Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2004
Publication Date: 2/20/2005
Citation: Lopez-Millan, A., Ellis, D.R., Grusak, M.A. 2005. Effect of zinc and manganese supply on the activities of superoxide dismutase and carbonic anhydrase in Medicago truncatula wild type and raz mutant plants. Plant Science. 168:1015-1022. Interpretive Summary: Micronutrient metals, like zinc, iron, copper and manganese, serve many important functions in plants. Unfortunately, we have not yet determined all aspects of how these metals are moved throughout the plant, or how they are utilized within various compartments of plant cells. We have been studying a unique plant mutant, of the species Medicago truncatula, that can accumulate high levels of zinc in its tissues, but which also exhibits symptoms of zinc deficiency. In order to understand whether the mutant is unable to properly move zinc or other metals into various cellular compartments, we analyzed the activity of several enzymes that require metals, and which are known to reside in different cellular compartments. Low activity of these enzymes has previously been used as an indicator of low tissue zinc availability. We hoped that this would indicate where an inadequate partitioning might be occurring in the mutant. We studied the activity of three types of superoxide dismutase enzymes, and a carbonic anhydrase enzyme, both in leaves and roots. We also grew plants on varying levels of zinc and manganese, in order to alter the levels of these metals in both normal and mutant plants. We found no major differences in enzyme activities between the mutant and normal plant, suggesting that compartmental availability of zinc was not altered in the mutant. Other factors must be responsible for the observed zinc deficiency symptoms in this plant. However, we also learned that the activity of carbonic anhydrase could be used as an indicator of elevated tissue zinc status in plants, at least up to normally observed tissue concentrations.
Technical Abstract: 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.