Is the regulation of nodulation conserved between species at the cell type level?

Nitrogen is an important limiting macronutrient in plants that regulates plant growth and development. Typically, plants that can take up more nitrogen (N) and utilise it effectively have a higher yield. This is often why farmers apply costly and environmentally expensive fertilisers to their soils. Legumes, such as peas or beans, are unusually able to form a symbiosis with N-fixing bacteria in order to gain otherwise unusable nitrogen from air in soil. These rhizobia are housed in 'nodules' in the roots, where N is exchanged for carbon compounds and a bacterial niche. If the ability to nodulate can be brought to agricultural plants that are not currently able to do so, this could lead to being able to develop the more N-efficient plants that are needed to cope with the ever-increasing global population. Nodulation in non-legume agricultural plants will also reduce our reliance on nitrogen fertilisers and will therefore make the agricultural industry more environmentally friendly and potentially more profitable, particularly for small farm holders.
A novel direction in the study of nodulation is to study one of the few non-legumes that produces nodules in rhizobial symbiosis, Parasponia andersonii. Recent evidence shows that nodulation regulatory genes in the genome of P. andersonii are highly conserved with model legumes such as Medicago truncatula, suggesting that the genetic mechanism of nodulation is also conserved. By understanding the conserved elements of the nodulation process, it will broaden the understanding of nodules and will help future work bring nodulation to other non-legume agricultural plants.