Key roles of microsymbiont amino acid metabolism in rhizobia-legume interactions

Abstract

Rhizobia are bacteria in the α-proteobacterial genera Rhizobium, Sinorhizobium, Mesorhizobium, Azorhizobium and Bradyrhizobium that reduce (fix) atmospheric nitrogen in symbiotic association with a compatible host plant. In free-living and/or symbiotically associated rhizobia, amino acids may, in addition to their incorporation into proteins, serve as carbon, nitrogen or sulfur sources, signals of cellular nitrogen status and precursors of important metabolites. Depending on the rhizobia-host plant combination, microsymbiont amino acid metabolism (biosynthesis, transport and/or degradation) is often crucial to the establishment and maintenance of an effective nitrogen-fixing symbiosis and is intimately interconnected with the metabolism of the plant. This review summarizes past findings and current research directions in rhizobial amino acid metabolism and evaluates the genetic, biochemical and genome expression studies from which these are derived. Specific sections deal with the regulation of rhizobial amino acid metabolism, amino acid transport, and finally the symbiotic roles of individual amino acids in different plant-rhizobia combinations.

Genetic and biochemical characterization of arginine biosynthesis in Sinorhizobium meliloti 1021

“…The latter gene is annotated as encoding a dehydrogenase of unknown function, but belonging to COG111, phosphoglycerate dehydrogenase and related dehydrogenases. S. meliloti encodes a chromosomal phosphoglycerate dehydrogenase (SerA) likely participating in serine biosynthesis (Dunn, 2015). The prototrophy of the argE mutant (see below), which may also lack the dehydrogenase, argues against the participation of SMa1838 in the production of serine or another metabolite required for growth.…”

“…L-Amino acid synthesis by rhizobia is generally crucial for the establishment and/or maintenence of a nitrogen-fixing symbiosis. In the alfalfa microsymbiont Sinorhizobium meliloti, mutants defective in the early steps of the arginine pathway (ornithine/arginine auxotrophs) are symbiotically defective, while those blocked in later steps (ornithine prototrophs/arginine auxotrophs) are effective, indicating that ornithine is especially important in symbiosis (Dunn, 2015).…”

“…Ornithine is produced in S. meliloti as an intermediate in arginine biosynthesis and as a product of arginine degradation (Dunn, 2015). Besides arginine, metabolites derived from ornithine include polyamines (Shaw et al, 2010), ornithine-containing lipids (Ló pez-Lara et al, 2005) and proline (Soto et al, 1994a, b).…”

“…Besides arginine, metabolites derived from ornithine include polyamines (Shaw et al, 2010), ornithine-containing lipids (Ló pez-Lara et al, 2005) and proline (Soto et al, 1994a, b). We hypothesize that a reduced ability to synthesize polyamines, which are required for the normal growth of rhizobia (Shaw 2011), explains why S. meliloti ornithine auxotrophs are symbiotically defective (Arteaga & Dunn, 2015;Dunn, 2015).…”

“…1) contains both ArgE and ArgJ (Dunn, 2015), which are non-homologous isofunctional enzymes, rather than isoforms derived from a common ancestral form (Omelchenko et al, 2010). Because ArgE is the only enzyme of the predicted pathway encoded on a megaplasmid, it was suggested to have an ancillary role in ornithine production (Barnett et al, 2001).…”