Fine-structural studies on spontaneous and induced fusion of higher plant protoplasts.

During the treatment of some plant tissues with cell-wall-degrading enzymes adjacent cells within the tissue fuse forming large multinucleate protoplasts. These have been termed spontaneous fusion bodies. The symplastic nature of plant tissues suggests that the retention of plasmodesmatal connexions might facilitate such spontaneous fusion. An electron-microscopic investigation of spontaneous fusion in tobacco-leaf and oat-root tissues has confirmed this suggestion. Enzymic degradation of the walls removes constrictions on the plasmodesmata, permitting their expansion, and as a result mixing of the cytoplasms of the fusing protoplasts can then occur. The fine structure of plasmodesmata and their relationship to the endoplasmic reticulum can be more easily studied in plasmodesmata which are undergoing expansion. It has been observed that the tubule which passes through the plasmodesma is in continuity with the endoplasmic reticulum membranes at either end. Models for plasmodesmatal structure are discussed in the light of this observation. The induced fusion of freely isolated protoplasts by sodium salts has been previously studied using the light microscope. Since it is difficult to follow the detailed mechanisms involved in the process, electron-microscopic methods have been employed in the present investigation. It appears that sodium nitrate first induces protoplast adhesion. This occasionally involves protrusions from the plasmalemma, not unlike microvilli. Following adhesion membrane fusion occurs, initially in localized regions, and then more generally. Eventually vacuolar fusion occurs facilitating complete cytoplasmic mixing. These findings are compared with events occurring during animal cell fusion and are discussed in relationship to a recent theoretical model for membrane fusion.