Volume changes in red beet vacuoles after hypotonic and hypertonic challenges: experimental and mathematical modelling approaches

SUTKA M; CHARA OSVALDO; ALLEVA K; AMODEO G

Montevideo

Congreso; 5th Southern Cone Biophysics Congress. 6th International Conference of Biological Physics.; 2007

IUPAP - SAB

In plants, vacuoles represent 90% of the cell volume and are responsible for both cell volume regulation and pH homeostasis. The objectiveof this work was to study the effects of HgCl2 and medium acidification on isolated vacuole volume changes, discriminating water fromsolute fluxes under different osmotic challenges. Two approaches were developed: experimental and mathematical modelling. In theexperimental approach, vacuoles were isolated from red beet root cells, equilibrated in an isotonic solution and then exposed to hypotonic orhypertonic conditions. The osmotic gradient was imposed manipulating final mannitol concentration and using a perfusion system. Vacuolevolume changes were individually recorded in a videomicroscopy setup. In some experiments, vacuoles were pre-incubated in the presenceof different HgCl2 concentrations or at different acidic pHs, both known to block osmotic water movement through pores. In themathematical approach, three theoretical models were developed. The models explore essentially the solute movement by diffusion(D_model), a regulatory volume mechanism (RV_model) or a combination of them (D-RV_model). In all of them, water movement wasconsidered to move osmotically. Our models are characterized by a system of ordinary non linear differential equations subject to a set ofinitial conditions. Simulations were performed using Euler method with a custom-made software developed in Visual Basic code. Simulationswere performed fitting the experimental data and comparing the models and, consequently, the tested hypothesis. Our results show thatunder hypotonic conditions simulations done using D_model fitted better than D-RV_model and RV_model. On the other hand,D-RV_model showed better fitting in hypertonic conditions. Therefore, our results indicate that red beet vacuoles respond differently toosmotic challenges: when exposed to hypotonic conditions, solute diffusion follows water osmotic fluxes. On the contrary, under hypertonicconditions besides solute diffusion process and water osmotic movement, a regulatory volume mechanism might be involved.