How inefficient rhizobia prolong their existence within nodules

Legumes are important food sources and therefore, the nitrogen fixing ability of legume–rhizobia symbioses have great potential to improve crop yields and/or reduce the use of nitrogenous fertilisers. Unfortunately the nitrogen fixing efficiency of many legume–rhizobial combinations is low. What restricts nodule efficiency? We believe that one answer lies in the neglected field of signal exchange within mature nodules. Indeed molecular determinants that permit rhizobia to persist for months within plants cells are still unknown. Here, we dissect acute infection that triggers nodulation from chronic infection in which the bacteria persist within nitrogen-fixing nodules. We suggest that defence responses are disabled in mature nodules and superseded by specialised mechanisms of bacterial population control.