INTRODUCTIONPlant cell walls are amazingly complex amalgams of carbohydrates, proteins, lignin, water, and incrusting substances such as cutin, suberin, and certain inorganic compounds that vary among plant species, cell types, and even neighboring cells. Developmental events and exposure to any of a number of abiotic and biotic stresses further increase this compositional and structural variation. Moreover, the dynamic nature and functions of plant cell walls in terms of growth and development, environmental sensing and signaling, plant defense, intercellular communication, and selective exchange interfaces are reflected in these variations. Much is currently known about the structure and metabolic regulation of the various cell wall components, but relatively little is known about their precise functions and intermolecular interactions.In this review, I will discuss the accumulated structural and regulatory data and the much more limited functional and intermolecular interaction information on five plant cell wall protein classes. These five protein classes, listed in Table 1, include the extensins, the glycine-rich proteins (GRPs), the proline-rich proteins (PRPs), the solanaceous lectins, and the arabinogalactan proteins (AGPs). These five proteins may be evolutionarily related to one another, most obviously because each of them, with the exception of the GRPs, contains hydroxyproline, and less obviously in the case of the GRPs because this class has nucleotide sequence similarity to the extensins. For completeness, I should mention that these are not the only cell wall proteins that are known. Others exist, such as cysteine-rich thionins, 28-and 70-kD water-regulated proteins, a histidine-tryptophan-rich protein, and many cell wall enzymes such as peroxidases, phosphatases, invertases, a-mannosidases, pmannosidases, p1,3-glucanases, p1,4-glucanases, polygalacturonase, pectin methylesterases, malate dehydrogenase, arabinosidases, a-galactosidases, pgalactosidases, pglucuronosidases, pxylosidases, proteases, and ascorbic acid oxidase (Varner and Lin, 1989). However, the above five classes generally represent the most abundant, and to date, the most well-studied and widely documented, plant cell wall proteins.Before describing these five wall protein classes, I should point out that research on these individual proteins has occurred in severa1 plant species, but relatively few examples exist where these cell wall proteins have been studied together in one plant, let alone in one particular plant organ or type of cell. Thus, data from one plant species are often extrapolated to represent the situation in other plant species. Although such extrapolations are usually valid, enough variations are now known that caution should be exercised in making or believing such claims. EXTENSINS StructureExtensins are a family of hydroxyproline-rich glycoproteins (HRGPs) found in the cell walls of higher plants. In dicots, extensins are particularly abundant and are generally characterized by the following: they are rich in h...

Abstract. Arabinogalactan-proteins (AGPs) are a family of extensively glycosylated hydroxyproline-rich glycoproteins that are thought to have important roles in various aspects of plant growth and development. After a brief introduction to AGPs highlighting the problems associated with defining and classifying this diverse family of glycoproteins, AGP structure is described in terms of the protein component (including data from molecular cloning), carbohydrate component, processing of AGPs (including recent data on glycosylphosphatidylinositol membrane anchors) and overall molecular shape. Next, the expression of AGPs is examined at several different levels, from the whole plant to the cellular levels, using a variety of experimental techniques and tools. Finally, AGP function is considered. Although the existing functional evidence is not incontrovertible, it does clearly droxyproline poor, lightly glycosylated and largely unreactive with Yariv reagent. It is worth remembering that it is human nature to group and classify things to facilitate their comprehension and discourse, whereas Mother Nature simply constructs biological entities, including AGPs, using material at hand with blatant, pedagogical disregard. Structure Protein moietyKnowledge of the protein moieties of AGPs has mostly come from purifying AGPs, deglycosylating them and analyzing their respective core proteins by amino acid analysis and, to a more limited extent, by sequence analysis ( fig. 2) [3 -10]. More recently, molecular cloning of several confirmed and putative AGP core proteins has CMLS, Cell. Mol. Life Sci. 58 (2001) 1399 -1417 1420-682X/01/101399-19 $ 1.50 + 0.20/0 © Birkhäuser Verlag, Basel, 2001 CMLS Cellular and Molecular Life Sciences point to roles for AGPs in vegetative, reproductive, and cellular growth and development as well as programmed cell death and social control. In addition and most likely inextricably linked to their functions, AGPs are presumably involved in molecular interactions and cellular signaling at the cell surface. Some likely scenarios are discussed in this context. AGPs also have functions of real or potential commercial value, most notably as emulsifiers in the food industry and as potential immunological regulators for human health. Several important questions remain to be answered with respect to AGPs. Clearly, elucidating the unequivocal functions of particular AGPs and relating these functions to their respective structures and modes of action remain as major challenges in the years ahead.

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 SP 5 EXTs, SP 5 /SP 4 EXTs, SP 4 EXTs, SP 4 /SP 3 EXTs, a SP 3 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.

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Summary• 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 ( G sink ).• In contrast to earlier reports, we observed massive upregulation of AGPs in saltstressed 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 → G sink , 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 β -D -glycosyl Yariv reagents might inhibit expansion growth by crosslinking monomeric AGPs, and thus mimic an AGP loss-of-function mutation.

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SummaryArabinogalactan proteins (AGPs) are a family of highly glycosylated, hydroxyproline-rich glycoproteins implicated in various aspects of plant growth and development. (b-D-glucosyl) 3 and (b-D-galactosyl) 3 Yariv phenylglycosides, commonly known as Yariv reagents, speci®cally bind AGPs in a non-covalent manner. Here (b-D-galactosyl) 3 Yariv reagent was added to Arabidopsis thaliana cell suspension cultures and determined to induce programmed cell death (PCD) by three criteria: (i) DNA fragmentation as detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) of DNA 3¢-OH groups; (ii) internucleosomal DNA fragmentation as visualized by genomic Southern blotting; and (iii) structural changes characteristic of PCD including cytoplasmic shrinkage and condensation, chromatin condensation and nuclear membrane blebbing. These ®ndings implicate AGP involvement in PCD in plants, presumably by perturbation of AGPs located at the plasma membrane± cell wall interface.

Arabinogalactan-proteins (AGPs) are complex glycoconjugates that are commonly found at the cell surface and in secretions of plants. Their location and diversity of structures have made them attractive targets as modulators of plant development but definitive proof of their direct role(s) in biological processes remains elusive. Here we overview the current state of knowledge on AGPs, identify key challenges impeding progress in the field and propose approaches using modern bioinformatic, (bio)chemical, cell biological, molecular and genetic techniques that could be applied to redress these gaps in our knowledge.