| Rhizobium | <bacteria> Gram-negative bacterium that fixes nitrogen in association with roots of some higher plants, notably legumes. Forms root nodules, in which it is converted to the nitrogen fixing bacteroid form. (18 Nov 1997) |
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| rhizobium leguminosarum | A species of gram-negative, aerobic bacteria that causes formation of root nodules on some, but not all, types of field pea, lentil, kidney bean, and clover. (12 Dec 1998) |
| rhizobium meliloti | A species of gram-negative, aerobic bacteria that causes formation of root nodules on some, but not all, types of sweet clover, alfalfa, and fenugreek. (12 Dec 1998) |