Expression studies of superoxide dismutases in nodules and leaves of transgenic alfalfa reveal abundance of iron-containing isozymes, posttranslational regulation, and compensation of isozyme activities

27 Pags.- 4 Figs.- 2 Tabls. The definitive version, with all 6 Figs., is available at: http://apsjournals.apsnet.org/loi/mpmiThe composition of antioxidant enzymes, especially superoxide dismutase (SOD), was studied in one nontransgenic and three transgenic lines of nodulated alfalfa plants. Transgenic lines overproduced MnSOD in the mitochondria of nodules and leaves (line 1-10), MnSOD in the chloroplasts (line 4-6), and FeSOD in the chloroplasts (line 10-7). In nodules of line 10-7, the absence of transgene-encoded FeSOD activity was due to a lack of mRNA, whereas in nodules of line 4-6 the absence of transgene-encoded MnSOD activity was due to enzyme inactivation or degradation. Transgenic alfalfa showed a novel compensatory effect in the activities of MnSOD (mitochondrial) and FeSOD (plastidic) in the leaves, which was not caused by changes in the mRNA levels. These findings imply that SOD activity in plant tissues and organelles is regulated, at least partially, at the posttranslational level. All four lines had low CuZnSOD activities and an abundant FeSOD isozyme, especially in nodules, indicating that FeSOD performs important antioxidant functions other than the scavenging of superoxide radicals generated in photosynthesis. This was confirmed by the detection of FeSOD cDNAs and proteins in nodules of other legumes such as cowpea, pea, and soybean. The cDNA encoding alfalfa nodule FeSOD was characterized and the deduced protein found to contain a plastid transit peptide. A comparison of sequences and other properties reveals that there are two types of FeSODs in nodules.This work was supported by grant 2FD97-1101 from the Comisión Interministerial de Ciencia y Tecnología and the European Commission and by grant PB98-0522 from the Dirección General de Investigación Científica (Spain). The Institute of Grassland and Environmental Research is grant aided by the Biotechnology and Biological Sciences Research Council. M. C. Rubio was the recipient of a predoctoral fellowship from the Ministry of Education and Culture (Spain). M. C. Rubio and J. Ramos also were supported by contracts on grant 2FD97-1101.Peer Reviewe