The effect of temperature and genistein concentration on lipo-chitooligosaccharide (LCO) production by wild-type and mutant strains of Bradyrhizobium japonicum

Lipo-chitooligosaccharides (LCOs), also known as nod factors, are the bacteria-to-plant signal molecules synthesized in response to the plant-to-bacteria signals, usually flavonoids. In Canada, low soil temperature is potentially a major factor limiting soybean growth and symbiotic nitrogen fixation. Low temperatures cause reductions in the amount of flavonoids, especially genistein, in soybean roots and also inhibit the expression of nod genes resulting in a delay in the onset of nodulation. The addition of genistein can overcome the earlier negative effects of low temperature. However, genistein is expensive and more cost-effective approaches to this problem may exist. We used UV mutagenesis to generate 10 mutants from Bradyrhizobium japonicum strain USDA 110, that expressed nod genes in the absence of plant-to-bacteria signal molecules (e.g. genistein) and they were further screened for their ability to synthesize LCO at three different temperatures (15, 17 and 25 °C). No detectable LCO was produced by these strains and mutants at 0 and 0.01 μM genistein. However, at higher genistein levels (0.1 and 1 μM) all mutants produced more LCO, at the three temperatures tested, than the wild type USDA 110 and 532C. Mutants Bj30054, Bj30056 and Bj30057 were most promising, producing the maximum concentration of LCO at low incubation temperatures (15 and 17 °C). Therefore, we envisage that under short season conditions that are characterized by low spring soil-temperatures these mutants (Bj30054, Bj30056 and Bj30057) would produce more LCO than USDA 110 or 532C, possibly leading to better nodulation and N2 fixation and eventually soybean yield.