Inhibition of the expression of Bradyrhizobium japonicum nod genes at low temperatures

The effect of temperature on the induction of nod gene expression in Bradyrhizobium japonicum by the isoflavone genistein was investigated. Experiments were conducted to examine the expression of a nodY-lacZ fusion of B. japonicum USDA110 under suboptimal incubation temperatures. The results of these studies indicated that higher genistein concentrations were required for maximum amounts of nodY-lacZ induction at 10 and 15°C than at 25°C. In addition, the time required to achieve maximal amounts of expression was significantly longer at the suboptimal temperatures. The effects of temperature on nod gene expression correlate with previously described inhibition of soybean nodulation that occurs at low root zone temperatures. The findings reported here indicate that this nodulation inhibition is in large part due to temperature effects on nod gene induction.