Effects of Increased 1-Aminocyclopropane-1-Carboxylate (ACC) Deaminase Activity in Bradyrhizobium sp. SUTN9-2 on Mung Bean Symbiosis under Water Deficit Conditions.

Bacteria exhibiting 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity, which inhibits the biosynthesis of ethylene in higher plants, promote plant growth through the degradation of ethylene precursors, such as ACC. ACC deaminase activity in Bradyrhizobium sp. SUTN9-2 was enhanced by genetic engineering and adaptive laboratory evolution (ALE)-based methods. The transferal of a plasmid containing the acdR and acdS genes into SUTN9-2 was genetic engineering improved, while the ALE method was performed based on the accumulation of an adaptive bacterial population that continuously grew under specified growth conditions for a long time. ACC deaminase enzyme activity was 8.9-fold higher in SUTN9-2:pMG103::acdRS and 1.4-fold higher in SUTN9-2 (ACCDadap) than in the wild-type strain. The effects of increased activity were examined in the host plant (Vigna radiata (L.) R.Wilczek SUT1). The improved strains enhanced nodulation in early stage of plant growth. SUTN9-2:pMG103::acdRS also maintained nitrogen fixation under water deficit conditions and increased the plant biomass after rehydration. Changes in nucleotides and amino acids in the AcdS protein of SUTN9-2 (ACCDadap) were then investigated. Some nucleotides predicted to be located in the ACC-binding site were mutated. These mutations may have increased ACC deaminase activity, which enhanced both symbiotic interactions and drought tolerance and promoted recovery after rehydration more than lower ACC deaminase activity. Adaptive evolution represents a promising strategy for further applications in the field.

Effect of rice cultivation systems on indigenous arbuscular mycorrhizal fungal community structure.

Arbuscular mycorrhizal fungi (AMF) in an agricultural ecosystem are necessary for proper management of beneficial symbiosis. Here we explored how the patterns of the AMF community in rice roots were affected by rice cultivation systems (the system of rice intensification [SRI] and the conventional rice cultivation system [CS]), and by compost application during growth stages. Rice plants harvested from SRI-managed plots exhibited considerably higher total biomass, root dry weight, and seed fill than those obtained from conventionally managed plots. Our findings revealed that all AMF sequences observed from CS plots belonged (only) to the genus Glomus, colonizing in rice roots grown under this type of cultivation, while rice roots sown in SRI showed sequences belonging to both Glomus and Acaulospora. The AMF community was compared between the different cultivation types (CS and SRI) and compost applications by principle component analysis. In all rice growth stages, AMF assemblages of CS management were not separated from those of SRI management. The distribution of AMF community composition based on T-RFLP data showed that the AMF community structure was different among four cultivation systems, and there was a gradual increase of Shannon-Weaver indices of diversity (H') of the AMF community under SRI during growth stages. The results of this research indicated that rice grown in SRI-managed plots had more diverse AMF communities than those grown in CS plots.

Genome analysis suggests that the soil oligotrophic bacterium Agromonas oligotrophica (Bradyrhizobium oligotrophicum) is a nitrogen-fixing symbiont of Aeschynomene indica.

Agromonas oligotrophica (Bradyrhizobium oligotrophicum) S58(T) is a nitrogen-fixing oligotrophic bacterium isolated from paddy field soil that is able to grow in extra-low-nutrient environments. Here, the complete genome sequence of S58 was determined. The S58 genome was found to comprise a circular chromosome of 8,264,165 bp with an average GC content of 65.1% lacking nodABC genes and the typical symbiosis island. The genome showed a high level of similarity to the genomes of Bradyrhizobium sp. ORS278 and Bradyrhizobium sp. BTAi1, including nitrogen fixation and photosynthesis gene clusters, which nodulate an aquatic legume plant, Aeschynomene indica, in a Nod factor-independent manner. Although nonsymbiotic (brady)rhizobia are significant components of rhizobial populations in soil, we found that most genes important for nodule development (ndv) and symbiotic nitrogen fixation (nif and fix) with A. indica were well conserved between the ORS278 and S58 genomes. Therefore, we performed inoculation experiments with five A. oligotrophica strains (S58, S42, S55, S72, and S80). Surprisingly, all five strains of A. oligotrophica formed effective nitrogen-fixing nodules on the roots and/or stems of A. indica, with differentiated bacteroids. Nonsymbiotic (brady)rhizobia are known to be significant components of rhizobial populations without a symbiosis island or symbiotic plasmids in soil, but the present results indicate that soil-dwelling A. oligotrophica generally possesses the ability to establish symbiosis with A. indica. Phylogenetic analyses suggest that Nod factor-independent symbiosis with A. indica is a common trait of nodABC- and symbiosis island-lacking strains within the members of the photosynthetic Bradyrhizobium clade, including A. oligotrophica.