Characterization and expression of plasma and tonoplast membrane aquaporins in primed seed of Brassica napus during germination under stress conditions

Two aquaporin genes were isolated from a cDNA library of canola (Brassica n apus L.). The first aquaporin, BnPIP1 of 1094 bp, encoding a putative polyp eptide of 287 amino acids with a predicted molecular mass of 30.4 kDa and a pI of 7.8, belongs to the family of plasma membrane intrinsic protein (PIP s) aquaporins. The B. napus aquaporin showed 85-94% identity to the Arabido psis thaliana PIPs. ABA priming of seed induced high levels of BnPIP1 trans cript which remained after subsequent re-drying of the seed. The second aqu aporin, Bn gamma-TIP2 of 1020 bp, encoded a putative polypeptide of 253 ami no acids with a predicted molecular mass of 25.8 kDa and a pI of 5.8. Bn ga mma-TIP2 showed 83-90% identity to gamma-TIP genes from a variety of plant species. Bn gamma-TIP2 was expressed only when radicle protrusion occurred in either untreated or primed seeds. Seeds primed with PEG or ABA germinate d earlier and showed a higher final percentage of germination than unprimed seed, particularly under salt and osmotic stresses at low temperature. Tra nscripts of both BnPIP1 and Bn gamma-TIP2 genes were present earlier during germination of primed seeds than non-primed seed. From these results, we c onclude that BnPIP1 is related to the water transportation required for enz ymatic metabolism of storage nutrients at the early stages of canola seed g ermination whereas Bn gamma-TIP2 expression is related to cell growth assoc iated with radicle protrusion. Priming induced the expression of BnPIP1 but had no effect on Bn gamma-TIP2.