Nature is rife with examples of mutually beneficial shaping strategies, like the co-evolution of species and animals feeding on, and simultaneously dispersing, plant seeds. The organisms involved in fixing nitrogen provide a particularly fascinating example of such a win-win relationship, which improves the chances of survival for both of the species involved.
Legumes, like all plants, need nitrogen to grow, and they usually get it from the soil in the form of ammonium. When the soil is deficient in ammonium, however, legumes team up with Rhizobium bacteria which can fix nitrogen in the context of a symbiotic relationship with the legume.
Legumes secrete special signal molecules that attract rhizobia and trigger them to secrete so-called nod factors. The nod factors, in turn, trigger the legumes to form nodules in their root systems, which can house bacteria. The legumes then provide nutrients to the rhizobia, and, in return, get the nitrogen that allows them to grow more quickly. The plants “police” the resulting root nodules, to ensure that the bacteria do not use nutrients without fulfilling their part of the bargain. Legumes reduce nodule growth and restrict nutrient flows to “cheating” bacteria and only select the strains that refrain from cheating to be allowed to enter nodules.
Legumes therefore mirror the steps that are required to sustain a shaping strategy: they engage symbiotic species, orchestrate a win-win relationship by exchanging nutrients, and dynamically adjust the resulting collaboration to ensure the overall health of the symbiosis.