Host and resident bacteria join forces to control fungi in plant roots


Date:
December 2, 2021
Source:
Max Planck Institute for Plant Breeding Research
Summary:
Researchers have discovered that diverse root-colonizing fungi
can benefit plants, but only when they are kept in check by the
host innate immune system and the bacteria residing in roots.



FULL STORY ==========================================================================
In nature, the roots of healthy plants are colonized by complex
microbial communities of bacteria and filamentous eukaryotes (i.e.,
fungi and oomycetes), the composition of which profoundly influences
plant health. Maintaining a microbial equilibrium in their roots is
very important for plants to remain healthy, however, the means by
which this is achieved by plants is still largely unknown. Now, in a
new study published in PNAS, Ste'phane Hacquard and his colleagues from
the Department of Plant-Microbe Interactions at the MPIPZ in Cologne,
Germany, shed light on the host and microbial factors that are required
to maintain a beneficial relationship between plant roots and their
diverse microbial partners.


==========================================================================
To tackle this research question, the first author of the study
Katarzyna W.

Wolinska used a complex microbial community comprising 183 bacteria
(B), 25 fungi (F), and 6 oomycetes (O) that were isolated from roots
of healthy Arabidopsis thaliana (Thale Cress) plants. She observed that
this complex BFO community was beneficial for plant growth compared to
sterile control plants grown in the absence of microbes. The authors
then hypothesized that inactivation of specific components of the plant
innate immune system -- the system responsible for tackling pathogen
infection -- would result in an altered microbial equilibrium in roots,
thereby affecting plant health.

Consistent with this hypothesis, the beneficial BFO community was
no longer beneficial in several of the immunocompromised mutant
plants. In particular, inactivation of two plant host genes involved
in tryptophan-derived specialized antimicrobials was sufficient to
turn the beneficial BFO community into a detrimental community that
negatively affected plant performance. The scientists then examined
the presence of abnormal microbial signatures in the roots of these immunocompromised plants and found that the major factor that could
explain growth differences across plants was the fungal load in their
roots. This observation led to the conclusion that the fungal burden
observed in the plant roots, in the absence of an intact immune system,
was likely the primary cause explaining the shift from a healthy to an unhealthy state.

To further explore whether the presence of fungi in the plant root
microbial community was indeed the direct cause of disease observed in
the plants, K. W.

Wolinska used the B, F, and O communities separately or in various
combinations (BO, FO, BF, BFO) and observed that the presence of the fungi
was indeed necessary to induce the unhealthy state of the plants. These
results indicate that the production of specialized anti-fungal molecules
from the host plant during tryptophan metabolism is key to maintain
a healthy fungal equilibrium in plant roots. Interestingly, these
anti-fungal molecules appeared to be insufficient in fully protecting
plants from fungi in the absence of bacteria, even in the presence of
a fully intact immune system.

According to the head of the study, Ste'phane Hacquard, "Our results
illustrate how host- and bacterium-encoded functions act in concert to
maintain fungi in check in Arabidopsisroots, thereby promoting plant
health and maintaining growth-promoting activities of multi-kingdom
microbial communities. The observation that the protective activity of
the bacterial community is as important as the host innate immune branch involving tryptophan-derived specialized metabolites for controlling fungi
is remarkable. It indicates that the plant immune system is insufficient
to fully protect plants from fungal burden, and that bacterial partners residing in roots provide an additional layer of protection, which is
needed for plant survival." These findings have important applications
for promoting plant health and turning potentially harmful fungi into beneficial isolates. By applying the knowledge gained in this study it
would now be conceivable to design mixed bacterial-fungal synthetic
communities that are expected to provide great fitness benefits to
the host.

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


========================================================================== Journal Reference:
1. Katarzyna W. Wolinska, Nathan Vannier, Thorsten Thiergart, Brigitte
Pickel, Sjoerd Gremmen, Anna Piasecka, Mariola
Piślewska-Bednarek, Ryohei Thomas Nakano, Youssef Belkhadir,
Paweł Bednarek, Ste'phane Hacquard. Tryptophan metabolism
and bacterial commensals prevent fungal dysbiosis in Arabidopsis
roots. Proceedings of the National Academy of Sciences, 2021; 118
(49): e2111521118 DOI: 10.1073/pnas.2111521118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211202141549.htm

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