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Characterization of Genomic Clones and Expression Analysis of the Three Types of Superoxide Dismutases During Nodule Development in Lotus japonicus

¹Department of Cell Biology, Institute of Biology, Leiden University, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands; ²Departamento de Nutrición Vegetal, Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas, Apdo 202, 50080 Zaragoza, Spain; ³Kazusa DNA Research Institute 2-6-7 Kazusa-Kamatari, Kisarazu, Chiba 292-0818, Japan; ⁴Centre for High Resolution Imaging and Processing, MSI/WTB Complex, School of Life Sciences, University of Dundee, Dundee DD1 5EH, U.K.

Superoxide dismutases (SODs) are metalloenzymes that play a primary role in the protection against oxidative stress in plants and other organisms. We have characterized four SOD genes in Lotus japonicus and have analyzed their expression in roots and four developmental stages of nodules. The expression of cytosolic CuZnSOD, at the mRNA, protein, and enzyme activity levels, decreases with nodule age, and the protein is localized in the dividing cells and infection threads of emergent nodules and in the infected cells of young nodules. The mitochondrial MnSOD was downregulated, whereas the bacteroidal MnSOD displayed maximal protein and enzyme activity levels in older nodules. Two additional genes, encoding plastidic (FeSOD1) and cytosolic (FeSOD2) FeSOD isoforms, were identified and mapped. The genes are located in different chromosomes and show differential expression. The FeSOD1 mRNA level did not change during nodule development, whereas FeSOD2 was upregulated. The distinct expression patterns of the SOD genes may reflect different regulatory mechanisms of the enzyme activities during nodule ontogeny. In particular, at the mRNA and activity levels, the virtual loss of cytosolic CuZnSOD in mature and old nodules, concomitant with the induction of FeSOD2, suggests that the two enzymes may functionally compensate each other in the cytosol at the late stages of nodule development.

Additional keywords: differential gene expression, model legume.