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 For its fertilizer needs, early corn made friends with
nitrogen-fixing soil microbes by leaking an enticing sugary
cocktail from its roots. The genetic recipe for this cocktail
was handed down from parent to offspring to ensure just the
right microbes came out to play.
But then the Green Revolution changed everything. Breeding tools
improved dramatically, leading to faster-growing,
higher-yielding hybrids than the world had ever seen. And
synthetic fertilizer application became de rigueur.
That’s the moment corn left its old microbe friends behind,
according to new research from the University of Illinois. And
it hasn’t gone back.
“Increasing selection for aboveground traits, in a soil setting
where we removed all reliance on microbial functions, degraded
microbial sustainability traits. In other words, over the course
of half a century, corn breeding altered its microbiome in
unsustainable ways,” says Angela Kent, professor in the
Department of Natural Resources and Environmental Sciences at
the University of Illinois and co-author of a new study in the
International Society of Microbial Ecology Journal.
Kent, along with co-authors Alonso Favela and Martin Bohn, found
modern corn varieties recruit fewer “good” microbes – the ones
that fix nitrogen in the soil and make it available for crops to
take up – than earlier varieties. Instead, throughout the last
several decades of crop improvement, corn has been increasingly
recruiting “bad” microbes. These are the ones that help
synthetic nitrogen fertilizers and other sources of nitrogen
escape the soil, either as potent greenhouse gases or in
water-soluble forms that eventually end up in the Gulf of Mexico
and contribute to oxygen-starved “dead zones.”
“When I was first analyzing our results, I got a little
disheartened,” says Favela, a doctoral student in the Program in
Ecology, Evolution, and Conservation Biology at Illinois and
first author on the study. “I was kind of sad we had such a huge
effect on this plant and the whole ecosystem, and we had no idea
we were even doing it. We disrupted the very root of the plant.”
To figure out how the corn microbiome has changed, Favela
recreated the history of corn breeding from 1949 to 1986 by
planting a chronological sequence of 20 off-patent maize lines
in a greenhouse.
“We have access to expired patent-protected lines that were
created during different time periods and environmental
conditions. We used that understanding to travel back in time
and look at how the associated microbiome was changing
chronologically,” he says.
As a source of microbes, Favela inoculated the pots with soil
from a local ag field that hadn’t been planted with corn or
soybeans for at least two years. Once the plants were 36 days
old, he sequenced the microbial DNA he collected from soil
adhering to the roots.
“We characterized the microbiome and microbial functional genes
related to transformations that occur in the nitrogen cycle:
nitrogen fixation, nitrification, and denitrification,” he says.
“We found more recently developed maize lines recruited fewer
microbial groups capable of sustainable nitrogen provisioning
and more microbes that contribute to nitrogen losses.”
Kent says breeding focused on aboveground traits, especially in
a soil context flooded with synthetic nitrogen fertilizers, may
have tweaked the sugary cocktail roots exude to attract
microbes.
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“Through that time period, breeders weren’t selecting
for maintenance of microbial functions like nitrogen fixation and
nitrogen mineralization because we had replaced all those functions
with agronomic management. As we started selecting for aboveground
features like yield and other traits, we were inadvertently
selecting against microbial sustainability and even actively
selecting for unsustainable microbiome features such as
nitrification and denitrification,” she says.
Now that it’s clear something has changed, can breeders bring good
microbes back in corn hybrids of the future?
Bohn, corn breeder and associate professor in the Department of Crop
Sciences at Illinois, thinks it’s very possible to “rewild” the corn
microbiome. For him, the answer lies in teosinte, a wild grass most
people would have to squint pretty hard at to imagine as the parent
of modern corn.
Like wild things everywhere, teosinte evolved in the rich context of
an entire ecosystem, forming close relationships with other
organisms, including soil microbes that made soil nutrients easier
for the plant to access. Bohn thinks it should be possible to find
teosinte genes responsible for creating the root cocktail that
attracts nitrogen-fixing microbes. Then, it’s just a matter of
introducing those genes into novel corn hybrids.
“I never thought we would go back to teosinte because it’s so far
removed from what we want in our current agricultural landscape. But
it may hold the key not only for encouraging these microbial
associations; it also may help corn withstand climate change and
other stresses,” Bohn says. “We actually need to go back to teosinte
and start investigating what we left behind so we can bring back
these important functions.”
Bringing back the ability for corn to recruit its own nitrogen
fixation system would allow producers to apply less nitrogen
fertilizer, leading to less nitrogen loss from the system overall.
“Farmers don't always know how much nitrogen they will need, so,
historically, they’ve dumped as much as possible onto the fields. If
we bring these characteristics back into corn, it might be easier
for them to start rethinking the way they manage nitrogen,” Bohn
says.
Kent adds that a little change could go a long way.
“If we could reduce nitrogen losses by even 10% across the growing
region of the Midwest, that would have huge consequences for the
environmental conditions in the Gulf of Mexico,” she says.
The article, “Maize germplasm chronosequence shows crop breeding
history impacts recruitment of the rhizosphere microbiome,” is
published in the International Journal of Microbial Ecology Journal
[https://doi.org/10.1038/s41396-021-00923-z].
The Department of Natural Resources and Environmental Sciences and
the Department of Crop Sciences are in the College of Agricultural,
Consumer and Environmental Sciences at the University of Illinois.
[Sources: Angela Kent, Alonso Favela,
Martin Bohn
News writer: Lauren Quinn] |
