IMPLICATION OF THE PLASMALEMMA H+-ATPASE IN SUGAR EXCHANGES BETWEEN VESSELS-ASSOCIATED CELLS AND XYLEM VESSELS, IN WALNUT STEMS

In winter and early spring, walnut trees exhibit variations of sugar content in the vascular sap, controlled by the exchanges between the vessels of the xylem and the vessel-associated cells (VACs) (Valentin et al., this issue). At the VAC/vessel interface, the recovery of sap sugars (influx) occurs via a H+/sugar symport. This symport seems to depend on the transmembrane pH gradient generated by a plasma membrane H+-ATPase. To be able to study the ATPase activity, we used a perfusion technique allowing the use of several effectors: fusicoccin (FC) and carbonyl cyanide m-chlorophenylhydrazone (CCCP). In walnut trees growing outside during winter, the uncoupler CCCP revealed a low pH gradient between the vessels of the xylem and the vessel associated cells. In these conditions fusicoccin, an activator of the H+-ATPase, had no effect on the perfusion solution, suggesting that the enzyme was lightly active. Close to bud break, a high pH gradient was measured and an acidification of the perfusion solution was observed in the presence of FC. For cold-deprived trees, kept in a greenhouse during winter, no significant difference in the pH gradient between winter and spring was observed. Moreover no modification of the sensitivity of the ATPase to FC was observed between both periods. These results allowed us to understand at least partially, how the H+-ATPase activity could control the sugars influx and play an important key role in physiological mechanisms, such as bud break and embolism refilling.
Abbreviations: VAC: vessel-associated cell; FC : fusicoccin; CCCP : carbonyl cyanide m-chlorophenylhydrazone.

IV International Walnut Symposium

G. Alves, T. Améglio, A. Lacointe, J.L. Julien, A. Guilliot, G. Petel

pH gradient, plasma membrane H+-ATPase, sugar fluxes, vessel-associated cell.

https://doi.org/10.17660/ActaHortic.2001.544.39