Isolation and functional characterization of three aquaporins from olive (Olea europaea L.)

To study the molecular bases of water transport in olive we characterized cDNAs from Olea europaea cv "Leccino" related to the aquaporin (AQP) gene family. A phylogenetic analysis of the corresponding polypeptides confirmed that they were part of water channel proteins localized in the plasma membrane and in the tonoplast. The full-length sequences were obtained by RACE-PCR and were named OePIP1.1, OePIP2.1 and OeTIP1.1. The OePIP2.1 and OeTIP1.1 encode functional water channel proteins, as indicated by expression assays in Xenopus laevis oocytes. OePIP1.1 and OePIP2.1 expression levels are high in roots and twigs and low in leaves. The highest hybridization signal of OeTIP1.1 was detected in twigs, while in roots and leaves the expression was low. To investigate the effect of abiotic stress on the transcript level of olive AQP genes, olive trees were subjected to drought treatment and the expression levels of the genes were measured by Northern-blot analysis. The transcript levels of each gene diminished strongly in plants submitted to drought stress, when soil moisture, twig water potential and twig hydraulic conductivity progressively decreased. The downregulation of AQP genes may result in reduced membrane water permeability and may limit loss of cellular water during periods of water stress. A possible role for AQPs on shoot embolism repair is discussed.