The promoter sequences of nodA,nodB,nodD were cloned based on the genomic information of Mesorhizobium loti MAFF303099 and fused with the reporter gene lacZ to construct the nod-lacZ fusion separately. Then these fusions were separately transformed into M. loti MAFF303099 and Rhizobium sp. NGR234 to detect the expression of these nod genes after induced by Lotus japonicus root extract (RE). The results showed that there was no expression of nodA when treated with root extracts.RE induced expression of nodB and nodD1,but showed upregulation of just 2-3 times. nodD2 expressed constitutively in M. loti MAFF303099.Neither of these induced the expression of nodA in M. loti MAFF303099 induced by 5 different flavonoid compounds separately,but the expression of nodA was detected in NGR234 strain treated by Luteolin.