Microfibril-Tip Growth and the Development of Pattern in Cell Walls

Abstract

No single model for the physical assembly of cellulose microfibrils is universally accepted at present, and it is likely that the subject will continue to be controversial for some time to come. Very important advances have been made since the publication of a major review of this subject by Shafizadeh and McGinnis (1971), but we seem no nearer to consensus (see, for example, Stöckmann, 1972; Atalla and Nagel, 1974; Preston, 1974; Palma et al., 1976; Robinson, 1977a; Brown and Willison, 1977; Burgess, 1979; Colvin, 1980). Burgess (1979) has stated that there are two fundamental hypotheses for microfibril assembly; (1) that glucan chains are first polymerized in a soluble form, and these intermediate molecules are later assembled into microfibrils (perhaps extracellularly, separated from the plasma membrane; perhaps via an intermediate hydrated “nascent fibril”); (2) that polymerization of glucan chains occurs directly at the free ends of preexisting microfibrils (i.e., that polymerization and crystallization are simultaneous, or virtually simultaneous). This simple statement should not, of course, be allowed to hide the existence of numerous variations, extensions, and modifications of these hypotheses, many of which are as viable now as when they were proposed. Take, for example, the shear-stress solution crystallization hypothesis proposed (and rejected) by Stöckmann (1972).