Cellulose Orientation in the Outer Epidermal Wall of Angiosperm Roots: Implications for Biosystematics

Focusing on orientation gives clue to relationships

Cellulose may well be the most abundant polysaccharide in the world; it occupies this proud position because it is a major component of plant cell walls. Deposition of cellulose must allow for cell elongation; changes in the orientation of the microfibrils within the wall are said to be important in allowing for and then resisting elongation. However, it may be more complex than this, as shown by Sven Kerstens and Jean-Pierre Verbelen at the University of Antwerp, Belgium (pp. 669-676). They selected 57 angiosperm species from 29 different families. In plants from each species, the cellulose in epidermal cells from the root elongation zone was stained using the fluorescent dye, Congo Red, and the orientation of the microfibrils determined by polarizing confocal microscopy. Three different patterns were observed. The least common is seen in the palms (Arecaceae), plants which lack a properly defined elongation zone, where microfibril orientation is apparently random. All the other species investigated exhibit a true elongation zone but, rather unexpectedly, two completely different patterns of microfibril organization are seen. In 23 out of the 27 families, the orientation is predominantly transverse to the root axis. However, in three families, Poaceae, Juncaceae and Cyperaceae (all from the Poales tribe within the monocots), the orientation of microfibrils in epidermal cells throughout the root elongation zone is parallel to the direction of growth. This is not seen in other plant families, even from within the tribe Poales. The authors suggest that we may need a re-think of the role of cellulose orientation in cell elongation. However, they add that there is a very clear link between current views on phylogeny within the Poales and the regulation of cellulose deposition. Bioinformatics has many roles, including the provision of information on taxonomic relationships. It also has many guises amongst which we may now include cellulose microfibril orientation.