Infectious bacteria force host plants to feed them, study finds
Insights into pathogen that threatens corn could inform future breeding


Date:
April 13, 2022
Source:
Ohio State University
Summary:
A species of bacteria that infect corn crops compel their hosts
to produce a feast of nutrients that keeps the pathogens alive
and thriving long before they start to kill the plant's cells,
new research shows.



FULL STORY ==========================================================================
A species of bacteria that infect corn crops compel their hosts to produce
a feast of nutrients that keeps the pathogens alive and thriving long
before they start to kill the plant's cells, new research shows.


==========================================================================
The study in young maize plants reveals that these bacteria not only
generate food for themselves in crops they inhabit, but also coax life-sustaining water from the plants.

Though the lab conditions don't represent exactly what happens in the
field, the research provides insights into fundamental processes central
to the ability of a pathogen to cause a corn disease prevalent in the
central and northeastern U.S. called Stewart's wilt. These bacteria
are also creating problems for rice and jackfruit crops in parts of the
Eastern hemisphere.

By confirming that a bacterial virulence factor, a protein called WtsE, initiates the mobilization of food and water into spaces where the
bacteria reside, the study lays groundwork for future breeding of plants
that can put the brakes on these bacterial survivalist tactics. Current breeding practices in corn are based on previous research that focused
on boosting the plants' immune response to these infectious bacteria,
a species known as Pantoea stewartii.

"No one has shown before that a dynamic flow of nutrients from plant
to bacteria supports proliferation of the bacteria during the initial
stages of infection. Our findings reveal a bacterial feeding frenzy,"
said lead study author David Mackey, professor of horticulture and crop
science at The Ohio State University.

"There have been no targeted efforts to control nutrient availability as
a means to control Pantoea stewartii, or other plant pathogenic bacteria
that rely on proteins similar to WtsE for their virulence. This opens up
an opportunity for us to look at the mechanism of how WtsE accomplishes
this. How is it manipulating the plant cells?" The research is published
today (April 13, 2022) in Cell Host & Microbe.



==========================================================================
The research focuses on a phase called biotrophy: After infecting a
plant, the bacteria initially parasitize living host cells and multiply dramatically. Only later do the bacteria start killing plant cells to
release further nutrients and cause disease.

In the field, corn flea beetles carry Pantoea stewartii and deposit the bacteria in wound sites the beetles create when feeding on crop stems
and leaves. Infections then spread unevenly from these sites.

For this study, Mackey and colleagues infiltrated maize seedlings with a powerful dose of the infectious bacteria, creating a series of uniformly infected leaves. This model system made it possible for the researchers
to determine that the release of nutrients and water preceded the death
of plant cells.

The team zeroed in on observing the actions of WtsE, one of a class
of proteins in pathogenic bacteria known as type III effectors. These
proteins are transported from the bacteria into infected plant cells to
both suppress plant immunity and, as discovered in the case of Pantoea stewartii, promote availability of water and food.

All of this activity takes place in the apoplast, a relatively dry
compartment on the interior of a plant tissue but outside of the plant
cells. That dryness is relevant, because one of WtsE's tricks is promoting
the availability of water in this space. A leading hypothesis has been
that this condition, called "water soaking," results from dying plant
cells spilling their contents into the apoplast when bacteria begin
their lethal attack.



========================================================================== "This is one of the main points we showed: Infection causes water
to accumulate in the apoplast well in advance of killing the plant
cells. It's an active process and it's dependent on the WtsE effector,"
said Mackey, also a professor of molecular genetics.

And then, once hydrated, the apoplast begins to fill with nutrients that function as sources of nitrogen and carbon for the bacteria -- sugars,
amino acids and organic acids that are generated and consumed in much
higher quantities than exist in the apoplast of a healthy plant.

The researchers confirmed the massive size of the feast by removing the bacteria from the plants and measuring how much carbon and nitrogen
they had taken up in a specific period of time -- which was 6 and 30
times higher, respectively, than that present in the apoplast of an
uninfected plant.

"It's not like the bacteria arrived and ate what was already available,"
Mackey said. "Plants are relinquishing sources of carbon and nitrogen into
the apoplast, where they are assimilated by the bacteria. Additionally,
the plant metabolic networks respond to the depletion by making more of
those compounds.

It's a really dynamic process and the WtsE effector drives that process."
The role of the effector was confirmed genetically -- mutant bacteria
lacking WtsE were unable to accomplish these same tasks.

With these findings, Mackey's lab is now positioned to pinpoint how
WtsE is able to coerce maize into doing its bidding -- and specifically,
which plant proteins the effector hijacks for help -- which could inform
future resistant breeding practices.

This work was supported by Ohio State's Translational Plant Sciences
Graduate Program, the National Institute of Food and Agriculture, the
U.S. Department of Agriculture, the National Science Foundation and the
Korean Rural Development Administration.

Co-authors include Irene Gentzel, Laura Giese, Gayani Ekanayake, Kelly
Mikhail and Wanying Zhao, all of Ohio State, and Jean-Christophe Cocuron
and Ana Paula Alonso of the University of North Texas.


========================================================================== Story Source: Materials provided by Ohio_State_University. Original
written by Emily Caldwell. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Irene Gentzel, Laura Giese, Gayani Ekanayake, Kelly Mikhail, Wanying
Zhao, Jean-Christophe Cocuron, Ana Paula Alonso, David
Mackey. Dynamic nutrient acquisition from a hydrated apoplast
supports biotrophic proliferation of a bacterial pathogen
of maize. Cell Host & Microbe, 2022; 30 (4): 502 DOI:
10.1016/j.chom.2022.03.017 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220413131143.htm

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