PLANT-CELLS CONTAIN 2 FUNCTIONALLY DISTINCT VACUOLAR COMPARTMENTS

The plant cell vacuole has multiple functions, including storage of pr oteins and maintenance of an acidic pH where proteases will have maxim al activity. It has been assumed that these diverse functions occur in the same compartment. Here, we demonstrate that antibodies to two dif ferent tonoplast intrinsic proteins, alpha-TIP and TIP-Ma27, label vac uole membranes of two different compartments within the same cell. The se compartments are functionally distinct, because barley lectin, a pr otein stored in root tips, is exclusively contained within the alpha-T IP compartment, while aleurain, a protease that serves as a marker for an acidified vacuolar environment, is exclusively contained within th e TIP-Ma27 compartment. As cells develop large vacuoles, the two compa rtments merge; this may represent a process by which storage products in the alpha-TIP compartment are exposed to the acidic lytic TIP-Ma27 compartment for degradation.