Since many aquaporins (AQPs) act as water channels, they are thought to play an important role in plant water relations. It is therefore of interest to study the expression patterns of AQP isoforms in order to further elucidate their involvement in plant water transport. We have monitored the expression patterns of all 35 Arabidopsis AQPs in leaves, roots and flowers by cDNA microarrays, specially designed for AQPs, and by quantitative real-time reverse transcriptase PCR (Q-RT-PCR). This showed that many AQPs are pre-dominantly expressed in either root or flower organs, whereas no AQP isoform seem to be leaf specific. Looking at the AQP subfamilies, most plasma membrane intrinsic proteins (PIPs) and some tonoplast intrinsic proteins (TIPs) have a high level of expression, while NOD26-like proteins (NIPs) are present at a much lower level. In addition, we show that PIP transcripts are generally down-regulated upon gradual drought stress in leaves, with the exception of AtPIP1;4 and AtPIP2;5, which are up-regulated. AtPIP2;6 and AtSIP1;1 are constitutively expressed and not significantly affected by the drought stress. The transcriptional down-regulation of PIP genes upon drought stress could also be observed on the protein level.

Aquaporins are water channel proteins that facilitate the movement of water and other small solutes across biological membranes. Plants usually have large aquaporin families, providing them with many ways to regulate the water transport. Some aquaporins are regulated post-translationally by phosphorylation. We have previously shown that the water channel activity of SoPIP2;1, an aquaporin in the plasma membrane of spinach leaves, was enhanced by phosphorylation at Ser115 and Ser274. These two serine residues are highly conserved in all plasma membrane aquaporins of the PIP2 subgroup. In this study we have purified and characterized two protein kinases phosphorylating Ser115 and Ser274 in SoPIP2;1. By anion exchange chromatography, the Ser115 kinase was purified from the soluble protein fraction isolated from spinach leaves. The Ca2+-dependent Ser274 kinase was purified by peptide affinity chromatography using plasma membranes isolated from spinach leaves. When characterized, the Ser115 kinase was Mg2+-dependent, Ca2+-independent and had a pH-optimum at 6.5. In accordance with previous studies using the oocyte expression system, site-directed mutagenesis and kinase and phosphatase inhibitors, the phosphorylation of Ser274, but not of Ser115, was increased in the presence of phosphatase inhibitors while kinase inhibitors decreased the phosphorylation of both Ser274 and Ser115. The molecular weight of the Ser274 kinase was approximately 50 kDa. The identification and characterization of these two protein kinases is an important step towards elucidating the signal transduction pathway for gating of the aquaporin SoPIP2;1.