CELLULOSE AS THE ANTI-GRAVITATIONAL POLYSACCHARIDE
Authors: Takayuki Hoson
Abstract: Plants show two principal responses to gravity. One is gravimorphogenesis, which enables plants to establish an appropriate body form. The other is gravity resistance to grow and develop against the gravitational force. Cellulose is involved in both gravity responses: in particular, cellulose is responsible for tension wood formation and gravity resistance in the basal supporting regions. Angiosperm trees form tension wood on the upper side of the leaning stems to increase the mechanical support when exposed to a gravitational stimulus. Tension wood has characteristic fibers with an inner gelatinous cell wall layer consisting mainly of cellulose microfibrils that are parallel to the long cell axis. In gravity resistance, plants enhance the strength of the whole body by increasing cell wall rigidity. The increase in cell wall rigidity in the growing regions may be brought about by modifications to the metabolism of certain matrix polysaccharides, such as xyloglucans and 1,3,1,4-β-glucans, in concert with the reorientation of cellulose microfibrils, whereas cellulose plays a central role in increasing cell wall rigidity in the basal supporting regions. When cellulose synthesis is stimulated in response to the gravitational force in these responses, the expression of cellulose synthase genes is upregulated. Thus, cellulose acts as the anti-gravitational polysaccharide in both gravimorphogenesis and gravity resistance.