Some plant cell wall polysaccharides contain small amounts of ester-linked hydroxycin — namic acid derivatives such as p-courmaric and ferulic acid (291). These ester-linked acid derivatives can undergo oxidative coupling to form dehydrodimers that may lead to cell wall polysaccharide cross-linking (291, 292) and may also be involved in the formation of polysaccharide-lignin complexes (293, 294). The ester-linked hydroxycinnamic acids are more abundant in the walls of the monocotyledonous group known as the commelinids, including the grass family (295) and such walls are particularly rich in hydroxycinnamic acids linked to the hemicellulose arabinoxylan (291), and to a lesser extent the hemicellu — lose xyloglucan. However, ester-linked hydroxycinnamic acids have also been shown to be linked to pectins in plants such as spinach and sugar beet (291,296-298). The types of link­ages associated with pectin include the following feruloylated arabinan and (1^4)-linked D-galactosyl oligosaccharides that presumably originate from side chains from RG-I: O — (6-O-frans-feruloyl)-p-D-galactopyranosyl-(1^4)-D-galactose, O-(2-O-trans-feruloyl)- a-L-arabinofuranosyl-(1^5)-L-arabinose, and O-a-L-arabinofuranosyl-(1^3)-O-(2- O-trans-feruloyl-a-L-arabinofuranosyl)-(1^5)-L-arabinose [see (291) and references therein] and O-[5-O-(feruloyl)-Ara]-(1^5)-[2-O-(feruloyl)-Ara]-(1^5)-Ara (299).

The mode ofsynthesis offeruloylated hemicellulose and pectin has received some, albeit, limited study. There is evidence that hemicellulosic arabinoxylan can be feruloylated by both feruloyl-glucose and feruloyl-CoA precursors/substrates, although the precise role of these substrates in the synthesis of feruloylated arabinoxylan within the cell (likely in the Golgi) or in the cell wall remains unclear (300, 301). A Golgi or sub-Golgi fraction from parsley suspension-cultured cells was able to transfer ferulic acid from feruloyl-CoA onto endogenous polysaccharide acceptors. However, since the identity of the polysaccharide(s) that was feruloylated was not determined (302), it is not clear whether the enzyme activity identified was involved in pectin or hemicellulose feruloylation.

Recently, Mitchell and coworkers (303), using a bioinformatics approach to identify genes highly expressed in cereals during the late stages of arabinoxylan synthesis, identified cereal Pfam family PF02458 genes, members of the CoA-acyl transferase superfamily, as candidate feruloyltransferases. However, enzymatic confirmation that these genes are actually involved in feruloylation has not been presented.