In poplars, two plant hormones boost each other in defense against
pathogenic fungi

Date:
May 3, 2022
Source:
Max Planck Institute for Chemical Ecology
Summary:
In contrast to previous assumptions, the defense hormones
salicylic acid and jasmonic acid do not always suppress each
other in regulating plant chemical defenses against pests and
pathogens. In trees, the interplay of both hormones can actually
increase plant resistance.



FULL STORY ==========================================================================
In contrast to previous assumptions, the defense hormones salicylic acid
and jasmonic acid do not always suppress each other in regulating plant chemical defenses against pests and pathogens. In trees, the interplay
of both hormones can actually increase plant resistance. This is the
conclusion researchers from the Max Planck Institute for Chemical Ecology
draw in a new study on poplars.

The scientists showed that higher levels of jasmonic acid were also
detectable in poplars that had been modified to produce increased levels
of salicylic acid or that had been treated with salicylic acid. Plants
that had higher concentrations of both hormones were also more resistant
to the rust fungus Melamspora larici-populina, with no negative effect on growth. Knowledge of the positive interaction of these hormones involved
in plant resistance could help to better protect poplars and other trees against pathogens.


==========================================================================
The function of plant hormones or phytohormones is to coordinate the
growth and development of plants. Moreover, they also control plant immune responses to microbial pathogens such as pathogenic fungi. Until now,
there has been a broad consensus in science that the signaling pathways
of the defense hormones salicylic acid and jasmonic acid act in opposite directions. Thus, if plants produce more salicylic acid, this would
inhibit the production of jasmonic acidand vice versa. Scientists have repeatedly shown this negative interplay in studies of the model plant Arabidopsis thaliana (thale cress) and many other annual herbs. "Contrary
to the assumption that the salicylic acid and jasmonic acid hormone
signaling pathways work in an opposite manner, we had already observed
in our earlier studies on poplar trees that both of these hormones
increase in response to infection by pathogenic fungi. Therefore, the
main research question was to determine the interaction between these
two defense hormones in poplar," Chhana Ullah, first author of the
publication, explains the starting point of the current study.

To study experimentally how salicylic acid levels affect the formation
of jasmonic acid, the scientists genetically modified experimental
plants of black poplar (Populus nigra) native to Germany so that they
produced higher amounts of salicylic acid than control plants. In
another experiment, they applied salicylic acid to the poplar leaves of genetically unmodified plants. "We manipulated salicylic acid levels in
poplar by genetic engineering and direct chemical application, after
which we conducted extensive chemical analyses of the plants with and
without fungal infection. This allowed us to separate the effects of
salicylic acid from other factors and show that it directly stimulates
jasmonic acid production," explains Chhana Ullah.

Plants that contained high levels of salicylic acid also had higher concentrations of jasmonic acid. In addition, these plants produced
more antimicrobial substances, known as flavonoids, even if there was
no infection with a pathogen. Further comparative studies with plants
that produced high levels of salicylic acid and control plants that
had each been infected with the rust fungus Melamspora larici-populina
showed that high levels of salicylic acid made poplars more resistant
to fungal attack.

Surprisingly, higher fungal resistance due to increased defenses did not negatively affect plant growth, as had been observed in Arabidopsis and
other annual herbs. In Arabidopsis, either salicylic acid or jasmonic
acid takes control of the immune response, while the other hormone
is suppressed.

Salicylic acid is produced in higher amounts after attack by biotrophic pathogens that do not kill plant tissue and feed on living plant material, while jasmonic acid is increased after attack by insects or necrotrophic pathogens that feed on dead plant tissue. "The negative interplay between
the defense hormones salicylic acid and jasmonic acid in plants like Arabidopsis enables the plant to prioritize protection against one kind
of enemy. Small herbs like Arabidopsis may benefit from such a narrow
focus because they lack the resources to defend against different kinds of enemies at once. This may also be the reason why Arabidopsis plants reduce their growth rate when in a defense mode," says Jonathan Gershenzon,
head of the Department of Biochemistry where the study was conducted.

In contrast to annual herbs such as thale cress, resources are usually
less limited for trees and other woody plants. Moreover, because of
their long lifespan, trees are often attacked simultaneously by different enemies, such as fungal and bacterial pathogens, leaf-eating caterpillars,
and wood-destroying insects. They may have evolved to use the salicylic
and jasmonic acid signaling pathways together for defense. The greater availability of resources in long- living woody plants may also be the
reason why high concentrations of salicylic acid do not affect plant
growth in poplars.

The researchers were surprised to find that high levels of salicylic
acid in poplars did not activate so-called pathogenesis-related (PR)
genes, although these are established markers for the salicylic acid
signaling pathway in Arabidopsis. "However, we found that the magnitude
of PR gene induction was positively correlated with the susceptibility
of poplar to rust. Apparently, the activation of PR genes in poplar is
not regulated by salicylic acid signaling, but by a different mechanism," Chhana Ullah explains.

The team of scientists led by Chhana Ullah still has to find out exactly
how the molecular mechanism of the positive interaction between salicylic
acid and jasmonic acid works in poplar. They also want to know which role
PR genes play in poplar and other woody plants. What is certain, however,
is that a fundamental knowledge of the positive interaction between
salicylic acid and jasmonic acid in poplar and other related trees could
make an important contribution to better protecting these plants from pest infestation and disease. Or, as Jonathan Gershenzon notes: "Poplars are
known as the trees of the people for their diversified uses by humans,
hence the genus name Populus: the Latin name for people. Incredibly fast-growing, poplars are cultivated as short-rotation woody crops and
are extremely important of the pulp and paper industry. They are also
desirable for biofuels." Improving their protection therefore serves
us all.


========================================================================== Story Source: Materials provided by
Max_Planck_Institute_for_Chemical_Ecology. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Chhana Ullah, Axel Schmidt, Michael Reichelt, Chung‐Jui Tsai,
Jonathan Gershenzon. Lack of antagonism between salicylic acid and
jasmonate signalling pathways in poplar. New Phytologist, 2022;
DOI: 10.1111/nph.18148 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/05/220503141350.htm

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