Nitrate induced regulation of nodule formation in soybean

Nodule formation was inhibited by exposing soybean plants to nitrate in plastic growth pouches. Exposure to 15 millimolar nitrate resulted in a 2.5-fold decrease in the number of nodules formed in the region of the primary root above the mark made at the time of inoculation to indicate the position of the root tip. Serial section analysis of Bradyrhizobium infections in this region revealed that infection initiation was inhibited approximately 3-fold by exposure to nitrate. Both initial cortical cell divisions and infection thread formation were inhibited. If exposure to nitrate was delayed for 18 hours after the time of inoculation, inhibition was much reduced. This indicates that most of the nitrate-sensitive events of infection were functionally complete within less than 18 hours. Exposure to nitrate for periods of 4 to 24 hours after inoculation, followed by transfer to no-nitrate conditions for the remainder of the time, did not result in substantial inhibition of nodule number. This indicates that the effects of nitrate on infection initiation can be almost entirely reversible. Split towel pouches were used to physically separate portions of the primary root exposed to nitrate and portions of the root exposed to rhizobia. In experiments where nitrate was applied either below or above the inoculated region of the primary root, the degree of inhibition of nodulation was not correlated with either the external concentration of nitrate in contact with root cells undergoing infection or with the internal concentration of nitrate in the infectible region of the root. These results indicate that nitrate itself may not directly inhibit infection initiation or induce host regulatory responses.