6. Aquaporins and plant development
Seeds play a crucial role in the reproductive cycle and dissemination
of higher plants. Most seeds are highly desiccated organs, and extensive
and well-defined water exchanges are associated with seed maturation
and germination, the latter process including seed imbibition and
subsequent embryo growth [104]. A fine regulation of aquaporin
expression during these processes has been described in many species,
including ice plant and Brassica napus [105,106]. In particular, TIP3s of
all plant species examined show seed-specific expression and their
abundance markedly decreased during germination [107–110]. TIP3
expression may accompany the massive deposition of storage proteins,
oligosaccharides and phytins in protein storage vacuoles during late
seed development [111]. In Arabidopsis and pea, mercury derivatives
reduced the rate of seed germination and seed imbibition, respectively
[110,112], suggesting a role of aquaporins in these processes. To date,
clear genetic evidence for a role of aquaporins in seed germination has
only been provided in rice using transgenic plants with loss- and gainof-
function of OsPIP1;3. Gene expression studies further indicated that
this aquaporin may mediate the effects of NO on seed germination
[113].
A strong link between aquaporin expression, cell expansion and
plant growth has emerged in recent years. For instance, the expression
pattern of the AtTIP1;1 promoter in Arabidopsis is correlated with
cell enlargement in roots, hypocotyls, leaves and flower stems [114],
and AtTIP1;1 expression was induced by the growth-promoting