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J. Membrane Biol. 210, 131142 (2006)DOI: 10.1007/s00232-005-0851-7Control of Volume and Turgor in Stomatal Guard CellsEnid A.C. MacRobbieDepartment of Plant Sciences, University of Cambridge, Downing St., Cambridge, CB2 3EA, UKReceived: 12 December 2005Abstract. Water loss from plants is determined by the
aperture of stomatal pores in the leaf epidermis, set
by the level of vacuolar accumulation of potassium
salt, and hence volume and turgor, of a pair of guard
cells. Regulation of ion uxes across the tonoplast,
the key to regulation of stomatal aperture, can only
be studied by tracer ux measurements. There are
two transport systems in the tonoplast. The rst is a
Ca2+-activated channel, inhibited by phenylarsine
oxide (PAO), responsible for the release of vacuolar
K+(Rb+) in response to the drought hormone,
abscisic acid (ABA). This channel is sensitive to
pressure, down-regulated at low turgor and up-regulated at high turgor, providing a system for turgor
regulation. ABA induces a transient stimulation of
vacuolar ion eux, during which the ux tracks the
ion content (volume, turgor), suggesting ABA reduces the set-point of a control system. The second
system, which is PAO-insensitive, is responsible for
an ion ux from vacuole to cytoplasm associated with
inward water ow following a hypo-osmotic transfer.
It is suggested that this involves an aquaporin as
sensor, and perhaps also as responder; deformation
of the aquaporin may render it ion-permeable, or,
alternatively, the deformed aquaporin may signal to
an associated ion channel, activating it. Treatment
with inhibitors of aquaporins, HgCl2 or silver sulfadiazine, produces a large transient increase in ion
release from the vacuole, also PAO-insensitive. It is
suggested that this involves the same aquaporin, either rendered directly ion-permeable, or signalling to
activate an associated ion channel.Key words: Aquaporin Guard cell Osmoregulation Turgor regulation Tonoplast ion channelsIntroductionMany plant cells regulate their turgor pressure to a
constant level in the face of changing external osmotic conditions. This is particularly important in
marine algae in the intertidal zone, which face large
changes in external osmotic conditions during air
exposure or following rain. After the initial water
ow induced by an external osmotic change, leading to a new steady state of internal osmotic
pressure, cell volume and turgor, in water equilibrium with the new external conditions, many algae
show altered ion uxes, leading to changes...