Prior exposure to powdery mildew makes plants more vulnerable to
subsequent disease

Date:
August 31, 2020
Source:
Washington University in St. Louis
Summary:
A key to surviving in the wild is fighting off infection -- and not
just once. For plants, as with humans, one infection may or may not
leave a plant with lasting immunity. Biologists conducted a series
of elegant experiments that capture how pathogen strains naturally
accumulate on plants over a growing season. Their findings reveal
the importance of understanding interactions among pathogens when
developing strategies for maintaining healthy crop populations.



FULL STORY ==========================================================================
Next time you head outside for a socially distant walk in between your
Zoom meetings, notice the rich diversity of plants along your path. As
we approach late summer, be sure to also notice the diversity of disease symptoms on those plants, including spots, blotches or fuzzy growth
caused by bacteria, viruses or fungi.


==========================================================================
A key to surviving in the wild is fighting off infection -- and not
just once.

As in humans, one infection may or may not leave a plant with lasting
immunity.

In fact, an early infection might make things worse. New research from
an international team including an assistant professor of biology at
Washington University in St. Louis shows that infection actually makes
a plant more susceptible to secondary infection -- in experiments and
in the wild. The findings are published in the Aug. 31 issue of Nature
Ecology & Evolution.

"We found that early infection facilitated later infection," said Rachel Penczykowski, assistant professor of biology in Arts & Sciences and
co-first author on the study. She performed the field experiments as
a postdoctoral researcher with Anna-Liisa Laine, senior author on the
paper, now at the University of Zu"rich.

"And the order in which pathogen strains infect a plant matters,"
Penczykowski said. "Some pathogen strains are especially likely to
facilitate infection by later-arriving strains." The findings --
obtained through a series of elegant experiments that capture how
pathogen strains naturally accumulate on plants over a growing season -
- reveal the importance of understanding interactions among pathogens
when developing strategies for maintaining healthy crop populations.



========================================================================== Early infection promotes later infection A common roadside weed, Plantago lanceolata is native to Europe, where this study took place, and Asia;
it is also commonly found in North America.

Infection by the pathogen Podosphaera plantaginis, a powdery mildew
fungus, is easy to spot with the naked eye.

In the wild, plant populations are exposed to and infected by multiple
powdery mildew strains over the course of their lifetime. The authors
wondered if prior exposure to one strain of powdery mildew affects the
plant's susceptibility to a second.

To simulate what would happen in the wild, the authors took young,
disease-free plants and brushed pathogen spores from one of four
pathogen strains onto a single leaf per plant. The rest of the leaves
were temporarily covered with a plastic bag.

The inoculated leaf was then covered with a spore-proof pouch for the
duration of the experiment, which prevented infection from spreading
between it and the other leaves. This method works because powdery
mildew produces a localized, leaf-surface infection that does not spread systemically in the plant.

Otherwise identical control plants received a sham inoculation instead
of powdery mildew spores.



==========================================================================
The plants were then placed in a common garden environment in a large
field (without locally occurring Plantago or powdery mildew), where they
were simultaneously exposed to all four pathogen strains.

Penczykowski and co-first author Fletcher Halliday, a current postdoctoral researcher in the Laine lab, found that none of the four strains of
powdery mildew inoculated onto plants protected the plants from a
secondary infection.

In fact, prior exposure to mildew made plants more susceptible to a
second powdery mildew infection compared to infection-nai"ve controls.

"If you look at each strain individually, some of the strains were better
than others at promoting later infection," Penczykowski said.

"Because crop plants may also be exposed to a diversity of pathogen
strains during a given growing season, understanding the ways in which different pathogen strains impact each other is important for developing sustainable disease control strategies in agricultural systems." Into the
wild Scientists sometimes place cohorts of healthy, greenhouse-grown
"sentinel plants" into field populations to measure the risk of pathogen infection. Doing this with sentinel plants allows researchers to control
for genetic background, age and condition.

To test how prior inoculation affected the probability of plants
becoming infected during epidemics in wild populations, the authors
inoculated plants as they did in the common garden experiment (again,
with uninoculated controls for comparison). Except this time, they moved
the potted sentinel plants into wild populations and waited for naturally occurring mildew spores to arrive.

The researchers found that previously infected sentinel plants acquired secondary mildew infections more often than control plants that had
never been infected. This was true even though the only way plants were catching the naturally occurring pathogen strains was through the wind.

"What we saw in both our common garden and our sentinel plant experiments
was that previously inoculated plants were more susceptible to later infection," Halliday said. "But could we detect the signature of pathogen strain facilitation in naturally infected wild plant populations? That
would require an intensive survey of wild plant-pathogen dynamics."
And into the wild the scientists went -- that is, using data from wild populations that were fortunately collected the previous year.

In an intensive survey of 13 field populations, the scientists tracked
mildew infection in wild plants over the course of two months. They
tagged plants as they found mildew infection; otherwise, they were
not manipulated in any way and had been growing in the field their
whole lives.

A small leaf area of each infected plant was cut and brought to the lab
to identify the mildew strains that infected the plants at different
times throughout the growing season.

The importance of being early Powdery mildew strains vary in their ability
to survive the winter and in the timing of their reproductive cycle.

Some strains arrive earlier in the growing season and are likely to be
the ones that had successfully overwintered and reproduced quickly.

Halliday dove into the genetic data compiled from the surveys of the 13
field populations and found that strains detected early in the season
commonly facilitated subsequent infections, and strains that arrived to
the populations later in the season benefited from that facilitation.

"The early-arriving strains are the ones that are driving the course
of epidemics and also affecting the diversity of pathogen strains that
assemble in plant populations," Halliday said.

"In other words, the strains that are ready to hit the ground running
in spring may impact both the ecological and evolutionary dynamics of plant-pathogen interactions," Penczykowski added.


========================================================================== Story Source: Materials provided by
Washington_University_in_St._Louis. Original written by Marta
Wegorzewska. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Fletcher W. Halliday, Rachel M. Penczykowski, Benoit Barre`s,
Jenalle L.

Eck, Elina Numminen, Anna-Liisa Laine. Facilitative priority
effects drive parasite assembly under coinfection. Nature Ecology &
Evolution, 2020; DOI: 10.1038/s41559-020-01289-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200831112349.htm

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