The Expression Pattern of the Tonoplast Intrinsic Protein gamma-TIP in Arabidopsis thaliana Is Correlated with Cell Enlargement

The vacuolar membrane (tonoplast) contains an abundant intrinsic protein with six membrane-spanning domains that is encoded by a small gene family. Different isoforms of tonoplast intrinsic protein (TIP) are expressed in different tissues or as a result of specific signals. Using promoter-β-glucuronidase (GUS) fusions and in situ hybridization, we have examined the expression of γ-TIP in Arabidopsis thaliana. GUS staining of plants transformed with promoter-GUS fusions showed that γ-TIP gene expression is high in recently formed tissues of young roots. In the shoot, γ-TIP gene expression was highest in the vascular bundles of stems and petioles, as well as in the stipules and in the receptacle of the flower. No GUS activity was detected in root or shoot meristems or in older tissues, suggesting temporal control of γ-TIP gene expression associated with cell elongation and/or differentiation. In situ hybridization carried out with whole seedlings confirmed that in root tips, γ-TIP mRNA was present only in the zone of cell elongation just behind the apical meristem. In seedling shoots, mRNA abundance was also found to be correlated with cell expansion. These results indicate that γ-TIP may be expressed primarily at the time when the large central vacuoles are being formed during cell enlargement.