Evaluation of a Rhizobium meliloti transconjugant for increased nodulation and biological nitrogen fixation in alfalfa

Graduation date: 1989Agrobacterium rhizogenes has been shown to cause root\ud proliferation and increased water use efficiency in\ud certain woody dicots. Root proliferation may be desirable\ud in herbaceous legumes as a means of increasing potential\ud sites for infection by Rhizobium species and subsequent\ud nodulation and nitrogen fixation. Thus, A. rhizogenes was\ud used to inoculate a series of forage legumes to evaluate\ud its ability to stimulate root activity. Growth chamber\ud experiments were conducted in which plants were grown in\ud plant tubes with nutrient agar. Plants were evaluated for\ud root development six weeks after inoculation. The preliminary\ud experiment showed that alfalfa treated with A.\ud rhizogenes 232 produced a significantly (159%) greater\ud number of roots, twice the root volume, 183% greater root\ud fresh weight, and 214% more root dry weight than the\ud uninoculated treatment. In white clover and field pea,\ud however, A. rhizogenes 232 did not produce significantly\ud increased root number or root mass. This experiment\ud indicated that A. rhizogenes was effective in stimulating\ud increased root initiating activity in alfalfa.\ud The positive effects of A. rhizogenes 232 on the\ud initial growth of alfalfa roots led to experiments to\ud evaluate A. rhizogenes's ability to stimulate root activity\ud and the effectiveness of a R. meliloti transconjugant\ud (pRi) in increasing nodulation and biological nitrogen\ud fixation.\ud The R. meliloti (pRi) transconjugant was formed by\ud introducing the Ri plasmid from A. rhizogenes into R.\ud meliloti. This R. meliloti transconjugant BL105-9 was\ud used to inoculate two alfalfa (Medicago sativa) cultivars\ud (Ladak 65 and Vernema) to evaluate its effectiveness for\ud increased nodulation and biological nitrogen fixation.\ud Strain effectiveness was determined following six weeks in\ud plant tubes containing nutrient agar in a growth chamber\ud experiment. Although the R. meliloti transconjugant\ud (BL105-9) did not produce significantly more root or shoot\ud mass than the R. meliloti parent (BL116), it did produce\ud significantly (72.8%) more nodules and 99.2% more nodule\ud dry weight. No significant difference in either total\ud plant nitrogen level or acetylene reduction rate was\ud observed between the R. meliloti transconjugant and the\ud parent strain. Thus, this experiment indicates that\ud introduction of the hairy root plasmid into R. meliloti\ud resulted in increased nodulation of alfalfa, but this\ud increased nodulation was not translated into increased\ud total plant nitrogen or increased dry matter production