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Soybean (lbc3), Parasponia, and Trema Hemoglobin Gene Promoters Retain Symbiotic and Nonsymbiotic Specificity in Transgenic Casuarinaceae: Implications for Hemoglobin Gene Evolution and Root Nodule Symbioses

The purpose of this study was to compare the control of expression of legume and nonlegume hemoglobin genes. We used the Casuarina glauca and Allocasuarina verticillata transformation system to examine the properties of the soybean (lbc3), Parasponia andersonii, and Trema tomentosa hemoglobin gene promoters in actinorhizal plants. Expression of the hemoglobin promoters gus genes was examined by fluorometric and histochemical assays. The fluorometric assays in various organs showed that the soybean and P. andersonii promoters were most active in nodules whereas the T. tomentosa promoter gave a very high activity in roots. The histochemical study showed that GUS activity directed by the soybean and the P. andersonii gus chimeric genes appeared mainly confined to the infected cells of the C. glauca and A. verticillata nodules. The T. tomentosa hemoglobin promoter was primarily expressed in the root's cortex and vascular tissue. The results indicate that the soybean, P. andersonii, and T. tomentosa hemoglobin promoters retain their cell-specific expression in transgenic members of the Casuarinaceae, suggesting a close relationship between legume, Ulmaceae member, and actinorhizal hemoglobin genes. The conservation of the mechanism for nodule-specific expression of soybean, P. andersonii, and C. glauca and A. verticillata hemoglobin genes is discussed in view of recent molecular phylogenetic data that suggest a single origin for the predisposition to form root nodule symbioses.