Description:
<p>This paper draws attention to an important unsolved problem in our understanding of the processes of ion transfer to plant cell vacuoles, which emerged from consideration of unexpected features of the measured kinetics of the transfer of tracer chloride from the outside solution to the vacuole of Characean cells. Over labelling times of 1-2 h the kinetics of vacuolar transfer of chloride are consistent with the two-phase model of the cell, of cytoplasm and vacuole in series, and the cytoplasmic exchange is characterized by a single rate constant for exchange. However, for shorter times the vacuolar transfer is biphasic, and there appear to be two components of vacuolar transfer, a fast component important at short times, and a slow component whose contribution increases with time. Half-cell labelling experiments, measuring the rate of transfer of tracer in the flowing cytoplasm, make it clear that the ground cytoplasm must be part of the slow phase; the view that the slow phase represents slowly exchanging organelles within the cytoplasm is not tenable. It is argued that the fast component is most likely to arise by transfer to the vacuole associated with action potentials on cutting the cell for analysis. There are two unexpected features of the kinetics of the slow component of vacuolar transfer. The first is the complete lack of discrimination between Cl- and Br- in the vacuolar transfer, in spite of strong discrimination in favour of Cl- at the plasmalemma. The vacuolar transfer is uphill, and if it arises from single ion transfer through an appropriate transporter, such lack of discrimination is unexpected. The property is even more surprising in the light of the second peculiarity of the kinetics, the relation between vacuolar transfer and influx, whether influx is manipulated by metabolic conditions, inhibitors or external concentration. Increased Cl- influx is associated with increased transfer of both Cl- and Br- to the vacuole. It is difficult to explain this in terms of independent processes of single ion transfer at the plasmalemma and tonoplast. An explanation in terms of transfer of salt-filled vesicles from cytoplasm to vacuole, by budding from the endoplasmic reticulum at a rate related to the influx to the cell, may be more consistent with the observed kinetics. The question of whether vesicle trafficking plays a role in vacuolar ion accumulation deserves to be re-examined by modern techniques. Finally some evidence is considered that might suggest (even more speculatively) that vesicle trafficking may also play a role in the dramatic stimulus-triggered losses of vacuolar solute in certain types of cell, such as stomatal guard cells and pulvinar motor cells. Again the question deserves to be investigated.</p>