T. Uchiumi1, Y. Shimoda1, T. Tsuruta1, Y. Mukoyoshi1, A. Suzuki1, S. Sato2, K. Senoo3,
1 Dept of Chemistry and Bioscience, Kagoshima University, Kagoshima 890-0065, Japan
2Kazusa DNA Research Institute, Chiba 292-0812, Japan
3Dept of Sustainable Resource Science, Mie University, Mie 514-8507, Japan
All plants are expected to have hemoglobins; nodule specific globins are called 'symbiotic globins' and globins not involved in symbiosis with rhizobia and Frankia are called 'nonsymbiotic globins'. Leguminous plants have both of the globin genes. However, symbiotic globin (leghemoglobin) accumulates specifically and abundantly in root nodules. The mechanism of controlling the expression of both the globin genes and the function of nonsymbiotic globins in leguminous plants have not been revealed.
Lotus japonicus has three symbiotic (LjLbl, 2, 3) genes and one nonsymbiotic (LjNSGl) globin gene. RT-PCR with specific primer sets for each globin genes showed that all the symbiotic globin genes were expressed specifically and strongly in root nodules. The expression of LjLbl, 2, 3 could not be detected in root, leaf, and stem of a mature plant, however, the low-level expression was detected in young seedlings. This suggests that symbiotic globin may have unknown function other than oxygen transporter for microsymbiont. LjNSGl was expressed at low level in root, leaf, and stem. The expression of LjNSG 1 was enhanced in root nodules, whereas, it was strongly repressed in the roots colonized by mycorrhizal fungi Glomus sp. R10. The expression of the nonsymbiotic globin gene (Mhb\ ) was also repressed in the mycorrhiza of Medicago sativa.
Nitric oxide (NO) is now known to be an "on signal" activating plant defenses against a pathogen (Klessig et al. 2000). Considering that heme can serve as a target of NO and that nonsymbiotic hemoglobin is localized in nuclei as same as NO synthase (Seregelyes et al. 2000), nonsymbiotic hemoglobin might play a critical role in establishing symbiosis between plants and mycorrhizal fungi.
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