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Xet marks the spot in plant evolution
One of the key processes in the elongation growth of plant cells is the slippage of cellulose microfibrils within the cellulose–xyloglucan network. Essential for this are xyloglucan hydrolases (XEHs) and xyloglucan endotransglycoslylases (XETs); the two types of enzyme are classified together as XTHs, as described by Van Sandt et al. (Antwerp, Belgium, pp. 39–51). Previous work by this group had shown that XET activity is correlated with cell elongation in all orders of vascular plants, including the 'primitive' Selaginellales; they had also shown the presence of a conserved XTH-coding amino acid sequence in Selaginella. The authors have now turned their attention to non-vascular plants, carrying out an extensive search for both XET activity and for XTH-like gene sequences. XET activity is present in bryophytes and again is generally correlated with cell growth. In accord with this, two cDNAs with XTH-like sequences were identified in the moss Physcomitrella patens. Going further down the complexity scale, XET activity was also detected in association with zones of growth in Chara (representing the Charophyta) and in Ulva (representing the Chlorophyta). However, no activity was detected in brown or red algae. Further, by using degenerate primers based on angiosperm XTH sequences, the authors identified a putative XTH-coding sequence in Chara; this sequence also had features characteristic of endoglucanases. The presence of both XET enzyme activity and XTH-like coding sequences in these, the simplest of the multicellular green plants, is particularly interesting because they do not possess xyloglucans in their cell walls. The likelihood is that these relatively primitive members of the enzyme family are capable of working on xylans and/or glucans in relation to cell growth. Overall, therefore, these data suggest that the evolutionary sequence culminating in the XET-based mechanisms in the cell walls of angiosperms originated towards the very base of the green plant lineage.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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