HG may contain regions that are substituted with (3 — D-xylose linked to C-3 of GalA (185,186, 193, 267). Such regions of xylosylated HG are referred to as xylogalacturonan (XGA) and have been most frequently identified in reproductive tissues of plants including apple (184, 193, 197), cotton and watermelon (185), and pea (342), but also in carrot (186). However, xylogalactruonan has also been detected in Arabidopsis leaves and stems (187), albeit it in lower levels than in reproductive and storage tissues such as apple and potato. No gene for XGA:xylosyltransferase (XGA:XylT) has been unambiguously identified. XGA:XylT activity was identified in studies of apiogalacturonan synthesis (341, 342). Although the product produced was not characterized in detail, at least some of the radioactive xylose appeared to be incorporated into apiogalacturonan and/or HG.

Interestingly, Nakamura and coworkers (263) has reported that in soybean some XGA maybe further elongated with p-1,4-linked xylose residues yielding p-1,4-xylans of up to seven xylosyl residues in length. Such results suggest that HG may, at least in soybean, be a primer or acceptor for a glycosyltransferase or a transglycosylase that establishes a link between pectin and the hemicellulose xylan. Such a linkage would be consistent with the characteristics of the Qual mutant (mutated in GAUT8) (260) and the irx8 mutant (mutated in GAUT 12) (132).