Fungal pathogen disables plant defense mechanism
The white mold fungus Sclerotinia sclerotiorum detoxifies the mustard oil
bomb in plants of the cabbage family
Date:
June 19, 2020
Source:
Max Planck Institute for Chemical Ecology
Summary:
Cabbage plants defend themselves against herbivores and pathogens
by deploying a defensive mechanism called the mustard oil
bomb. Researchers have now been able to show that this defense
is also effective against the widespread fungus Sclerotinia
sclerotiorum. However, the pathogen uses at least two different
detoxification mechanisms that enable the fungus to successfully
spread on plants defended in this way.
FULL STORY ========================================================================== Cabbage plants defend themselves against herbivores and pathogens
by deploying a defensive mechanism called the mustard oil bomb: when
the plant tissue is damaged, toxic isothiocyanates are formed and can effectively fend off attackers. Researchers at the Max Planck Institute
for Chemical Ecology and the University of Pretoria have now been able
to show in a new study that this defense is also effective to some
extent against the widespread and detrimental fungus Sclerotinia
sclerotiorum. However, the pathogen uses at least two different
detoxification mechanisms that enable the fungus to successfully spread
on plants defended in this way. The metabolic products thus formed are non-toxic to the fungus, allowing it to grow on these plants.
========================================================================== Sclerotinia sclerotiorum is a devastating fungal pathogen that can
infect more than 400 different plant species. The main symptom of the
disease called Sclerotinia wilt or white mold is wilting. Visible are
also the white, cotton- like fungal spores that overgrow plant leaves
and stalks. In agriculture, rapeseed cultivation is particularly at
risk. The plant disease can affect other members of the cabbage family,
and also potatoes, legumes and strawberries.
Scientists at the Max Planck Institute for Chemical Ecology in Jena have
long been studying the glucosinolates and isothiocyanates that constitute
the special defense mechanism of cabbage family plants, which include
rapeseed, radishes and mustard. "We wanted to find out how successful
plant pathogens overcome the plant defense and colonize these plants. We therefore asked ourselves whether widespread fungal pathogens have
strategies to adapt to the chemical defenses of plants of the cabbage
family," Jingyuan Chen, the first author of the study, explains.
The researchers were able to show experimentally that the defense based
on glucosinolates is actually effective against fungal attacks. However,
they also discovered two different strategies of the white mold fungus to detoxify the defensive substances: The first is a general detoxification pathway that binds glutathione to the isothiocyanate toxins. This
type of detoxification of organic poisons is quite common in insects
and even mammals. The second and far more effective way to render the isothiocyanates harmless is to hydrolyse them, i.e. to cleave them enzymatically with a water molecule. The researchers wanted to identify
the enzymes and corresponding genes underlying this detoxification
mechanism. Genes that enable the successful detoxification of these
substances had already been described in bacteria. They are called
Sax genes after experiments with the model plant Arabidopsis thaliana:
Survival in Arabidopsis eXtracts.
"We based our search on the known bacterial SaxA proteins to select
candidate genes for further investigations. We then tested whether these
genes are actually expressed in greater quantities in fungi exposed to the toxins, and whether the resulting protein can render the toxins harmless," explains Daniel Vassa~o, one of the study leaders. Using high-resolution analytical methods, the scientists were able to identify and quantify
the metabolites produced by the fungus during detoxification. They
also used mutants of the fungus in which the SaxA-encoding gene had
been knocked out for comparison. This revealed that the Sax protein
of the white mold fungus is active against a range of isothiocyanates,
allowing it to colonize different plants of the cabbage family.
Mutants lacking the gene for this detoxification pathway were dramatically reduced in their capacity to tolerate isothiocyanates. "However, it
was surprising to see that these mutants up-regulated their general
pathway of detoxification, although this did not compensate for the
mutation," says Jingyuan Chen. Glutathione conjugation cannot detoxify isothiocyanates nearly as effectively as hydrolysis can. Although it seems
to be metabolically more expensive for the fungus, this general pathway
is always present as it helps the fungus to detoxify a huge variety of
poisons. "It is possible that this general pathway protects the fungus initially, while the machinery required for the more specialized pathway
is assembled after an initial exposure to the toxin and can take over
later in the infection," says Daniel Vassa~o.
In further experiments, the researchers want to investigate whether
other fungi that successfully infect plants of the cabbage family also
detoxify isothiocyanates via the same pathway, and whether unrelated
fungal species are also able to degrade these toxins. "Then we will know whether this widespread detoxification is due to repeated evolution in
fungi colonizing mustards, or is a feature which has been conserved over
time and is therefore found in many fungal lines," Jonathan Gershenzon, director of the Department of Biochemistry where the research was
conducted, concludes.
========================================================================== Story Source: Materials provided by
Max_Planck_Institute_for_Chemical_Ecology. Note: Content may be edited
for style and length.
========================================================================== Journal Reference:
1. Jingyuan Chen, Chhana Ullah, Michael Reichelt, Franziska Beran,
Zhi-Ling
Yang, Jonathan Gershenzon, Almuth Hammerbacher, Daniel
G. Vassa~o. The phytopathogenic fungus Sclerotinia sclerotiorum
detoxifies plant glucosinolate hydrolysis products via an
isothiocyanate hydrolase. Nature Communications, 2020; 11 (1)
DOI: 10.1038/s41467-020-16921-2 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2020/06/200619104310.htm
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