Cellulose, an abundant, crystalline polysaccharide, is central to plant morphogenesis and to many industries. Chemical and ultrastructural analyses together with map-based cloning indicate that the RSW1 locus of Arabidopsis encodes the catalytic subunit of cellulose synthase. The cloned gene complements the rsw1 mutant whose temperature-sensitive allele is changed in one amino acid. The mutant allele causes a specific reduction in cellulose synthesis, accumulation of noncrystalline beta-1,4-glucan, disassembly of cellulose synthase, and widespread morphological abnormalities. Microfibril crystallization may require proper assembly of the RSW1 gene product into synthase complexes whereas glucan biosynthesis per se does not.

The influence of the light microscopical stains, Calcofluor white and Congo red, on the process of chitin microfibril formation of the chrysoflagellate alga Poterioochromonas stipitata was studied with light and electron microscopy. There is a concentration-dependent inhibition of lorica formation with both dyes. In the presence of the inhibitors malformed loricae are made, which do not show the usual ultrastructure and arrangement of the chitin microfibrils. Instead of long, laterally associated microfibrils, short rods or irregular networks of subelementary (15-25 A) fibrils are found. Microfibril assembly obviously takes place on the accessible outside of the plasma membrane. There must be a gap between the polymerization and microfibril formation reactions, allowing the stains to bind to the polymerized subunits. Thus, later association of these units to form microfibrils is disturbed. The microfibril-orienting mechanism also depends on normal microfibril formation. A model summarizing these hypotheses is suggested.

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