The mechanism by which events in the angiosperm cell wall are communicated to the cytoplasm is not well characterized. A family of five Arabidopsis wall-associated kinases (WAKs) have the potential to provide a physical and signaling continuum between the cell wall and the cytoplasm. The WAKs have an active cytoplasmic protein kinase domain, span the plasma membrane, and contain an N terminus that binds the cell wall. We show here that WAK s are expressed at organ junctions, in shoot and root apical meristems, in expanding leaves, and in response to wall disturbances. Leaves expressing an antisense WAK gene have reduced WAK protein levels and exhibit a loss of cell expansion. WAKs are covalently bound to pectin in the cell wall, providing evidence that the binding of a structural carbohydrate by a receptor-like kinase may have significance in the control of cell expansion. INTRODUCTIONIn animal, fungal, and algal systems, the physical connection and the communication between the extracellular matrix (ECM) and the cell plays a fundamental role in cell growth and division (Fowler and Quatrano, 1997;Lukashev and Werb, 1998;Tsai, 1998). Similarly, the plant cell wall forms an ECM of carbohydrate and protein that provides structure for individual cells and whole organs. The cell wall must be dynamic as cells divide and elongate, and modulation of its composition and architecture is required during its synthesis and after it has been deposited (Cosgrove, 1997;Reiter, 1998). The wall must therefore be considered in the context of modulating plant development (Kohorn, 2000). Communication between the cytoplasm and the cell wall is necessary and evident because events like cell expansion (Cosgrove, 1997) and pathogen infection (Hammond-Kosack and Jones, 1996) lead to altered biosynthesis and modification of cell wall components and downstream cytoplasmic events such as systemic acquired resistance. How the dynamics and synthesis of the cell wall are coordinated with cytoplasmic events is largely uncharacterized.Developing cells have walls that are composed of cellulose, hemicellulose, pectin, and proteins. Cellulose is directly secreted by cellulose synthase into the ECM, where it assembles with hemicelluloses and pectins, which are produced in the endomembrane system and secreted by vesicles. The cell wall also includes endoglucanases (Hayashi et al., 1984; Zuo et al., 2000), xyloglucan endotransglycosylases (Fry et al., 1992;Vissenberg et al., 2000), expansins (McQueen-Mason et al., 1992; Cho and Cosgrove, 2000), and a number of other glycosyl transferases that alter carbohydrate linkages and modify secreted cell wall components. Other cell wall proteins, some of which are heavily glycosylated, have been proposed as structural cell wall components or have been implicated in mediating multiple aspects of plant development (reviewed in: Showalter, 1993; Cosgrove, 1997;Kohorn, 2000). These include the families of proline-rich proteins, glycine-rich proteins, hydroxyprolinerich glycoproteins, and arabinogalactan proteins...
The mechanism by which events in the angiosperm cell wall are communicated to the cytoplasm is not well characterized. A family of five Arabidopsis wall-associated kinases (WAKs) have the potential to provide a physical and signaling continuum between the cell wall and the cytoplasm. The WAKs have an active cytoplasmic protein kinase domain, span the plasma membrane, and contain an N terminus that binds the cell wall. We show here that WAK s are expressed at organ junctions, in shoot and root apical meristems, in expanding leaves, and in response to wall disturbances. Leaves expressing an antisense WAK gene have reduced WAK protein levels and exhibit a loss of cell expansion. WAKs are covalently bound to pectin in the cell wall, providing evidence that the binding of a structural carbohydrate by a receptor-like kinase may have significance in the control of cell expansion. INTRODUCTIONIn animal, fungal, and algal systems, the physical connection and the communication between the extracellular matrix (ECM) and the cell plays a fundamental role in cell growth and division (Fowler and Quatrano, 1997;Lukashev and Werb, 1998;Tsai, 1998). Similarly, the plant cell wall forms an ECM of carbohydrate and protein that provides structure for individual cells and whole organs. The cell wall must be dynamic as cells divide and elongate, and modulation of its composition and architecture is required during its synthesis and after it has been deposited (Cosgrove, 1997;Reiter, 1998). The wall must therefore be considered in the context of modulating plant development (Kohorn, 2000). Communication between the cytoplasm and the cell wall is necessary and evident because events like cell expansion (Cosgrove, 1997) and pathogen infection (Hammond-Kosack and Jones, 1996) lead to altered biosynthesis and modification of cell wall components and downstream cytoplasmic events such as systemic acquired resistance. How the dynamics and synthesis of the cell wall are coordinated with cytoplasmic events is largely uncharacterized.Developing cells have walls that are composed of cellulose, hemicellulose, pectin, and proteins. Cellulose is directly secreted by cellulose synthase into the ECM, where it assembles with hemicelluloses and pectins, which are produced in the endomembrane system and secreted by vesicles. The cell wall also includes endoglucanases (Hayashi et al., 1984; Zuo et al., 2000), xyloglucan endotransglycosylases (Fry et al., 1992;Vissenberg et al., 2000), expansins (McQueen-Mason et al., 1992; Cho and Cosgrove, 2000), and a number of other glycosyl transferases that alter carbohydrate linkages and modify secreted cell wall components. Other cell wall proteins, some of which are heavily glycosylated, have been proposed as structural cell wall components or have been implicated in mediating multiple aspects of plant development (reviewed in: Showalter, 1993; Cosgrove, 1997;Kohorn, 2000). These include the families of proline-rich proteins, glycine-rich proteins, hydroxyprolinerich glycoproteins, and arabinogalactan proteins...
A highly virulent cotton wilt pathogen, Fusarium oxysporum f. sp. vasinfectum VCG0114 (race 4) was found in West Texas in 2017, after being known in California since 2001. Isolates obtained from wilted plants collected in 2017 from Texas, in 2015 from China, and during 2001 to 2014 from California and isolates from historical collections including the race 4 reference isolate were characterized by soil-infestation pathogenicity assays, DNA sequence analysis, and vegetative compatibility analysis. All obtained F. oxysporum f. sp. vasinfectum isolates belonged to VCG0114. All of these isolates, except one isolate from China, caused disease in a soil-infestation assay without nematodes. Thus, they belong to the nematode-independent pathotype. Texas isolates were significantly more virulent than were isolates from China or California on Gossypium barbadense ‘Pima S-7’. Four different genotypes (N, T, MT, and MiT) were identified based on the transposable element Tfo1 insertion into the PHO gene and independent MULE or MITE insertions into the Tfo1 transposon. Some significant differences in virulence were detected among the genotypes in some locations. No differences in pathogenicity were observed between the California and China collection isolates on Pima S-7, and the virulence of the major genotypes was similar on the Gossypium hirsutum cultivar ‘Stoneville 474’ or the Barbren 713 germplasm line. Simple polymerase chain reaction (PCR) methods were developed to specifically determine and detect the four genotypes within VCG0114. A specific PCR method to detect all VCG0114 isolates was also developed. These methods will facilitate the timely identification of infested fields and seed lots and the elucidation of evolutionary relationships among the isolates. This should help to closely monitor the movement of the pathogen and reduce dissemination of these devastating pathogens.
Plant type II arabinogalactan (AG) polysaccharides are attached to arabinogalactan proteins (AGPs) at hydroxyproline residues, and they are very diverse and heterogeneous structures. The AG consists of a β-(1→3)-linked galactan backbone with β-(1→6)-galactan side chains that are modified mainly with arabinose, but they may also contain glucuronic acid, rhamnose or other sugars. Here, we studied the positions of fucose substitutions in AGPs, and we investigated the functions of this fucosylation. Monosaccharide analysis of Arabidopsis leaf AGP extracts revealed a significant reduction in L-Fucose content in the fut4 mutant, but not in the fut6 mutant. In addition, Fucose was reduced in the fut4 mutant in root AGP extracts and was absent in the fut4/fut6 mutant. Curiously, in all cases reduction of fucose was accompanied with a reduction in xylose levels. The fucosylated AGP structures in leaves and roots in wild type and fut mutant plants were characterised by sequential digestion with AG specific enzymes, analysis by Polysaccharide Analysis using Carbohydrate gel Electrophoresis, and Matrix Assisted Laser Desorption/Ionisation (MALDI)-Time of Flight Mass spectrometry (MS). We found that FUT4 is solely responsible for the fucosylation of AGPs in leaves. The Arabidopsis thaliana FUT4 and FUT6 genes have been previously proposed to be non-redundant AG-specific fucosyltransferases. Unexpectedly, FUT4 and FUT6 enzymes both fucosylate the same AGP structures in roots, suggesting partial redundancy to each other. Detailed structural characterisation of root AGPs with high energy MALDI-Collision Induced Dissociation MS and NMR revealed an abundant unique AG oligosaccharide structure consisting of terminal xylose attached to fucose. The loss of this structure in fut4/fut6 mutants explains the reduction of both fucose and xylose in AGP extracts. Under salt-stress growth conditions the fut4/fut6 mutant lacking AGP fucosylation exhibited a shorter root phenotype than wild type plants, implicating fucosylation of AGPs in maintaining proper cell expansion under these conditions.
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