Water-deficit stress poses unique challenges to plant cells dependent on a hydrostatic skeleton and a polysaccharide-rich cell wall for growth and development. How the plant cell wall is adapted to loss of water is of interest in developing a general understanding of water stress tolerance in plants and of relevance in strategies related to crop improvement. Drought tolerance involves adaptations to growth under reduced water potential and the concomitant restructuring of the cell wall that allow growth processes to occur at lower water contents. Desiccation tolerance, by contrast, is the evolution of cell walls that are capable of losing the majority of cellular water without suffering permanent and irreversible damage to cell wall structure and polymer organization. This minireview highlights common features and differences between these two water-deficit responses observed in plants, emphasizing the role of the cell wall, while suggesting future research avenues that could benefit fundamental understanding in this area.

Plant roots of many species produce thousands of cells that are released daily into the rhizosphere. These cells are commonly termed border cells because of their major role in constituting a biotic boundary layer between the root surface and the soil. In this study, we investigated the occurrence and ultrastructure of such cells in Arabidopsis (Arabidopsis thaliana) using light and electron microscopy coupled to high-pressure freezing. The secretion of cell wall molecules including pectic polysaccharides and arabinogalactan-proteins (AGPs) was examined also using immunofluorescence microscopy and a set of anticarbohydrate antibodies. We show that root tips of Arabidopsis seedlings released cell layers in an organized pattern that differs from the rather randomly dispersed release observed in other plant species studied to date. Therefore, we termed such cells border-like cells (BLC). Electron microscopical results revealed that BLC are rich in mitochondria, Golgi stacks, and Golgi-derived vesicles, suggesting that these cells are actively engaged in secretion of materials to their cell walls. Immunocytochemical data demonstrated that pectins as well as AGPs are among secreted material as revealed by the high level of expression of AGPepitopes. In particular, the JIM13-AGP epitope was found exclusively associated with BLC and peripheral cells in the root cap region. In addition, we investigated the function of BLC and root cap cell AGPs in the interaction with rhizobacteria using AGP-disrupting agents and a strain of Rhizobium sp. expressing a green fluorescent protein. Our findings demonstrate that alteration of AGPs significantly inhibits the attachment of the bacteria to the surface of BLC and root tip.Many plants can produce large numbers of metabolically active root ''border'' cells that are programmed to separate from each other and to be released from the root tip periphery into the external environment (Brigham et al., 1995a;Hawes et al., 1998). Root border cells are defined as cells that are released into solution within seconds when root tips are placed into water (Hawes et al., 2000(Hawes et al., , 2003. In the absence of free water, these cells remain adherent to the root tips. The border cells of most species remain viable even after separation from the root tips (Hawes et al., 2000) and can survive independently from roots in vitro as well as in natural field conditions. Interestingly, border cells can undergo cell division in vitro and develop into callus tissue (Hawes and Lin, 1990).As to the role of the border cells, Brigham et al. (1995b) have proposed that these cells are differentiated tissue of the root system that modulates the environment of the plant root by producing specific substances to be released into the rhizosphere. Proteins synthesized in border cells exhibit profiles that are distinct, qualitatively and quantitatively, from those synthesized in the root tips (Brigham et al., 1995b). These important differences in protein expression profiles are correlated with similarly distinct...

During plant sexual reproduction, pollen germination and tube growth require development under tight spatial and temporal control for the proper delivery of the sperm cells to the ovules. Pollen tubes are fast growing tip-polarized cells able to perceive multiple guiding signals emitted by the female organ. Adhesion of pollen tubes via cell wall molecules may be part of the battery of signals. In order to study these processes, we investigated the cell wall characteristics of in vitro-grown Arabidopsis (Arabidopsis thaliana) pollen tubes using a combination of immunocytochemical and biochemical techniques. Results showed a well-defined localization of cell wall epitopes. Low esterified homogalacturonan epitopes were found mostly in the pollen tube wall back from the tip. Xyloglucan and arabinan from rhamnogalacturonan I epitopes were detected along the entire tube within the two wall layers and the outer wall layer, respectively. In contrast, highly esterified homogalacturonan and arabinogalactan protein epitopes were found associated predominantly with the tip region. Chemical analysis of the pollen tube cell wall revealed an important content of arabinosyl residues (43%) originating mostly from (1/5)-a-L-arabinan, the side chains of rhamnogalacturonan I. Finally, matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis of endo-glucanase-sensitive xyloglucan showed mass spectra with two dominant oligosaccharides (XLXG/XXLG and XXFG), both being mono O-acetylated, and accounting for over 68% of the total ion signals. These findings demonstrate that the Arabidopsis pollen tube wall has its own characteristics compared with other cell types in the Arabidopsis sporophyte. These structural features are discussed in terms of pollen tube cell wall biosynthesis and growth dynamics.

Water is a major limiting factor in growth and reproduction in plants. The ability of tissues to survive desiccation is commonly found in seeds or pollen but rarely present in vegetative tissues. Resurrection plants are remarkable as they can tolerate almost complete water loss from their vegetative tissues such as leaves and roots. Metabolism is shut down as they dehydrate and the plants become apparently lifeless. Upon rehydration these plants recover full metabolic competence and 'resurrect'. In order to cope with desiccation, resurrection plants have to overcome a number of stresses as water is lost from the cells, among them oxidative stress, destabilization or loss of membrane integrity and mechanical stress. This review will mainly focus on the effect of dehydration in angiosperm resurrection plants and some of the strategies developed by these plants to tolerate desiccation. Resurrection plants are important experimental models and understanding the physiological and molecular aspects of their desiccation tolerance is of great interest for developing drought-tolerant crop species adapted to semi-arid areas.

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A variety of Southern African resurrection plants were surveyed using high-throughput cell wall profiling tools. Species evaluated were the dicotyledons, Myrothamnus flabellifolia and Craterostigma plantagineum; the monocotyledons, Xerophyta viscosa, Xerophyta schlecterii, Xerophyta humilis and the resurrection grass Eragrostis nindensis, as well as a pteridophyte, the resurrection fern, Mohria caffrorum. Comparisons were made between hydrated and desiccated leaf and frond material, with respect to cell wall composition and polymer abundance, using monosaccharide composition analysis, FT-IR spectroscopy and comprehensive microarray polymer profiling in combination with multivariate data analysis. The data obtained suggest that three main functional strategies appear to have evolved to prepare plant cell walls for desiccation. Arabinan-rich pectin and arabinogalactan proteins are found in the resurrection fern M. caffrorum and the basal angiosperm M. flabellifolia where they appear to act as 'pectic plasticizers'. Dicotyledons with pectin-rich walls, such as C. plantagineum, seem to use inducible mechanisms which consist of up-regulating wall proteins and osmoprotectants. The hemicellulose-rich walls of the grass-like Xerophyta spp. and the resurrection grass E. nindensis were found to contain highly arabinosylated xylans and arabinogalactan proteins. These data support a general mechanism of 'plasticising' the cell walls of resurrection plants to desiccation and implicate arabinose-rich polymers (pectin-arabinans, arabinogalactan proteins and arabinoxylans) as the major contributors in ensuring flexibility is maintained and rehydration is facilitated in these plants.

Brefeldin A (BFA), a specific inhibitor of Colgi-mediated secretion in animal cells, has been used to study the organization of the secretory pathway and the function of the Colgi apparatus in plant cells. To this end, we have employed a combination of electron microscopical, immunocytochemical, and biochemical techniques to investigate the effects of this drug on the architecture of the Colgi apparatus as well as on the secretion of proteins and complex cell wall polysaccharides in sycamore maple (Acerpseudoplatanus) suspension-cultured cells. We have used 2.5 and 7.5 pg/mL of BFA, which is comparable to the 1 to 10 pg/mL used in experiments with animal cells. Electron micrographs of high-pressure frozen and freeze-substituted cells show that although BFA causes swelling of the endoplasmic reticulum cisternae, unlike in animal cells, it does not induce the disassembly of sycamore maple Colgi stacks. Instead, BFA induces the formation of large clusters of Colgi stacks, an increase in the number of trans-like Colgi cisternae, and the accumulation in the cytoplasm of very dense vesicles that appear to be derived from trans Colgi cisternae. These vesicles contain large amounts of xyloglucan (XC), the major hemicellulosic cell wall polysaccharide, as shown by immunocytochemical labeling with anti-XC antibodies. All of these structural changes disappear within 120 min after removal of the drug. In vivo labeling experiments using [3H]leucine demonstrate that protein secretion into the culture medium, but not protein synthesis, is inhibited by approximately 80% in the presence of BFA. In contrast, the incorporation of [3Hlfucose into N-linked glycoproteins, which occurs in transColgi cisternae, appears to be affected to a greater extent than the incorporation of [3H]xylose, which has been localized to media1 Colgi cisternae. BFA also affects secretion of complex polysaccharides as evidenced by the approximate 50% drop in incorporation of [3H]xylose and [3H]fucose into cell wall hemicelluloses. Taken together, these findings suggest that at concentrations of 2.5 to 7.5 pg/mL BFA causes the following major changes in the secretory pathway of sycamore maple cells: (a) it inhibits the transport of secretory proteins to the cell surface by about 80% and of hemicelluloses by about 50%; (b) it changes the patterns of glycosylation of N-linked glycoproteins and hemicelluloses; (c) it reduces traffic

Virtually nothing is known about the mechanisms and enzymes responsible for the glycosylation of arabinogalactan proteins (AGPs). The glycosyltransferase 37 family contains plant-specific enzymes, which suggests involvement in plantspecific organs such as the cell wall. Our working hypothesis is that AtFUT4 and AtFUT6 genes encode ␣(1,2)fucosyltransferases (FUTs) for AGPs. Multiple lines of evidence support this hypothesis. First, overexpression of the two genes in tobacco BY2 cells, known to contain nonfucosylated AGPs, resulted in a staining of transgenic cells with eel lectin, which specifically binds to terminal ␣-linked fucose. Second, monosaccharide analysis by high pH anion exchange chromatography and electrospray ionization mass spectrometry indicated the presence of fucose in AGPs from transgenic cell lines but not in AGPs from wild type cells. Third, detergent extracts from microsomal membranes prepared from transgenic lines were able to fucosylate, in vitro, purified AGPs from BY2 wild type cells. Susceptibility of [ 14 C]fucosylated AGPs to ␣(1,2)fucosidase, and not to ␣(1,3/4)fucosidase, indicated that an ␣(1,2) linkage is formed. Furthermore, dearabinosylated AGPs were not substrate acceptors for these enzymes, indicating that arabinosyl residues represent the fucosylation sites on these molecules. Testing of several polysaccharides, oligosaccharides, and glycoproteins as potential substrate acceptors in the fucosyl transfer reactions indicated that the two enzymes are specific for AGPs but are not functionally redundant because they differentially fucosylate certain AGPs. AtFUT4 and AtFUT6 are the first enzymes to be characterized for AGP glycosylation and further our understanding of cell wall biosynthesis.