“…However, it has remained unclear whether the accompanying water flow across the guard cell plasma membrane and tonoplast can simply be mediated by the lipid phase of membranes or whether aquaporins, channel proteins that facilitate transmembrane water transport, are involved. In fact, information on aquaporin expression in guard cells has remained scarce (Sarda et al, 1997;Sun et al, 2001;Fraysse et al, 2005;Shope and Mott, 2006;Yang et al, 2006;Heinen et al, 2014). Plasma membrane intrinsic proteins (PIPs) represent the most abundant aquaporins in the plant plasma membrane (Santoni et al, 2003), and transcriptome analysis indicated expression of 8 and 12 PIP isoforms in stomata of Arabidopsis thaliana and stomatal complexes of maize (Zea mays), respectively (Leonhardt et al, 2004;Heinen et al, 2014).…”