TONOPLAST INVAGINATIONS AND INTRAVACUOLAR VESICLES IN CULTURED CALLUS CELLS OF STEVIA REBAUDIANA

The tonoplast of cells within meristematic regions was observed to possessinvaginations,of variable size and form,in cultured callus of Stevia rebaudiana on differentiating medium.The invaginations of the tonoplast,which occur in cells at differentphases in vacuolation,are numerous in some cells but fewer in others,and sometimes canoccupy a considerable volume of the vacuole.These structures may be free of apparentcontents; may possess numerous vesicles and/or complex membranous system; or maycontain a larger membrane-bound vesicle including a number of smaller internal vesiclesor complex internal membranes.Another phenomenon observed in these cells is the presence of vesicular structures that are free within the vacuole.These intravacuolar vesicles, usually composed of either two closely approximated limiting membranes and anumber of smaller internal vesicles,or multiple concentric membranes,are morphologically similar to the tonoplast invagination and its contents.It is suggested that the intrava-cuolar vesicles develop from invagination of the tonoplast which become detached intothe vacuole.The results for acid phosphatase (Acpase) localization demonstrate that thetonoplast and its invaginations generally lack Acpase activity while the intravacuolarvesicles have Acpase activity,indicating that these intravacuolar vesicles are functionallyanalogous to the lysosomes and therefore constitute part of the lytic compartment inthese cells.It is concluded that the occurrence of the intravacuolar vesicles containingAcpase may be involved in the spatial separation of lytic enzyme from stored metabolitesin the central vacuole.