Ants are less attracted to the extrafloral nectar of plants with symbiotic, nitrogen-fixing rhizobia

Many plants maintain symbiotic relationships with multiple partners that do not interact directly, but are connected through their common host. Understanding the functional interplay of symbionts associated with the same host remains an important challenge in biology. Here we show nitrogen-fixing rhizobia alter the plant chemistry and defensive strategy of lima bean (Phaseolus lunatus) by differentially affecting direct and indirect defenses against herbivores. We inoculated lima bean plants (R+) with a natural rhizobium strain and measured nutritive and defensive plant traits for young, intermediate, and mature leaves in comparison to rhizobia-free (R-) controls. Furthermore, we experimentally induced indirect defense (extrafloral nectar; EFN) and subsequently counted ants attracted to each plant. Rhizobia increased cyanogenesis, a constitutive direct chemical defense against herbivores, but decreased inducible EFN production to 0.5mg sugar g^-1 dw in plants with rhizobia, relative to 1.6mg sugar g^-1dw in rhizobia-free controls. R+ plants attracted significantly fewer ants (mean= 0.9 ants) than R- plants (mean= 2.6 ants). The fundamentally different rhizobia-mediated effects on simultaneously expressed defensive plant traits indicate rhizobia can have significant bottom-up effects on higher trophic levels. Lower ant recruitment in R+ plants likely resulted from decreased EFN, which may be the side-effect of a carbon tradeoff within the plant between EFN and rhizobia. Our results show belowground symbionts can play a critical and underestimated role in determining complex aboveground interactions.