Subcompartment localization of the side chain xyloglucan-synthesizing enzymes within Golgi stacks of tobacco suspension-cultured cells.

Xyloglucan is the dominant hemicellulosic polysaccharide of the primary cell wall of dicotyledonous plants that plays a key role in plant development. It is well established that xyloglucan is assembled within Golgi stacks and transported in Golgi-derived vesicles to the cell wall. It is also known that the biosynthesis of xyloglucan requires the action of glycosyltransferases including α-1,6-xylosyltransferase, ß-1,2-galactosyltransferase and α-1,2-fucosyltransferase activities responsible for the addition of xylose, galactose and fucose residues to the side chains. There is, however, a lack of knowledge on how these enzymes are distributed within subcompartments of Golgi stacks. We have undertaken a study aiming at mapping these glycosyltransferases within Golgi stacks using immunogold-electron microscopy. To this end, we generated transgenic lines of tobacco (Nicotiana tabacum) BY-2 suspension-cultured cells expressing either the α-1,6-xylosyltransferase, AtXT1, the ß-1,2-galactosyltransferase, AtMUR3, or the α-1,2-fucosyltransferase AtFUT1 of Arabidopsis thaliana fused to green-fluorescent protein (GFP). Localization of the fusion proteins within the endomembrane system was assessed using confocal microscopy. Additionally, tobacco cells were high pressure-frozen/freeze-substituted and subjected to quantitative immunogold labelling using anti-GFP antibodies to determine the localization patterns of the enzymes within subtypes of Golgi cisternae. The data demonstrate that: (i) all fusion proteins, AtXT1-GFP, AtMUR3-GFP and AtFUT1-GFP are specifically targeted to the Golgi apparatus; and (ii) AtXT1-GFP is mainly located in the cis and medial cisternae, AtMUR3-GFP is predominantly associated with medial cisternae and AtFUT1-GFP mostly detected over trans cisternae suggesting that initiation of xyloglucan side chains occurs in early Golgi compartments in tobacco cells.

Identification of pectin methylesterase 3 as a basic pectin methylesterase isoform involved in adventitious rooting in Arabidopsis thaliana.

• Here, we focused on the biochemical characterization of the Arabidopsis thaliana pectin methylesterase 3 gene (AtPME3; At3g14310) and its role in plant development. • A combination of biochemical, gene expression, Fourier transform-infrared (FT-IR) microspectroscopy and reverse genetics approaches were used. • We showed that AtPME3 is ubiquitously expressed in A. thaliana, particularly in vascular tissues. In cell wall-enriched fractions, only the mature part of the protein was identified, suggesting that it is processed before targeting the cell wall. In all the organs tested, PME activity was reduced in the atpme3-1 mutant compared with the wild type. This was related to the disappearance of an activity band corresponding to a pI of 9.6 revealed by a zymogram. Analysis of the cell wall composition showed that the degree of methylesterification (DM) of galacturonic acids was affected in the atpme3-1 mutant. A change in the number of adventitious roots was found in the mutant, which correlated with the expression of the gene in adventitious root primordia. • Our results enable the characterization of AtPME3 as a major basic PME isoform in A. thaliana and highlight its role in adventitious rooting.

Analysis of LuPME3, a pectin methylesterase from Linum usitatissimum, revealed a variability in PME proteolytic maturation.

Pectin methylesterase (PME) catalyses the de-methylesterification of pectin in plant cell walls during cell elongation. (1) Pectins are mainly composed of α(1, 4)-D-galacturonosyl acid units that are synthesised in a methylesterified form in the Golgi apparatus to prevent any interaction with Ca2+ ions during their intracellular transport. (2) The highly methylesterified pectins are then secreted into the apoplasm (3) and subsequently de-methylesterified in muro by PMEs. This can either induce the formation of pectin gels through the Ca2+ crosslinking of neighbouring non-methylesterified chains or create substrates for pectin-degrading enzymes such as polygalacturonases and pectate lyases for the initiation of cell wall loosening. (4) PMEs belong to a large multigene family. Sixty-six PME-related genes are predicted in the Arabidopsis genome. (1) Among them, we have recently shown that AtPME3 (At3g14310), a major basic PME isoform in A. thaliana, is ubiquitously expressed in vascular tissues and play a role in adventitious rooting. (5) In flax (Linum usitatissimum), three genes encoding PMEs have been sequenced so far, including LuPME3, the orthologue of AtPME3. Analysis of the LuPME3 isoform brings new insights into the processing of these proteins.