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

SUTKA, M; CHARA, O; ALLEVA, K; OZU, M; DORR, R; AMODEO, G.

Montevideo, Uruguay

Congreso; 6th Internacional Conference of Biological Physics, ICBP and 5th Southern Cone Biophysics Congress.; 2007

Comite organizador ad hoc

  In plants, vacuoles represent 90% of the cell volume and are responsible for both cell volume regulation and pH homeostasis. The objective of this work was to study the effects of HgCl2 and medium acidification on isolated vacuole volume changes, discriminating water from solute fluxes under different osmotic challenges. Two approaches were developed: experimental and mathematical modelling. In the experimental approach, vacuoles were isolated from red beet root cells, equilibrated in an isotonic solution and then exposed to hypotonic or hypertonic conditions. The osmotic gradient was imposed manipulating final mannitol concentration and using a perfusion system. Vacuole volume changes were individually recorded in a videomicroscopy setup. In some experiments, vacuoles were pre-incubated in the presence of different HgCl2 concentrations or at different acidic pHs, both known to block osmotic water movement through pores. In the mathematical 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 was considered to move osmotically. Our models are characterized by a system of ordinary non linear differential equations subject to a set of initial conditions. Simulations were performed using Euler method with a custom-made software developed in Visual Basic code. Simulations were performed fitting the experimental data and comparing the models and, consequently, the tested hypothesis. Our results show that under 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 to osmotic challenges: when exposed to hypotonic conditions, solute diffusion follows water osmotic fluxes. On the contrary, under hypertonic conditions besides solute diffusion process and water osmotic movement, a regulatory volume mechanism might be involved.