Tonoplast intrinsic protein isoforms as markers for vacuolar functions

Plant cell vacuoles may have storage or lytic functions, but biochemical ma rkers specific for the tonoplasts of functionally distinct vacuoles are poo rly defined. Here, we use antipeptide antibodies specific for the tonoplast intrinsic proteins alpha-TIP, gamma-TIP, and delta-TIP in confocal immunof luorescence experiments to test the hypothesis that different TIP isoforms may define different vacuole functions. Organelles labeled with these antib odies were also labeled with anti-pyrophosphatase antibodies, demonstrating that regardless of their size, they had the expected characteristics of va cuoles. Our results demonstrate that the storage vacuole tonoplast contains delta-TIP, protein storage vacuoles containing seed-type storage proteins are marked by alpha- and delta- or alpha- and delta- plus gamma-TIP, wherea s vacuoles storing vegetative storage proteins and pigments are marked by d elta-TIP alone or delta- plus gamma-TIP. In contrast, those marked by I-TIP alone have characteristics of lytic vacuoles, and results from other resea rchers indicate that alpha-TIP alone is a marker for autophagic vacuoles. I n root tips, relatively undifferentiated cells that contain vacuoles labele d separately for each of the three TIPs have been identified. These results argue that plant cells have the ability to generate and maintain three sep arate vacuole organelles, with each being marked by a different TIP, and th at the functional diversity of the vacuolar system may be generated from di fferent combinations of the three basic types.