From North to South: a latitudinal look at legume nodulation processes

Legumes and some nodulation processes evolved about 55-60 Ma. Since then they have radiated from their origin at either side of the Tethys seaway, to high latitudes in both the northern and southern hemispheres. In many cases this has involved different tribes and genera, and different nodule processes, but with the common feature that almost all legumes in the higher latitudes are potentially nodulated and, with the exception of some herbaceous species of Chamaecrista, nodulated caesalpinioid legumes are rare. This is not true of tropical regions where all three sub-families are found, with many of their species lacking the ability to nodulate. Whether or not this is related to the availability of combined nitrogen is a matter of current discussion. This review will consider the range of nodulation phenotypes (infection, morphology, structure) and show how many of these are confined to one or other hemisphere. How this might relate to the different genera and species of endophytic (nitrogen fixing) rhizobia in relation to soil conditions will also be discussed. Better knowledge of the wide variation in nodulation parameters is important for understanding the ecology of different regions and for management purposes. Nodule characters are of great potential use in defining taxonomic groupings in legumes.