Expression and inhibition of aquaporins in germinating Arabidopsis seeds

Extensive and kinetically well-defined water exchanges occur during germination of seeds. A putative role for aquaporins in this process was investigated in Arabidopsis. Macro-arrays carrying aquaporin gene-specific tags and antibodies raised against aquaporin subclasses revealed two distinct aquaporin expression programs between dry seeds and young seedlings. High expression levels of a restricted number of tonoplast intrinsic protein (TIP) isoforms (TIP3;1 and/or TIP3;2, and TIP5;1) together with a low expression of all 13 plasma membrane aquaporin (PIP) isoforms was observed in dry and germinating materials. In contrast, prevalent expression of aquaporins of the TIP1, TIP2 and PIP subgroups was induced during seedling establishment. Mercury (5 microM HgCl(2)), a general blocker of aquaporins in various organisms, reduced the speed of seed germination and induced a true delay in maternal seed coat (testa) rupture and radicle emergence, by 8-9 and 25-30 h, respectively. Most importantly, mercury did not alter seed lot homogeneity nor the seed germination developmental sequence, and its effects were largely reversed by addition of 2 mM dithiothreitol, suggesting that these effects were primarily due to oxidation of cell components, possibly aquaporins, without irreversible alteration of cell integrity. Measurements of water uptake in control and mercury-treated seeds suggested that aquaporin functions are not involved in early seed imbibition (phase I) but would rather be associated with a delayed initiation of phase III, i.e. water uptake accompanying expansion and growth of the embryo. A possible role for aquaporins in germinating seeds and more generally in plant tissue growth is discussed.