Water pores—water
pours?
The discovery of
aquaporins, specific protein channels that facilitate water flux through plant
cell membranes, was relatively recent and we are far from understanding their
role within plant water relations. A truly international team from
France,
Japan
and
Austria
(Fleurat-Lessard
et al., pp. 457–460)
briefly reviews the evidence that the distribution of aquaporins in the
tonoplast and plasma membrane may be related to the ability to manipulate
rapidly cell turgor and/or cell volume. The paper focuses on the cells of
soybean (
Glycine max) root nodules. Nodules are the sites of symbiotic N
2
fixation, a process that requires very low oxygen tension. Control of O
2
diffusion in the nodule is thus very important and it has been suggested that
this control involves rapid changes in the shape and volume of the cells in the
nodule inner cortex. This, in turn, implicates aquaporins and thus the authors
have examined the distribution of plasma membrane and tonoplast aquaporins.
Antibodies raised against two different aquaporins from the plasma membrane
(PIP1 and PIP2) were conjugated to gold and were then used to detect their
target proteins in ultra-thin sections of root nodules. The micrographs clearly
show that the highest concentrations of both PIP1 and PIP2 were in the inner
cortex and endodermis while the lowest were in the infected cells; pericycle
cells showed intermediate levels. The authors also repeated earlier experiments
in carrying out immunolocalization of a tonoplast aquaporin,
g-TIP, showing firstly that the protein was
very abundant and secondly that its distribution pattern was very similar to
that of PIP1 and PIP2. The authors comment that the density of aquaporins is
similar to that of other plant cells in which rapid water fluxes occur,
suggesting that rapid fluxes also occur in the nodule cortex. However, neither
this nor a role in regulating O2 diffusion has been directly
demonstrated.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
