This paper deals with the enzymes controlling the extent and pattern of methylesterification in pectins within the primary walls of plant cells. It also reviews the consequences of methyl-esterification for gel formation within the cell wall and for the resistance of the wall to mechanical stress. Methyl ester groups are added to pectic galacturonans by pectin methyltransferase (PMT) enzymes during pectin synthesis. Later, within the cell wall, some methyl esters may be removed by pectin methylesterase (PME) enzymes. These enzymes activities were examined in three systems, in each of which growing or dividing cells were compared with inactive cells: suspension-cultured cells of flax, mung bean hypocotyls and poplar cambium In each system the pectins in the walls of actively growing or dividing cells were more highly methyl-esterified than those of inactive cells. Microsomal PMT activity was characterised throughout the growth cycle of suspension-cultured flax cells, The total PMT activity was maximal during the phase of rapid growth and declined when growth ceased. It was stimulated by exogenous pectins, the optimum type depending on pH. Several basic, neutral and acidic isoforms were solubilised. A number of PME isoforms were present in all three plant systems. In mung bean hypocotyls and poplar stems, neutral isoforms predominated in active cells and basic isoforms in non-growing cells. Three mung bean PMEs were characterised and the most basic one sequenced. It is suggested that as the cells pass beyond the stage of active growth, the pectins are less methyl-esterified at the point when they are exported into the wall and causing stronger bounding of the basic PME isoforms which predominate at that growth stage. The interactions of non-esterified pectic carboxyls with cations control the gelation of pectins and the mechanical properties of the gels. A new 'cable' model is presented for the structure of calcium pectate gels at the high concentrations typical of cell walls. The cable model is based on conformational analysis of galacturonans by solid-state NMR, and incorporates not only the accepted 21 helical 'egg-box' structures but also 3(1) helical and intermediate regions. Similar NMR experiments on cell walls revealed still more complex gels with methyl-esterified chain regions participating in both junction zones and inter-junction segments. Because of this structural complexity the stability of cation binding covers a wide range within and between cell walls. The chelating agents most often used to extract pectins have a high enough affinity for calcium ions to remove them completely from cell walls, although imidazole is weaker than the rest. SIMS microscopy of nax hypocotyls showed that calcium ions, bound to low-ester galacturonan segments, were concentrated in the epidermal cell walls and particularly at the tricellular junctions. Since the tricellular junctions are stressed by turgor pressure and contain only these pectins, they are a good example of in muro load-bearing by mechanically strong pectate gels. However low-ester pectins with a high affinity for calcium ions also appear to stiffen the Vigna epidermal cell wall longitudinally and may contribute to the cessation of growth as the hypocotyl matures.
