Plant aquaporin homologs can be classified
according to their sequence into up to seven subfamilies [8,9],
which may also correspond to distinct sub-cellular localizations. The
so-called Plasma membrane Intrinsic Proteins (PIPs), which localize to
the plasma membrane mostly, can be further divided into PIP1 and
PIP2 subclasses. The Tonoplast Intrinsic Proteins (TIPs) are targeted to
the vacuolar membrane (tonoplast).