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Fertilizer destroys plant microbiome's ability to safeguard against disease

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A brand-new study of the function microbial communities play on the leaves of plants suggests that fertilizing crops may make them more susceptible to illness.

University of California, Berkeley, biologists found that spraying tomatoes with microbes from healthy tomatoes secured them from disease-causing germs, however that fertilizing the tomatoes beforehand negated the protection, causing a boost in the population of pathogenic microbes on the plants' leaves.

University of California - Berkeley
19.12.18

A brand-new study of the function microbial communities play on the leaves of plants suggests that fertilizing crops may make them more susceptible to illness.

University of California, Berkeley, biologists found that spraying tomatoes with microbes from healthy tomatoes secured them from disease-causing germs, however that fertilizing the tomatoes beforehand negated the protection, causing a boost in the population of pathogenic microbes on the plants' leaves.

While the scientists do not yet understand whether the increased number of bad bacteria on the leaves actually makes the tomatoes sick, the study clearly reveals that fertilizer throws the community of microbes on the leaves off-balance. That possibly might 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, notably, the microbiome-mediated defense of natural plant/microbe interactions," stated senior author Britt Koskella, a UC Berkeley assistant professor of integrative biology.

The fertilizer effect was not the only surprise from the research study, Koskella said. She and co-author Maureen Berg, 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 advantageous microorganisms sprayed on the leaves was typically more efficient in protecting the plants from infection than greater doses. Berg sprayed leaves with a synthetic microbial community made up of 12 types of bacteria taken from the natural microbiome of healthy tomatoes.

"We discovered that the most protective community was the most dilute, the least concentrated, the lowest dose," she stated. "This was totally nonintuitive. A medium dosage provided medium protection and the greatest dosage was the least protective."

Probiotics for plants

The reasons are uncertain, however the findings are essential due to the fact that organic farmers are speaking about spraying crops with probiotics to encourage better growth and disease protection, in the exact same way that people take in probiotics including "good" microbes in hopes of enhancing their health.

"The fact that we saw this lower-dose/higher-protection effect recommends it is not as easy as simply tossing on more microbes," Koskella stated. "There is a great deal of work to be done comprehending how to apply a plant probiotic."

Koskella focuses on plants' above-ground microbiomes, or the phyllosphere, a poorly comprehended community compared to the well-studied below-ground microbiome related to plant roots, the rhizosphere. Researchers are discovering unsuspected activity within phyllosphere microbes, including that a few of the bacteria fix nitrogen from the air like root-associated bacteria|germs. Lots of research 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 applies for the above-ground microbiome.

Her experiments are relevant to the issue of treating crops with probiotics, and could help respond to concerns such as: What is the ideal mix of bacteria for a given plant? What is the best way to use 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 trigger tomato speck, a major issue that's treated with pesticides. The new microbial community on the tomatoes did, in fact, safeguard the plants from colonization by pathogens, though the microbial communities acquired from some tomato fields worked better than the microbiomes from other fields.

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

Artificial|Synthetic microbial communities

Surprisingly, when they varied the concentration of microbes sprayed on the leaves, they discovered that in most 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 exact same outcome: low, diluted doses were more protective against Pseudomonas than were high, concentrated doses.

Berg duplicated the experiment to confirm the perplexing findings, but during one subsequent trial she chose 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 eliminated the protective effects previously observed.

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

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

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

Nonetheless, Koskella and Berg stated, 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 actually been fertilizing crops for so long it would shock me if we have not already seen effects of long-term fertilization on how plants communicate with their microbes," she stated. "There are a lot of studies that show domesticated plants tend to have extremely different microbial communities than their wild relatives."

The huge questions are, does that affect the plant's general health, and why?

Author: University of California - Berkeley
Source: https://phys.org/news/2018-07-fertilizer-microbiome-ability-disease.html

Note by AIRS: Likewise with plants: human being as being part of the nature are exposed to same considerations what happens to our microbiome with all those nutrients, cosmetics and environmental factors affecting the integrity and balanced state of our health. The 2nd Airs Conference will dig here for answers.