Role of the Lutoidic Tonoplast in the Control of the Cytosolic Homeostasis within the Laticiferous Cells of Hevea

Abstract

Lutoids, the vacuo-lysosomes of the Hevea latex cells, compartmentalize, in vivo, numerous ions such as H⁺, Mg⁺⁺, Ca⁺⁺, Pi, citrate, some of them strongly toxic for the cytosolic metabolism. Evidence is given for the correlation of the in vivo compartimentation of some of these ions inside the lutoids with the latex production by Hevea.

Two opposing H⁺ pumps were localized on the lutoidic tonoplast; the one is a Mg⁺⁺-dependent ATPase, the other a NADH-consuming redox system (cytochrome c: artificial acceptor). The functioning of these H⁺ pumps may account for a major part of the transtonoplastic ΔpH variations, and therefore the cytosolic pH control, which probably regulates the highly pH-dependent latex metabolism.

The resulting proton-motive force energizes the accumulation and compartmentation of the inhibiting ions inside the lutoïds, and ensures the control of the «detoxification» and ionic equilibrium of the cytoplasm of the laticiferous cells.

Treatment of Hevea bark with ethrel, an ethylene generator which «stimulates» latex production, induces an increase in the H⁺-pumping ATPase activity, resulting in the activation of the metabolism in the latex cells.

All the results reviewed lead us to propose that the lutoids play a double role as a «biophysical pH-STAT and a «detoxicating trap», thus controlling the cytosolic homeostasis.