A Medicago truncatula line that persistently fixes nitrogen in response to nitrogen fertiliser

The legume-rhizobial symbiosis enables the fixing of atmospheric nitrogen by a naturally occurring biological process that takes place in specialised root nodules. However, the symbiosis comes at a high carbon cost to the host and in the presence of nitrogen fertiliser, the plant strongly inhibits this process, both by inhibiting the development of nitrogen fixing nodules and by inhibiting the fixing process in nodules that are already present.

Medicago truncatula is a model legume plant and is also used as a forage crop. We hypothesise that natural variation exists in M. truncatula in the extent of inhibition of symbiotic nitrogen fixation by externally supplied fertiliser.

We inoculated a variety of M. truncatula lines with rhizobia and discovered a line that showed persistent nitrogen fixation (PNF), even in the presence of high nitrate levels. While the control line showed complete inhibition of symbiotic nitrogen fixation, the PNF line still acquired 46% of nitrogen from biological fixation.

Long-distance signalling regulates the nodulation process, and we hypothesise that this is perturbed in the PNF line. We tested this using split root grafts between the control and PNF lines, where the stock and grafted roots were grown on substrates with and without nitrate. The results showed that systemic long-distance signalling was altered in the PNF line.

We are currently identifying the genetic basis of this natural variation to reveal the mechanism of the PNF trait. Our work will allow the breeding of persistently nitrogen fixing cultivars even in the presence of soil nitrogen fertiliser.