Is the legume nodule a modified root or stem or an Organ sui generis?

The legume nodule, which houses nitrogen-fixing rhizobia, is a unique plant organ. Its homology with lateral roots has been inferred by a comparison with other nitrogen-fixing nodules, especially those formed on actinorhizal plants in response to Frankia inoculation or on Parasponia roots following inoculation with Bradyrhizobium species. These nodules are clearly modified lateral roots in terms of their structure and development. However, legume nodules differ from lateral roots and these other nodules in their developmental origin, anatomy, and patterns of gene expression, and, consequently, several other evolutionary derivations, including from stems, wound or defense responses, or the more ancient vesicular-arbuscular mycorrhizal symbiosis, have been postulated for the legume nodule. In this review, we first present a broad view of the legume family showing the diversity of nodulation occurrence and types in the different subfamilies and particularly within the subfamily Papilionoideae. We then define the typological and molecular criteria used to discriminate the basic organs - root, stem, leaf - of the plant. Finally, we discuss the possible origins of the legume nodule in terms of these typological and molecular bases.