Tonoplast Intrinsic Protein Isoforms as Markers for Vacuolar Functions

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Abstract
Plant cell vacuoles may have storage or lytic functions, but biochemical markers specific for the tonoplasts of functionally distinct vacuoles are poorly defined. Here, we use antipeptide antibodies specific for the tonoplast intrinsic proteins α-TIP, γ-TIP, and δ-TIP in confocal immunofluorescence experiments to test the hypothesis that different TIP isoforms may define different vacuole functions. Organelles labeled with these antibodies were also labeled with antipyrophosphatase antibodies, demonstrating that regardless of their size, they had the expected characteristics of vacuoles. Our results demonstrate that the storage vacuole tonoplast contains δ-TIP, protein storage vacuoles containing seed-type storage proteins are marked by α- and δ- or α- and δ-plus γ-TIP, whereas vacuoles storing vegetative storage proteins and pigments are marked by δ-TIP alone or δ-plus γ-TIP. In contrast, those marked by γ-TIP alone have characteristics of lytic vacuoles, and results from other researchers indicate that α-TIP alone is a marker for autophagic vacuoles. In root tips, relatively undifferentiated cells that contain vacuoles labeled separately for each of the three TIPs have been identified. These results argue that plant cells have the ability to generate and maintain three separate vacuole organelles, with each being marked by a different TIP, and that the functional diversity of the vacuolar system may be generated from different combinations of the three basic types.