Xylem-specific gene expression in loblolly pine

Conflicting genomic signals affect phylogenetic inference in four species of North American pines

Adaptive evolutionary processes in plants may be accompanied by episodes of introgression, parallel evolution and incomplete lineage sorting that pose challenges in untangling species evolutionary history. Genus Pinus (pines) is one of the most abundant and most studied groups among gymnosperms, and a good example of a lineage where these phenomena have been observed. Pines are among the most ecologically and economically important plant species. Some, such as the pines of the southeastern USA (southern pines in subsection Australes), are subjects of intensive breeding programmes. Despite numerous published studies, the evolutionary history of Australes remains ambiguous and often controversial. We studied the phylogeny of four major southern pine species: shortleaf (Pinus echinata), slash (P. elliottii), longleaf (P. palustris) and loblolly (P. taeda), using sequences from 11 nuclear loci and maximum likelihood and Bayesian methods. Our analysis encountered resolution difficulties similar to earlier published studies. Although incomplete lineage sorting and introgression are two phenomena presumptively underlying our results, the phylogenetic inferences seem to be also influenced by the genes examined, with certain topologies supported by sets of genes sharing common putative functionalities. For example, genes involved in wood formation supported the clade echinata-taeda, genes linked to plant defence supported the clade echinata-elliottii and genes linked to water management properties supported the clade echinata-palustris The support for these clades was very high and consistent across methods. We discuss the potential factors that could underlie these observations, including incomplete lineage sorting, hybridization and parallel or adaptive evolution. Our results likely reflect the relatively short evolutionary history of the subsection that is thought to have begun during the middle Miocene and has been influenced by climate fluctuations.

Scanning SNPs from a large set of expressed genes to assess the impact of artificial selection on the undomesticated genetic diversity of white spruce

A scan involving 1134 single-nucleotide polymorphisms (SNPs) from 709 expressed genes was used to assess the potential impact of artificial selection for height growth on the genetic diversity of white spruce. Two case populations of different sizes simulating different family selection intensities (K = 13% and 5%, respectively) were delineated from the Quebec breeding program. Their genetic diversity and allele frequencies were compared with those of control populations of the same size and geographic origin to assess the effect of increasing the selection intensity. The two control populations were also compared to assess the effect of reducing the sampling size. On one hand, in all pairwise comparisons, genetic diversity parameters were comparable and no alleles were lost in the case populations compared with the control ones, except for few rare alleles in the large case population. Also, the distribution of allele frequencies did not change significantly (P ≤ 0.05) between the populations compared, but ten and nine SNPs (0.8%) exhibited significant differences in frequency (P ≤ 0.01) between case and control populations of large and small sizes, respectively. Results of association tests between breeding values for height at 15 years of age and these SNPs supported the hypothesis of a potential effect of selection on the genes harboring these SNPs. On the other hand, contrary to expectations, there was no evidence that selection induced an increase in linkage disequilibrium in genes potentially affected by selection. These results indicate that neither the reduction in the sampling size nor the increase in selection intensity was sufficient to induce a significant change in the genetic diversity of the selected populations. Apparently, no loci were under strong selection pressure, confirming that the genetic control of height growth in white spruce involves many genes with small effects. Hence, selection for height growth at the present intensities did not appear to compromise background genetic diversity but, as predicted by theory, effects were detected at a few gene SNPs harboring intermediate allele frequencies.

Association mapping of quantitative disease resistance in a natural population of loblolly pine (Pinus taeda L.)

Genetic resistance to disease incited by necrotrophic pathogens is not well understood in plants. Whereas resistance is often quantitative, there is limited information on the genes that underpin quantitative variation in disease resistance. We used a population genomic approach to identify genes in loblolly pine (Pinus taeda) that are associated with resistance to pitch canker, a disease incited by the necrotrophic pathogen Fusarium circinatum. A set of 498 largely unrelated, clonally propagated genotypes were inoculated with F. circinatum microconidia and lesion length, a measure of disease resistance, data were collected 4, 8, and 12 weeks after inoculation. Best linear unbiased prediction was used to adjust for imbalance in number of observations and to identify highly susceptible and highly resistant genotypes ("tails"). The tails were reinoculated to validate the results of the full population screen. Significant associations were detected in 10 single nucleotide polymorphisms (SNPs) (out of 3938 tested). As hypothesized for genes involved in quantitative resistance, the 10 SNPs had small effects and proposed roles in basal resistance, direct defense, and signal transduction. We also discovered associated genes with unknown function, which would have remained undetected in a candidate gene approach constrained by annotation for disease resistance or stress response.

Gene expression in Eucalyptus branch wood with marked variation in cellulose microfibril orientation and lacking G-layers

In response to gravitational stresses, angiosperm trees form tension wood in the upper sides of branches and leaning stems in which cellulose content is higher, microfibrils are typically aligned closely with the fibre axis and the fibres often have a thick inner gelatinous cell wall layer (G-layer). Gene expression was studied in Eucalyptus nitens branches oriented at 45 degrees using microarrays containing 4900 xylem cDNAs, and wood fibre characteristics revealed by X-ray diffraction, chemical and histochemical methods. Xylem fibres in tension wood (upper branch) had a low microfibril angle, contained few fibres with G-layers and had higher cellulose and decreased Klason lignin compared with lower branch wood. Expression of two closely related fasciclin-like arabinogalactan proteins and a beta-tubulin was inversely correlated with microfibril angle in upper and lower xylem from branches. Structural and chemical modifications throughout the secondary cell walls of fibres sufficient to resist tension forces in branches can occur in the absence of G-layer enriched fibres and some important genes involved in responses to gravitational stress in eucalypt xylem are identified.

Characterization of the structure, expression and function of Pinus radiata D. Don arabinogalactan-proteins

A synthetic phenylglycoside (beta-GlcY) that interacts specifically with arabinogalactan-proteins (AGPs), a class of plant cell surface proteoglycans, has been used to study the spatial distribution of AGPs in the xylem tissue of radiata pine. These studies demonstrated that AGPs were located in the compound middle lamella (CML) of the newly developed tracheid. Abundant, low salt extractable AGPs were purified from xylem tissue. Monosaccharide analysis showed that arabinose and galactose were the main sugars present. Linkage analysis showed that most of the arabinose was in the furanose form, at the terminal and 5-linked positions, and the majority of the galactose was in the pyranose form at the terminal 3-, 6- and 3,6-linked positions; a linkage composition typical of AGPs. The AGPs had an abundance of characteristic amino acid residues including alanine, hydroxyproline, proline, and serine. Separation of the AGPs using reversed-phase high performance liquid chromatography showed that one main fraction was eluted, which tested positive for AGPs by dot-blot analysis using anti-AGP monoclonal antibodies. Sedimentation equilibrium analysis showed that this main fraction contained a 226 kDa species. We have examined the function of AGPs in tracheid differentiation using an established radiata pine callus culture system grown on media containing beta-GlcY. The effect of beta-GlcY on the cultures was to reduce the overall tracheid differentiation rate in a concentration dependent manner, ultimately resulting in cell death. These studies provide further evidence that AGPs play an important role in tracheid differentiation, and thus may be an important biological target for improving wood quality.

Microarray gene expression profiling of developmental transitions in Sitka spruce (Picea sitchensis) apical shoots

The apical shoot drives the yearly new stem growth of conifer trees, is the primary site for the establishment of chemical and physical defences, and is important in establishing subsequent perennial growth. This organ presents an interesting developmental system, with growth and development progressing from a meristematic tip through development of a primary vascular system, to a base with fully differentiated and lignified secondary xylem on the inside and bark tissue with constitutive defence structures such as resin, polyphenolic phloem parenchyma cells, and sclereids on the outside. A spruce (Picea spp.) microarray containing approximately 16.7K unique cDNAs was used to study transcript profiles that characterize the developmental transition in apical shoots of Sitka spruce (Picea sitchensis) from their vegetative tips to their woody bases. Along with genes involved in cell-wall modification and lignin biosynthesis, a number of differentially regulated genes encoding protein kinases and transcription factors with base-preferred expression patterns were identified, which could play roles in the formation of woody tissues inside the apical shoot, as well as in regulating other developmental transitions associated with organ maturation. Preferential expression of known conifer defence genes, genes encoding defence-related proteins, and genes encoding regulatory proteins was observed at the apical shoot tip and in the green bark tissues at the apical shoot base, suggesting a commitment to constitutive defence in the apical shoot that is co-ordinated with rapid development of secondary xylem.

The proteome of maritime pine wood forming tissue

Wood is one of our most important natural resources. Surprisingly, we know hardly anything about the details of the process of wood formation. The aim of this work was to describe the main proteins expressed in wood forming tissue of a conifer species (Pinus pinaster Ait.). Using high resolution 2-DE with linear pH gradient ranging from 4 to 7, a total of 1039 spots were detected. Out of the 240 spots analyzed by MS/MS, 67.9% were identified, 16.7% presented no homology in the databases, and 15.4% corresponded to protein mixtures. Out of the 57 spots analyzed by MALDI-MS, only 15.8% were identified. Most of the 175 identified proteins play a role in either defense (19.4%), carbohydrates (16.6%) and amino acid (14.9%) metabolisms, genes and proteins expression (13.1%), cytoskeleton (8%), cell wall biosynthesis (5.7%), secondary (5.1%) and primary (4%) metabolisms. A summary of the identified proteins, their putative functions, and behavior in different types of wood are presented. This information was introduced into the PROTICdb database and is accessible at http://cbib1.cbib.u-bordeaux2.fr/Protic/Protic/home/index.php. Finally, the average protein amount was compared with their respective transcript abundance as quantified through EST counting in a cDNA-library constructed with mRNA extracted from wood forming tissue.

Salt stress upregulates periplasmic arabinogalactan proteins: using salt stress to analyse AGP function

Arabinogalactan proteins (AGPs) are implicated in cell expansion by unknown mechanisms, thus AGP content and cell-expansion rate might be correlated. We used Yariv reagent to quantify release rates and distribution of AGP at the cell surface of tobacco BY-2 cells: plasma membrane (M); soluble periplasmic AGPs released by cell rupture (S); cell wall (W); and growth medium (Gsink). In contrast to earlier reports, we observed massive upregulation of AGPs in salt-stressed cells, and hence the absence of a simple, direct cause-and-effect relationship between growth rate and AGP release. There was a more subtle connection. A dynamic flux model, M-->S-->W-->Gsink, indicated that turnover was nondegradative, with little free diffusion of AGPs trapped in the pectic matrix of nonadapted cells where transmural migration of high molecular-weight AGPs occurred mainly by plug flow (apposition and extrusion). In contrast, however, an up to sixfold increased AGP release rate in the slower-growing salt-adapted cells indicated a greatly increased rate of AGP diffusion through a much more highly porous pectic network. We hypothesize that classical AGPs act as pectin plasticizers. This explains how beta-D-glycosyl Yariv reagents might inhibit expansion growth by crosslinking monomeric AGPs, and thus mimic an AGP loss-of-function mutation.

Poplar genes encoding fasciclin-like arabinogalactan proteins are highly expressed in tension wood

•  Fifteen poplar cDNA encoding fasciclin-like arabinogalactan proteins (PopFLAs) were finely characterized, whereas the presence of arabinogalactan proteins (AGPs) was globally assessed during wood formation. •  PopFLAs transcript accumulation was analysed through EST distribution in cDNA libraries, semi-quantitative RT-PCR, microarray experiment and Northern blot analysis. Similarly, AGPs contents were globally quantified by rocket electrophoresis. AGPs accumulation was further examined by Western blotting and immunocytolocalization. •  Ten PopFLAs were specifically expressed in tension wood (TW) and not expressed in the cambial zone. Rocket electrophoresis revealed important AGPs accumulation in TW xylem. An anti-AGPs specific antibody recognized two proteins preferentially present in the cell wall-bound fraction from TW. Immunocytochemistry revealed a strong labelling close to the inner part of the G-layer of TW fibres. •  PopFLAs are expressed in xylem and many are up-regulated in TW. It is suggested that some PopFLAs accumulating at the inner side of the G-layer may have a specific function in the building of this layer. PopFLAs expression may therefore be linked to the specific mechanical properties of TW.

Identification of quantitative trait loci influencing wood property traits in loblolly pine (Pinus taeda L.). III. QTL Verification and candidate gene mapping

A long-term series of experiments to map QTL influencing wood property traits in loblolly pine has been completed. These experiments were designed to identify and subsequently verify QTL in multiple genetic backgrounds, environments, and growing seasons. Verification of QTL is necessary to substantiate a biological basis for observed marker-trait associations, to provide precise estimates of the magnitude of QTL effects, and to predict QTL expression at a given age or in a particular environment. Verification was based on the repeated detection of QTL among populations, as well as among multiple growing seasons for each population. Temporal stability of QTL was moderate, with approximately half being detected in multiple seasons. Fewer QTL were common to different populations, but the results are nonetheless encouraging for restricted applications of marker-assisted selection. QTL from larger populations accounted for less phenotypic variation than QTL detected in smaller populations, emphasizing the need for experiments employing much larger families. Additionally, 18 candidate genes related to lignin biosynthesis and cell wall structure were mapped genetically. Several candidate genes colocated with wood property QTL; however, these relationships must be verified in future experiments.

The promoter of a cytosolic glutamine synthetase gene from the conifer Pinus sylvestris is active in cotyledons of germinating seeds and light-regulated in transgenic Arabidopsis thaliana

We have isolated and characterized a genomic clone encoding Scots pine (Pinus sylvestris) cytosolic glutamine synthetase (GS). The clone contains the 5' end half of the gene including part of the coding region and 980 bp upstream of the translation initiation codon. The major transcription start site (+1) was mapped around 180 nucleotides upstream of the translation initiation codon. Sequence analysis of the 5'-upstream region of the gene reveals the presence of putative regulatory elements including a poly-CT consensus sequence, a purine-rich tandem repeat and two AT-rich regions. Fusions of the upstream gene region to uidA were shown to be transiently expressed in the cotyledons of germinating pine seeds transformed by microprojectile bombardment. Stable transformation of Arabidopsis thaliana revealed the shoot apical meristem as the major region of heterologous permanent expression in Arabidopsis, in agreement with the expression of the GS gene in Pinus. Moreover, quantitative data derived from fluorometric beta-glucuronidase assays in control and continuous light-grown transgenic Arabidopsis plants indicate that the isolated upstream region of the gene contains regulatory sequences involved in the response to light.

An arabinogalactan protein(s) is a key component of a fraction that mediates local intercellular communication involved in tracheary element differentiation of zinnia mesophyll cells

Local intercellular communication is involved in tracheary element (TE) differentiation of zinnia (Zinnia elegans L.) mesophyll cells and mediated by a proteinous macromolecule, which was designated xylogen. To characterize and isolate xylogen, a bioassay system to monitor the activity of xylogen was developed, in which mesophyll cells were embedded in microbeads of agarose gel at a low (2.0-4.3x10(4) cells ml(-1)) or high density (8.0-9.0x10(4) cells ml(-1)) and microbeads of different cell densities were cultured together in a liquid medium to give a total density of 2.1-2.5x10(4) cells ml(-1). Without any additives, the frequency of TE differentiation was much smaller in the low-density microbeads than in the high-density microbeads. This low level of TE differentiation in the low-density microbeads was attributable to the shortage of xylogen. When cultures were supplemented with conditioned medium (CM) prepared from zinnia cell suspensions undergoing TE differentiation, the frequency of TE differentiation in the low-density microbeads increased remarkably, indicating the activity of xylogen in the CM. The xylogen activity in CM was sensitive to proteinase treatments. Xylogen was bound to galactose-specific lectins such as Ricinus communis agglutinin and peanut agglutinin, and precipitated by beta-glucosyl Yariv reagent. These results indicate that xylogen is a kind of arabinogalactan protein.

Sequences upstream and downstream of two xylem-specific pine genes influence their expression

The identification of regulatory elements conferring high levels of expression in differentiating pine xylem will be valuable for genetic engineering of wood properties and will contribute to our understanding of gene regulation in this important group of forest trees. We examined the roles of both upstream and downstream elements in regulating the expression of two genes with preferential expression in developing xylem of loblolly pine. Gene constructs containing a PtX3H6, PtX14A9, or CaMV 35S promoter, the uidA gene encoding beta-glucuronidase, and a PtX3H6, PtX14A9, or NOS terminator were used to transform tobacco and hybrid poplar. When combined with the NOS terminator, neither pine promoter conferred xylem-specific expression in tobacco. When combined with the PtX3H6 promoter, an element at the 3' end of PtX3H6 reduced GUS expression resulting in preferential expression in vascular tissues. This silencing effect was not observed when the pine terminator was tested in conjunction with the CaMV 35S promoter. The PtX14A9 terminator did not increase tissue specificity. In leaves of transgenic poplar, both pine promoters conferred preferential GUS expression in veins when combined with the NOS terminator. The PtX3H6 terminator greatly decreased expression in leaves and stems when combined with the PtX3H6 promoter but only slightly altered expression when combined with the CaMV 35S promoter. An element at the 3' end of PtX14A9 increased GUS expression in veins when used in conjunction with either the PtX14A9 or CaMV35S promoter.

Exploiting secondary growth in Arabidopsis. Construction of xylem and bark cDNA libraries and cloning of three xylem endopeptidases

The root-hypocotyl of Arabidopsis produces a relatively large amount of secondary vascular tissue when senescence is delayed by the removal of inflorescences, and plants are grown at low population density. Peptidase zymograms prepared from isolated xylem and phloem revealed the existence of distinct proteolytic enzyme profiles within these tissues. cDNA libraries were constructed from isolated xylem and bark of the root-hypocotyl and screened for cDNAs coding for cysteine, serine, and aspartic peptidases. Three cDNAs, two putative papain-type cysteine peptidases (XCP1 and XCP2) and one putative subtilisin-type serine peptidase (XSP1), were identified from the xylem library for further analysis. Using RNA gel blots it was determined that these peptidases were expressed in the xylem and not in the bark. Quantitative reverse transcriptase-polymerase chain reaction confirmed the RNA gel-blot results and revealed high levels of XCP1 and XCP2 mRNA in stems and flowers of the infloresence. A poly-histidine-tagged version of XCP1 was purified from Escherichia coli by denaturing metal-chelate chromatography. Following renaturation, the 40-kD recombinant XCP1 was not proteolytically active. Activation was achieved by incubation of recombinant XCP1 at pH 5.5 and was dependent on proteolytic processing of the 40-kD inactive polypeptide to a 26-kD active peptidase.

Uclacyanins, stellacyanins, and plantacyanins are distinct subfamilies of phytocyanins: plant-specific mononuclear blue copper proteins

The cDNAs encoding plantacyanin from spinach were isolated and characterized. In addition, four new cDNA sequences from Arabidopsis ESTs were identified that encode polypeptides resembling phytocyanins, plant-specific proteins constituting a distinct family of mononuclear blue copper proteins. One of them encodes plantacyanin from Arabidopsis, while three others, designated as uclacyanin 1, 2, and 3, encode protein precursors that are closely related to precursors of stellacyanins and a blue copper protein from pea pods. Comparative analyses with known phytocyanins allow further classification of these proteins into three distinct subfamilies designated as uclacyanins, stellacyanins, and plantacyanins. This specification is based on (1) their spectroscopic properties, (2) their glycosylation state, (3) the domain organization of their precursors, and (4) their copper-binding amino acids. The recombinant copper binding domain of Arabidopsis uclacyanin 1 was expressed, purified, and shown to bind a copper atom in a fashion known as "blue" or type 1. The mutant of cucumber stellacyanin in which the glutamine axial ligand was substituted by a methionine (Q99M) was purified and shown to possess spectroscopic properties similar to uclacyanin 1 rather than to plantacyanins. Its redox potential was determined by cyclic voltammetry to be +420 mV, a value that is significantly higher than that determined for the wild-type protein (+260 mV). The available structural data suggest that stellacyanins (and possibly other phytocyanins) might not be diffusible electron-transfer proteins participating in long-range electron-transfer processes. Conceivably, they are involved in redox reactions occurring during primary defense responses in plants and/or in lignin formation.

Arabinogalactan-proteins from Nicotiana alata and Pyrus communis contain glycosylphosphatidylinositol membrane anchors

Arabinogalactan-proteins (AGPs) are a class of proteoglycans found in cell secretions and plasma membranes of plants. Attention is currently focused on their structure and their potential role in growth and development. We present evidence that two members of a major class of AGPs, the classical AGPs, AGPNa1 from styles of Nicotiana alata and AGPPc1 from cell suspension cultures of Pyrus communis, undergo C-terminal processing involving glycosylphosphatidylinositol membrane anchors. The evidence is that (i) the transmembrane helix at the C terminus predicted from the cDNA encoding these proteins is not present-the C-terminal amino acid is Asn87 and Ser97 for AGPNa1 and AGPPc1, respectively; (ii) both AGP protein backbones are substituted with ethanolamine at the C-terminal amino acid; and (iii) inositol, glucosamine, and mannose are present in the native AGPs. An examination of the deduced amino acid sequences of other classical AGP protein backbones shows that glycosylphosphatidylinositol-anchors may be a common feature of this class of AGPs.

PLANT CELL WALL PROTEINS

The nature of cell wall proteins is as varied as the many functions of plant cell walls. With the exception of glycine-rich proteins, all are glycosylated and contain hydroxyproline (Hyp). Again excepting glycine-rich proteins, they also contain highly repetitive sequences that can be shared between them. The majority of cell wall proteins are cross-linked into the wall and probably have structural functions, although they may also participate in morphogenesis. On the other hand, arabinogalactan proteins are readily soluble and possibly play a major role in cell-cell interactions during development. The interactions of these proteins between themselves and with other wall components is still unknown, as is how wall components are assembled. The possible functions of cell wall proteins are suggested based on repetitive sequence, localization in the plant body, and the general morphogenetic pattern in plants.

Proteoglycans and related components in plant cells

After the context is set by a brief description of the plant cell surface, emphasis is placed on one class of cell surface components, the arabinogalactan proteins. An expansion of knowledge regarding the structure, expression, and function of these proteoglycans has been initiated and is being sustained through new experimental approaches, including the development of monoclonal antibody probes and the cloning of cDNAs corresponding to core polypeptides. An examination of the structure of both the polypeptide and carbohydrate components of arabinogalactan proteins is presented with emphasis placed on recently deduced core polypeptide sequences. Information about the biosynthesis and turnover of arabinogalactan proteins is incomplete, especially with regard to the carbohydrate component. Although functions of arabinogalactan proteins have not been clearly identified, regulated expression and several other lines of evidence point to involvement in plant reproductive development, pattern formation, and somatic embryogenesis, as well as in the underlying processes of cell division, cell expansion, and cell death. Arabinogalactan proteins are compared with animal proteoglycans and mucins, and the results of searches for plant analogues of other animal extracellular matrix components are examined.

Cloning and developmental/stress-regulated expression of a gene encoding a tomato arabinogalactan protein

Arabinogalactan proteins (AGPs) represent a major class of plant hydroxyproline-rich glycoproteins (HRGPs) and are components of cell walls and plasma membranes. AGPs are thought to play roles in cell differentiation, development, and cell-cell interactions. Using a synthetic DNA oligonucleotide based upon an amino acid sequence motif common to AGPs from Lolium, rose, and carrot (i.e., Hyp-Ala-Hyp-Ala-Hyp), we have isolated and sequenced the first AGP gene from a partial Sau3A tomato genomic library packaged in bacteriophage charon 35. The deduced 215 amino acid protein contains 20% Ala, 22% Pro, 10% Gly, and 11% Ser and consists of two Pro-Ala-Pro-Ala-Pro pentapeptide repeats and 16 Ala-Pro dipeptide repeats, consistent with known AGP amino acid compositions and sequences. Comparison of the genomic sequence to a reverse transcribed PCR product and tomato cDNA confirmed the AGP gene is expressed and contains one large intervening sequence. RNA blot hybridization analysis in tomato indicates this AGP gene is strongly expressed in stem and flower, moderately expressed in root and green fruit, and weakly expressed in leaves and red fruit as a 980 nucleotide transcript. Five-day-old seedlings also express this transcript; however, this expression is not regulated by light. More significantly, a gradient of AGP gene expression is observed in tomato stems, ranging from high levels of expression in young internodes to low levels of expression in old internodes. Wounding serves to down-regulate expression in young and old internodes. Heat shock also affects AGP gene expression in stems by transiently down-regulating mRNA levels.

Arabinogalactan-proteins: a class of extracellular matrix proteoglycans involved in plant growth and development

Arabinogalactanproteins (AGPs) are proteoglycans of the extracellular matrix o f most plants. Since the late 1980s, AGPs have attracted widespread attention from plant biologists following reports of their involvement in plant development. In particular, the use of monoclonal antibodies to carbohydrate epitopes of AGPs has demonstrated stage- and tissue-specificity and has led to suggestions that they are involved in tissue morphogenesis. The recent cloning of the genes for several AGP protein backbones allows us to consider new strategies to address their function. Here, we summarize our knowledge of AGPs and consider parallels with animal proteoglycans as a possible framework for future work.