.
Once considered outlandish, the idea that plants help their relatives is taking root
By Elizabeth Pennisi
For people, and many other animals, family matters. Consider how many
jobs go to relatives. Or how an ant will ruthlessly attack intruder
ants but rescue injured, closely related nestmates. There are good
evolutionary reasons to aid relatives, after all. Now, it seems, family
feelings may stir in plants as well.
A Canadian biologist planted the seed of the idea more than a decade
ago, but many plant biologists regarded it as heretical—plants lack the
nervous systems that enable animals to recognize kin, so how can they
know their relatives? But with a series of recent findings, the notion
that plants really do care for their most genetically close peers—in a
quiet, plant-y way—is taking root. Some species constrain how far their
roots spread, others change how many flowers they produce, and a few
tilt or shift their leaves to minimize shading of neighboring plants,
favoring related individuals.
"We need to recognize that plants not
only sense whether it's light or dark or if they've been touched, but
also whom they are interacting with," says Susan Dudley, a plant
evolutionary ecologist at McMaster University in Hamilton, Canada, whose
early plant kin recognition studies sparked the interest of many
scientists.
Beyond broadening views of plant behavior, the new work may have a
practical side. In September 2018, a team in China reported that rice
planted with kin grows better, a finding that suggested family ties can
be exploited to improve crop yields. "It seems anytime anyone looks for
it, they find a kin effect," says André Kessler, a chemical ecologist at
Cornell University.
From termites to people, kin-specific behaviors have evolved over and
over in animals, showing there is a strong advantage to helping
relatives pass on shared genes. Dudley reasoned that the same
evolutionary forces should apply to plants. Not long after researchers
proved that plants can distinguish "self" from "nonself" roots, she
tested whether they could also pick out and favor kin. She grew American
searocket (Cakile edentula), a succulent found on North
American beaches, in pots with relatives or with unrelated plants from
the same population. With strangers, the searocket greatly expanded its
underground root system, but with relatives, it held these competitive
urges in check, presumably leaving more room for kin roots get nutrients
and water. The claim, published in 2007, shocked colleagues. A few
sharply criticized the work, citing flawed statistics and bad study
design.
Since then, however, other researchers have confirmed her findings. Recently, working with Moricandia moricandioides,
a Spanish herb, Rubén Torices and his colleagues at the University of
Lausanne in Switzerland and the Spanish National Research Council
demonstrated cooperation in flowering. After growing 770 seedlings in
pots either alone or with three or six neighbors of varying relatedness,
the team found the plants grown with kin put out more flowers, making
them more alluring to pollinators. The floral displays were especially
big in plants in the most crowded pots of relatives, Torices and his
colleagues reported on 22 May 2018 in Nature Communications.
Torices, now at King Juan Carlos University in Madrid, calls the kin
effects "altruistic" because each individual plant gives up some of its
ultimate seedmaking potential to expend more energy making flowers. In
the end, he suspects, more seeds are fertilized overall in the closely
related pots.
Doubts linger. Is a plant identifying genetic kin, or simply
recognizing that its neighbor is more or less similar to itself? "I do
not think that there has been convincing evidence for kin recognition in
plants yet," says Hélène Fréville, a population biologist studying
crops at the Montepellier outpost of the French National Institute for
Agricultural Research.
Sagebrush bushes (Artemisia tridentata) have provided some
strong clues, however. When injured by herbivores, these plants release
volatile chemicals that stimulate neighboring sagebrush to make
chemicals toxic to their shared enemies. Ecologist Richard Karban at the
University of California, Davis, wondered whether kin were
preferentially warned. His group had already found that sagebrush plants
roughly fall into two "chemotypes," which mainly emit either camphor or
another organic compound called thujone when their leaves are damaged.
The team showed that the chemotypes are heritable, making them a
potential kin recognition signal. In 2014, the researchers reported that
when volatiles from a plant of one chemotype were applied to the same
type of plant, those plants mounted stronger antiherbivore defenses and
had much less insect damage than when the volatiles were applied to a
plant of the other chemotype—a hint of a kin effect.
The mustard Arabidopsis thaliana has provided another clue. About 8 years ago, Jorge Casal, a plant biologist at the University of Buenos Aires, noticed that Arabidopsis
plants growing next to relatives shift the arrangement of their leaves
to reduce shading of their neighbors, but don't do that when the
neighbors are unrelated. How they sense the presence of relatives was a
mystery, however.
The plants do have light sensors, and in 2015, Casal's team
discovered that the strength of reflected light striking nearby leaves
signaled relatedness and triggered the rearrangements. Relatives tend to
sprout leaves at the same height, bouncing more light onto each other's
leaves. By shifting leaves to reduce how much they shade each other,
the relatives cumulatively grow more vigorously and produce more seeds,
his team found. "There is no other case of kin recognition in plants
where the cue, the receptors, and the fitness consequences have been
established," Casal says.
Since then, he has shown that when sunflower kin are planted close
together, they, too, arrange themselves to stay out of one another's
way. The sunflowers incline their shoots alternately toward one side of
the row or the other, Casal and his colleagues reported in 2017 in the Proceedings of the National Academy of Sciences.
Taking advantage of the effect, they planted 10 to 14 related plants
per square meter—an unheard of density for commercial growers—and got up
to 47% more oil from plants that were allowed to lean away from each
other than plants forced to grow straight up.
Chui-Hua Kong, a chemical ecologist at the China Agricultural
University in Beijing, is exploiting a similar effect to boost rice
yields. His lab studies rice varieties that give off weed-killing
chemicals in their roots. Right now, they don't have high enough yields
to replace commonly grown varieties that require herbicides. But in
3-year-long field tests, kin-recognizing versions of these
self-protective rice varieties produced a 5% increase in yield when
grown with kin, rather than unrelated plants, Kong and colleagues
reported in late September 2018 in New Phytologist. To test the
approach on a larger scale, he and his colleagues are planting "kin"
seedlings of the weed-killing strain together in paddy fields in South
China.
Brian Pickles, an ecologist at the University of Reading in the
United Kingdom, proposes that kin recognition could even help forests
regenerate. By tracing flows of nutrients and chemical signals between
trees connected by underground fungi, he showed that the firs
preferentially feed their kin and warn them about insect attacks. The
finding suggested a family of firs would grow faster than unrelated
trees.
To some biologists, the emerging picture of communicating,
cooperating plants is still based on thin evidence. Laurent Keller, an
evolutionary biologist at the University of Lausanne who has shown that
some apparent signs of kin recognition in Arabidopsis can
instead stem from innate differences among the plants, calls for more
rigor in studies. "People have started to realize that it is important
to think carefully about the design of the experiment to rule out other
potential explanations," he says.
Keller is keeping an open mind and predicts stronger evidence of
plant kin recognition will emerge. Karban is already convinced. "We are
learning that plants are capable of so much more sophisticated behavior
than we had thought," he says. "It's really cool stuff."
.
No hay comentarios:
Publicar un comentario