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Expression of a Class 1 Hemoglobin Gene and Production of Nitric Oxide in Response to Symbiotic and Pathogenic Bacteria in Lotus japonicus

¹Graduate School of Science and Technology, Kagoshima University, Kagoshima 890-0065, Japan; ²National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305--8602, Japan; ³Frontier Science Research Center and ⁴Department of Chemistry and Bioscience, Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan; ⁵Department of Agricultural Sciences, Faculty of Agriculture, Saga University, Saga 840-8502, Japan

Symbiotic nitrogen fixation by the collaboration between leguminous plants and rhizobia is an important system in the global nitrogen cycle, and some molecular aspects during the early stage of host-symbiont recognition have been revealed. To understand the responses of a host plant against various bacteria, we examined expression of hemoglobin (Hb) genes and production of nitric oxide (NO) in Lotus japonicus after inoculation with rhizobia or plant pathogens. When the symbiotic rhizobium Mesorhizobium loti was inoculated, expression of LjHb1 and NO production were induced transiently in the roots at 4 h after inoculation. In contrast, inoculation with the nonsymbiotic rhizobia Sinorhizobium meliloti and Bradyrhizobium japonicum induced neither expression of LjHb1 nor NO production. When L. japonicus was inoculated with plant pathogens (Ralstonia solanacearum or Pseudomonas syringae), continuous NO production was observed in roots but induction of LjHb1 did not occur. These results suggest that modulation of NO levels and expression of class 1 Hb are involved in the establishment of the symbiosis.