Download The Plant Secretory Pathway: An Essential

Sang-Jin Kim1 and Federica Brandizzi1,2,3,*
1
Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, USA
Michigan State University-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI, USA
3
Department of Plant Biology, Michigan State University, East Lansing, MI, USA.
*Corresponding author: E-mail, [email protected]; Fax, (517) 353-9168
(Received September 4, 2013; Accepted December 12, 2013)
2
For building and maintaining the complex structure of the
surrounding wall throughout their life, plant cells rely on the
endomembrane system, which functions as the main provider and transporter of cell wall constituents. Efforts to
understand the mechanisms of synthesis and transport of
cell wall materials have been generating valuable information for diverse practical applications. Nonetheless, the identity of the endomembrane components necessary for the
transport of cell wall enzymes and polysaccharides is not
well known. Evidence indicates that plant cells can accomplish secretion of cell wall constituents through multiple
pathways during development or under stress conditions
and, that compared with other eukaryotes, they rely on a
highly diversified toolkit of proteins for membrane traffic.
This suggests that production of the cell wall in plants
consists of intricate and highly regulated pathways. In this
review, we summarize important discoveries that have
allowed the activities of the plant secretory pathway to
be linked to the production and deposition of cell wallsynthesizing enzymes and polysaccharides.
Keywords: Endocytosis Exocyst complex Microtubuleassociated cellulose synthase complex (MASC) Pectin Secretion trans-Golgi network (TGN).
Abbreviations: BR1,
brassinosteroid-insensitive1;
CESA,
cellulose synthase; CSC, cellulase synthase complex; ER, endoplasmic reticulum; GFP, green fluorescent protein; HYGR,
hygromycin B phosphotransferase; MASC, microtubuleassociated cellulose synthase complex; MTD, mannitol dehydrogenase; PGIP, polygalacturonase inhibitor protein;
PMEI, pectin methylesterase inhibitor protein; PVC, prevacuolar compartment; SCAMP, secretory carrier membrane
protein; SNARE, soluble N-ethylmaleimide-sensitive factor attachment protein receptor; SCV, secretory vesicle cluster; TE,
tracheary element; TGN, trans-Golgi network; VHA-a1,
vacuolar-type H+-ATPase subunit a1; YFP, yellow fluorescent
protein.
Introduction
The plant secretory pathway consists of numerous functionally
interlinked organelles. The first organelle of the secretory pathway is the endoplasmic reticulum (ER) in which proteins are
synthesized and assembled for export to the Golgi apparatus. It
is conventionally accepted that the Golgi apparatus, which in
plants is made up of numerous, motile and polarized stacks of
membranous compartments called cisternae, collects membranes and lumenal content from the ER for further processing
and sorting to distal compartments which include the transGolgi network (TGN), vacuoles and the plasma membrane
(Matheson et al. 2006, Foresti and Denecke 2008). Forward
membrane transport in the endomembrane system is counterbalanced by endocytosis, which ensures membrane recycling
but also perception of external stimuli. The plasma membrane
interfaces the cell content with the external environment,
which is largely occupied by a cell wall. The plant cell wall
consists of a complex structure of carbohydrates and proteins,
and it confers mechanical strength to the plant during development and stress resistance under biotic and abiotic stress
conditions. Because of the accumulation of polysaccharides,
plant cell walls are considered a valuable carbon source for
biofuel production (Somerville 2007).
The complex plant cell wall structure is built and maintained
by diverse proteins involved in cell wall synthesis, modification
and secretion. The major structural and functional constituents
of the walls are hemicelluloses, cellulose, pectin and lignin, whose
relative content varies depending on the species, tissue and cell
development and growth stages (Pauly and Keegstra 2010, Fry
2011). The Golgi and plasma membranes are the two main sites
where non-lignin cell wall constituents are synthesized. Cellulose
is synthesized at the plasma membrane by a cellulose synthase
complex (CSC), which consists of diverse types of cellulose synthases (CESAs) that are exported to the plasma membrane after
formation of the CSC (Haigler and Brown 1986). Hemicelluloses
and pectins are synthesized in the Golgi by sequential
Special Focus Issue – Mini Review
The Plant Secretory Pathway: An Essential Factory for Building
the Plant Cell Wall
Plant Cell Physiol. 55(4): 687–693 (2014) doi:10.1093/pcp/pct197, available online at www.pcp.oxfordjournals.org
! The Author 2014. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists.
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modification of the side chain in the various Golgi cisternae. For
example, the synthesis of the basic backbone of xyloglucan, a
major hemicellulose component, is initiated from cis-Golgi by
cellulose synthase-like C4 (CSLC4) and xylosyltransferase (XT1)
(Cocuron et al. 2007). In the medial-Golgi, b-1,2-galactosyltransferase (MUR3) adds galactosyl residue at the xylosyl side chain,
and fucosylation of xyloglucan by fucosyltransferase (FUT1) takes
place in the trans-Golgi (Chevalier et al. 2010). Similar to xyloglucan, homogalacturonan, a predominant form of pectin, is also
considered to be synthesized in the cis-Golgi by galactosyltransferases (GAUT-1 and GAUT-7) and methylesterified in the
medial- or trans-Golgi by methyltransferases (Atmodjo et al.
2011, Zhang and Staehelin 1992).
The transport of glycosyltransferases and polysaccharides to
the plasma membrane is generally considered to be mediated by
the default ER–Golgi–post-Golgi–plasma membrane traffic
route. However, multiple lines of evidence support that nonconventional secretory routes may be operating through vesicle-dependent and independent pathways within the plant
endomembrane system (An et al. 2006, Cheng et al. 2009,
Hatsugai et al. 2009, Wang et al. 2010, Zhang et al. 2011). For
example, secretion of two cytoplasmic proteins, mannitol
dehydrogenase (MTD) and hygromycin B phosphotransferase
(HYGR), has been shown to be resistant to brefeldin A, a wellknown endocytosis and secretion inhibitor. Furthermore, immunoelectron microscopy with specific antibodies failed to detect
MTD and HYGR in vesicle-like structures (Cheng et al. 2009,
Zhang et al. 2011). Although further confirmation is required,
these results suggest that secretion of MTD and HYGR may not
be vesicle dependent. Secretion of HYGR was reported to require
Golgi-localized synaptotagmin 2 (SYT2). Although the role of
SYT2 in secretion of HYGR is not yet defined, it will be interesting
to test whether secretion of MTD also depends on SYT2.
Another deviation for the conventional secretion route is exemplified by the release of cargo into the extracellular space by
organelle fusion as has been shown for vacuoles in order to
secrete caspase-like proteases and proteasome subunits into
the apoplast under pathogen infection (Hatsugai et al. 2009).
Decadal efforts to characterize the plant endomembrane
system have discovered several factors involved in secretion
of proteins and polysaccharides in the wall; however, it is still
unclear how crucial steps of secretion of diverse material are
achieved. In this review, we will report on progress in the study
of secretion of cell wall polysaccharides and enzymes involved
in cell wall synthesis and modification (summarized in Fig. 1).
Secretion of Cellulose Synthase Complex and
Cell Wall-Modifying Enzymes
Intracellular traffic depends on intermediate compartments
as well as on a large number of proteins that ensure fidelity
and directionality of each traffic route. Among these,
soluble N-ethylmaleimide-sensitive factor attachment protein
receptors (SNAREs) are essential proteins for endomembrane
688
traffic, and they are required for membrane fusion which is
accomplished upon formation of a trans-SNARE complex between SNAREs on target and donor membranes. At least 65
SNAREs exist in Arabidopsis (Kim and Brandizzi 2012), and each
member of the diversified SNARE subfamilies has been suggested to have a specific role during development and stress
conditions as well as a possible redundant function (Sanderfoot
2007). One of the most enigmatic SNAREs in plant cells is
SYP61, which is a Qc-SNARE found in the AtVPS45 complexes
in the TGN of Arabidopsis (Bassham et al. 2000). The SYP61
compartment was found to be involved in the traffic of auxin
transporters and the plasma membrane-localized receptors
such as brassinosteroid-insensitive1 (BRI1) as an early endosome (Robert et al. 2008). Intriguingly, SYP61 has been shown
also to be related to retrograde trafficking of vacuolar sorting
receptors from the pre-vacuolar compartment (PVC) (Niemes
et al. 2010). To solve a seemingly complex role for the SYP61
compartments, an interesting proteomic analysis of the vesicles
containing SYP61 has been recently performed (Drakakaki et al.
2012). The study identified numerous proteins known to localize at the TGN, the PVC and the plasma membrane. The presence of the plasma membrane SNARE SYP121 in the SYP61
compartment suggests that SYP61 could be involved in the
transport of SYP121, whose focal accumulation at the plasma
membrane was found to be important during a biotic stress
response (Kwon et al. 2008). In addition to SYP121, three CESAs
(CESA1, CESA3 and CESA6) were also identified in the SYP61
compartment (Drakakaki et al. 2012). CESAs are known to
localize in two types of compartments. One of these is the
vacuolar-type H+-ATPase subunit a1 (VHA-a1)/SYP61 compartment as deduced by co-localization of VHA-a1 with
CESA3 (Crowell et al. 2009). The role of the VHA-a1/SYP61
compartment in the transport of CSCs to plasma membrane
is not clear in terms of whether it serves as an endosome or as a
secretory vesicle. However, co-localization of CESA6 with TGN
markers (SYP41 and SYP42) and decreased fluorescence of
CESA3 in the VHA-a1/SYP61 compartment after treatment
with the protein synthesis inhibitor cycloheximide suggests
that the VHA-a1/SYP61 compartment may originate from
the TGN and function as a secretory vesicle (Crowell et al.
2009, Gutierrez et al. 2009). Nonetheless, it cannot be ruled
out that the co-localization of CESA3 and CESA6 with TGNlocalized SNAREs may be the consequence of endocytic
processes. For example, the BRI1 compartments have been
reported as a mixture of newly synthesized and endocytosed
compartments. When cells were treated with cycloheximide
and a specific V-ATPase inhibitor (concanamycin A) to block
endocytic transport to the tonoplast, decreased signal of the
BRI1–green fluorescent protein (GFP) was observed (Dettmer
et al. 2006), suggesting that the BRI1 compartments are also
partially generated from the secretory pathway.
A different type of transport intermediate is represented by
the microtubule-associated cellulose synthase compartment
(MASC), which is considered as a population of microtubuleassociated vesicles containing CSCs (Crowell et al. 2009).
Plant Cell Physiol. 55(4): 687–693 (2014) doi:10.1093/pcp/pct197 ! The Author 2014.
Secretion and synthesis of plant cell walls
Fig. 1 Diagram showing the routes and players for the secretion of pectin, and cell wall- synthesizing and modifying enzymes. JIM5, lowmethylesterified pectin; JIM7, high-methylesterified pectin; PMEI1, inhibitor of pectin methylesterase1; CSC, cellulose synthase complex; SVC,
secretory vesicle cluster; MASC, microtubule-associated CSC; red vesicles, VHA-a1/SYP61 compartment; purple vesicles, vesicles for site-directed
secretion of mucilage or during TE development; blue vesicles, endosomes of low-methylesterified pectin; yellow vesicles, SVC; orange
vesicles, MASC.
The MASCs are probably the compartments that redistribute
CSCs along the microtubule, presumably after endocytosis
(Paredez et al. 2006, Crowell et al. 2009). Evidence indicates
that MASCs are unlikely to be involved in secretion since the
delivery of newly synthesized CESA3 to the plasma membrane
is not dependent on the integrity of the microtubule to
which MASCs are normally bound (Gutierrez et al. 2009).
Furthermore, populations of microtubule-associating GFP–
CESA3 compartments were found to co-localize only partially
with SYP61 or to associate transiently with the VHA-a1 compartment (Crowell et al. 2009, Gutierrez et al. 2009). Increased
yellow fluorescent protein (YFP)–CESA6-positive compartments bound to microtubules were observed under treatment
with the cellulose synthesis inhibitor isoxaben (Gutierrez et al.
2009). When oryzalin and isoxaben were used, YFP–CESA6
signal was not detected, presumably as a consequence of
oryzalin-mediated depolymerization of microtubules.
Furthermore, CESA6 was found to interact with m2, one of
the subunits of ADAPTOR PROTEIN COMPLEX2 important
for clathrin-mediated endocytosis, implicating that endocytosis
of CSCs depends on a clathrin-mediated endocytosis (Bashline
et al. 2013). Although transient association between MASCs
and VHA-a1 compartments has been observed (Crowell et al.
2009), these results suggest that the endocytosed MASCs are
not involved in the secretion of newly synthesized CSCs to the
plasma membrane. Thus, although evidence supports that
MASCs are not the terminal compartments for transport to
the plasma membrane but rather cellular recycling or redistributing compartments, further experimental evidence is needed
to define their precise cellular role.
It has been suggested that secretion of pectin methylesterase
inhibitor protein (PMEI1) and a polygalacturonase inhibitor protein (PGIP2) relies on machinery that is different from that of the
CSC to the plasma membrane. As deduced in the SYP61 vesicle
proteomic analysis (Drakakaki et al. 2012), membrane fusion between vesicles containing CSC and plasma membrane are probably dependent upon SYP121 SNARE complexes. However,
secretion of PMEI1 and PGIP2 was not affected by an inhibitor
of the SYP121-dependent pathway, and secretion of PMEI1 was
discovered to rely on the glycosylphosphatidylinositol (GPI)
anchor (De Caroli et al. 2011), suggesting that a SYP121-independent pathway is responsible for the secretion of PME1 and PGIP2.
Secretion of Polysaccharides
As discussed above, secretion of the CSC is mediated by
the VHA-a1/SYP61 compartment, even though it is not clear
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whether the VHA-a1/SYP61 compartment containing the CSC
functions as a secretory vesicle or as an endosome. Because of
the difficulty in detecting polysaccharides using a method similar to that used for vesicular proteomic analysis, secretion of
polysaccharides has been investigated mostly using immunoelectron microscopy with antibodies recognizing carbohydrate
epitopes. With this approach, hemicellulose and pectin have
been clearly shown to localize in the apoplast (a site of deposition), Golgi (a site of synthesis) and vesicular structures (for
delivery) (Moore et al. 1991, Zhang and Staehelin 1992,
McFarlane et al. 2008, Chevalier et al. 2010). The presence of
polysaccharides in the vesicular compartment was further
investigated by Toyooka et al. (2009) through the analysis of
the secretory carrier membrane protein 2 (NtSCAMP2) in tobacco. NtSCAMP2 is localized in the TGN, plasma membrane,
cell plate and secretory vesicle cluster (SVC). The SVC containing NtSCAMP2 was suggested to be responsible for the secretion of cargo generated in the Golgi via clathrin coat-mediated
budding. This is supported by co-localization of NtSCAMP2 and
pectin in the SVCs as deduced by immunoelectron microscopy
analyses with JIM7 antibody for the detection of pectin, and
with an NtSCAMP2 antibody, as well as by evidence of membrane fusion events between SVCs and the plasma membrane
(Toyooka et al. 2009). In addition, the plasma membranelocalized NtSCAMP2 fused to reversible photo-switching
fluorescent protein (Dronpa) was found to move to punctate
structures, when the Dronpa-fused NtSCAMP2 in the plasma
membrane was selectively activated. The identity of the punctate structures is not characterized in terms of whether they are
TGNs for the next round of secretion or late endosomes for the
degradation (Toyooka and Matsuoka 2009). The protein structure of SCAMPs is well conserved in eukaryotes (Law et al.
2012). The biochemical role of NtSCAMP2 in the SVCs has
not been investigated yet, but several reports suggest that
SCAMP2 in human is involved in regulated granule exocytosis.
SCAMPs are composed of three distinct domains: (i) N- and
C-terminal variable domains; (ii) four transmembrane domains;
and (iii) E-peptides that connect the second and third transmembrane domains (Hubbard et al. 2000). The highly conserved E-peptide of SCAMP2 in human has been proved to
form a membrane fusion complex with small GTPase (Arf6),
phospholipase D1 (PLD1), SYT1 and syntaxin 1 (Syn1) (Liu et al.
2002, Liu et al. 2005), suggesting that it regulates exocytosis via
membrane fusion machinery. However, interacting partners of
SCAMP2 in plants remain to be identified. In addition to the
traditional secretion of cell wall materials from the Golgi to the
plasma membrane, cell wall polysaccharides such as pectin and
xyloglucan were also found to be endocytosed to the cell plate
with other plasma membrane proteins during cytokinesis
(Baluška et al. 2005, Dhonukshe et al. 2006).
Cytokinesis is a process dividing a mother cell into two
daughter cells. In plants, a cell plate forms in the middle of
the mother cell and expands out to the plasma membrane
for the completion of the cell division process. Cell plate formation requires complex vesicle trafficking through the
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endomembrane system. The presence of NtSCAMP2 in the
cell plate (Toyooka and Matsuoka 2009, Toyooka et al. 2009)
may implicate that de novo synthesized cell wall materials
could be transported to the cell plate for formation of the
new wall. In addition, secretion of newly synthesized proteins
to the cell plate was reported to be essential during cytokinesis,
while endocytosis may be not (Reichardt et al. 2007). However,
at least for pectin, endocytosis-mediated transport seems to be
more critical during cell plate formation. It has been shown that
low-methylesterified pectins are distributed in the cis- and
medial-Golgi and at the cell wall, whereas high-methylesterified
pectins are present in the medial- and trans-Golgi, in secretory
vesicles and at the cell wall, suggesting that pectins undergo
maturation while they are delivered to the trans-Golgi, and that
high-methylesterified pectins are secreted (Zhang and Staehelin
1992). Then, the high-methylesterified pectins are de-esterified
by pectin methylesterase in the cell wall (Pelloux et al. 2007).
Such spatial maturation of pectins has been determined using
immunoelectron microscopy with JIM5 (low-methylesterified
pectin) and JIM7 (high-methylesterified pectin) antibodies
(Knox et al. 1990, Zhang and Staehelin 1992). Application of
these two types of antibodies to label cell plates during cytokinesis showed high levels of staining by the JIM5 antibody and
low or absence of JIM7 staining in the cell plate. The cell plate
was also reactive to antibodies detecting dimers of rhamnogalacturonan II, a type of pectin, which are known to form by
borate cross-linking in the cell wall matrix (O’Neill et al. 2001,
Baluška et al. 2005, Dhonukshe et al. 2006). The absence of JIM7
labeling in the cell plate implicates that secretory vesicles containing JIM7-positive pectin are not involved in cell plate
formation, further suggesting that JIM7-positive pectin and
JIM5-positive pectin are transported by different types of secretory vesicles, and presumably their pathways do not overlap.
Furthermore, co-localization of NtSCAMP2 and JIM7-labeled
pectin was found in the SVCs, but only NtSCAMP2 was
shown to localize in the cell plate (Dhonukshe et al. 2006,
Toyooka et al. 2009). This suggests that there may be a specific
cargo receptor of polysaccharides that recruits cargo into
differentiated NtSCAMP2 SVCs for secretion to plasma membranes during cytokinesis.
Site-Sirected Secretion of Polysaccharides
Golgi to plasma membrane secretion can occur by both nonsite-directed and site-directed routes. For example, auxin efflux
carrier 1 (PIN1) has a characteristic polar distribution in the
plasma membrane that is maintained by endocytosis in the guanidine exchange factor (GNOM)-positive endosomes after nonsite-directed transport to the plasma membrane (Dhonukshe
et al. 2008). However, secretion of pectic polysaccharide in the
seed coat cell of Arabidopsis seems to be site directed. Cortical
microtubules in the seed coat cells are relatively abundant in the
mucilage pocket near the plasma membrane, where active secretion of mucilage occurs (McFarlane et al. 2008). Decreased mucilage secretion of temperature-sensitive microtubule mutant
Plant Cell Physiol. 55(4): 687–693 (2014) doi:10.1093/pcp/pct197 ! The Author 2014.
Secretion and synthesis of plant cell walls
(mor1-1) seeds under restrictive temperature has been observed,
even though mor1-1 seeds have normal mucilage pockets, similar
to wild-type seeds (McFarlane et al. 2008). Although there is no
direct evidence that pectin-containing vesicles move along the
cortical microtubules and dock to the plasma membrane, it is
probable that the high level of cortical microtubules defines a
region of the cell that may guide vesicles to active sites for
secretion, presumably where membrane fusion machinery complexes are located.
In general, trafficking of vesicles to their target membrane
requires a number of proteins, such as SNAREs, vesicle coat
proteins, GTPases and tethering factors. Tethering factors are
proteins that have a role in bridging between vesicles and target
membranes before the SNARE-mediated vesicle fusion. One of
the tethering complexes involved in exocytosis is the exocyst
complex. Exocyst complexes in Saccharomyces cerevisiae have
eight subunits: Sec3p, Sec5p, Sec6p, Sec8p, Sec10p, Sec15p,
Exo70p and Exo84p (TerBush et al. 1996). It has been shown
that exocyst complexes in yeast are important for polar secretion (Finger et al. 1998). These eight subunits are also found
in Arabidopsis, suggesting a conserved role for the exocyst
complexes.
Using reverse genetics, a number of exocyst mutants have
been characterized in Arabidopsis. Interestingly, several of the
exocyst components characterized to date have been identified
for their important role in the polar growth of specific cell
types, such as pollen tubes and root hairs, where active tip
growth occurs. Pollen tube elongation requires highly organized
vesicle trafficking at the tip of the pollen tube, where highmethylesterified pectin is mainly deposited during tip growth.
The sec8 mutant in Arabidopsis has defects in pollen germination, as well as pollen tube elongation, implicating that a loss
of one component in the exocyst complex results in inefficient
polar secretion (Cole et al. 2005). One of the exocyst components, Exo70, has 23 members in Arabidopsis, whereas only one
Exo70 is present in mammals and yeast, suggesting that the
Exo70 family in Arabidopsis could be regulated in a development- or tissue-specific manner, but also that each member
could have distinct roles during secretion. Several Exo70 family
members have been investigated, and the exo70a1 mutant
showed a reduced apical dominance, similar to the sec8
mutant, which is required for polar cell growth in root hairs
and stigmatic papillae (Synek et al. 2006). Recently, Exo70A1
was also identified to be involved in tracheary element (TE)
development (Li et al. 2013). The TE in the exo70a1 mutant
showed an irregular pattern of secondary cell wall thickening,
incomplete perforation between two neighboring TEs, and an
increased number of Golgi bodies and large vesicular compartments. It is plausible that the abnormal ultrastructure in the TE
of the exo70a1 mutant could be due to a lack of site-directed
vesicle transport, although distribution of Exo70A1 during TE
development has been not investigated yet. Another Exo70
family member, Exo70E2, has been identified in the distinct
vesicles involved in exocytosis, called EXPOs (Wang et al.
2010). Wang et al. (2010) also found by immunoelectron
microscopy that Exo70A1 is localized to EXPOs, implying that
EXPOs could be important during TE development. So far, only
cytosolic S-adenosylmethionine synthase 2 (SAM2) has been
identified as a cargo of EXPOs. Considering the cytosolic localization of SAM2, EXPOs may be generated from cytoplasm,
similar to autophagosomes. In addition, EXPOs are not stained
by the endocytic dye FM4-64 (Wang et al. 2010), suggesting that
EXPOs do not originate from endocytosis or the TGN.
Identification of cargo using purification and proteomic analysis of EXPOs and investigation of a role for EXPOs will increase
our understating of non-conventional secretion during TE
development.
Biotic stress has been known to induce active vesicle transport to the site of infection, and SYP121 (PEN1) was shown to
accumulate at the site of infection, presumably through the
action of both secretory and endocytic routes (Kwon et al.
2008, Nielsen et al. 2012), where it may mediate vesicle fusion
at the site of infection. The focal distribution of SYP121 SNARE
under biotic stress is somehow connected to the accumulation
of exocyst complexes at the same site. Specifically, other Exo70
members, Exo70B2 and Exo70H1, were reported to be involved
in biotic stress resistance, presumably by tethering the vesicles
at the site of infection. Exo70B2 was also found to interact
weakly with SNAP33, a SNARE in the SYP121 SNARE complex
(Pečenková et al. 2010), indicating that an exocyst complex
containing Exo70B2 tethers vesicles containing the SNAP33
SNARE complex under biotic stress.
Evidence showing the importance of the exocyst complex in
cell wall deposition was also reported in another study, which
investigated pectin deposition in the seeds of sec8 and exo70a1
mutants and identified reduced pectin accumulation in the
seed coat (Kulich et al. 2010). These results support that exocyst
complexes participate in the exocytosis of cell wall polysaccharides in the plasma membrane. Therefore, it is necessary to
investigate in more detail the role of the cytoskeleton, recruitment of the tethering factor and the identity of vesicles in order
to better understand the mechanism of site-directed secretion.
Conclusions and Perspectives
The understanding of cell wall synthesis has been growing
rapidly at a biosynthetic level, but the characterization of the
mechanisms leading to secretion of cell wall components has
been moving at a much slower pace, most probably because of
the very complex nature of the plant endomembrane system.
This is exemplified by the studies highlighting the heterogeneous nature of the TGN and the SYP61 compartments,
which have indicated multifunctionality for the organelles as
carriers and sorting compartments. The mechanisms leading to
polarized secretion, and the nature of the secretory vesicles and
associated secretory machinery, are largely unknown. Large
gaps also exist in the understanding of the mechanisms that
regulate secretion at qualitative and quantitative levels. As
demonstrated through the proteomics analyses of the SYP61
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compartment, organelle isolation offers a real perspective to
acquire meaningful insights on the nature of the contents of
the transport intermediates as well as on the machinery responsible for secretion. Therefore, we anticipate that proteomics
analyses of vesicles containing specific cargos such as CESAs,
PMEI1, PGIP2 and SAM2 during development or under diverse
environmental conditions will provide valuable information to
define the identity and role of the transport intermediates and
associated protein machinery and will answer fundamental
questions on the mechanisms adopted by cells to build the
wall during growth and development as well as in responses
to biotic and abiotic stress.
Funding
This work was supported by the Chemical Sciences,
Geosciences and Biosciences Division, Office of Basic Energy
Sciences, Office of Science, US Department of Energy [DEFG02-91ER20021] for infrastructure; the DOE Great Lakes
Bioenergy Research Center [DOE Office of Science BER DEFC02-07ER64494].
Disclosures
The authors have no conflicts of interest to declare.
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