Metabolic transitions of rhizobia and application to engineering N2­-fixation in cereals

Abstract: Colonization by bacteria of the zone surrounding plant roots (rhizosphere) is crucial to plant productivity, with plants secreting 10-30% of total photosynthate to engineer the rhizosphere to their advantage. Microarray and metabolic analysis has been used to dissect the composition of the pea root secretome and map the transcriptional response of bacterial to secreted metabolites. This has led to identification of the master regulator of attachment of Rhizobium leguminosarum to pea roots. During subsequent infection of legumes, the metabolic repertoire of rhizobia is dramatically restricted with reduced metabolic diversity in mature bacteroids (differentiated form of rhizobia).  Bacteroids fail to assimilate the ammonium they produce from N₂-fixation and instead secrete it to the plant. They also become symbiotic auxotrophs for a number of amino acids and cofactors. Considering the metabolic exchanges in rhizobia has suggested approaches to engineer N₂-fixation into synthetic symbioses between various bacteria and cereals, which are currently being pursued.