Intracellular feruloylation of pectic polysaccharides

The pectic polysaccharides of spinach cell walls carry feruloyl groups on arabinose and galactose residues. The following experiments were designed to discover whether the arabinose residues are feruloylated intra-or extracellularly. Cultured spinach cells started to incorporate exogenous [(3)H]arabinose into polymers at a linear rate after a lag period of approx. 3-4 min, although radioactive polysaccharides and extensin did not start to appear outside the plasmalemma until after an approx. 25-min lag. In the same cells, polysaccharide-bound feruloyl-[(3)H]arabinose units starded to accumulate radioactivity at a linear rate after a lag period of approx. 4-5 min. Therefore, arabinose residues of polysaccharides began to be feruloylated while still intracellular. The rate of formation of polysaccharide-bound feruloyl-[(3)H]arabinose units did not appreciably increase after 25 min, showing that any additional extracellular feruloylation of the polysaccharide was relatively slow. This conclusion was supported by two different types of pulse-chase experiments, one of which was designed to detect feruloylation of polysaccharides up to 6 d after synthesis.