A fasciclin-like arabinogalactan-protein (FLA) mutant of Arabidopsis thaliana, fla1, shows defects in shoot regeneration

BACKGROUND: The fasciclin-like arabinogalactan-proteins (FLAs) are an enigmatic class of 21 members within the larger family of arabinogalactan-proteins (AGPs) in Arabidopsis thaliana. Located at the cell surface, in the cell wall/plasma membrane, they are implicated in many developmental roles yet their function remains largely undefined. Fasciclin (FAS) domains are putative cell-adhesion domains found in extracellular matrix proteins of organisms from all kingdoms, but the juxtaposition of FAS domains with highly glycosylated AGP domains is unique to plants. Recent studies have started to elucidate the role of FLAs in Arabidopsis development. FLAs containing a single FAS domain are important for the integrity and elasticity of the plant cell wall matrix (FLA11 and FLA12) and FLA3 is involved in microspore development. FLA4/SOS5 with two FAS domains and two AGP domains has a role in maintaining proper cell expansion under salt stressed conditions. The role of other FLAs remains to be uncovered. METHOD/PRINCIPAL FINDINGS: Here we describe the characterisation of a T-DNA insertion mutant in the FLA1 gene (At5g55730). Under standard growth conditions fla1-1 mutants have no obvious phenotype. Based on gene expression studies, a putative role for FLA1 in callus induction was investigated and revealed that fla1-1 has a reduced ability to regenerate shoots in an in vitro shoot-induction assay. Analysis of FLA1p:GUS reporter lines show that FLA1 is expressed in several tissues including stomata, trichomes, the vasculature of leaves, the primary root tip and in lateral roots near the junction of the primary root. CONCLUSION: The results of the developmental expression of FLA1 and characterisation of the fla1 mutant support a role for FLA1 in the early events of lateral root development and shoot development in tissue culture, prior to cell-type specification.     

Arabidopsis thaliana FLA4 functions as a glycan‐stabilized soluble factor via its carboxy‐proximal Fasciclin 1 domain

Fasciclin‐like arabinogalactan proteins (FLAs) are involved in numerous important functions in plants but the relevance of their complex structure to physiological function and cellular fate is unresolved. Using a fully functional fluorescent version of Arabidopsis thaliana FLA4 we show that this protein is localized at the plasma membrane as well as in endosomes and soluble in the apoplast. FLA4 is likely to be GPI‐anchored, is highly N‐glycosylated and carries two O‐glycan epitopes previously associated with arabinogalactan proteins. The activity of FLA4 was resistant against deletion of the amino‐proximal fasciclin 1 domain and was unaffected by removal of the GPI‐modification signal, a highly conserved N‐glycan or the deletion of predicted O‐glycosylation sites. Nonetheless these structural changes dramatically decreased endoplasmic reticulum (ER)‐exit and plasma membrane localization of FLA4, with N‐glycosylation acting at the level of ER‐exit and O‐glycosylation influencing post‐secretory fate. We show that FLA4 acts predominantly by molecular interactions involving its carboxy‐proximal fasciclin 1 domain and that its amino‐proximal fasciclin 1 domain is required for stabilization of plasma membrane localization. FLA4 functions as a soluble glycoprotein via its carboxy‐proximal Fas1 domain and its normal cellular trafficking depends on N‐ and O‐glycosylation.     

Genome-wide identification, classification and expression analysis of genes encoding putative fasciclin-like arabinogalactan proteins in Chinese cabbage (Brassica rapa L.)

Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), have both predicted AGP-like glycosylated regions and putative fasciclin (FAS) domains, which may function in cell adhesion and communication. Previous studies have identified 21, 27, and 34 FLAs in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and wheat (Triticum aestivum), respectively. In this study, we identified 33 FLAs in the annotated genome of Chinese cabbage (Brassica rapa ssp. pekinensis line Chiifu-401-42). Sequence analysis indicated that FAS domains each contain two highly conserved regions, named H1 and H2, and that 17 FLAs from B. rapa (BrFLAs) possess both of these regions. Prediction of glycosylphosphatidylinositol (GPI) modification sites suggested that 15 BrFLAs were GPI-anchored to the plasma membrane. Additionally, 25 BrFLAs may have been duplicated during the processes that shaped the triplicated genome of the mesopolyploid B. rapa. Expression analyses indicated that BrFLA1, BrFLA11, BrFLA13, BrFLA28 and BrFLA32 were specifically expressed in inflorescence. Meanwhile, BrFLA9 (homologous to AtFLA12) is specifically expressed in stem, and BrFLA6/22 (homologous to AtFLA11) is also highly expressed in stem, suggesting BrFLA6/9/22 may have the same functions as AtFLA11/12 in A. thaliana. Taken together, the identification and bioinformatic analysis of FLAs in B. rapa will open the way for studying their biological functions in plant growth and development as well as evolutionary history of this gene family from A. thaliana to B. rapa.     

Genome-wide identification of fasciclin-like arabinogalactan proteins in jute and their expression pattern during fiber formation

Molecular Biology Reports - Fasciclin-like arabinogalactan proteins (FLAs), a class of arabinogalactan proteins (AGPs) are involved in plant growth and development via cell communication and...     

The classical arabinogalactan protein gene family of arabidopsis

Arabinogalactan proteins (AGPs) are extracellular proteoglycans implicated in plant growth and development. We searched for classical AGPs in Arabidopsis by identifying expressed sequence tags based on the conserved domain structure of the predicted protein backbone. To confirm that these genes encoded bona fide AGPs, we purified native AGPs and then deglycosylated and deblocked them for N-terminal protein sequencing. In total, we identified 15 genes encoding the protein backbones of classical AGPs, including genes for AG peptides-AGPs with very short backbones (10 to 13 amino acid residues). Seven of the AGPs were verified as AGPs by protein sequencing. A gene encoding a putative cell adhesion molecule with AGP-like domains was also identified. This work provides a firm foundation for beginning functional analysis by using a genetic approach.     

基于cas9/gRNA创制毛果杨PtrFLA31/34突变体

束状阿拉伯半乳聚糖蛋白（FLA）对植物生长发育都起着重要作用。基于生物信息学分析,我们识别毛果杨FLA家族有46个成员;进化分析显示毛果杨FLA家族分为A、B、C、D 4个亚族,且PtrFLA31和PtrFLA34处在进化树的一个小分支。半定量RT-PCR分析表明,PtrFLA31和PtrFLA34在毛果杨木质组织特异性、高水平表达。为了鉴定PtrFLA31和PtrFLA34在木材形成上的功能,我们用Cas9/gRNA技术敲除PtrFLA31/34,获得3株毛果杨fla31/34敲除双基因突变体材料。     

Focus Issue on Plant Cell Walls: A Bioinformatics Approach to the Identification,Classification, and Analysis of Hydroxyproline-Rich Glycoproteins

Hydroxyproline-rich glycoproteins (HRGPs) are a superfamily of plant cell wall proteins that function in diverse aspects of plant growth and development. This superfamily consists of three members: hyperglycosylated arabinogalactan proteins (AGPs), moderately glycosylated extensins (EXTs), and lightly glycosylated proline-rich proteins (PRPs). Hybrid and chimeric versions of HRGP molecules also exist. In order to “mine” genomic databases for HRGPs and to facilitate and guide research in the field, the BIO OHIO software program was developed that identifies and classifies AGPs, EXTs, PRPs, hybrid HRGPs, and chimeric HRGPs from proteins predicted from DNA sequence data. This bioinformatics program is based on searching for biased amino acid compositions and for particular protein motifs associated with known HRGPs. HRGPs identified by the program are subsequently analyzed to elucidate the following: (1) repeating amino acid sequences, (2) signal peptide and glycosylphosphatidylinositol lipid anchor addition sequences, (3) similar HRGPs via Basic Local Alignment Search Tool, (4) expression patterns of their genes, (5) other HRGPs, glycosyl transferase, prolyl 4-hydroxylase, and peroxidase genes coexpressed with their genes, and (6) gene structure and whether genetic mutants exist in their genes. The program was used to identify and classify 166 HRGPs from Arabidopsis (Arabidopsis thaliana) as follows: 85 AGPs (including classical AGPs, lysine-rich AGPs, arabinogalactan peptides, fasciclin-like AGPs, plastocyanin AGPs, and other chimeric AGPs), 59 EXTs (including SP₅ EXTs, SP₅/SP₄ EXTs, SP₄ EXTs, SP₄/SP₃ EXTs, a SP₃ EXT, “short” EXTs, leucine-rich repeat-EXTs, proline-rich extensin-like receptor kinases, and other chimeric EXTs), 18 PRPs (including PRPs and chimeric PRPs), and AGP/EXT hybrid HRGPs.The genomics era has produced vast amounts of biological data that await examination. In order to “mine” such data effectively, a bioinformatics approach can be utilized to identify genes of interest, subject them to various in silico analyses, and extract relevant biological information on them from various public databases. Examination of such data produces novel insights with respect to the genes in question and can be used to facilitate and guide further research in the field. Such is the case here, where bioinformatics tools were developed to identify, classify, and analyze members of the Hyp-rich glycoprotein (HRGP) superfamily encoded by the Arabidopsis (Arabidopsis thaliana) genome.HRGPs are a superfamily of plant cell wall proteins that are subdivided into three families, arabinogalactan proteins (AGPs), extensins (EXTs), and Pro-rich proteins (PRPs), and extensively reviewed (Showalter, 1993; Kieliszewski and Lamport, 1994; Nothnagel, 1997; Cassab, 1998; José-Estanyol and Puigdomènech, 2000; Seifert and Roberts, 2007). However, it has become increasingly clear that the HRGP superfamily is perhaps better represented as a spectrum of molecules ranging from the highly glycosylated AGPs to the moderately glycosylated EXTs and finally to the lightly glycosylated PRPs. Moreover, hybrid HRGPs, composed of HRGP modules from different families, and chimeric HRGPs, composed of one or more HRGP modules within a non-HRGP protein, also can be considered part of the HRGP superfamily. Given that many HRGPs are composed of repetitive protein sequences, particularly the EXTs and PRPs, and many have low sequence similarity to one another, particularly the AGPs, BLAST searches typically identify only a few closely related family members and do not represent a particularly effective means to identify members of the HRGP superfamily in a comprehensive manner.Building upon the work of Schultz et al. (2002) that focused on the AGP family, a new bioinformatics software program, BIO OHIO, developed at Ohio University, makes it possible to search all 28,952 proteins encoded by the Arabidopsis genome and identify putative HRGP genes. Two distinct types of searches are possible with this program. First, the program can search for biased amino acid compositions in the genome-encoded protein sequences. For example, classical AGPs can be identified by their biased amino acid compositions of greater then 50% Pro (P), Ala (A), Ser (S), and Thr (T), as indicated by greater than 50% PAST. Similarly, arabinogalactan peptides (AG peptides) are identified by biased amino acid compositions of greater then 35% PAST, but the protein (i.e. peptide) must also be between 50 and 90 amino acids in length. Likewise, PRPs can be identified by a biased amino acid composition of greater then 45% PVKCYT. Second, the program can search for specific amino acid motifs that are commonly found in known HRGPs. For example, SP₄ pentapeptide and SP₃ tetrapeptide motifs are associated with EXTs, a fasciclin H1 motif is found in fasciclin-like AGPs (FLAs), and PPVX(K/T) (where X is any amino acid) and KKPCPP motifs are found in several known PRPs (Fowler et al., 1999). In addition to searching for HRGPs, the program can analyze proteins identified by a search. For example, the program checks for potential signal peptide sequences and glycosylphosphatidylinositol (GPI) plasma member anchor addition sequences, both of which are associated with HRGPs (Showalter, 1993, 2001; Youl et al., 1998; Sherrier et al., 1999; Svetek et al., 1999). Moreover, the program can identify repeated amino acid sequences within the sequence and has the ability to search for bias amino acid compositions within a sliding window of user-defined size, making it possible to identify HRGP domains within a protein sequence.Here, we report on the use of this bioinformatics program in identifying, classifying, and analyzing members of the HRGP superfamily (i.e. AGPs, EXTs, PRPs, hybrid HRGPs, and chimeric HRGPs) in the genetic model plant Arabidopsis. An overview of this bioinformatics approach is presented in Figure 1. In addition, public databases and programs were accessed and utilized to extract relevant biological information on these HRGPs in terms of their expression patterns, most similar sequences via BLAST analysis, available genetic mutants, and coexpressed HRGP, glycosyl transferase (GT), prolyl 4-hydroxylase (P4H), and peroxidase genes in Arabidopsis. This information provides new insight to the HRGP superfamily and can be used by researchers to facilitate and guide further research in the field. Moreover, the bioinformatics tools developed here can be readily applied to protein sequences from other species to analyze their HRGPs or, for that matter, any given protein family by altering the input parameters.Open in a separate windowFigure 1.Bioinformatics workflow diagram summarizing the identification, classification, and analysis of HRGPs (AGPs, EXTs, and PRPs) in Arabidopsis. Classical AGPs were defined as containing greater than 50% PAST coupled with the presence of AP, PA, SP, and TP repeats distributed throughout the protein, Lys-rich AGPs were a subgroup of classical AGPs that included a Lys-rich domain, and chimeric AGPs were defined as containing greater than 50% PAST coupled with the localized distribution of AP, PA, SP, and TP repeats. AG peptides were defined to be 50 to 90 amino acids in length and containing greater than 35% PAST coupled with the presence of AP, PA, SP, and TP repeats distributed throughout the peptide. FLAs were defined as having a fasciclin domain coupled with the localized distribution of AP, PA, SP, and TP repeats. Extensins were defined as containing two or more SP₃ or SP₄ repeats coupled with the distribution of such repeats throughout the protein; chimeric extensins were similarly identified but were distinguished from the extensins by the localized distribution of such repeats in the protein; and short extensins were defined to be less than 200 amino acids in length coupled with the extensin definition. PRPs were identified as containing greater than 45% PVKCYT or two or more KKPCPP or PVX(K/T) repeats coupled with the distribution of such repeats and/or PPV throughout the protein. Chimeric PRPs were similarly identified but were distinguished from PRPs by the localized distribution of such repeats in the protein. Hybrid HRGPs (i.e. AGP/EXT hybrids) were defined as containing two or more repeat units used to identify AGPs, extensins, or PRPs. The presence of a signal peptide was used to provide added support for the identification of an HRGP but was not used in an absolute fashion. Similarly, the presence of a GPI anchor addition sequence was used to provide added support for the identification of classical AGPs and AG peptides, which are known to contain such sequences. BLAST searches were also used to provide some support to our classification if the query sequence showed similarity to other members of an HRGP subfamily. Note that some AGPs, particularly chimeric AGPs, and PRPs were identified from an Arabidopsis database annotation search and that two chimeric extensins were identified from the primary literature as noted in the text.     

Putative fasciclin-like arabinogalactan-proteins (FLA) in wheat (Triticum aestivum) and rice (Oryza sativa): identification and bioinformatic analyses

Putative plant adhesion molecules include arabinogalactan-proteins having fasciclin-like domains. In animal, fasciclin proteins participate in cell adhesion and communication. However, the molecular basis of interactions in plants is still unknown and none of these domains have been characterized in cereals. This work reports the characterization of 34 wheat (Triticum aestivum) and 24 rice (Oryza sativa) Fasciclin-Like Arabinogalactan-proteins (FLAs). Bioinformatics analyses show that cereal FLAs share structural characteristics with known Arabidopsis FLAs including arabinogalactan-protein and fasciclin conserved domains. At least 70% of the wheat and rice FLAs are predicted to be glycosylphosphatidylinositol-anchored to the plasma membranes. Expression analyses determined from the relative abundance of ESTs in the publicly available wheat EST databases and from RNA gel blots indicate that most of these genes are weakly expressed and found mainly in seeds and roots. Furthermore, most wheat genes were down regulated by abiotic stresses except for TaFLA9 and 12 where cold treatment induces their expression in roots. Plant fasciclin-like domains were predicted to have 3-D homology with FAS1 domain of the fasciclin I insect neural cell adhesion molecule with an estimated precision above 70%. The structural analysis shows that negatively charged amino acids are concentrated along the β1-α3-α4-β2 edges, while the positively charged amino acids are concentrated on the back side of the folds. This highly charged surface distribution could provide a way of mediating protein–protein interactions via electrostatic forces similar to many other adhesion molecules. The identification of wheat FLAs will facilitate studying their function in plant growth and development and their role in stress response.Nucleotide sequence data reported are available in the DBJ/EMBL/GenBank databases under the accession numbers: TaFLA1, DQ872374; TaFLA2, DQ872375; TaFLA3, DQ872376; TaFLA4, DQ872377; TaFLA5, DQ872378; TaFLA6, DQ872379; TaFLA7, DQ872380; TaFLA8, DQ872381; TaFLA9, DQ872382; TaFLA10, DQ872383; TaFLA11, DQ872384; TaFLA12, DQ872385; TaFLA13, DQ872386; TaFLA14, DQ872387; TaFLA15, DQ872388; TaFLA16, DQ872389; TaFLA17, DQ872390; TaFLA18, DQ872391; TaFLA19, DQ872392; TaFLA20, DQ872393; TaFLA21, DQ872394; TaFLA22, DQ872395; TaFLA23, DQ872396; TaFLA24, DQ872397; TaFLA25, DQ872398; TaFL26, DQ872399; TaFLA27, DQ872400; TaFLA28, DQ872401; TaFLA29, DQ872402; TaFLA30, DQ872403; TaFLA31, DQ872404; TaAGP1, DQ872405; TaFLA33, DQ872406; TaFLA34, DQ872407. If requested the database will withhold release of data until publication.An erratum to this article can be found at     

Arabinogalactan proteins are essential in somatic embryogenesis of Daucus carota L.

Daucus carota L. cell lines secrete a characteristic set of arabinogalactan proteins (AGPs) into the medium. The composition of this set of AGPs changes with the age of the culture, as can be determined by crossed electrophoresis with the specific AGP-binding agent, β-glucosyl Yariv reagent. Addition of AGPs isolated from the medium of a non-embryogenic cell line to an expiant culture initiated the development of the culture to a non-embryogenic cell line. Without addition of AGPs or with addition of carrot-seed AGPs an embryogenic cell line was established. Three-month-old embryogenic cell lines usually contain less than 30% of dense, highly cytoplasmic cells, i.e. the embryogenic cells, but when carrot-seed AGPs were added this percentage increased to 80%. Addition of carrot-seed AGPs to a two-year-old, non-embryogenic cell line resulted in the re-induction of embryogenic potential. These results show that specific AGPs are essential in somatic embryogenesis and are able to direct development of cells.     

Using genomic resources to guide research directions. The arabinogalactan protein gene family as a test case

Arabinogalactan proteins (AGPs) are extracellular hydroxyproline-rich proteoglycans implicated in plant growth and development. The protein backbones of AGPs are rich in proline/hydroxyproline, serine, alanine, and threonine. Most family members have less than 40% similarity; therefore, finding family members using Basic Local Alignment Search Tool searches is difficult. As part of our systematic analysis of AGP function in Arabidopsis, we wanted to make sure that we had identified most of the members of the gene family. We used the biased amino acid composition of AGPs to identify AGPs and arabinogalactan (AG) peptides in the Arabidopsis genome. Different criteria were used to identify the fasciclin-like AGPs. In total, we have identified 13 classical AGPs, 10 AG-peptides, three basic AGPs that include a short lysine-rich region, and 21 fasciclin-like AGPs. To streamline the analysis of genomic resources to assist in the planning of targeted experimental approaches, we have adopted a flow chart to maximize the information that can be obtained about each gene. One of the key steps is the reformatting of the Arabidopsis Functional Genomics Consortium microarray data. This customized software program makes it possible to view the ratio data for all Arabidopsis Functional Genomics Consortium experiments and as many genes as desired in a single spreadsheet. The results for reciprocal experiments are grouped to simplify analysis and candidate AGPs involved in development or biotic and abiotic stress responses are readily identified. The microarray data support the suggestion that different AGPs have different functions.     

The Arabidopsis SOS5 locus encodes a putative cell surface adhesion protein and is required for normal cell expansion

Cell surface proteoglycans have been implicated in many aspects of plant growth and development, but genetic evidence supporting their function has been lacking. Here, we report that the Salt Overly Sensitive5 (SOS5) gene encodes a putative cell surface adhesion protein and is required for normal cell expansion. The sos5 mutant was isolated in a screen for Arabidopsis salt-hypersensitive mutants. Under salt stress, the root tips of sos5 mutant plants swell and root growth is arrested. The root-swelling phenotype is caused by abnormal expansion of epidermal, cortical, and endodermal cells. The SOS5 gene was isolated through map-based cloning. The predicted SOS5 protein contains an N-terminal signal sequence for plasma membrane localization, two arabinogalactan protein-like domains, two fasciclin-like domains, and a C-terminal glycosylphosphatidylinositol lipid anchor signal sequence. The presence of fasciclin-like domains, which typically are found in animal cell adhesion proteins, suggests a role for SOS5 in cell-to-cell adhesion in plants. The SOS5 protein was present at the outer surface of the plasma membrane. The cell walls are thinner in the sos5 mutant, and those between neighboring epidermal and cortical cells in sos5 roots appear less organized. SOS5 is expressed ubiquitously in all plant organs and tissues, including guard cells in the leaf.     

Arabinogalactan proteins are required for apical cell extension in the moss Physcomitrella patens

Cell biological, structural, and genetic approaches have demonstrated the presence of arabinogalactan proteins (AGPs) in the moss Physcomitrella patens and provided evidence for their function in cell expansion and specifically in the extension of apical tip-growing cells. Inhibitor studies indicated that apical cell expansion in P. patens is blocked by synthetic AGP binding beta-glucosyl Yariv reagent (betaGlcYR). The anti-(1-->5)-alpha-L-arabinan monoclonal antibody LM6 binds to some AGPs in P. patens, to all plasma membranes, and to the cell wall surface at the most apical region of growing protonemal filaments. Moreover, LM6 labeling of cell walls at the tips of apical cells of P. patens was abolished in the presence of betaGlcYR, suggesting that the localized movement of AGPs from the plasma membrane to the cell wall is a component of the mechanism of tip growth. Biochemical and bioinformatic analyses were used to identify seven P. patens ESTs encoding putative AGP core proteins from homology with Arabidopsis thaliana, Brassica napus, and Oryza sativa sequences and from peptide fragments isolated from betaGlcYR-precipitated AGPs. Gene knockout by homologous recombination of one of these genes, P. patens AGP1, encoding a classical AGP core protein, resulted in reduced cell lengths in protonemal filaments, indicating a role for AGP1 in apical cell expansion in P. patens.     

Identification of motifs for cell adhesion within the repeated domains of transforming growth factor-beta-induced gene, betaig-h3

betaig-h3 is a transforming growth factor-beta-inducible cell adhesion molecule that has four characteristic homologous repeated domains. We made recombinant betaig-h3 proteins, which were highly active in mediating human corneal epithelial (HCE) cell adhesion and spreading. The 2nd and the 4th repeated domains were sufficient to mediate HCE cell adhesion. A sequence analysis showed that aspartic acid (Asp) and isoleucine (Ile) of the 2nd and the 4th domains are highly conserved in many fasciclin 1 homologous (fas-1) domains. Substitution mutational study identified these two amino acids are essential for cell adhesion. Synthetic peptides containing Asp and Ile, NKDIL and EPDIM derived from the 2nd and the 4th domains, respectively, almost completely blocked cell adhesion mediated by not only wild type betaig-h3 but also each of the 2nd and the 4th domains. These peptides alone were fully active in mediating cell adhesion. In addition, we demonstrated the functional receptor for betaig-h3 is alpha(3)beta(1) integrin. These results, therefore, establish the essential motifs within the 2nd and the 4th domains of betaig-h3, which interact with alpha(3)beta(1) integrin to mediate HCE cell adhesion to betaig-h3 and suggest that other proteins containing Asp-Ile in their fas-1 domains could possibly function as cell adhesion molecules.     

Identification of three potent hydroxyproline O‐galactosyltransferases in Arabidopsis

Arabinogalactan proteins (AGPs) are plant‐specific extracellular glycoproteins implicated in a variety of processes during growth and development. AGP biosynthesis involves O‐galactosylation of hydroxyproline (Hyp) residues followed by a stepwise elongation of the complex sugar chains. However, functionally dominant Hyp O‐galactosyltransferases, such that their disruption produces phenocopies of AGP‐deficient mutants, remain to be identified. Here, we purified and identified three potent Hyp O‐galactosyltransferases, HPGT1, HPGT2 and HPGT3, from Arabidopsis microsomal fractions. Loss‐of‐function analysis indicated that approximately 90% of the endogenous Hyp O‐galactosylation activity is attributable to these three enzymes. AGP14 expressed in the triple mutant migrated much faster on SDS‐PAGE than when expressed in wild‐type, confirming a considerable decrease in levels of glycosylation of AGPs in the mutant. Loss‐of‐function mutant plants exhibited a pleiotropic phenotype of longer lateral roots, longer root hairs, radial expansion of the cells in the root tip, small leaves, shorter inflorescence stems, reduced fertility and shorter siliques. Our findings provide genetic evidence that Hyp‐linked arabinogalactan polysaccharide chains are critical for AGP function and clues to how arabinogalactan moieties of AGPs contribute to cell‐to‐cell communication during plant growth and development.     

Cell surface arabinogalactan-proteins and their relation to cell proliferation and viability

Summary We have used high-pressure freezing followed by freeze substitution (HPF/FS) to preserve in vivo grown lily pollen tubes isolated from the style. The results indicated that HPF/FS (i) allows excellent preservation of the pollen tubes, (ii) maintains in situ the stylar matrix secreted by the transmitting tract cells, and (iii) preserves the interactions that exist between pollen tubes. Particular attention has been given to the structure of the pollen tube cell wall and the zone of adhesion. The cell wall is composed of an outer fibrillar layer and an inner layer of material similar in texture and nature to the stylar matrix and that is not callose. The stylar matrix labels strongly for arabinogalactan proteins (AGPs) recognized by monoclonal antibody JIM13. The zone of adhesion between pollen tubes contains distinct matrix components that are not recognized by JIM13, and apparent cross-links between the two cell walls. This study indicates that HPF/FS can be used successfully to preserve in vivo grown pollen tubes for ultrastructural investigations as well as characterization of the interactions between pollen tubes and the stylar matrix.Abbreviations AGPs arabinogalactan proteins - FS freeze substitution - HPF high-pressure freezing