That’s the case for Xanthomonas, the organism that
causes bacterial leaf spot disease in tomato and pepper plants. Like
many microbes with short generation times, it can evolve at
lightning speed to acquire beneficial traits, such as the ability to
elude its host’s defense system.
New research from the University of Illinois shows one Xanthomonas
species, X. euvesicatoria (Xe), has evolved to avoid detection by
the immune system of tomato plants.
“It’s part of the evolutionary warfare between plants and pathogens,
where the plant has some defense trait and then some portion of the
pathogen population evolves to escape it. The plant has to develop
or acquire a new defense trait, but the process is much slower in
plants compared to microbes. This study is a great example of that
ongoing battle in progress. It tells us we can’t completely rely on
this trait to combat bacterial spot disease caused by Xe,” says
Sarah Hind, assistant professor in the Department of Crop Sciences
at Illinois and co-author on a pair of recent studies published in
Molecular Plant-Microbe Interactions and Physiological and Molecular
Plant Pathology.
The tomato defense system keeps tabs on Xanthomonas and other
bacteria with immune receptors that chemically detect flagella, the
long whip-like tail structures that allow bacteria to move or “swim”
through soil and plant tissues. Hind and her colleagues used
laboratory and genomic modeling techniques to show one of tomato’s
receptors, FLS3, no longer works to detect flagellin proteins in Xe.
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Their work shows Xe’s flagellin proteins have changed by just one
amino acid, but it’s enough to escape detection by tomato’s FLS3
receptors.
Graduate student and study co-author Maria Malvino says, “It was
surprising to see that only one amino acid change was making all the
difference. It made us wonder how binding between flagellin and FLS3
could be so dramatically altered.”
The fact that Xe can sneak past tomato’s defenses means farmers can
rely even less on inherent disease resistance. Instead, they’ll have
to combat the disease in other ways, such as spraying copper-based
pesticides.
In some locations, including the Midwest region and in Illinois
specifically, Xe isn’t as much of a problem as two other Xanthomonas
species, X. perforans and X. gardneri (Xp and Xg). Tomato can still
hold its own against these species for the time being, but Hind is
concerned Xe will share its evasion strategy.
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“X. euvesicatoria [Xe] had been the predominant
strain for a long time, but within the last decade or two, it's
become less prominent and has been overtaken by another species, Xp,”
she says. “Xp and Xe are really genetically close, and it’s been
shown that they can share their genetic material with each other. So
it wouldn't be out of the realm of possibility that that Xe’s
evasion strategy could make its way into Xp and provide the same
advantage against tomato.”
Hind says the tendency of these bacteria to defeat host defenses
through rapid evolution makes breeding for disease resistance
difficult in tomato.
“It’s like whack-a-mole for breeders. It takes a long time to
release a resistant variety. Often, by the time they go to release a
new one, the pathogen population shifts,” Hind says. “And when you
add to that the difficulty of maintaining all the desirable traits
of a tomato, it’s a tough situation. Again, that leaves us relying
on fungicides and copper treatments to keep tomato production
profitable here in Illinois.”
The first article, “Influence of flagellin polymorphisms, gene
regulation, and responsive memory on the motility of Xanthomonas
species that cause bacterial spot disease of solanaceous plants,” is
published in Molecular Plant-Microbe Interactions [DOI:
10.1094/MPMI-08-21-0211-R]. Authors include Maria Malvino, Amie Bott,
Cory Green, Tanvi Majumdar, and Sarah Hind.
The second article, “Utilizing Tajima’s D to identify potential
microbe-associated molecular patterns in Xanthomonas euvesicatoria
and X. perforans,” is published in Physiological and Molecular Plant
Pathology [DOI: 10.1016/j.pmpp.2021.101744]. Authors include Julius
Pasion and Sarah Hind. Funding was provided by USDA’s National
Institute of Food and Agriculture.
The Department of Crop Sciences is in the College of Agricultural,
Consumer and Environmental Sciences at the University of Illinois
Urbana-Champaign.
[Sources: Sarah Hind & Maria
Malvino
News writer: Lauren Quinn]
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