Plant aquaporins and cytosolic acidification: controlling the transcellular water pathway

GABRIELA AMODEO

Asilomar

Workshop; 4th PanAmerican Plant Membrane Biology Workshop; 2012

PanAmerican Plant Membrane Biology

In roots, the radial flow of water is at its best described by the contributions of three different parallel pathways: apoplastic, symplastic and transcellular [1]. According to this ‘composite transport model’, the relative contribution of these pathways to overall water uptake may vary substantially, implying a putative dominant role for the radial component of water transport in terms of modulating water uptake. These approximations lead the transcellular pathway, i.e. water moving through cell membranes as a key versatile point for allowing rapid adjustments to control water flow when exposed to environmental changes that might affect plant survival.  Plant aquaporins (AQPs) are seen as ‘‘cellular plumbers’’, regulating the movement of water across membranes [2]. They can increase the cell membrane osmotic water permeability coefficient (Pf) by up to 20-fold. Consequently, signal transduction or metabolic control affecting AQPs might significantly affect root water transport [3]. In particular, the plant plasma membrane holds one of the largest groups of aquaporins known as PIP aquaporins. Interestingly, all PIP members share very high identity remaining concentrated in only two main clusters: PIP1 and PIP2, but also disclose two intriguing aspects: i) the potential of modulating whole membrane water permeability by co-expression of both types, distinguished for showing a differential capacity to reach the plasma membrane; and ii), the faculty to reduce water permeation through the pore after cytosolic acidification, as a consequence of a gating process. Regarding the first item, most of PIP2 members show high water transport capacity when expressed alone in heterologous systems but an increasing number of some PIP1 do not straightforwardly reach the plasma membrane [4]. On the other hand, both PIP1 and PIP2 share conserved residues in their sequence that sense cytosolic pH, providing the faculty to reduce water permeation through the pore by acidification, as a consequence of a channel gating process. This mechanism was explained by the protonation of a conserved histidine residue located on the intracellular-exposed loop D of PIPs [5], which finally lead to the proposal of a structural mechanism for pH dependence of PIP gating, based on the X-ray structures of the closed and open conformations of a PIP aquaporins [6]. In order to contribute in the understanding of aquaporins and adaptation to salt environments we studied the transcelullar water pathway in roots of Beta vulgaris, a highly adapted plant to salt environments. Our working hypothesis is that cytosolic pH and PIP co-expression might enhance plasticity to the membrane water transport capacity if they jointly trigger any cooperative interaction. Isolated plasma membrane vesicles from Beta vulgaris storage root showed atypically high water permeability that can be shut down by acidic pH [7]. Co-expression  of highly root expressed  aquaporins (BvPIP1;1 and BvPIP2;2) in a heterologous system -Xenopus oocytes-  not only enhance plasma membrane water permeability showing a seven-fold increment but also modulates aquaporin pH inhibitory response [8]. This pH dependent behavior shows that PIP1-PIP2 co-expression accounts for a different pH sensitivity by shifting the inhibitory response if compared to BvPIP2;2 expressed alone. These results show that PIP co-expression impacts modulating the membrane water permeability through the pH regulatory response, enhancing in this way membrane versatility to adjust its water transfer capacity. We examined the effect of salt treatment in Beta vulgaris and observed that changes in aquaporin expression level seem not the critical point in plant adaptation. We propose that the main first barrier in the plant cell, i.e. the plasmalemma holds strongly regulated aquaporins that also share a high capacity to adjust water permeability employing direct mechanisms that increase versatility in the requested response.