Résumé
The functional characterization of numerous transport systems across the vacuolar membrane (tonoplast) has provided a better understanding of the versatile role played by the vacuole in plant cells. The tonoplast H+-ATPase and the H+-pyrophosphatase pump protons into the vacuole making this organelle inside positive and acidic (Hedrich et al., 1989; Taiz, 1992). Maintaining an electrochemical gradient of protons, in addition to its role in cytoplasmic pH regulation, energizes various transport processes across the tonoplast. Several types of ions channels have been identified by the patch clamp technique, but the selectivity of most of them and hence their physiological role remain elusive (Hedrich and Schroeder, 1989). They probably represent major transport pathways for ions such as K+ and Cl-, the main determinants of cell turgor, and NO3- which can be stored in and mobilized from the vacuole. Calcium-permeable channels as well as a Ca++/H+ antiporter occur in plant vacuoles (Alexandre et al., 1990; Blackfordet al., 1990). They reflect the role that the vacuole as a major calcium store plays in signal transduction in plants. The tonoplast also carries Na+/H+ exchangers and Cd+ transporters that can account for the detoxifying role of the vacuole during saline and heavy metal stress, respectively (Taiz, 1992).