Cell walls are crucial for development, signal transduction, and disease resistance in plants. Cell walls are made of cellulose, hemicelluloses, and pectins. Xyloglucan (XG), the principal load-bearing hemicellulose of dicotyledonous plants, has a terminal fucosyl residue. A 60-kilodalton fucosyltransferase (FTase) that adds this residue was purified from pea epicotyls. Peptide sequence information from the pea FTase allowed the cloning of a homologous gene, AtFT1, from Arabidopsis. Antibodies raised against recombinant AtFTase immunoprecipitate FTase enzyme activity from solubilized Arabidopsis membrane proteins, and AtFT1 expressed in mammalian COS cells results in the presence of XG FTase activity in these cells.

Nucleotide sugars are the universal sugar donors for the formation of polysaccharides, glycoproteins, proteoglycans, glycolipids, and glycosylated secondary metabolites. At least 100 genes encode proteins involved in the formation of nucleotide sugars. These nucleotide sugars are formed using the carbohydrate derived from photosynthesis, the sugar generated by hydrolyzing translocated sucrose, the sugars released from storage carbohydrates, the salvage of sugars from glycoproteins and glycolipids, the recycling of sugars released during primary and secondary cell wall restructuring, and the sugar generated during plant-microbe interactions. Here we emphasize the importance of the salvage of sugars released from glycans for the formation of nucleotide sugars. We also outline how recent studies combining biochemical, genetic, molecular and cellular approaches have led to an increased appreciation of the role nucleotide sugars in all aspects of plant growth and development. Nevertheless, our understanding of these pathways at the single cell level is far from complete.

UDP-xylose (Xyl) is an important sugar donor for the synthesis of glycoproteins, polysaccharides, various metabolites, and oligosaccharides in animals, plants, fungi, and bacteria. UDP-Xyl also feedback inhibits upstream enzymes (UDP-glucose [Glc] dehydrogenase, UDP-Glc pyrophosphorylase, and UDP-GlcA decarboxylase) and is involved in its own synthesis and the synthesis of UDP-arabinose. In plants, biosynthesis of UDP-Xyl is catalyzed by different membrane-bound and soluble UDP-GlcA decarboxylase (UDP-GlcA-DC) isozymes, all of which convert UDP-GlcA to UDP-Xyl. Because synthesis of UDP-Xyl occurs both in the cytosol and in membranes, it is not known which source of UDP-Xyl the different Golgi-localized xylosyltransferases are utilizing. Here, we describe the identification of several distinct Arabidopsis genes (named AtUXS for UDP-Xyl synthase) that encode functional UDP-GlcA-DC isoforms. The Arabidopsis genome contains fiveUXS genes and their protein products can be subdivided into three isozyme classes (A–C), one soluble and two distinct putative membrane bound. AtUxs from each class, when expressed inEscherichia coli, generate active UDP-GlcA-DC that converts UDP-GlcA to UDP-Xyl. Members of this gene family have a large conserved C-terminal catalytic domain (approximately 300 amino acids long) and an N-terminal variable domain differing in sequence and size (30–120 amino acids long). Isoforms of class A and B appear to encode putative type II membrane proteins with their catalytic domains facing the lumen (like Golgi-glycosyltransferases) and their N-terminal variable domain facing the cytosol. Uxs class C is likely a cytosolic isoform. The characteristics of the plant Uxs support the hypothesis that unique UDP-GlcA-DCs with distinct subcellular localizations are required for specific xylosylation events.

In search of cell death-inducing proteins, we found a xyloglucanase (BcXYG1) that induced strong necrosis and a resistance response in dicot plants. Expression of the gene was strongly induced during the first 12 h post inoculation, and analysis of disease dynamics using PathTrack showed that a strain overexpressing produced early local necrosis, supporting a role of BcXYG1 as an early cell death-inducing factor. The xyloglucanase activity of BcXYG1 was not necessary for the induction of necrosis and plant resistance, as a mutant of BcXYG1 lacking the xyloglucanase enzymatic activity retained both functions. Residues in two exposed loops on the surface of BcXYG1 were found to be necessary for the induction of cell death but not to induce plant resistance. Further analyses showed that BcXYG1 is apoplastic and possibly interacts with the proteins of the plant cell membrane and also that the BcXYG1 cell death-promoting signal is mediated by the leucine-rich repeat receptor-like kinases BAK1 and SOBIR1. Our findings support the role of cell death-inducing proteins in establishing the infection of necrotrophic pathogens and highlight the recognition of fungal apoplastic proteins by the plant immune system as an important mechanism of resistance against this class of pathogens.

). † Authors that contributed equally to this work. SummarySpecies of the genus Citrus accumulate large quantities of flavanones that affect fruit flavor and have been documented to benefit human health. Bitter species, such as grapefruit and pummelo, accumulate bitter flavanone-7-O-neohesperidosides responsible, in part, for their characteristic taste. Non-bitter species, such as mandarin and orange, accumulate only tasteless flavanone-7-O-rutinosides. The key flavor-determining step of citrus flavanone-glycoside biosynthesis is catalyzed by rhamnosyltransferases; 1,2 rhamnosyltransferases (1,2RhaT) catalyze biosynthesis of the bitter neohesperidosides, while 1,6 rhamnosyltransferases (1,6RhaT) catalyze biosynthesis of the tasteless rutinosides. We report on the isolation and functional characterization of the gene Cm1,2RhaT from pummelo which encodes a citrus 1,2RhaT. Functional analysis of Cm1,2RhaT recombinant enzyme was conducted by biotransformation of the substrates using transgenic plant cell culture. Flavanones and flavones, but not flavonols, were biotransformed into 7-O-neohesperidosides by the transgenic BY2 tobacco cells expressing recombinant Cm1,2RhaT. Immunoblot analysis established that 1,2RhaT protein was expressed only in the bitter citrus species and that 1,6RhaT enzyme, whose activity was previously documented in non-bitter species, was not cross-reactive. Expression of Cm1,2RhaT at the RNA level was prominent in young fruit and leaves, but low in the corresponding mature tissue, thus correlating well with the developmental pattern of accumulation of flavanone-neohesperidosides previously established. Phylogenetic analysis of the flavonoid glycosyltransferase gene family places Cm1,2RhaT on a separate gene cluster together with the only other functionally characterized flavonoid-glucoside rhamnosyltransferase gene, suggesting a common evolutionary origin for rhamnosyltransferases specializing in glycosylation of the sugar moieties of flavonoid glucosides.