Effects of Temperature on Competition for Nodulation in Phylogenetically Different Bradyrhizobium Strains

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  • Yokoyama Tadashi
    Natl. Inst. Agro-Biol. Resour.:(present address) Tokyo Univ. Agric. Technol

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  • Bradyrhizobium属根粒菌のダイズへの根粒形成における温度の影響
  • Bradyrhizobiumゾク コンリュウキン ノ ダイズ エ ノ コンリュウ ケイセイ ニ オケル オンド ノ エイキョウ

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Abstract

To characterize nodulation genes of soybean Bradyrhizobium which have geographically different origins, we studied the effects of temperature on competition for nodulation among B. japonicum, B. elkanii and Thai soybean Bradyrhizobium in terms of growth rates, competitive nodule occupancies and intensities of nod gene expression. Cell densities evaluated by OD_<600> of B. elkanii USDA31 and USDA76 were superior to those of B. japonicum USDA110 and USDA122 at 28℃ for 43 h after starting the growth test and at 35℃ for 28.5 h, respectively. Cell density of Thai soybean Bradyrhizobium TARC64 was also superior to that of B. japonicum tested at 35℃. Equal volumes of cell density of B. japonicum USDA110, USDA122, USDA123, 5033, B. elkanii USDA31, USDA76, and Thai soybean Bradyrhizobium TARC64 were mixed with each other and inoculated to soybean seeds (Glycine max cv. Enrei) in a mayonnaise bottle filled with 300 mL sterilized vermiculite. Soybean seeds in the bottles were kept in 3 different temperature combinations of 23℃ (day) 18℃ (night), 25℃-25℃, and 34℃-28℃ for 21 days. At combinations of 23℃-18℃ and 25℃-25℃, percents of nodule occupancies of B. japonicum were 55.9 and 46.6, respectively. In the case of B. elkanii, percents of nodule occupancies were 15.5 and 28.0, respectively. However, at the combination of 34℃-28℃, the percent of nodule occupancy of B. japonicum drastically decreased to 8.1, whereas that of B. elkanii rapidly increased to 90.2. To evaluate intensities of nod gene expression among the different strains under different temperature conditions from 20 to 40℃, pZB32 plasmids carrying nodY-lacZ fusion were introduced into USDA110, USDA76, and TARC64. B. japonicum USDA110 carrying pZB32 showed the highest nod gene induction activity at 23℃. In the case of B. elkanii carrying pZB32, the strain had the highest nod gene induction activity at 35℃. TARC64 carrying pZB32 also showed the highest nod gene induction activity at 35℃. These results indicate that Bradyrhizobium has preferential temperatures for nod gene induction.

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