The Plant Journal (2019) 100, 661–676
doi: 10.1111/tpj.14479
Craterostigma plantagineum cell wall composition is
remodelled during desiccation and the glycine-rich protein
CpGRP1 interacts with pectins through clustered arginines
Niklas U. Jung1, Valentino Giarola1, Peilei Chen1, John Paul Knox2
and Dorothea Bartels1,*
1
Institute of Molecular Physiology and Biotechnology of Plants (IMBIO), Faculty of Natural Sciences, University of Bonn,
Kirschallee 1, Bonn D-53115, Germany, and
2
Centre for Plant Sciences, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK
Received 21 March 2019; revised 27 June 2019; accepted 23 July 2019; published online 27 July 2019.
*For correspondence (e-mail dbartels@uni-bonn.de).
SUMMARY
Craterostigma plantagineum belongs to the desiccation-tolerant angiosperm plants. Upon dehydration,
leaves fold and the cells shrink which is reversed during rehydration. To understand this process changes in
cell wall pectin composition, and the role of the apoplastic glycine-rich protein 1 (CpGRP1) were analysed.
Cellular microstructural changes in hydrated, desiccated and rehydrated leaf sections were analysed using
scanning electron microscopy. Pectin composition in different cell wall fractions was analysed with monoclonal antibodies against homogalacturonan, rhamnogalacturonan I, rhamnogalacturonan II and hemicellulose epitopes. Our data demonstrate changes in pectin composition during dehydration/rehydration which is
suggested to affect cell wall properties. Homogalacturonan was less methylesterified upon desiccation and
changes were also demonstrated in the detection of rhamnogalacturonan I, rhamnogalacturonan II and hemicelluloses. CpGRP1 seems to have a central role in cell adaptations to water deficit, as it interacts with pectin
through a cluster of arginine residues and de-methylesterified pectin presents more binding sites for the proteinpectin interaction than to pectin from hydrated leaves. CpGRP1 can also bind phosphatidic acid (PA)
and cardiolipin. The binding of CpGRP1 to pectin appears to be dependent on the pectin methylesterification
status and it has a higher affinity to pectin than its binding partner CpWAK1. It is hypothesised that changes
in pectin composition are sensed by the CpGRP1CpWAK1 complex therefore leading to the activation of
dehydration-related responses and leaf folding. PA might participate in the modulation of CpGRP1 activity.
Keywords: desiccation tolerance, resurrection plant, Craterostigma plantagineum, cell wall, glycine-rich protein, pectin, leaf folding, lipid binding, phosphatidic acid.
Linked article: This paper is the subject of a Research Highlight article. To view this Research Highlight article
visit https://doi.org/10.1111/tpj.14582.
INTRODUCTION
Most land plants are not able to survive prolonged periods
of water shortage and even a mild water deficit can lead to
irreversible damage and plant death (Zhang and Bartels,
2018). Water plays a crucial role in the maintenance of cell
turgor, transport of solutes and nutrients and it mediates
hydrophobic and hydrophilic interactions essential for
macromolecular structures. Plants, as sessile organisms,
face dehydration periods and have different strategies to
reduce water loss and adapt to low water availability (Verslues and Juenger, 2011).
Desiccation tolerance is rare in vegetative tissues of vascular plants, but it is common in seeds and pollen. Only
approximately 300 species of vascular plants have been
reported to survive extensive dehydration during their vegetative growth phase. These plants are generally called resurrection plants (Porembski, 2000). Most angiosperm
resurrection plants are native to central and southern
Africa, Australia, south America and India (Gaff, 1971; Gaff
and Bole, 1986). The resurrection plant Craterostigma plantagineum (Cp) has been extensively studied (Giarola et al.,
2017). Lindernia brevidens (Lb) and Lindernia subracemosa
(Ls) are closely related to C. plantagineum (Rahmanzadeh
et al., 2005) and their genome sequences have recently
been deciphered (VanBuren et al., 2018). The interest for
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License,
which permits use, distribution and reproduction in any medium, provided the original work is properly cited and
is not used for commercial purposes.
661
662 Niklas U. Jung et al.
comparative studies of these plants are different degrees
in desiccation tolerance. L. brevidens is desiccation-tolerant like C. plantagineum (Phillips et al., 2008) but L. subracemosa is desiccation-sensitive (Seine et al., 1995).
Cell walls are the outermost structures of plant cells and
essential to maintain cell and organ integrity and functionality. Mechanical stress is built up when the cell wall folds
up during dehydration. When the vacuole shrinks and the
cell contents are drawn inwards, tension builds up
between the plasma membrane and the rigid cell wall
(Levitt, 1987). The protection of the plasma membrane during dehydration is essential for the plant to survive. Iljin
(1957) proposed that any plant could tolerate desiccation if
the mechanical stress can be limited. In desiccation-sensitive species the cell wall loses its integrity upon desiccation. When resurrection plants encounter desiccation the
leaves start to fold and the cell volume is reduced (Farrant,
2000; Farrant et al., 2003; Willigen et al., 2003). Previous
studies showed dehydration-induced changes in cell wall
architecture, cell wall composition and variations of hemicellulose polysaccharides and pectin-associated arabinans
in resurrection plants (Vicre et al., 1999, 2004; Moore et al.,
2006, 2008). It has been proposed that high levels of pectic-arabinans, arabinogalactan-proteins and arabinoxylans
ensure the required cell wall plasticity upon dehydration
for the resurrection plant Myrothamnus flabellifolia (Moore
et al., 2013). Changes in xyloglucan and modifications of
pectin structures have been reported for Craterostigma
wilmsii (Vicre et al., 1999, 2004).
Glycine-rich proteins (GRPs) have a high-glycine-content
with glycine residues arranged in (Gly)n-X repeats. In addition, a cysteine-rich region, an oleosin domain, RNA-recognition motifs, a cold-shock domain or zinc-finger motifs are
found in GRPs. Although several GRPs have been characterised, the function of the glycine-rich domains in these
proteins is poorly understood (Czolpinska and Rurek,
2018). GRPs are classified according to the arrangement of
the glycine-rich repeats and the presence of additional
domains (Sachetto-Martins et al., 2000; Fusaro et al., 2001;
Bocca et al., 2005; Mangeon et al., 2010). Class I GRPs
share a high-glycine-content region with (GGX)n repeats. A
C-terminal cysteine-rich region is present in the class II
GRPs. The class III GRPs have a lower glycine content and
may have an additional oleosin domain. Class IV GRPs are
known as RNA-binding GRPs with either an RNA-recognition motif or a cold-shock domain and in some cases GRPs
have additional zinc-finger motifs. Class V GRPs are similar
to class I GRPs but show mixed patterns of glycine repeats.
The expression patterns and the subcellular localisation of
the different proteins within the GRP-superfamily are
highly diverse thus suggesting that these proteins have different functions (Mangeon et al., 2010). GRPs with an
apoplastic signal peptide have been proposed to be an
important component of cell wall structures (Condit and
Meagher, 1986, 1987; Keller et al., 1988). For example, the
French bean PvGRP1.8 protein is part of the cell wall and
plays a role in the protoxylem repair system (Ringli et al.,
2001). According to microarray results GRPs could also be
implicated in maintaining protoxylem structures
(Yokoyama and Nishitani, 2006). Glycine-rich proteins have
been proposed to connect the secondary cell wall thickenings between protoxylem elements (Ryser et al., 2004).
The AtGRP9 protein from Arabidopsis thaliana interacts
with a cinnamyl alcohol dehydrogenase (AtCAD5) and may
be involved in lignin biosynthesis (Chen et al., 2007).
Besides class I GRPs also GRPs from other classes are
plant cell wall components. The class V glycine-rich protein
1 (BhGRP1) from Boea hygrometrica was proposed to be
important for cell wall integrity during dehydration,
whereas the class II GRP NtCIG1 protein from tobacco was
proposed to enhance callose deposition in cell walls (Ueki
and Citovsky, 2002; Wang et al., 2009).
The C. plantagineum glycine-rich protein 1 (CpGRP1)
belongs to class II GRPs. CpGRP1 is highly abundant in the
apoplast of desiccated leaves and interacts with the
C. plantagineum cell wall-associated protein kinase 1
(CpWAK1) (Giarola et al., 2016). WAKs contain an extracellular pectin binding domain and an intracellular serine/
threonine protein kinase domain. WAKs have been suggested to link the cytoplasm to the extracellular matrix and
to activate signalling pathways in response to pectin
changes (He et al., 1999; Anderson et al., 2001; Kohorn and
Kohorn, 2012). The CpGRP1CpWAK1 complex may play a
role in sensing dehydration-induced cell wall changes and
thus activate dehydration-induced signalling pathways
(Giarola et al., 2016). A similar complex is known from
A. thaliana where the cysteine-rich region of AtGRP3 interacts with the cell wall-associated kinase 1 (AtWAK1). This
complex has been proposed to be involved in pathogen
defence mechanisms (Park et al., 2001).
Phosphatidic acid (PA) belongs to the group of phospholipids and consists of a glycerol backbone, a saturated fatty
acid, an unsaturated fatty acid and a phosphate group and
has different roles in plants (Hou et al., 2016). Phospholipase C and phospholipase D are involved in the synthesis
of PA from phosphatidylinositol-(4,5)-bisphosphate (PIP2),
phosphatidylcholine
and
phosphatidyl-ethanolamine.
These reactions are crucial for the maintenance of the PA
pool, which serves as a source for the biosynthesis of other
phospholipids (Munnik, 2001; Ufer et al., 2017). PA can also
function as signalling molecule in response to environmental cues. PA was shown to bind to other proteins and thus
modify the activity of proteins (Hou et al., 2016).
Changes in the C. plantagineum cell wall during a desiccation/rehydration cycle were investigated with a set of
monoclonal antibodies that recognise different epitopes of
homogalacturonan (HG), rhamnogalacturonan I (RG-I),
rhamnogalacturonan II (RG-II) and hemicelluloses. Our
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
Modifications of pectin in response to desiccation 663
findings suggest that C. plantagineum cell walls undergo
reversible remodelling of pectic polymers upon desiccation. A cluster of arginine residues within the CpGRP1 protein enables its binding to pectin and PA. HG
methylesterification is reduced upon dehydration thus promoting CpGRP1 binding to pectin. Changes in the pectin
organisation provide plasticity to the cell wall which is
required for the extensive folding and expansion of cell
walls during desiccation/rehydration.
RESULTS
Morphological characterisation of leaf structures
Microscopic views of the C. plantagineum leaf surfaces
and transverse sections during a desiccation/rehydration
cycle are shown in Figure 1(a,b). Untreated and rehydrated
leaf tissues are almost identical suggesting that leaves can
fully recover from desiccation-induced cellular changes
after 48 h of rehydration. In contrast to the hydrated and
rehydrated samples, the epidermis of the desiccated leaf is
extensively folded. In this compact structure leaf glands
are trapped in the epidermal folds. Leaf folding mainly
occurs during late dehydration when the relative water
content is below 60%. Cell walls in desiccated tissues were
slightly thicker than cell walls in untreated and 48 h-rehydrated tissues as shown by cell wall staining (Figure 1c,d).
These findings suggest that changes in pectin composition
between the different samples might be involved in cell
wall adaptations to water-stress conditions.
Pectin and hemicellulose profiles determined in a
desiccation/rehydration cycle
Panels of monoclonal antibodies allow the monitoring of
changes in cell wall polysaccharides (Table 1). In enzymelinked immunosorbent assays (ELISAs), JIM5, JIM7, LM20
and LM19 were used to analyse differences in the
methylesterification pattern of C. plantagineum HG fractions upon desiccation and rehydration (Table 2). The abundance of the pectin and hemicellulose epitopes is correlated
with colour intensity. JIM5, JIM7 and LM20 displayed a
stronger binding to the untreated and rehydrated (1 and 2)
samples than to the desiccated sample. LM19, which detects
fully de-methylesterified HG, displayed a stronger signal to
desiccated samples than to untreated samples. These
results indicate changes in the HG methylesterification status during the desiccation/rehydration cycle and suggest
homogalacturonan synthesis in the recovery process.
Changes in the abundance of RG-II were analysed using the
42-6 antibody which binds RG-II monomers, crosslinked RGII and an unknown pectic component (Table 1). The antibody bound more strongly to the untreated and rehydrated
samples than to the desiccated samples (Table 2).
LM25, LM15, LM11, LM6 and LM5 monoclonal antibodies were used to analyse changes in the hemicelluloses
and the rhamnogalacturonan I (Tables 1 and 2). LM25 and
LM15 indicate strong signals for xyloglucan in desiccated
leaves which is reversed during rehydration. LM11, which
detects xylan, bound stronger to the desiccated samples,
whereas LM6, which detects (1?5)-a-arabinan, displayed a
slightly weaker binding to pectins of desiccated leaves
than to pectins of untreated leaves. No significant changes
were observed in the binding of the LM5 antibody which
detects (1?4)-b-galactan. Results obtained with RG-I, RG-II
and hemicellulose antibodies suggest changes in the
C. plantagineum cell wall architecture during the desiccation/rehydration cycle.
Interaction between CpGRP1 and pectin
The glycine-rich cell wall protein CpGRP1 interacts with the
CpWAK1 kinase protein in the apoplast and it has been
hypothesised that CpGRP1 binds to pectin (Giarola et al.,
2016). The glycine-rich domain of CpGRP1 contains a positively charged arginine cluster through which it may interact with cell wall polysaccharides, such as pectins
(Figure 2a,b). Three different approaches have been used
to analyse a possible interaction of CpGRP1 and pectins:
Blue-native page gel-shift assays, ELISA and dot-blot
assays using either commercial pectin (from citrus peel;
Sigma-Aldrich, USA) or the 1,2-cyclohexanediaminetetraacetic acid (CDTA)-pectin fraction isolated from C. plantagineum or closely related species. For binding
experiments the full-length recombinant CpGRP1 protein
or fragments of the CpGRP1 protein corresponding to Nor C-terminal domains or to the mutated N-terminal
domain (two arginines mutated to glycine, a352g_c358g)
were used (Figure 2). When the CpGRP1 full-length protein
was incubated with commercial pectin and calcium, the
electrophoretic mobility was retarded compared to the
CpGRP1 protein in a native page (Figure 3). A similar
mobility shift was observed for the N-terminal fragment,
but not for the C-terminal fragment. The mutated N-terminal fragment did also show a mobility shift in the presence
of pectin and calcium but in contrast to the non-mutated
fragment, two protein bands were detected. No mobility
shift for any of the proteins was observed in the absence
of calcium. The interaction between CpGRP1 and pectins
was further investigated using pectins extracted from
C. plantagineum leaves (Figure 4a). Microtiter plate wells
were coated with CDTA-soluble pectins or KOH-soluble cell
wall fractions and then incubated with the CpGRP1 fulllength protein, the N-terminal or the mutated N-terminal
polypeptide. No complex was observed between CpGRP1
and the KOH cell wall fractions. The strongest interaction
was observed with the N-terminal polypeptide, followed
by the mutated N-terminal polypeptide and the full-length
protein. Each of the tested proteins showed the highest
value for the CDTA-pectin fraction isolated from desiccated
C. plantagineum leaves. No differences were observed in
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
664 Niklas U. Jung et al.
Figure 1. Morphological characterisation of Craterostigma plantagineum leaf structures using scanning electron microscopy. Surface images (a), transverse section images (b) and images of cell walls stained with silver nitrate (c) are shown. Micrographs were taken in a desiccation/rehydration cycle. From left to right:
Untreated (RWC = 100%), Partially dehydrated (RWC = 50%), Desiccated (RWC = 2%), 24 h after rehydration (Rehydrated 1) and 48 h after rehydration (Rehydrated 2). Representative micrographs are shown. Three different samples were used for the evaluation. (d) Thickness of C. plantagineum cell walls in a desiccation/rehydration cycle. Images from (c) were used for the evaluation. Cell wall thickness increases slightly during dehydration and the starting point (untreated
samples) is restored after 48 h of rehydration. U = Untreated, PD = Partially dehydrated, D = Desiccated, R 1 = Rehydrated 1, R 2 = Rehydrated 2. The asterisk
indicates the levels of significance in comparison with the untreated sample (one-way ANOVA, HolmSidak method): *P < 0.05.
the presence or absence of calcium. In addition, pectin
fractions from the two closely related species differing in
desiccation tolerance, L. brevidens and L. subracemosa,
were prepared and the CpGRP1 binding to those fractions
was analysed (Figure 4b). The CpGRP1 full-length protein
and the N-terminal polypeptide showed the strongest interaction with C. plantagineum pectin, followed by pectin
from L. brevidens and L. subracemosa. To confirm that
these differences are due to the pectin composition and
not due to the amount of isolated HG the galacturonic acid
(GA) content was determined for all different fractions. No
significant differences in the GA content were detected
which supports comparable HG contents (Table 3). In a
second step the JIM5, JIM7, LM20 and LM19 antibodies
were used to analyse the L. brevidens and L. subracemosa
HG fractions. The methylesterification profile of HG for the
desiccation-tolerant plant L. brevidens was similar to the
one of C. plantagineum (Table 4). For L. subracemosa the
signal intensity detected with the antibodies in the rehydrated samples was lower than the signal in the untreated
sample suggesting that this plant is not able to restore cell
wall architecture during rehydration which is consistent
with the phenotype (Table 4).
To confirm the ELISA analyses a series of dot-blot experiments was carried out. The interaction of the CpGRP1 protein and pectin was either detected using a 6x His-tag
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
Modifications of pectin in response to desiccation 665
Table 1 Monoclonal antibodies used in this study for pectin and hemicellulose characterization
HGa
RG-IIb
RG-Ib
Hemicelluloses
Antibody
Specificity
Reference
JIM5
JIM7
LM20
LM19
42-6
LM6
LM5
LM25
LM15
LM11
Partially or de-methylesterified HG
Partially methylesterified HG
Methylesterified HG
Fully de-methylesterified HG
B-RG-II, RG-II monomers, unknown pectic fragment
(1?5)-a-arabinan
(1?4)-b-galactan
Xyloglucan (XXLG, XLLG)
Xyloglucan (XXXG)
(1?4)-b-xylan
Knox et al. (1990)
Knox et al. (1990)
Verhertbruggen et al. (2009)
Verhertbruggen et al. (2009)
Zhou et al. (2018)
Willats et al. (1998)
Jones et al. (1997)
Pedersen et al. (2012)
Marcus et al. (2008)
McCartney et al. (2005)
a
Homogalacturonan.
Rhamnogalacturonan.
b
antibody or the JIM5 antibody (Figure 5a). The dot-blot
analyses confirmed the interaction between the CpGRP1
full-length protein and the two N-terminal protein fragments with pectins. BSA (Carl Roth, Karlsruhe, Art.-Nr.
8076.2) and the LEA-like 11-24His recombinant protein
(Petersen et al., 2012) were used as negative controls and
did not show any interaction. Figure 5(b) presents the species-specific interactions between the CpGRP1 full-length
protein, the N-terminal fragment and the mutated N-terminal fragment with the CDTA fractions of C. plantagineum,
L. brevidens and L. subracemosa. Mutation of arginines in
the N-terminal fragment led again to weaker interactions
with pectins. Quantification of signal intensities of the dot
blots fully supports the ELISA results (Figure 5c). The
binding of CpGRP1 and CpGRP1 protein fragments to
pectins from the desiccated samples was stronger than
the binding to pectins from the untreated or rehydrated
samples in all experiments. The highest signal intensity
was obtained for pectin isolated from C. plantagineum. To
investigate and compare the CpGRP1pectin interaction,
the pectin binding of two other apoplastic proteins,
CpWAK1 and the C. plantagineum germin-like protein 1
(CpGLP1), was investigated. Figure 6 demonstrates that
the CpGRP1 binds much stronger to pectin than CpWAK1
or CpGLP1.
Pre-treatment of pectin with CAPS-buffered solutions
(pH 7–11) or with 0.1 M sodium carbonate reduced the
extent of HG methylesterification. CpGRP1 full-length protein bound more strongly to de-methylesterified than to
methylesterified pectin from C. plantagineum (Figure 7a).
Table 2 Analysis of changes in the cell wall composition of Craterostigma plantagineum leaves in a desiccation/rehydration cycle
0.00 0.50 1.00 1.50 2.00 2.50
JIM5, JIM7 and LM20 detect varying levels of methylesterification in HG. LM19 detects fully de-methylesterified HG. The 42-6 antibody
detects RG-II, crosslinked RG-II and an unknown pectic epitope. LM6 and LM5 detect pectic arabinan and pectic galactan, epitopes present
in the RG-I, respectively. LM25 and LM15 detect xyloglucan and LM11 detects xylan. For specificity of different antibodies see Table 1. The
colour scale in relation to absorbance values is shown bottom left. Results for HG were generated analysing the CDTA fractions in 1:5 dilutions and results for RG-I and RG-II were generated analysing the KOH fractions in 1:5 dilutions. Values shown are means of three biological
replicates SD. The letters indicate the levels of significance (one-way ANOVA, HolmSidak method): a,b,c,d P < 0.05; ABCD P < 0.01;
aA = significantly different from ‘Untreated’ sample, bB = significantly different from ‘Desiccated’ sample, cC = significantly different from
‘Rehydrated 1’ (24 h) sample, dD = significantly different from ‘Rehydrated 2’ (48 h) sample.
a
Homogalacturonan.
b
Rhamnogalacturonan.
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
666 Niklas U. Jung et al.
We hypothesize that the de-methylesterification of HG provides more binding sites for the CpGRP1 protein. Highly
methylesterified commercial apple pectin (Apple Pectin
Powder; Solgar, USA) was used to confirm the effectiveness of the de-methylesterification procedure as indicated
by the specificities of the LM19 and LM20 probes. Figure 7(b) shows that untreated apple pectin is recognised
by the LM20 antibody but not by LM19, whereas the
sodium carbonate-treated and hence de-methylesterified
apple pectin is recognised by the LM19 antibody and not
by LM20.
As lipids are a major component of cell membranes and
are modified during desiccation/rehydration in C. plantagineum (Gasulla et al., 2013) it was tested whether
CpGRP1 may also bind to lipids. CpGRP1 binding to different lipids was analysed in protein-lipid overlay assays (Figure 8a). The LEA-like 11–24 protein was used as a positive
control and it was used with or without an additional Histag to account for possible His-tag background binding
(Petersen et al., 2012). The CpGRP1 full-length recombinant protein showed strong binding to PA and cardiolipin
(CL), but not to any other lipid tested (Figure 8b). The Nterminal fragment of CpGRP1 and the full-length CpGRP1
showed similar binding to PA, whereas the binding of the
C-terminal fragment of CpGRP1 to PA was very weak.
These results indicate that the binding of CpGRP1 to PA is
mainly mediated by the N-terminal region of the protein.
Mutations of two arginines in this region led to weaker
binding, suggesting the involvement of positively charged
arginines in the CpGRP1PA interaction.
DISCUSSION
Pectin fractions are remodelled upon dehydration
Folding of cell walls and changing of texture and chemical
composition is an adaptation to desiccation in resurrection
plants (Bartels and Hussain, 2011). Intensive cell wall folding of desiccated cells is also in place in C. plantagineum
as shown by scanning electron microscopy (SEM) (Figure 1). Similar observations have been made for L. brevidens and L. subracemosa, which are closely related to
C. plantagineum and C. wilmsii (Farrant, 2000; Phillips
et al., 2008). Although the leaves of the desiccation-sensitive plant L. subracemosa are folded upon dehydration,
they became brown and necrotic and did not recover after
rewatering, which points to alterations in the composition
of the cell wall of C. plantagineum. To address this question pectins and hemicelluloses associated with cell wall
flexibility were analysed.
C. plantagineum showed a lower degree of methylesterification in HG upon dehydration, which is reversed during
rehydration (Table 2). L. brevidens did show a similar
methylesterification status as C. plantagineum, which is in
agreement with the fact that this plant is also desiccation
tolerant. In contrast, no significant changes in the HG fraction were detected for the desiccation-sensitive plant
L. subracemosa upon rehydration (Table 4). The degree of
methylesterification may be of importance for desiccation
tolerance, at least among the Linderniaceae. HG is thought
to be synthesised in the Golgi apparatus in a highly
methylesterified form and is de-methylesterified in the cell
Figure 2. (a) Craterostigma plantagineum CpGRP1 amino acid sequence and protein domains. The different protein domains of CpGRP1 are indicated by
coloured boxes above the sequence. Red: predicted signal peptide; blue: Semi-repetitive glycine-rich region; the two different amino acid tandem repeat motifs
are shown with solid or dashed arrows respectively. Yellow: Cysteine-rich region; the six cysteine residues are marked by black triangles. Different protein fragments are marked by dotted lines. Additionally, the full-length CpGRP1 recombinant protein and the other polypeptides carried six histidine residues. (b) Electrostatic surface model of the recombinant CpGRP1 protein. Clustered arginines in the glycine-rich domain of the protein build up a negative cluster. Arginines at
positions 118 and 120 were mutated to glycine (red boxes). The image in (a) was modified from Giarola et al. (2016).
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
Modifications of pectin in response to desiccation 667
Figure 3. Analysis of CpGRP1pectin interaction by Blue-native page gel-shift assay. The full-length recombinant CpGRP1 protein, a CpGRP1 N-terminal protein
fragment and the mutated N-terminal protein fragment were used for this analysis. The proteins were detected with CpGRP1 or His-tag antibodies and SDS-gels
were stained using Coomassie brilliant blue.
Figure 4. Quantification of the proteinpectin interaction using the CpGRP1 full-length protein, the N-term. fragment and the mutated N-term. fragment and
pectin isolated from (a) Craterostigma plantagineum, (b) Lindernia brevidens and Lindernia subracemosa during dehydration and rehydration. All signals shown
were detected using 1:5 dilutions of CDTA fractions. No signals were detected in KOH fractions. The LEA-like 11–24 protein was incubated with commercial pectin and 2 mM Ca2+ and used as a negative control (Neg.). The CpGRP1 full-length protein was incubated with commercial pectin and Ca2+ and used as a positive
control (Pos.). U = Untreated, D = Desiccated, R 1 = Rehydrated 1 (24 h), R 2 = Rehydrated 2 (48 h). Values shown are means of three biological replicates SD.
The star indicates the levels of significance in comparison with the untreated sample (one-way ANOVA, HolmSidak method): *P < 0.05; **P < 0.01.
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
668 Niklas U. Jung et al.
Table 3 Quantification of galacturonic acid content in the different CDTA-pectin fractions in lg ll1
Craterostigma plantagineum
Lindernia brevidens
Lindernia subracemosa
Untreated
Desiccated
Rehydrated 1
Rehydrated 2
2.01 0.31
1.81 0.24
2.05 0.25
2.12 0.22
1.55 0.26
1.92 0.21
2.00 0.24
1.58 0.27
1.65 0.27*
1.62 0.41
1.60 0.21
2.02 0.20
Values shown are means of three biological replicates standard deviation (SD). The star indicates the levels of significance in comparison
to the untreated sample (one-way ANOVA, HolmSidak method): *P < 0.05.
Table 4 Analysis of changes in cell wall methylesterification of Lindernia brevidens (Lb) and Lindernia subracemosa (Ls) leaves in a desiccation/rehydration cycle
0.00 0.50 1.00 1.50 2.00 2.50
JIM5, JIM7 and LM20 detect varying levels of methylesterification in HG. LM19 detects fully de-methylesterified HG. For specificity of different antibodies see Table 1. The colour scale in relation to absorbance values is shown bottom left. Results were generated analysing the
CDTA fractions in 1:5 dilutions. Values shown are means of three biological replicates standard deviation (SD). The letters indicate the
levels of significance (one-way ANOVA, HolmSidak method): a,b,c,d P < 0.05; ABCD P < 0.01; aA = significantly different from ‘Untreated’
sample, bB = significantly different from ‘Desiccated’ sample, cC = significantly different from ‘Rehydrated 1’ (24 h) sample, dD = significantly different from ‘Rehydrated 2’ (48 h) sample.
a
Homogalacturonan.
wall (Zhang and Staehelin, 1992; Staehelin and Moore,
1995; Sterling, 2001). Thus, the de-methylesterification of
HG in the cell wall is a one-way process and there is no
evidence of methylesterification of pectin in cell walls. The
increasing binding of antibodies detecting methylesterified
HG in rehydrated samples suggests that pectin is synthesised de novo in C. plantagineum and L. brevidens during
the recovery process (Tables 2 and 4).
Pectin de-methylesterification is catalysed by a large
enzyme family of pectin methylesterases (PMEs) (Fleischer
et al., 1999; Le Gall et al., 2015). Little information is known
about the activities of PMEs in C. plantagineum. PMEs are
thought to be crucial in influencing cell wall properties and
to be important for the modulation of apoplastic Ca2+ concentrations (Micheli, 2001; Wu and Jinn, 2010; Wu et al.,
2010, 2018). Ca2+ mediates the interaction between
de-methylesterified pectin chains leading to the formation
of “egg-box” pectin structures (Grant et al., 1973; Jarvis,
1984; Moore et al., 1986; Lloyd, 1991). An increase in
apoplastic Ca2+ was observed in C. wilmsii upon dehydration suggesting enhanced cell wall strength in Craterostigma upon dehydration (Vicre et al., 1999).
RG-II is crosslinked by borate (Kobayashi et al., 1996).
The interaction between RG-II, borate and HG is important
for the physical and biochemical properties of the cell wall
(O’Neill et al., 2001). The antibody 42-6 detects RG-II monomers, borate-crosslinked RG-II and an unknown pectic
component (Zhou et al., 2018). We found significant
changes in RG-II during desiccation/rehydration (Table 2).
The decrease in the 42-6 signals in response to dehydration suggests pectin remodelling which could affect cell
wall rigidity.
Xyloglucan connects cellulose fibrils and contributes to
cell wall extensibility (Moore et al., 1986; Fry, 1989). Higher
abundance of xyloglucan and de-methylesterified pectins
are both known to strengthen the cell wall. This fits well
with the increase in xyloglucan and de-methylesterified
pectins in desiccated C. plantagineum leaves (Table 2) and
C. wilmsii (Vicre et al., 1999). Increased detection of xylan,
mostly present in secondary cell walls, was observed upon
dehydration in C. plantagineum. Higher xylan contents
may contribute to cell wall strength, as xylan together with
xyloglucan connects cellulose fibrils. A. thaliana plants
with reduced xylan contents have weakened cell walls
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
Modifications of pectin in response to desiccation 669
Figure 5. Evaluation of CpGRP1 interaction with Craterostigma plantagineum, Lindernia brevidens and Lindernia subracemosa pectin fractions during dehydration and rehydration by dot-blot analyses. (a) Binding of CpGRP1 full-length, N-terminal fragment and mutated N-terminal fragment to commercial pectin. BSA
and the LEA-like 11-24 protein were used as negative controls. In the left-hand column the protein was spotted on the membrane first and then incubated with
pectin, the right-hand column represents the opposite experiment, the pectin was spotted first and then the protein was added. (b) Interaction of CpGRP1 fulllength, N-terminal fragment. and mutated N-terminal fragment with CDTA fractions. From each fraction 1 lg of pectin was spotted. Quantification of signals
from (b) is shown in (c). U = Untreated, PD = Partially dehydrated, D = Desiccated, R 2 = Rehydrated 2 (48 h), Cp = Craterostigma plantagineum, Lb = Lindernia
brevidens, Ls = Lindernia subracemosa. Values shown are means of three technical replicates SD.
(Brown et al., 2007; Wu et al., 2009). (1–>4)-b-galactan, the
most flexible component of the cell wall, decreases the
ability of pectin molecules to crosslink and also (1–>5)-aarabinan side chains are motile (Ha et al., 1996; Jones
et al., 1997). These components do not change or even
decrease upon dehydration in C. plantagineum (Table 2).
Our results demonstrate major changes in cell wall components. The flexible cell wall components are not changing or are even decreasing upon drying. Conversely,
components associated with stiffer and stronger cell walls
increase. These findings are in agreement with the results
published by Vicre et al. (1999) but are different to the
results by Jones and McQueen-Mason (2004). They found
an increase in cell wall flexibility and a higher activity of
expansins. We propose that the C. plantagineum cell wall
folds in a distinct way and this process is tightly influenced
by different factors. Some connections are strengthened,
while more flexibility is added to others. Different mechanisms need to be in place to protect the cells from being
damaged by the mechanical tension during the dehydration process.
CpGRP1 binds to de-methylesterified pectin through
clustered arginines
Glycine-rich proteins have been connected to cell wall
properties of plants (Condit and Meagher, 1986, 1987;
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
670 Niklas U. Jung et al.
Figure 6. Quantification of proteinpectin interaction using the CpGRP1
full-length recombinant protein, the CpWAK1 extracellular recombinant protein fragment and the CpGLP1 full-length recombinant protein together with
CDTA-pectin extracts from C. plantagineum leaves. Values shown are
means of three biological replicates SD. The asterisk indicates the levels
of significance in comparison to the control sample (one-way ANOVA,
HolmSidak method): **P < 0.01.
Keller et al., 1988). In C. plantagineum the class II glycinerich protein CpGRP1 accumulates in the apoplast of dehydrated leaf tissues (Giarola et al., 2015). The glycine-rich
protein 1 from the resurrection plant Boea hygrometrica
(BhGRP1) was proposed to be correlated with cell wall flexibility (Wang et al., 2009). The A. thaliana glycine-rich protein 3 (AtGRP-3), structurally similar to CpGRP1, interacts
with a cell wall-associated protein kinase 1 (AtWAK1). This
interaction is involved in plant-pathogen defence mechanisms and requires the C-terminal cysteine-rich domain
(Park et al., 2001). Giarola et al. (2016) showed that
CpGRP1 binds to a WAK protein of C. plantagineum,
CpWAK1, and proposed that the CpGRP1-CpWAK1 complex could be involved in dehydration-induced mechanisms. WAKs can bind to cell wall pectins (Kohorn and
Kohorn, 2012) but a GRPpectin interaction has not been
characterised so far. Here we showed that the CpGRP1 protein binds to pectins which requires the glycine-rich
domain of CpGRP1 (Figure 3). The substitution of two
arginines within the glycine-rich domain by glycines was
sufficient to reduce the proteinpectin interaction (Figures 3, 4 and 5). The involvement of an arginine cluster in
pectin binding was also shown for Phaseolus vulgaris
polygalacturonase inhibitor proteins (Spadoni et al., 2006).
The glycine-rich domain of the CpGRP1 protein is important for pectin binding. The repetitive character of this
Figure 7. Quantification of CpGRP1pectin interaction after pectin demethylesterification. (a) ELISA to quantify GRP1pectin interaction after
pre-treatment of coated homogalacturonan with CAPS-buffers (pH 7 11) or
with sodium carbonate. (b) Binding of LM19 and LM20 to different concentrations of apple pectin with and without sodium carbonate pre-treatment.
Values shown are means of three biological replicates SD. The asterisk
indicates the levels of significance in comparison with the untreated sample
(one-way ANOVA, HolmSidak method): **P < 0.01.
domain leads to multiple arginines with a distinct spacing.
This spacing could be important for the spatial organisation of a positive amino acid cluster, which plays a crucial
role in pectin binding. Similarly, the binding to PA seems
to be mediated through the N-terminal domain of the protein and the arginine cluster. Multiple binding capacities of
CpGRP1 suggests that this protein may coordinate different ligands involved in cell wall/membrane modifications
during the folding process.
The role of Ca2+ in the proteinpectin binding is not
clear. The gel-shift assay suggests Ca2+ to be important for
the interaction but the ELISAs showed that Ca2+ was not
essential for the interaction between CpGRP1 and pectin
(Figures 3 and 4). One explanation could be that gel shift
assays are less sensitive than ELISAs and therefore in gel
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
Modifications of pectin in response to desiccation 671
Figure 8. Proteinlipid overlay assays using the recombinant CpGRP1 full-length protein, the N. term. polypeptide, the mutated N-term. polypeptide and the Cterm. polypeptide. Anti-His antibody was used to detect the different protein fragments. The LEA-like 11-24 protein (with and without His-tag) served as a positive control and was detected with an antibody specific to LEA-like 11-24. (a) Investigation of CpGRP1 binding properties to different lipids: phospatidylcholine
(PC), phosphatidic acid (PA), diacylglycerol 18:3 (di18:3-DAG), diacylglycerol 16:0 (di16:0-DAG), monogalactosyldiacylglyceride (MGDG), digalactosyldiacylglyceride (DGDG), phosphatidylglycerin (PG), phosphatidylethanolamine (PE) and cardiolipin (CL). (b) Evaluation of differences in PA binding strength of protein
fragments using filters containing 10, 5, 1, 0.5 and 0.1 ng of PA.
shift assays Ca2+ might be required to crosslink pectins
and therefore increases the strength of the interaction. In
A. thaliana a Ca2+-induced modification of the pectin structure supports the interaction with AtWAK1 (Decreux and
Messiaen, 2005). Ca2+ seems not to be necessary for
CpGRP1pectin binding in vitro, but a role of Ca2+ in
planta cannot be excluded.
To compare the CpGRP1pectin interaction with other
apoplastic proteins, CpWAK1 and CpGLP1 were used.
CpWAK1 and CpGLP1 were chosen, because they are
exposed to pectin in a similar way as CpGRP1 due to their
localisation. Both proteins are able to bind pectin (Figure 6)
but the interaction of CpGRP1 with pectin is much stronger. To exclude the pectin binding of CpGRP1, CpWAK1
and CpGLP1 is just due to a high electrostatic charge, two
other proteins with high lysine and arginine frequencies
were used as additional controls (Figure 5). The recombinant LEA-like 11-24 protein (Petersen et al., 2012) has a
lysine frequency of 9.1% which is more than double compared with CpGRP1, which has a lysine frequency of 3%
and an arginine frequency of 7.4%. BSA (UniProtKB accession number P02769) has a lysine frequency of 9.9% and
an arginine frequency of 4.3%. Despite the high lysine and
arginine frequencies LEA-like 11–24 and BSA did not show
any interaction to pectin, proving a high electrostatic
charge alone is not sufficient for a proteinpectin interaction and a particular spatial arrangement of the amino acid
residues is required. The strong pectin binding of CpGRP1
seems to be specific, as other apoplastic proteins or proteins with a high electrostatic charge did not show any or a
weak proteinpectin binding.
After identifying the protein domains important for pectin binding, we investigated the pectin fractions with
regards to CpGRP1 binding. Both the CpGRP1 full-length
protein and the N-term. fragment showed a slightly stronger binding to pectin samples isolated from desiccated
C. plantagineum leaves than to pectins isolated from
untreated leaves (Figure 4a). This could be due to the
methylesterification level of HG which decreased upon
dehydration. Previous reports demonstrated the importance of de-methylesterified HG stretches for protein binding (Spadoni et al., 2006; Chevalier et al., 2019). Pectin
with a lower degree of methylesterification provides more
binding sites for proteins. The small N-term. fragment
showed always a higher signal in ELISA pectin binding
assays than the full-length CpGRP1 protein (Figure 4a,b).
Likely, more protein molecules can bind to the
de-methylesterified pectin and this explains the stronger
signal. To demonstrate a link between the degree of pectin
methylesterification and protein binding, the C. plantagineum pectin was treated with solutions of different pH
values to stepwise de-methylesterify the HG. Our results
demonstrate that de-methylesterification of HG provides
more binding sites for CpGRP1 (Figure 7).
Reversible methylesterification of HG in the C. plantagineum cell wall during dehydration might be
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
672 Niklas U. Jung et al.
Figure 9. Model depicting the interaction between
the Craterostigma plantagineum glycine-rich protein 1 (CpGRP1), the C. plantagineum cell wall associated protein kinase 1 (CpWAK1) and cell wall
pectins and their role in regulating growth and
dehydration-stress responses (modified from Giarola et al., 2016). CpGRP1 is a part of the pectin
matrix. Under non-stress conditions cell wall pectins (jagged blue lines) with a high degree of
methylesterification (red dots) are bound to
CpWAK1 and are involved in cell expansion. Upon
dehydration, CpGRP1 is more abundant. Clustered
arginines in the N-terminal domain of CpGRP1 are
involved in binding PA and de-methylesterified
stretches of pectin, which are more abundant upon
dehydration. The binding of CpGRP1 to CpWAK1 is
most likely mediated by cysteines in the C-terminal
domain of CpGRP1. CpGRP1 might be either bound
to PA or pectin with both competing for CpGRP1
binding. Ca2+ levels increase upon dehydration
which may lead to the formation of egg-box structures resulting in an increase in rigidity of the cell
walls.
responsible for the reduction of mechanical stress and
might provide more binding sites for proteins. However,
the protein-binding experiments using pectin from L. brevidens and L. subracemosa suggest that other factors in the
structure of pectins also contribute to proteinpectin interactions. No significant changes were detected in the
degree of methylesterification between C. plantagineum,
L. brevidens and L. subracemosa, but the binding of the
CpGRP1 protein to the pectin fractions from the three species was different (Figures 4 and 5). There seems to be a
correlation between desiccation tolerance and the
CpGRP1-binding capacity to pectin, as the signal is strongest for the C. plantagineum pectin, weaker for the L. brevidens pectin and even weaker for the pectin isolated from
the non-tolerant L. subracemosa. The reasons for these differences are unknown.
Pectins and hemicelluloses of the C. plantagineum cell
wall are modified, especially the HG and xyloglucan. These
changes strengthen the cell wall. A higher proportion of
de-methylesterified HG provides more binding sites for the
CpGRP1 protein. CpGRP1 interacts through arginines with
HG or PA and via cysteines in the C-terminal part of
CpGRP1 with the extracellular domain of CpWAK1. These
results are now integrated in a model how CpGRP1,
CpWAK1, pectins and PA may interact and what are their
possible roles in regulating cell shrinkage and expansion
(Figure 9). We propose, that CpGRP1 is an essential factor
in cell wall adaptations to desiccation.
EXPERIMENTAL PROCEDURES
Cultivation of plants
C. plantagineum Hochst., L. brevidens Skan, and L. subracemosa
De Wild plants were grown as described in Bartels et al. (1990)
and Phillips et al. (2008). Plants were subjected to desiccation and
rehydration treatments and the hydration status of leaves was
determined by calculating the relative water content (RWC) with
the formula: RWC [%] = (Initial weight dry weight)/(full turgor
weight dry weight) 9 100 (Bernacchia et al., 1996). For dehydration, fully-grown plants were dehydrated in pots until partial dehydration (PD, 50%) or desiccation (D, 2%) samples. Completely
desiccated plants were rehydrated for either 24 or 48 h. Leaves
from dehydrated and rehydrated plants were ground in liquid
nitrogen with a mortar and a pestle and stored at 80°C.
Molecular biology techniques and DNA sequencing
analysis
Molecular biology techniques were performed according to Green
and Sambrook (2012). DNA sequencing was carried out by GATC
Biotech (https://www.gatc-biotech.com/en/index.html) and primer
synthesis by Eurofins MWG Operon (http://www.eurofinsgenomics.
eu). All primers used are listed in Table S1.
Scanning electron microscopy
C. plantagineum leaves were frozen in liquid nitrogen and sputtered with palladium for 2 min using a sputter-coater (SCD 040;
Balzer, http://www.oerlikon.com/balzers). Leaves were fixed on the
sample holder and analysed under the electron microscope at
9100 and 9400 magnification (Cambridge Stereoscan S 200; Cambridge Instrument Company, UK).
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
Modifications of pectin in response to desiccation 673
Cell walls were stained as follows: leaf material was cut with
a razor blade and immediately immersed in cold FAA (10% (v/v)
formalin, 10% (v/v) acetic acid, 30% (v/v) water and 50% (v/v)
ethanol) solution for at least 24 h at 4°C. After fixation samples
were incubated for 30 min in 85% (v/v) ethanol, 50% (v/v) ethanol/50% (v/v) acetone and finally in 100% (v/v) acetone solutions.
Samples were embedded using the Agar Low Viscosity Kit
(Plano GmbH, Wetzlar, https://www.plano-em.de). Cell wall structures were stained with a 30% (w/v) silver nitrate solution for
10 min. After staining samples were incubated in 10% (v/v) HCl
for 5 min before analysing the surface under the SEM (Blockface imaging).
Critical point drying (CPD 020; Balzers, http://www.oerlikon.
com/balzers) was performed according to Svitkina et al. (1984)
before analysing with the SEM.
Protein analyses
The sequence encoding the N-terminal fragment (aa 22–120) and
the C-terminal fragment (aa 121–156) of CpGRP1 (GenBank
accession number ALQ43973.1) was amplified with primers from
the pET28 CpGRP1His plasmid to add a NcoI site and a XhoI
site at the 50 and 30 ends, respectively (CpGRP1_NTERM_R/T7
promoter and CpGRP1_CTERM_F/T7 terminator, Table S1). The
sequence encoding the extracellular domain of CpWAK1 (aa 31–
315) was amplified with primers from a CpWAK1 cDNA clone
(GenBank accession number KT893872.1; Giarola et al., 2016) to
add a XhoI site at the 30 end (pJET1.2 fwd and CpWAK1_XhoI_R,
Table S1). A NcoI site is already present in the CpWAK1
sequence. The sequence encoding the C. plantagineum germinlike protein 1 (CpGLP1, Dulitz, 2016) without the signal peptide
(aa 27–226) was excised from a pAD vector using EcoRI and SalI
restriction enzymes. N- and C-terminal fragments of CpGRP1 as
well as the CpWAK1 extracellular fragment were cloned between
the NcoI and the XhoI sites of the pET28a expression vector
(NOVAGEN, http://www.novagen.com) to create the corresponding protein-His-tag translation fusion constructs (pET28
CpGRP1_N-terminalHis, pET28 CpGRP1_C-terminalHis, and
pET28 CpWAK1_extracellularHis, respectively). The CpGLP1 fragment was cloned between EcoRI and SalI sites of the pET28a
vector to generate the pET28 CpGLP1_His fusion construct. Overexpression constructs were transformed into BL21 (DE3) Escherichia coli cells (Amersham Pharmacia Biotech, https://www.ge
lifesciences.com) and protein overexpression was induced by
adding 1 mM isopropyl-1-thio-b-D-galaytopyranoside (IPTG). The
recombinant proteins were purified from bacteria 5 h after IPTG
€ hrig,
induction using affinity chromatography (Kirch and Ro
2010). Purified protein fragments were concentrated using Amicon Ultracel-10K centrifugal concentrators (MILLIPORE, http://
www.millipore.com) and desalted with PD10-columns (http://
www.gelifesciences.com) before freeze-drying. Freeze-dried proteins were used for Blue-native page gel-shift assays, ELISA pectin binding and dot-blot assays.
The CpGRP1 sequence coding for two arginine residues (arginine [118] and [120]) in the CpGRP1 N-terminal fragment was
mutated with primers (CpGRP1_NTERM_a352g_c358g_F and
CpGRP1_NTERM_a352g_c358g_R. Table S1) using the QuikChange II site-directed mutagenesis kit according to the manufacturer0 s instructions (Agilent, https://www.agilent.com). Separation
of proteins was performed as described in He (2011) based on the
method first described by Laemmli (1970). The separation of the
CpGRP1 C-terminal fragment was performed as described by
Scha€gger and von Jagow (1987) using a peptide page. Proteins of
SDS-PAGE gels were visualised with Coomassie brilliant blue
R250 (0.1% (w/v) Coomassie R250, 50% (v/v) methanol, 10% (v/v)
glacial acetic acid, 40% (v/v) water). Immunoblot analyses were
performed according to Towbin et al. (1979). CpGRP1 or 6x-HisTag polyclonal antibodies (http://www.thermofisher.com) were
used at a dilution of 1:5000. Detection of proteins was performed
using the ECL Western Blotting detection kit (GE Healthcare, http://
www.gehealthcare.com). Signals were visualised using the Azure
Biosystems c300 chemiluminescent detection system (http://
www.biozym.com).
Extraction of cell wall components
Cell wall components were obtained by following extractions as
described by Cornuault et al. (2014). The 1,2-cyclohexanediaminetetraacetic acid (CDTA) fraction was obtained by vortexing
1 mg freeze-dried material in 1 ml 50 mM CDTA pH 7.5 for 1 h. The
KOH fraction was obtained by vortexing the pellet from the CDTA
extraction in 1 ml 4 M KOH, 1% (w/v) NaBH4 solution for 1 h. Acetic
acid (80%) was used to neutralise the pH of the KOH fraction after
extraction. All samples were stored at 20°C until use.
Enzyme-linked immunosorbent assay
Isolated CDTA and KOH fractions were incubated in microtiter
plates (NUNC-Immuno MicroWell 96 well solid plates, flat bottom, http://www.sigmaaldrich.com) overnight at 4°C. Plate wells
were washed vigorously six times with PBS (137 mM NaCl,
2.7 mM KCl, 10 mM Na2HPO4, 2 mM KH2PO4) and then blocked
using 200 ll per well of 4% (w/v) non-fat milk powder in PBS
for 2 h at room temperature. The plates were washed nine times
with PBS and padded dry. Primary antibodies were added and
incubated overnight at 4°C. The plates were washed 12 times
with PBS and shaken dry. Then 150 ll secondary peroxidasecoupled anti-rabbit or anti-rat IgG antibodies (http://www.sigmaa
ldrich.com) were added at a 5000-fold dilution in 4% milk powder/PBS for 1 h at room temperature. After washing 12 times
with PBS, microtitre plates were developed using 150 ll of substrate solution (0.1 M sodium acetate buffer, pH 6, 0.1% (w/v)
tetramethyl benzidine, 0.006% (v/v) H2O2) in each well. The
enzyme reaction was stopped by adding 40 ll of 2.5 M H2SO4 to
each well, and the absorbance at 450 nm was determined for
each well.
Analysis of the cell wall composition. The experiment was performed as described in Cornuault et al. (2014) with minor
changes. Nine different rat monoclonal antibodies (mAB) were
used in this study: JIM5, JIM7, LM20, LM19, LM25, LM15, LM11,
LM5 and LM6 and one rabbit mAb: 42-6 (Table 1). CDTA and KOH
1:5 dilutions were used for ELISAs as they showed the most
appropriate signal intensity (Figure S1). Ten-fold dilution of hybridoma cell culture supernatants in 4% (w/v) non-fat milk powder/
PBS and a 1:10 000 dilution for 42-6 were used as primary antibodies (150 ll each well).
CpGRP1, CpWAK1, CpGLP1 pectin binding assay. After incubating CDTA and KOH fractions overnight, the recombinant proteins
were dissolved in 4% (w/v) non-fat milk powder at a concentration
of 1 lg ll1 and incubated in the plates overnight at 4°C. The protein was detected using a 6x-His-Tag polyclonal antibody (http://
www.thermofisher.com) at a 1:5000 or 1:10 000 dilution for
CpGRP1, 1:10 000 for CpWAK1 and 1:10 000 for CpGLP1. Pre-treatment of pectin was performed by incubating the plates with
50 mM CAPS-buffered solution at alkaline pH (adjusted between 7
and 11 with HCl and KOH) or with 0.1 M sodium carbonate.
© 2019 The Authors.
The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd,
The Plant Journal, (2019), 100, 661–676
674 Niklas U. Jung et al.
Determination of GA
The GA content was determined according to Blumenkrantz and
Asboe-Hansen (1973) and Verma et al. (2014). All CDTA and KOH
fractions were analysed for their GA content by m-hydroxydiphenyl method, using GA as a standard (Figure S2).
Blue-native page gel-shift assays
For gel-shift assays 0.5 lg of CpGRP1 were incubated with 0.5 lg of
citrus pectin (http://www.sigmaaldrich.com) and 2 mM CaCl2 for 4 h
at room temperature. The samples were mixed with 59 sample
buffer (15.5 ml 1 M TrisHCl, pH 6.8; 2.5 ml of 1% bromophenol blue
solution; 7 ml of water; 25 ml of glycerol) and separated in 15%
polyacrylamide gels without SDS and without stacking gel. A Tris/
boric acid buffer (89 mM Tris, 89 mM boric acid; pH 9.25) was used
for gel preparation and gel electrophoresis. Gels were either stained
with Coomassie brilliant blue or incubated in 50 mM Tris/1% (w/v)
SDS buffer prior to immunoblotting. Proteins were detected using a
6x-His-Tag polyclonal antibody (http://www.thermofisher.com) at a
1:5000 dilution.
Buchholz and T. Dieckmann for providing plant material, H-J.
Ensikat for help with the SEM, S. Raj and P. Do€ rmann for help
with lipid-binding experiments and M. Kobayashi (Graduate
School of Agriculture, Kyoto University, Kyoto) for providing the
42-6 antibody. S. Manduzo and A. Sergeeva helped with providing the expression constructs for the recombinant CpGLP1 and
CpWAK1, respectively. The work was supported by the Deutsche
Forschungsgemeinschaft (BA712-18-1). N. Jung is a member of
the GRK2064.
CONFLICT OF INTEREST
The authors confirm that there is no conflict of interest to
declare.
AUTHOR CONTRIBUTIONS
NJ and PC planned and designed the research and conducted the experiments. NJ wrote the manuscript. DB, PK
and VG designed the research, supervised the work and
corrected the manuscript.
Dot-blot pectin binding assays
SUPPORTING INFORMATION
Dot-blot assays were either based on proteins or pectins immobilised on a nitrocellulose membrane. Dots of 1.5 ll of either polygalacturonic acid (PGA) or of the CDTA-pectin fraction or of the
CpGRP1 recombinant protein were spotted on a nitrocellulose
membrane and allowed to dry. Membranes were blocked with 4%
(w/v) non-fat milk powder in TBST for 2 h and were then washed
with TBST three times for 5 min. The membranes were incubated
with CpGRP1 recombinant protein (1 lg ml1; membranes with
PGA and CDTA spots) or pectin (2 mg ml1; membranes with
CpGRP1 spots) in TBST overnight at 4°C followed by three washing steps. The membranes were incubated with 6x-His-tag antibody (1:5000 dilution) or JIM5 antibody (1:10 dilution) overnight at
4°C and then washed again three times. Immunodetection was
performed as described above and quantification of dot intensity
was done using ImageJ (http://www.imagej.net).
Additional Supporting Information may be found in the online version of this article.
Protein-lipid overlay assay
Protein-lipid overlay assays were performed according to Deak
et al. 1999 and Ufer et al. 2017 to analyse lipid-binding properties
of CpGRP1.
Electrostatic surface modelling
The Phyre2 (http://www.sbg.bio.ic.ac.uk/phyre2/html/page.cgi?id=in
dex) (Kelley et al., 2015) web service was used to create a PDB-file
from the CpGRP1 protein sequence, which was then used for electrostatic surface modelling using the website http://www.charmm-gui.
org/?doc=input/pbeqsolver (Im et al., 1998; Jo et al., 2008a,b).
ACCESSION NUMBERS
The GenBank accession numbers used here are KT893871.
1 (CpGRP1), KT893872.1 (CpWAK1) and JQ067608.1 (LEAlike 11–24). The UniProtKB accession numbers used here
are A0A0S2ZYI4 (CpGRP1), A0A0S2ZYM7 (CpWAK1),
P02769 (BSA) and O23764 (CpLEA-like 11-24).
ACKNOWLEDGEMENTS
Part of this work was inspired by discussions with J. Renaut
and colleagues within the SMARTWALL project. We thank C.
Figure S1. Standard-curve to calculate the galacturonic acid (GA)
content in the different pectin fractions.
Figure S2. LM25 binding to different dilutions of the KOH fraction
extracted from C. plantagineum to determine the most appropriate dilution for ELISA studies.
Table S1. List of primers used in this study.
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