Cellulose microfibril orientation in Oocystis solitaria: proof that microtubules control the alignment of the terminal complexes

In the green alga Oocystis solitaria microtubules control the regular deposition of cellulose microfibrils. Although it has frequently been suggested that the influence of the cortical microtubules is mediated through the alignment of structures in the plasma membrane, e.g. the cellulose-synthesizing enzymes, experimental proof is lacking. In Oocystis the putative cellulose-synthesizing units, the so-called terminal complexes, can be visualized following freeze-fracture. With respect to the synthesis of a given layer of microfibrils two distinct situations are observable: terminal complex doublets occur before the start of cellulose formation, but are subsequently separated into single terminal complexes by pressure exerted by the crystallizing microfibrils. In order to investigate the effect of anti-microtubular substances on the orientation of the terminal complexes, the state of cellulose deposition at the time of drug application was marked by short (15–30 min) treatment with Congo Red, which causes a morphological change in the terminal complexes. The characteristic alignment of the terminal complexes, both doublets and fragmented single ones, is severely disturbed in cells treated with the herbicide amiprophosmethyl, which is known to interfere with the action of microtubules. The results provide strong evidence that microtubules control the alignment of the putative cellulose-forming units in Oocystis. The observed pattern of interference indicates that the microtubules most probably achieve their control by imposing fluidity channels on the membrane and not via direct links with the terminal complexes.