It is not known whether RG-II is synthesized on preexisting HG that is synthesized by GAUT1 or related GalATs, or whether a unique GalAT is responsible for synthesizing the HG backbone of RG-II. It is also not know if the side chains of RG-II are synthe­sized as individual oligosaccharides and transferred in bulk onto HG, or whether in­dividual glycosyltransferases transfer each distinct glycosyl residue individually onto the growing non-reducing end of the maturing RG-II molecule. The results of Egelund et al. (274) on the proposed RG-II:XylTs would be in agreement with the latter model (see below).

5.4.9.1 RG-II:apiosyltransferase (RG-II:ApiT)

As mentioned above it is possible that the apiosyltransferase activity identified during the studies of apiogalacturonan synthesis in Lemna is involved in RG-II synthesis. However, no gene for RG-II ApiT has been identified. Interestingly, when the gene reported to encode the UDP-apiose or UDP-D-apiose/UDP-D-xylose synthase was downregulated in Nicotiana benthamiana by virus-induced gene silencing of NbAXSl, the result was a reduction in the amount of RG-II in the walls (358). These results provide evidence that UDP-apiose is the substrate for incorporation of apiose into RG-II and that the UDP-D-apiose/UDP-D-xylose synthase gene encodes the enzyme that synthesizes the required substrate for RG-II synthesis.