Effect of zinc and manganese supply on the activities of superoxide dismutase and carbonic anhydrase in Medicago truncatula wild type and raz mutant plants

The novel raz (requires additional zinc) mutant of Medicago truncatula can accumulate high Zn concentrations in all tissues when compared to wild type plants, yet still develops Zn deficiency symptoms, suggesting that total Zn in tissues may not be physiologically available. The objectives of this study were first to determine whether there are differences in the biochemical Zn availability between wild type and raz mutant plants based on the activity of carbonic anhydrase and superoxide dismutase and second to assess the suitability of using the activities of these two Zn-requiring enzymes as indicators of utilizable Zn in a wide range of Zn tissue concentrations. In leaf extracts, CA and total SOD activities as well as the distribution of the tSOD activity among the different isoforms were similar in both genotypes. In roots, there were no significant differences observed in total SOD activities between genotypes; however, CuZnSOD activities were lower in raz than in wild type plants when grown at high Zn concentrations. Based on these results, availability of Zn in raz leaves does not seem to be altered; however, in roots of the raz mutant, Zn availability is restricted or limited in comparison to the wild type plants, especially when raz roots accumulate high levels of Zn. With increasing total Zn tissue concentrations, CA activity increased linearly and then reached a plateau in both leaves and roots, whereas CuZnSOD also increased linearly in leaves but no significant correlation was found in roots, suggesting that CA is a better indicator of tissue Zn status.