filaments. The HG nanofilaments appear to
be absent in the periclinal walls, which sug-
gests that HG forms an amorphous mesh, as
depicted in current cell wall models (Fig. 5B)
( 6 ). The nanofilament orientation is compa-
tible with the springlike behavior of the epi-
dermis, because filaments would stiffen the
tissue along their axis and permit reversible
lateral extension ( 34 ).
In our model, wall thickening and cell ex-
pansion are inevitably linked, even though
thickening is not always observed in rapidly
expanding cells ( 4 ). Additionally, the model
focuses on the most recently deposited ma-
terial, assuming that previous layers adapt to
expansion, and thus clearly demonstrates the
importance of cell wall remodeling in per-
mitting cell expansion.
Although the basis for the proposal of the
expanding beam model forArabidopsispave-
ment cells is an observation of HG nanofila-
ments, similar self-expansion models may well
apply to other types of extracellular matrix
polymers in which organized structures could
be changed after synthesis ( 35 ). For example,
the carrageenan of red algae ( 36 )andthe
polarized, calcium-dependent alginates in
brown algae ( 37 ) have the requisite attributes.
Thus, biochemical processes similar to those
that act upon pectins could underlie growth
in organisms that have cell walls but do not
have HG. Our observations could inspire the
development of smart materials with the reg-
ulated capacity to expand. Localized swelling
of the extracellular matrix in nonwalled orga-
nisms, such as animals, may even alter mor-
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ACKNOWLEDGMENTS
The dSTORM was performed at the Medical Research Council
Laboratory of Molecular Biology, Cambridge, and we thank
N. Barry and J. Howe for their support. We thank A. Gurzadyan
for the critical evaluation of our FEM model and H. Höfte for
help with the fundraising and discussion of the results.
Funding:A.P. has received support from the French National
Research Agency (ANR) GoodVibration ANR-17-CE13-0007 and
the European Union in the framework of the Marie-Curie FP7
COFUND People Program, through the award of an AgreenSkills+
fellowship (under grant agreement 201310). The Institut
Jean-Pierre Bourgin (IJPB) benefits from the support of Saclay Plant
Sciences-SPS (ANR-17-EUR-0007). The Microscopy Facility at the
Sainsbury Laboratory is supported by the Gatsby Charitable
Foundation, and work at the Sainsbury Laboratory was supported by
the Gatsby Charitable Foundation (through fellowship GAT3395/
DAA). The Meyerowitz Laboratory has support from the Howard
Hughes Medical Institute. This work has benefited from the support of
IJPB's Plant Observatory technological platforms.Author
contributions:K.T.H.: software and visualization; K.T.H. and A.P.:
investigation, methodology, and formal analysis; R.W. carried out the
cryo-SEM; A.P., E.M.M., K.T.H., and R.W.: writing, original draft, and
writing, review and editing; A.P. with the help of other authors:
conceptualization.Competing interests:E.M.M. is a member of the
Management Board of the Sainsbury Laboratory at Cambridge
University. The authors declare no other competing interests.Data
and materials availability:All data, codes, and materials used
in this study are available. Please contact A.P. for the material request.
All data are available in the manuscript, the supplementary
materials, or at Mendeley ( 38 ). All codes used in this study are
available on Zenodo ( 39 – 42 ).
SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/367/6481/1003/suppl/DC1
Materials and Methods
Figs. S1 to S11
References ( 43 , 44 )
MDAR Reproducibility Checklist
Movies S1 to S7
Audio S1 and S2
View/request a protocol for this paper fromBio-protocol.15 September 2019; accepted 24 January 2020
10.1126/science.aaz5103Haaset al.,Science 367 , 1003–1007 (2020) 28 February 2020 5of5
Fig. 5. Cell wall model including HG nanofilaments.(A) HG nanofilaments as quaternary structures
composed of helical chains uniaxially arranged on a hexagonal or a rectangular net for (left) methylated and
(right) demethylated HG with unit cell areas (orange polygons) Ahand Ar, respectively. Ex,y, expansion
in a lateral dimension; blue circles, oxygen; small gray circles, carbon; magenta circles, methyl hydrogen
(black arrow); large orange circles, calcium ions. Color scheme is based on the monochrome diagram of
Walkinshaw and Arnott ( 19 , 20 ). (B) The cell wall models (left) without and (right) with HG nanofilaments. (1)
Cellulose microfibrils (blue) embedded in pectin matrix containing (2) HGs methylated and demethylated
(violet and green segments) and (3) and (4) rhamnogalacturonans type II and I (red). Updated cell wall model
contains (5) methylated (violet) and (6) demethylated (green) HG nanofilaments. (C) The expanding beam
model. (Left) Young wall composed of (5) methylated HG. (Right) Local HG nanofilament demethylation
causes asymmetric cell wall expansion and lobe formation (green).
RESEARCH | RESEARCH ARTICLE