Fumarate and cytosolic pH coordinate the synthesis or comsuption of C4 organic acid in Arabidopsis thaliana

ARIAS, C. L.; ANDREO, C. S.; DRINCOVICH, M. F.; GERRARD WHEELER, M. C.

Mar del Plata

Congreso; XXIX Reunión Argentina de Fisiología Vegetal; 2012

Sociedad Argentina de Fisiología Vegetal

A. thaliana is a plant species that accumulates high levels of organic acids and uses them as carbon, energy and reducing power sources. Among the enzymes that metabolize these compounds one of the most important is malic enzyme (ME), which is present as a multigene family. In this regard, NADP-ME2 is located in the cell cytosol of all Arabidopsis organs being able to catalyze both the forward (oxidative decarboxylation of malate) and reverse (reductive carboxylation of pyruvate) ME reactions. Particularly, both NADP-ME2 activities are regulated by fumarate. However, it is unclear which one is prevailing in vivo and whether this metabolite may influence the direction of the reaction in any condition. In the present work, an analysis at several pHs was performed using the recombinant enzyme. The results indicated a pH-dependence of the fumarate modulation with opposite behavior on the two activities analyzed. In contrast to the high activation by fumarate observed at pH 7.5, the decarboxylation is not longer activated at pH 7. Instead, in the latter situation, fumarate acted as an inhibitor. Furthermore, the degree of activation for the carboxylation is close 10 times higher at pH 7 than at pH 7.5. Thus, at pH 7 a strong preference of the reverse reaction over the forward one was observed, while at pH 7.5 the opposite is true. Thereby, the concerted and specific fumarate regulation of both reactions by changes in the cytosolic pH would direct the net flux through this key metabolic point, leading to the synthesis or the degradation of C4 compounds according to the physiological context. The cellular conditions are a reflection of the highly varying environments at which plants are exposed. In this regard, a possible mechanism involving diurnal-nocturnal pH fluctuations, NADP-ME2 and the cytosolic fumarase isoform that convert ME substrate into ME modulator is discussed.