Auxotrophy in rhizobia revisited

What determines symbiotic nitrogen fixation efficiency in rhizobium: recent insights into Rhizobium leguminosarum

Symbiotic nitrogen fixation (SNF) by rhizobium, a Gram-negative soil bacterium, is an essential component in the nitrogen cycle and is a sustainable green way to maintain soil fertility without chemical energy consumption. SNF, which results from the processes of nodulation, rhizobial infection, bacteroid differentiation and nitrogen-fixing reaction, requires the expression of various genes from both symbionts with adaptation to the changing environment. To achieve successful nitrogen fixation, rhizobia and their hosts cooperate closely for precise regulation of symbiotic genes, metabolic processes and internal environment homeostasis. Many researches have progressed to reveal the ample information about regulatory aspects of SNF during recent decades, but the major bottlenecks regarding improvement of nitrogen-fixing efficiency has proven to be complex. In this mini-review, we summarize recent advances that have contributed to understanding the rhizobial regulatory aspects that determine SNF efficiency, focusing on the coordinated regulatory mechanism of symbiotic genes, oxygen, carbon metabolism, amino acid metabolism, combined nitrogen, non-coding RNAs and internal environment homeostasis. Unraveling regulatory determinants of SNF in the nitrogen-fixing protagonist rhizobium is expected to promote an improvement of nitrogen-fixing efficiency in crop production.

Proteomic analysis of free-living Bradyrhizobium diazoefficiens: highlighting potential determinants of a successful symbiosis

BACKGROUND

Strain CPAC 7 (=SEMIA 5080) was recently reclassified into the new species Bradyrhizobium diazoefficiens; due to its outstanding efficiency in fixing nitrogen, it has been used in commercial inoculants for application to crops of soybean [Glycine max (L.) Merr.] in Brazil and other South American countries. Although the efficiency of B. diazoefficiens inoculant strains is well recognized, few data on their protein expression are available.

RESULTS

We provided a two-dimensional proteomic reference map of CPAC 7 obtained under free-living conditions, with the successful identification of 115 spots, representing 95 different proteins. The results highlighted the expression of molecular determinants potentially related to symbiosis establishment (e.g. inositol monophosphatase, IMPase), fixation of atmospheric nitrogen (N2) (e.g. NifH) and defenses against stresses (e.g. chaperones). By using bioinformatic tools, it was possible to attribute probable functions to ten hypothetical proteins. For another ten proteins classified as "NO related COG" group, we analyzed by RT-qPCR the relative expression of their coding-genes in response to the nodulation-gene inducer genistein. Six of these genes were up-regulated, including blr0227, which may be related to polyhydroxybutyrate (PHB) biosynthesis and competitiveness for nodulation.

CONCLUSIONS

The proteomic map contributed to the identification of several proteins of B. diazoefficiens under free-living conditions and our approach-combining bioinformatics and gene-expression assays-resulted in new information about unknown genes that might play important roles in the establishment of the symbiosis with soybean.

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

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.