permaculture

[Lawrence London <lfljvenaura@gmail.com>]

https://www.sciencedaily.com/releases/2018/07/180726162736.htm

Fertilizer destroys plant microbiome's ability to protect against disease
Healthy microbiomes on plant leaves protect against pathogens, though not
on fertilized plants Date: July 26, 2018Source: University of California -
BerkeleySummary: Despite enthusiasm for spraying probiotics on crops to
ensure healthy microbiomes, little is known about what a healthy
above-ground biome, or phyllosphere, looks like. Recent experiments show
that both natural microbiomes and synthetic biomes constructed from normal
populations are protective against pathogens, though sometimes low doses
work better than high doses. Surprisingly, fertilizing the plant allowed
pathogens to multiply on leaves despite a healthy phyllosphere.
A growth chamber packed with tomato plants sprayed with bacteria to
simulate different leaf microbiomes and then challenged with a plant
pathogen to measure how well the microbiomes protect the plant from disease.
*Credit: Britt Koskella photo.*

A new study of the role microbial communities play on the leaves of plants
suggests that fertilizing crops may make them more susceptible to disease.

University of California, Berkeley, biologists found that spraying tomatoes
with microbes from healthy tomatoes protected them from disease-causing
bacteria, but that fertilizing the tomatoes beforehand negated the
protection, leading to an increase in the population of pathogenic microbes
on the plants' leaves.

While the researchers don't yet know whether the increased number of bad
bacteria on the leaves actually makes the tomatoes sick, the study clearly
shows that fertilizer throws the community of microbes on the leaves
off-balance. That potentially could allow disease-causing organisms to
enter the plant.

"When we change the nutrient environment that plants are in, we are
fundamentally altering the plant-microbiome interaction and also,
importantly, the microbiome-mediated protection of natural plant/microbe
interactions," said senior author Britt Koskella, a UC Berkeley assistant
professor of integrative biology.

The fertilizer effect was not the only surprise from the study, Koskella
said. She and co-author Maureen Berg, a graduate student, were
investigating how the density of the microbial community on the leaves
affected the plants' resistance to disease and discovered that a lower dose
of beneficial microbes sprayed on the leaves was often more effective in
protecting the plants from infection than higher doses. Berg sprayed leaves
with an artificial microbial community composed of 12 species of bacteria
taken from the natural microbiome of healthy tomatoes.

"We found that the most protective community was the most dilute, the least
concentrated, the lowest dose," she said. "This was completely
nonintuitive. A medium dose gave medium protection and the highest dose was
the least protective."

*Probiotics for plants*

The reasons are unclear, but the findings are important because organic
farmers are talking about spraying crops with probiotics to encourage
better growth and disease protection, in the same way that humans consume
probiotics containing "good" microbes in hopes of improving their health.

"The fact that we saw this lower-dose/higher-protection effect suggests it
is not as simple as just throwing on more microbes," Koskella said. "There
is a lot of work to be done understanding how to apply a plant probiotic."

She and Berg will report their findings in the Aug. 6 print edition of the
journal *Current Biology*; the article will be posted online July 26.

Koskella focuses on plants' above-ground microbiomes, or the phyllosphere,
a poorly understood community compared to the well-studied below-ground
microbiome associated with plant roots, the rhizosphere. Researchers are
finding unsuspected activity within phyllosphere microbes, including that
some of the bacteria fix nitrogen from the air like root-associated
bacteria. Many studies have demonstrated that microbial communities in the
roots can promote plants' nutrient uptake, growth and resistance to
disease, and Koskella is investigating whether this also holds true for the
above-ground microbiome.

Her experiments are relevant to the issue of treating crops with
probiotics, and could help answer questions such as: What is the right mix
of bacteria for a given plant? What is the best way to apply this proper
mix?

To investigate these questions, Koskella and Berg began by sampling the
natural leaf microbes of healthy tomatoes grown in outdoor fields at UC
Davis.

They then sprayed the mix on sterile tomato plants in growth chambers at UC
Berkeley and, one week later, injected the leaves with Pseudomonas syringae
bacteria, which cause tomato speck, a major problem that's treated with
pesticides. The new microbial community on the tomatoes did, in fact,
protect the plants from colonization by pathogens, though the microbial
communities obtained from some tomato fields worked better than the
microbiomes from other fields.

"This phyllosphere microbial community, much like our own skin, is a first
line of defense against disease, so we expected to see protection, though
we didn't know for sure," Koskella said.

*Artificial microbial communities*

Surprisingly, when they varied the concentration of microbes sprayed on the
leaves, they found that in many cases low dosages worked better than high
dosages.

To find out why, they constructed an artificial microbial community
composed of 12 of the species found on natural plants -- basically, the 12
that grew best in culture. When they sprayed various dosages of the
synthetic community on tomatoes, they got the same result: low, diluted
doses were more protective against Pseudomonas than were high, concentrated
doses.

Berg repeated the experiment to confirm the puzzling findings, but during
one subsequent trial she decided to fertilize the droopy plants first. In
that trial, none of the microbiome doses were protective against
Pseudomonas. When they repeated the trial with and without fertilization,
they confirmed that application of fertilizer abolished the protective
effects previously observed.

In each experiment, they judged protection against pathogens by recording
the relative population of Pseudomonas compared to the other, mostly
beneficial microbes, since a healthy microbiome should effectively compete
with a pathogen and knock it down to low levels.

Koskella has suspicions about why fertilizer alters the microbiome, top
among them the possibility that the nutrients make healthier leaves, which
keeps all the microbes happy and obviates the need for the good microbes to
out-compete the bad microbes. She and her group are now pursuing
experiments to test that hypothesis.

They still have no idea why probiotic treatment at low dosages works better
than high doses, but hope that future research can solve this mystery and
help guide the suitable application of probiotics in agriculture.

Nevertheless, Koskella and Berg said, the impact of fertilizer on the leaf
and stem microbiome should lead biologists to explore fertilizer's effect
on the root microbiome as well, and on the general health of the plant.

"We have been fertilizing crops for so long it would surprise me if we
haven't already seen consequences of long-term fertilization on how plants
interact with their microbes," she said. "There are a lot of studies that
show domesticated plants tend to have very different microbial communities
than their wild relatives."

The big questions are, does that affect the plant's overall health, and why?
------------------------------

*Story Source:*

Materials
<http://news.berkeley.edu/2018/07/26/fertilizer-destroys-plant-microbiomes-ability-to-protect-against-disease/>
provided by *University of California - Berkeley* <http://www.berkeley.edu>.
Original written by Robert Sanders. *Note: Content may be edited for style
and length.*
------------------------------

*Journal Reference*:

   1. Maureen Berg, Britt Koskella. *Nutrient- and Dose-Dependent
   Microbiome-Mediated Protection against a Plant Pathogen*. *Current
   Biology*, 2018; DOI: 10.1016/j.cub.2018.05.085
   <http://dx.doi.org/10.1016/j.cub.2018.05.085>

------------------------------

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University of California - Berkeley. "Fertilizer destroys plant
microbiome's ability to protect against disease: Healthy microbiomes on
plant leaves protect against pathogens, though not on fertilized plants."
ScienceDaily. ScienceDaily, 26 July 2018. <
www.sciencedaily.com/releases/2018/07/180726162736.htm>.