Determination of essential mechanisms in rhizobia that sustain free-living conditions and a functional nitrogen-fixing symbiosis with legumes

Rhizobia are soil-bacteria known by their remarkable ability to thrive as oligotrophs and establish agriculturally important nitrogen-fixing symbiosis with leguminous plants. As part of this sophisticated lifestyle, rhizobia must survive and proliferate in the dynamic soil environment before engaging in symbiosis. Symbiosis is characterized by a dramatic number of developmental steps that will allow rhizobia to proliferate inside the legume, in root organs termed nodules. As an endosymbiont, rhizobia will develop into nitrogen-fixing bacteroids capable of converting atmospheric nitrogen into ammonia for the plant. In return, the legume will provide nutrients and a safe space to proliferate. The capacity to rapidly respond to changes within their environment and colonize their legume host has been crucial for the evolutionary success of rhizobia. Accordingly, throughout their evolutionary history, rhizobia have acquired a myriad of gene functions that allow them to cope with the environment. Thus, rhizobia have become a unique model to address how organisms adapt to the environment and establish beneficial interactions with their community. We addressed how rhizobia manage to thrive as oligotrophic soil bacteria. Through a systemswide genetic approach based on Tn5 transposon sequencing (TnSeq) in the rhizobial model Sinorhizobium meliloti, we uncovered the complete set of essential genes for growth under variations in nutrient availability. We identified that from the large genome of S. meliloti (6128 genes), only 5.14% (320 essential genes) encode the most fundamental processes. Moreover, we found that these core-essential genes serve as a molecular chassis on which essential components for specific conditions build on top, hence, reflecting the extensive genomic arsenal that rhizobia have. Furthermore, we show that S. meliloti adapts its cell cycle regulation as a result of the nutrients present in the media. Our results provide novel insights into the mechanisms used by rhizobia to persist as rhizosphere ...