Evolving a Rhizobium for non-legume nodulation
Naturally occurring rhizobia in the soil form nodules on legumes and not on non-legumes with the exception of Parasponia, a member of the Ulmaceae family (1). Induction of legume nodules by rhizobia involves a multi step interaction, following an exchange of signals between the Rhizobium strain and its host (2,3,4). Parasponia nodulation resembles the nitrogen-fixing root nodulation formed between other non-legumes and actinomycetes of the genus Frankia. In non-legumes, nodule formation requires the initiation of cell division within the root cortex followed by the induction of a lateral root below the site of cell division The final structure is a lateral root swollen on either side of the central vascular tissue by cortical cells filled with bacteria actively engaged in nitrogen fixation. An important finding was that some strains of rhizobia can nodulate both legumes and Parasponia suggesting that the widely differing nodulation processes are largely plant determined and require the correct trigger(s) from rhizobia or Frankia.