Mapping of Tonoplast Intrinsic Proteins in Maturing and Germinating Arabidopsis Seeds Reveals Dual Localization of Embryonic TIPs to the Tonoplast and Plasma Membrane

We have mapped the expression of the tonoplast intrinsic protein (TIP) gene family members in Arabidopsis seeds by fluorescent protein tagging of their genomic sequences and confocal microscopy. Three isoforms (TIP1;1, TIP2;1, and TIP2;2) have distinct patterns of expression in maternal tissues (outer integument and placento-chalazal region). Two isoforms, TIP3;1 and the previously uncharacterized TIP3;2, are the only detectable TIPs in embryos during seed maturation and the early stages of seed germination. Throughout these developmental stages, both isoforms co-locate to the tonoplast of the protein storage vacuoles, but also appear to label the plasma membrane. Plasma membrane labeling is specific to TIP3;1 and TIP3;2, is independent of the position of the fluorescent protein tag, and appears to be specific to early seed maturation and early germination stages. We discuss these results in the context of the predicted distribution of aquaporins in Arabidopsis seeds.