Euphyllophytic pteridophytes

All vascular plants possessing megaphylls (leaves with true veins) instead of microphylls are called 'euphyllophytes' . Living euphyllophytic pteridophytes are Equisetum, Psilotum and Tmesipteris (which are all eusporangiate) and ferns (both eusporangiate and leptosporangiate, the latter being later-diverging). They contain RG-II remarkably similar to that of angiosperms, though in some groups (ferns and Psilotum but not Equisetum) Rha is replaced by MeRha (Matsunaga et al. 2004). No functional consequences of the Rha/ MeRha swap were detectable.

It has recently been found that, apparently unique outside the Poales, horsetails (Equisetum) contain MLG (Fry et al. 2008b; S0rensen et al. 2008). Equisetum MLG broadly resembles that of the Poales, except that the major product of lichenase is the tetrasaccharide, not the trisaccharide, and that laminaribiose is also a noticeable lichenase product. Equisetum MLG resembles poalean MLG in having appreciable quantities of a nonasaccharide repeat unit, but appears to lack the dodecasaccharide. Equisetum possesses not only the polysaccharide MLG but also a novel endo-Iransglucosylase whose favoured action appears to be cleaving MLG and grafting a segment of it on to a xyloglucan (acceptor substrate) chain (Fry et al., unpublished).

For such an enzyme to act in vivo, Equisetum would need also to possess xyloglucan. This it clearly does, on the basis of Driselase and XEG digestion products; however, these digests include several unusual oligosaccharides, indicating that Equisetum xyloglucan has unique structural features.

Man residues are more abundant in eusporangiate pteridophyte walls (extreme in the case of Psilotum) than in leptosporangiate fern walls (Popper & Fry 2004). Conversely, proanthocyanidins are more abundant in leptospo-rangiate fern AIR than in eusporangiate species.

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