Abstract
Bradyrhizobia are known symbiotic partners of soybean. However, some soybean cultivars restrict nodulation by some Bradyrhizobium bacterial strains. These restrictions are related to compatibility between the Rj genes of soybean cultivars and nodulation types of inoculated bacteria. The objective of this study was to determine nodulation incompatibility of Type B strains with Rj3 soybean cultivars. Newly isolated B. elkanii strains BLY3-8 and BLY6-1 from Myanmar and specific strain Bradyrhizobium elkanii USDA33, which are incompatible with Rj3 soybean cultivars, and B. japonicum USDA110 were used as inoculants to check compatibility or incompatibility with Rj3 soybean cultivars. Nitrogen fixation activity was measured by the acetylene reduction method. Ethylene concentration (reduction of acetylene) was determined by flame ionization gas chromatography. According to the inoculation test results, USDA110 was compatible with all soybean cultivars because it formed effective nodules (Figure S1 in Appendix) and possessed nitrogenase activity. Similarly, B. elkanii strains BLY3-8, BLY6-1, and USDA33 were highly compatible with non-Rj and Rj4-gene harboring soybean cultivars because they had the ability to form functional nodules and possessed nitrogenase activity. Inversely, BLY3-8, BLY6-1, and USDA33 were incompatible with Rj3 soybean cultivars because they produced ineffective nodules. Consequently, the ratio of ineffective nodule number to total nodule number was >0.5. Therefore, nodule formation by the newly isolated B. elkanii strains BLY3-8 and BLY6-1 was restricted by the Rj3 soybean cultivars potentially making them useful as specific strains to detect the Rj3 gene in soybean cultivars.
Highlights
Nodulation and symbiotic nitrogen fixation are important for soybean cultivation
These results show that the Rj2Rj3 and Rj3 soybean cultivars restricted nodule formation by B. elkanii strains BLY3-8 and BLY6-1
As a continuing study based on the initial findings, B. elkanii strains BLY3-8 and BLY6-1 were tested for nodule formation on various soybean cultivars harboring different Rj genes
Summary
Nodulation and symbiotic nitrogen fixation are important for soybean cultivation. Symbiotic nitrogen fixation provides 40% - 70% of the total nitrogen requirement of soybean (65 to >160 kg∙N∙ha–1) [1]. Symbiotic nitrogen fixation is highly specific, as a particular species or strain of Rhizobia can perform the symbiotic association with only a specific leguminous species or cultivar [2]. This specificity involves molecular recognition of host plants and bacteria, through the exchange of signaling compounds that induce nodule formation and nitrogen fixation [3] [4]. Bradyrhizobium strains can be divided into nodulation Types A, B, and C based on compatibility of the bradyrhizobia with Rj gene soybean cultivars [6] [7]. Types B and C strains have restricted nodule formation on Rj2Rj3 and Rj4 genotype cultivars, respectively. Htwe et al [8] reported that strain Types A, B, and C account for 74%, 22%, and 4% of Myanmar Bradyrhizobium strains, respectively
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