Rhizobium alters inducible defenses in broad bean, Vicia faba

Conversion of inorganic nitrogen by mutualistic nitrogen-fixing bacteria is essential for plant growth and reproduction, as well as the development of chemical and mechanical defenses. It is unclear, however, how these bacteria alter cooccurring symbioses at higher trophic levels; e.g., extrafloral nectary (EFN) induction, in response to herbivory, to attract defensive mutualists. We hypothesized that plants colonized by nitrogen-fixing bacteria would mount a larger inducible, defensive response than plants lacking symbioses, as defensive traits are costly. We predicted that bean plants, Vicia faba L., harboring Rhizobium leguminosarum bv. viciae Frank would produce more EFNs upon leaf damage, than plants lacking the symbionts, as EFN induction in V. faba is resource-dependent. Here we report that V. faba colonized by R. leguminosarum produced similar numbers of EFNs as did plants without symbionts. Plants with symbionts, however, produced significantly fewer EFNs over 1 week in response to leaf damage, than those without leaf damage. As such, nitrogen-fixing bacteria may not always benefit the host plant, but rather, the utility of these bacteria may be dependent on the prevailing ecological conditions.