RESUMEN
We have isolated and characterized a Lotus japonicus gene (Ljsbp) encoding a putative polypeptide with striking homology to the mammalian 56-kDa selenium-binding protein (SBP). cDNA clones homologous to LjSBP were also isolated from soybean, Medicago sativa, and Arabidopsis thaliana. Comparative expression studies in L japonicus and A. thaliana showed that sbp transcripts are present in various tissues and at different levels. Especially in L japonicus nodules and seedpods and A. thaliana siliques, sbp expression appears to be developmentally up-regulated. sbp Gene transcripts were localized by in situ hybridization in the infected cells and vascular bundles of young nodules, while in mature nodules, low levels of expression were only detected in the parenchymatous cells. Expression of sbp transcripts in young seedpods and siliques was clearly visible in vascular tissues and embryos, while in embryos, low levels of expression were detected in the root epidermis and the vascular bundles. Polyclonal antibodies raised against a truncated LjSBP recombinant protein recognized a polypeptide of about 60 kDa in nodule extracts. Immunohistochemical experiments showed that accumulation of LjSBP occurred in root hairs, in the root epidermis above the nodule primordium, in the phloem of the vasculature, and abundantly in the infected cells of young nodules. Irrespective of the presence of rhizobia, expression of SBP was also observed in root tips, where it was confined in the root epidermis and protophloem cells. We hypothesize that LjSBP may have more than one physiological role and can be implicated in controlling the oxidation/reduction status of target proteins, in vesicular Golgi transport, or both.