in a lab incubator. When the plants
reached about half a meter tall, she placed them in pots outside, surrounding some with leaf litter from a healthy cacao
tree, some with litter from other kinds of trees and some with
no litter at all.
After two weeks, she brought the plants back into the
greenhouse to analyze their microbiomes. She found nearly
300 kinds of endophytes, which she, Mejía and colleagues
reported last year in Proceedings of the Royal Society B.
The microbiome membership differed between the litter
treatments. Plants in pots with either kind of leaf litter possessed less diverse microbiomes than those without litter,
probably because the microbes in the litter quickly took over
Healthy cacao pods (left) contain cacao beans encased in white flesh.
The fungus that causes black pod rot (right) is a constant threat to
When pressed against a plate of nutrients that support methane-eating
microbes, a soybean leaf leaves behind spots of growing bacteria. Those
bacteria may help grow more robust plants.