and rotted fruit or flower spots in a variety of plants ranging
from avocados to zinnias.
Microbes for conservation
Back in Hawaii, after that disheartening hike to the P. kaalaensis
graveyard, Zahn pondered how to protect native plants in wild
environments such as Oahu’s mountains.
In people, Zahn considered, antibiotics can damage normal
gut microbe populations, leaving a person vulnerable to infection by harmful microbes. P. kaalaensis got similar treatment in
the greenhouse, where it received regular dosing of fungicide.
In retrospect, Zahn realized, that treatment probably left the
plants bereft of their natural microbiome and weakened their
immune systems, leaving them vulnerable to mildew infection
once dropped into the jungle.
For people on antibiotics, probiotics — beneficial bacteria —
can help restore balance. Zahn thought a similar strategy, a sort
of plant probiotic, could help protect P. kaalaensis in future
attempts at moving it outside.
For a probiotic, Zahn looked to a P. kaalaensis cousin,
Phyllostegia hirsuta, which can survive in the wild. He put
P. hirsuta leaves in a blender and sprayed the slurry over
P. kaalaensis growing in an incubator.
Then, Zahn placed a leaf infected with powdery mildew into
the incubator’s air intake. The mint plants treated with the
P. hirsuta slurry experienced delayed, less severe infections
compared with untreated plants, Zahn and Amend, also at
the University of Hawaii at Manoa, reported last year in PeerJ.
The probiotic had worked.
Zahn used DNA sequencing to identify the microbes in
the slurry. Many of the microbiome members probably ben-
efit P. kaalaensis, but he thinks he’s found a major protector:
a yeast called Pseudozyma aphidis that lives on leaves. “This
yeast normally just passively absorbs nutrients from the
environment,” Zahn says. “But given the right victim, it will
turn into a vicious spaghetti monster.” When mildew spores
land nearby, the yeast grows tentacle-like filaments that
appear to envelop and feed on the mildew.
Emboldened by his results, Zahn trekked back to the jungle
and planted six slurry-treated plants in April 2016. They survived for about two years, but by May 2018, they were all dead.
“It was still a huge win,” says Nicole Hynson, a community
ecologist also at Manoa. After all, P. kaalaensis without probiotics last only months. And the probiotics approach might
apply beyond one little Hawaiian mint, Hynson adds: “We’re
really at the beginning of thinking how we might use the microbiome to address plant restoration.”
Zahn has since moved to Utah Valley University in Orem,
where he’s hoping to help endangered cacti with microbes.
Meanwhile, he’s left the Phyllostegia project in the hands of
Jerry Koko, a graduate student in Hynson’s lab. Koko is studying how the yeast and some root-based fungi protect the plant.
Hynson says their goal is to build “a superplant.” With probiotics on both roots and shoots, an enhanced P. kaalaensis should
be well-equipped to grow strong and resist mildew. In greenhouse experiments so far, Koko says, the plants with both types
of beneficial fungi seem to sport fewer, smaller powdery mildew
patches than plants that received no probiotic treatment.
While the restoration of a little flowering plant, or a few
more bushels of soybeans, may seem like small victories, they
could herald big things for plant microbiomes in conservation as well as agriculture. The farmers and conservationists
of the future may find themselves seeding and tending not just
plants, but their microscopic helpers, too. s
s Natalie Christian et al. “Exposure to the leaf litter micro-
biome of healthy adults protects seedlings from pathogen
damage.” Proceedings of the Royal Society B. July 12, 2017.
s Geoffrey Zahn and Anthony S. Amend. “Foliar microbiome
transplants confer disease resistance in a critically-
endangered plant.” PeerJ. November 10, 2017.
Plant Microbe Effects
Legumes and other
Rhizobia Provide the plant with extra
nitrogen, which boosts
photosynthesis and helps the
plant produce bitter, nitrogen-
containing alkaloids that
defend against plant-eating
Maize (Zea mays) Glomus
Increase the plant’s root
length and water uptake
Dang shen (Codonopsis
pilosula), a medicinal
herb also known as
poor man’s ginseng
Improve plant size, chlorophyll
content and photosynthesis
Stachybotrys Reduce the severity of leaf
rust caused by the fungus
Help the plant withstand
Botanical beneficiaries Bacteria and fungi that live in and on
plants can help their hosts in several ways. Here’s a sampling.
Powdery mildew attacks Phyllostegia kaalaensis (left), preventing the
plant from taking hold in the wild. But a beneficial yeast (rods, right)
found on the leaf of another species of Phyllostegia sends out filaments
that attack the mildew (blob in inset) and protect the plants.