Effect of organic N source on bacterial growth, lipo-chitooligosaccharide production, and early soybean nodulation by Bradyrhizobium japonicum

Production of Bradyrhizobium japonicum inoculants is problematic because high inoculation rates are necessary but expensive, while production of rhizobial Nod factors (lipo-chitooligosaccharides (LCOs)), key signal molecules in the establishment of legume–rhizobia symbioses, may be inhibited at high culture cell densities. We conducted experiments to determine the effects of growth medium N source on B. japonicum growth, LCO production, and early nodulation of soybean. We found that 1.57 mmol ammonium nitrate·L–1 resulted in less rhizobial growth and rhizobial capacity to produce LCOs (on a per cell basis) than did 0.4 g yeast extract·L–1, which contained the same amount of N as the ammonium nitrate. Increasing yeast extract to 0.8 g·L–1 increased rhizobial growth and LCO production on a volume basis (per litre of culture) and did not affect cell capacity to produce LCOs; however, at 1.4 g yeast extract·L–1 per cell, production was reduced. A mixture of 0.8 g yeast extract·L–1 and 1.6 g casein hydrolysate·L–1 resulted in the greatest bacterial growth and LCO production on a volume basis but reduced LCO production per cell. Changes in organic N level and source increased production of some of the measured LCOs more than others. LCO production was positively correlated with cell density when expressed on a volume basis; however, it was negatively correlated on a per cell basis. We conclude that although quorum sensing affected Nod factor production, increased levels of organic N, and specific compositions of organic N, increased LCO production on a volume basis. Greenhouse inoculation experiments showed that the medium did not modify nodule number and N fixation in soybean, suggesting that it could have utility in inoculant production.Key words: Nod factor, casein hydrolysate, yeast extract, quorum sensing.