Bacteroid oxalate oxidase and soluble oxalate in nodules of faba beans (Vicia fabaL.) submitted to water restricted conditions: possible involvement in nitrogen fixation

Abstract

The inhibitory effect exerted by water stress on acetylene reduction activity (ARA) by nodulated roots of faba beans (Vicia faba L.) was correlated with a 40% decline in the organic acid pool of nodule cytosol. Oxalate concentration was lowered (-55%) whereas a stimulation of the bacteroid oxalate oxidase concomitantly occurred. This enzyme was characterized by an optimal activity at pH 8 but, as in higher plants, exhibited a K m for oxalate of 1.4 mM and an inhibition by substrate excess. Oxalate provided to bacteroid incubations supported C 2 H 2 reduction up to 2.5 mM whereas higher concentrations were strongly inhibitory. In contrast, purified symbiosomes incubated with oxyleghaemoglobin reduced C 2 H 2 in the presence of oxalate concentrations up to 10 mM. The peribacteroid membrane (PBM), in controlling the oxalate flux to the bacteroids avoided the substrate inhibition which would limit its efficiency. Thus, oxalate present in high concentration in faba bean nodules could play a role as complementary substrate for bacteroids slowing down the nitrogen fixation decline induced by water restricted conditions.