The role of pH gradients across the plasmalemma of Catharanthus roseus and its involvement in the release of alkaloids

During growth, Catharanthus roseus cells exhibit an acidification of the culture medium that may be controlled by Ca2+. With a view to enhance the productivity of alkaloids by plant cells, the effect of extracellular pH modifications on the excretion processes has been investigated. Ca2+ dependent proton pumping leads to the release of various lipophilic amine-like compounds (benzylamine, methylamine, nicotine) initially accumulated by the cells, but also facilitates the excretion of endogenous ajmalicine. Once released in the medium, these compounds are however taken up again by the cells, probably as the charged form. For the alkaloid contained in C. roseus some evidence suggests that the diffusible form comes from the cytosolic compartment and not from the storage vacuoles. This appears to be a major production limitation to the use of pH gradients in order to favour alkaloid excretion.