Differential localization of tonoplast intrinsic proteins on the membrane of protein body Type II and aleurone grain in rice seeds.

Tonoplast intrinsic proteins (TIPs) belong to an aquaporin family of proteins that function as water-transport channels. In this study, we isolated and characterized three novel rice cDNAs for OsTIP1, OsTIP2, and OsTIP3 that are homologous to rice gamma-TIP cDNA. Northern blot hybridization analyses revealed that rice gamma-TIP was expressed in all plant organs. OsTIP1 was expressed in mature seed embryos and during early seed germination. OsTIP2 was expressed exclusively in roots. OsTIP3 was specifically expressed in seeds. These results suggest that the OsTIP1, OsTIP2, and OsTIP3 genes encode discrete, functionally specialized TIPs. Immunocytochemical analysis in rice endosperm cells revealed that rice gamma-TIP was localized only on the protein body type II (PB-II) membranes, whereas OsTIP3 was localized on the PB-II and the aleurone grain membranes. Although both the PB-II and the aleurone grain are derived from vacuoles, these results suggest that they may be derived from different types of vacuoles.