For better or for worse: cooperation and competition in the legume-rhizobium symbiosis

摘要

Thanks to mutualisms, many species acquire new biological functions, extend their ecological range, and generally increase their fitness. However, each partner has to allocate resources towards its own reproduction and the service it provides to the other. Therefore, even within mutualistic interactions, conflicts of interest can arise from resource allocation trade-offs and lead to mutualism breakdown (Sachs & Simms, 2006). Stabilizing mechanisms are required to prevent such breakdown (Herre et aL,1999). In this issue of New PhytoLogist, Triesen (pp. 1096—1 111) focuses on the symbiosis involving leguminous plants and nitrogen-fixing bacteria called rhizobia that develop in their root nodules. This symbiosis implies large costs for both partners(nitrogen fixation for rhizobia and nodule organogenesis for legumes) but also large benefits: rhizobia can expect a large fitness output (Denison & Kiers, 2011) and legumes gain access to an exclusive source of nitrogen. Triesen took up the challenge ofinvestigating the quantitative relationships between these costs and benefits and questioned the existence of conflicts of interest in this mutualism.