The water to solute permeability ratio governs the osmotic volume dynamics in beetroot vacuoles

OSVALDO CHARA; MOIRA SUTKA; MARCELO OZU; VICTORIA VITALI; AMODEO, GABRIELA

San Miguel de Tucumán

Congreso; III Latin American Federation of Biophysical Societies (LAFeBS) ? IX IberoAmerican Congress of Biophysics ? XLV Reunion Anual SAB 2016; 2016

Plant cell vacuoles occupy up to 90% of the cell volume and are constantlysubjected to water and solute exchange. The osmotic flowand vacuole volume dynamics relies on the vacuole membrane -thetonoplast- and its capacity to regulate its permeability to both water andsolutes. The osmotic permeability coefficient (Pf) is the parameter thatbetter characterizes the water transport when submitted to an osmoticgradient. Usually, Pf determinations are made in vitro from the initialrate of volume change, when a fast (almost instantaneous) osmolalitychange occurs. When aquaporins are present, it is accepted that initialvolume changes are only due to water movements. However, in livingcells osmotic changes are not necessarily abrupt but gradually imposed.Under these conditions, water flux might not be the only relevant drivingforce shaping the vacuole volume response. In this study, we quantitativelyinvestigated volume dynamics of isolated Beta vulgaris rootvacuoles under progressively applied osmotic gradients at different pH,a condition that modifies the tonoplast Pf. We followed the vacuole volumechanges while simultaneously determining the external osmolalitytime-courses and analyzing these data with mathematical modelling.Our findings indicate that in these conditions, vacuole volume changeswould not depend on the membrane elastic properties, nor on the nonosmoticvolume of the vacuole, but on water and solute fluxes across thetonoplast. We found that the volume of the vacuole at the steady state isdetermined by the ratio of water to solute permeabilites (Pf/Ps), whichin turn is ruled by pH. The dependence of the permeability ratio on pHcan be interpreted in terms of the degree of aquaporin inhibition andthe consequently solute transport modulation. This is relevant in plantorgans such as root, leaves, cotyledons or stems that perform extensiverhythmic growth movements, which involve considerable cell volumechanges within seconds to hours.