Alpine plant spins its own flavonoid wool

Date:
June 17, 2021
Source:
University of Cambridge
Summary:
Like the movie version of Spider-Man who shoots spider webs from
holes in his wrists, a little alpine plant has been found to eject
cobweb-like threads from tiny holes in specialized cells on its
leaves. It's these tiny holes that have taken plant scientists
by surprise because puncturing the surface of a plant cell would
normally cause it to explode like a water balloon.



FULL STORY ==========================================================================
Like the movie version of Spider-Man who shoots spider webs from holes
in his wrists, a little alpine plant has been found to eject cobweb-like threads from tiny holes in specialised cells on its leaves. It's these
tiny holes that have taken plant scientists by surprise because puncturing
the surface of a plant cell would normally cause it to explode like a
water balloon.


==========================================================================
The small perennial cushion-shaped plant with bright yellow flowers,
Dionysia tapetodes, is in the primula family and naturally occurs
in Turkmenistan and north-eastern Iran, and through the mountains of Afghanistan to the border of Pakistan. What makes it unusual is its
leaves, which are covered in long silky fibres that resemble fine cobwebs called 'woolly farina'.

Quite a few of its primula relatives have leaves coated with a fine powder consisting almost entirely from flavone, which is a class of flavonoid.

Flavonoids are small specialised molecules involved in plant metabolism
and are recognised for their anti-inflammatory and antioxidant
properties. But this Dionysia species does not have flavone powder on
its leaves, instead it has very fine wool just 1-2 microns thick --
far thinner than a human hair, which is about 75 microns.

As part of an ongoing collaboration between the University of Cambridge's
plant science research institute Sainsbury Laboratory Cambridge University (SLCU) and Cambridge University Botanic Garden (CUBG), Dionysia was
selected from the Botanic Garden's living collection of 8,000 cultivated
plant species to be analysed at SLCU's Microscopy Core Facilities.

"The woolly farina threads seem to cover the entire leaf surface with
long threads even connecting leaf-to-leaf," said Paul Aston, who is the
Botanic Garden Alpine and Woodland Supervisor. "Nobody knew what this
wool was or how it was made and so we thought that this would be an
interesting specimen to study. There are many things that plants make
that we do not yet know about - - this is especially true for alpine
plants where we see many unusual adaptations to the harsh high altitude environments they live in." Samples were analysed using advanced light
and electron microscopes, which revealed the micron-diameter wool had
distinct parallel grooves running along their length. But the most
surprising observation was how the wool was emerging from the leaves.



==========================================================================
"The leaves are covered in tiny hairs called trichomes. Each trichome
has a spherical shaped glandular cell at the end -- like a stalk with a
single round cell at the tip -- and we could see the threads emerging
straight out of the glandular cell," said Dr Raymond Wightman, who is
the Microscopy Core Facility Manager in the University of Cambridge's
Sainsbury Laboratory. "But we know that plant cells are surrounded by a
cell wall that protects and retains pressure within the cell. Poking holes through the cell membrane and cell wall would cause the cell to burst --
like puncturing a water balloon." So how were the threads getting out
without exploding the cell? Using a powerful electron microscope at the Cambridge Advanced Imaging Centre (CAIC), they sectioned the glandular
cells and when they zoomed in could see tiny gaps in the cells just
large enough to thread a single woolly farina fibre through.

"The plant manufactures the fibre inside the cell and then threads it
through the gaps that are just wide enough for it," said Dr Matthieu
Bourdon who is a researcher at SLCU and co author of the report published
in BMC Plant Biology.

"There is a distinct opening in the plasma membrane, cell wall and
cuticle creating a hole that forms a tight seal around the fibre --
we could even see wax on the cell surface acting like a plug to seal
any gaps. We observed multiple fibres being extruded from individual
glandular cells at specific spots across its surface. The plant must
be concentrating the flavone building blocks within the cells at
these specific exit sites to be able to produce the elongating fibre."
Dr Wightman also analysed the chemistry of the fibres to find what they
are made from using the Institute's Raman microscope, but the wool's
complex structure required further analysis using specialist equipment
and skills from the University's Yusuf Hamied Department of Chemistry.

"The analysis of the woolly farina sample was challenging because of the
small size of the sample and the similarity of the chemicals that it was
made up of, said Dr Josephine Gaynord, PhD graduate from the University
of Cambridge's Department of Chemistry and who undertook the further
analysis using advanced chromatography, mass spectrometry and nuclear
magnetic resonance (NMR) spectroscopy techniques. "It helped that we
knew the majority of the sample was flavone, a chemical that we could
buy and compare to the woolly farina sample.

Thanks to some excellent support from the NMR team in the Department
of Chemistry we were able to run bespoke analysis and provide possible structures for the modified flavones that were present. It would be
very interesting to follow this work up in the future." Wightman said
they were expecting the fibres to be made up of flavonoids similar
to the powdery coating on the leaves of some other primula species,
but were intrigued by how this species was able to turn flavones into
such stable wool-like fibres. "We found the wool produced by Dionysia
tapetodes has a special chemical structure that is a mix of flavone and
flavone derivatives that may use hydrogen bonding between molecules to
form the elongated fibres.

This means inside the cell these flavones need to be mixed precisely
while being added to the end of the fibre so that they exit the gap as one continuous thread -- like squeezing a continuous line of toothpaste from
a tube." "While it is not known what purpose the woolly farina serves,
it is thought it could be a protective measure offering tolerance to
freezing, drought and/or blocking high UV," said Simon Wallis, Alpine
and Woodland Assistant at CUBG.

"This latter theory is supported by observations that we have made from
our alpine collection, comparing the wool-producing Dionysia tapetodes
with a subset of Dionysia tapetodes that do not have woolly farina and are
more susceptible to sun scorching." The team is interested in further exploring the properties of these fibres to determine if they might be
a useful biomaterial.

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


========================================================================== Journal Reference:
1. Matthieu Bourdon, Josephine Gaynord, Karin H. Mu"ller, Gareth Evans,
Simon Wallis, Paul Aston, David R. Spring, Raymond
Wightman. Microscopy and chemical analyses reveal flavone-based
woolly fibres extrude from micron-sized holes in glandular
trichomes of Dionysia tapetodes. BMC Plant Biology, 2021; 21 (1)
DOI: 10.1186/s12870-021-03010-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/06/210617101224.htm

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