Argonaute5 and its associated small RNAs modulate the transcriptional response during the rhizobia-Phaseolus vulgaris symbiosis.

Both plant- and rhizobia-derived small RNAs play an essential role in regulating the root nodule symbiosis in legumes. Small RNAs, in association with Argonaute proteins, tune the expression of genes participating in nodule development and rhizobial infection. However, the role of Argonaute proteins in this symbiosis has been overlooked. In this study, we provide transcriptional evidence showing that Argonaute5 (AGO5) is a determinant genetic component in the root nodule symbiosis in Phaseolus vulgaris. A spatio-temporal transcriptional analysis revealed that the promoter of PvAGO5 is active in lateral root primordia, root hairs from rhizobia-inoculated roots, nodule primordia, and mature nodules. Transcriptional analysis by RNA sequencing revealed that gene silencing of PvAGO5 affected the expression of genes involved in the biosynthesis of the cell wall and phytohormones participating in the rhizobial infection process and nodule development. PvAGO5 immunoprecipitation coupled to small RNA sequencing revealed the small RNAs bound to PvAGO5 during the root nodule symbiosis. Identification of small RNAs associated to PvAGO5 revealed miRNAs previously known to participate in this symbiotic process, further supporting a role for AGO5 in this process. Overall, the data presented shed light on the roles that PvAGO5 plays during the root nodule symbiosis in P. vulgaris.

Paenibacillus farraposensis sp. nov., isolated from a root nodule of Arachis villosa.

Strain UY79T was isolated from a root nodule of Arachis villosa, collected at the Esteros de Farrapos National Park, Río Negro, Uruguay. Cells were non-motile Gram-variable rods with central to subterminal oval to ellipsoidal endospores that swell the sporangia. Growth was observed in the range of 15-42 °C (optimum, 30 °C), pH 5.0-9.0 (optimum, pH 7.0-8.0) and with up to 3 % (w/v) NaCl (optimum, 1-2 %). Strain UY79T was facultative anaerobic, catalase-positive and oxidase-negative. According to the results of 16S rRNA gene sequence analysis, UY79T belongs to the genus Paenibacillus and is closely related to P. ottowii MS2379T, P. peoriae BD-57T, P. polymyxa ATCC 842T and P. brasilensis PB172T, exhibiting 99.4, 99.0, 99.0 and 98.9% sequence identity, respectively. Average nucleotide identity and digital DNA-DNA hybridization values with the most closely related type strains were 74.3-88.6% and 38.2-48.7 %, respectively. Major fatty acids (>10 %) were anteiso-C15:0, iso-C15:0, and C16 : 0. Menaquinones MK-7 and MK-6 were the only isoprenoid quinones detected. Major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and an unidentified glycolipid. Spermidine was the predominant polyamine. The DNA G+C content based on the draft genome sequence was 46.34 mol%. Based on the current polyphasic study, UY79T represents a novel species of the genus Paenibacillus, for which the name Paenibacillus farraposensis sp. nov. is proposed. The type strain is UY79T (=CCM 9147T=CGMCC 1.19038T).

Early Molecular Dialogue Between Legumes and Rhizobia: Why Are They So Important?

Legume-rhizobia symbiosis has a considerable ecological relevance because it replenishes the soil with fixed-nitrogen (e.g., ammonium) for other plants. Because of this benefit to the environment, the exploitation of the legume-rhizobia symbiosis can contribute to the development of the lower input, sustainable agriculture, thereby, reducing dependency on synthetic fertilizers. To achieve this goal, it is necessary to understand the different levels of regulation of this symbiosis to enhance its nitrogen-fixation efficiency. A different line of evidence attests to the relevance of early molecular events in the establishment of a successful symbiosis between legumes and rhizobia. In this chapter, we will review the early molecular signaling in the legume-rhizobia symbiosis. We will focus on the early molecular responses that are crucial for the recognition of the rhizobia as a potential symbiont.

In BPS1 Downregulated Roots, the BYPASS1 Signal Disrupts the Induction of Cortical Cell Divisions in Bean-Rhizobium Symbiosis.

BYPASS1 (BPS1), which is a well-conserved gene in plants, is required for normal root and shoot development. In the absence of BPS1 gene function, Arabidopsis overproduces a mobile signalling compound (the BPS1 signal) in roots, and this transmissible signal arrests shoot growth and causes abnormal root development. In addition to the shoot and root meristem activities, the legumes also possess transient meristematic activity in root cortical cells during Rhizobium symbiosis. We explored the role of Phaseolus vulgaris BPS1 during nodule primordium development using an RNA-interference (RNAi) silencing approach. Our results show that upon Rhizobium infection, the PvBPS1-RNAi transgenic roots failed to induce cortical cell divisions without affecting the rhizobia-induced root hair curling and infection thread formation. The transcript accumulation of early nodulin genes, cell cyclins, and cyclin-dependent kinase genes was affected in RNAi lines. Interestingly, the PvBPS1-RNAi root nodule phenotype was partially rescued by exogenous application of fluridone, a carotenoid biosynthesis inhibitor, which was used because the carotenoids are precursors of BPS1 signalling molecules. Furthermore, we show that the PvBPS1 promoter was active in the nodule primordia. Together, our data show that PvBPS1 plays a vital role in the induction of meristematic activity in root cortical cells and in the establishment of nodule primordia during Phaseolus-Rhizobium symbiosis.

High-quality permanent draft genome sequence of Bradyrhizobium sp. Ai1a-2; a microsymbiont of Andira inermis discovered in Costa Rica.

Bradyrhizobium sp. Ai1a-2 is is an aerobic, motile, Gram-negative, non-spore-forming rod that was isolated from an effective nitrogen fixing root nodule of Andira inermis collected from Tres Piedras in Costa Rica. In this report we describe, for the first time, the genome sequence information and annotation of this legume microsymbiont. The 9,029,266 bp genome has a GC content of 62.56% with 247 contigs arranged into 246 scaffolds. The assembled genome contains 8,482 protein-coding genes and 102 RNA-only encoding genes. This rhizobial genome was sequenced as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) project proposal.

High-quality permanent draft genome sequence of Bradyrhizobium sp. Tv2a.2, a microsymbiont of Tachigali versicolor discovered in Barro Colorado Island of Panama.

Bradyrhizobiumsp. Tv2a.2 is an aerobic, motile, Gram-negative, non-spore-forming rod that was isolated from an effective nitrogen-fixing root nodule of Tachigali versicolor collected in Barro Colorado Island of Panama. Here we describe the features of Bradyrhizobiumsp. Tv2a.2, together with high-quality permanent draft genome sequence information and annotation. The 8,496,279 bp high-quality draft genome is arranged in 87 scaffolds of 87 contigs, contains 8,109 protein-coding genes and 72 RNA-only encoding genes. This rhizobial genome was sequenced as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) project.

Taxonomy of rhizobia and agrobacteria from the Rhizobiaceae family in light of genomics.

Phylogenomic analyses showed two major superclades within the family Rhizobiaceae that corresponded to the Rhizobium/Agrobacterium and Shinella/Ensifer groups. Within the Rhizobium/Agrobacterium group, four highly supported clades were evident that could correspond to distinct genera. The Shinella/Ensifer group encompassed not only the genera Shinella and Ensifer but also a separate clade containing the type strain of Rhizobium giardinii. Ensifer adhaerens (Casida A(T)) was an outlier within its group, separated from the rest of the Ensifer strains. The phylogenomic analysis presented provided support for the revival of Allorhizobium as a bona fide genus within the Rhizobiaceae, the distinctiveness of Agrobacterium and the recently proposed Neorhizobium genus, and suggested that R. giardinii may be transferred to a novel genus. Genomics has provided data for defining bacterial-species limits from estimates of average nucleotide identity (ANI) and in silico DNA-DNA hybridization (DDH). ANI reference values are becoming the gold standard in rhizobial taxonomy and are being used to recognize novel rhizobial lineages and species that seem to be biologically coherent, as shown in this study.

Genome sequence of Ensifer arboris strain LMG 14919(T); a microsymbiont of the legume Prosopis chilensis growing in Kosti, Sudan.

Ensifer arboris LMG 14919(T) is an aerobic, motile, Gram-negative, non-spore-forming rod that can exist as a soil saprophyte or as a legume microsymbiont of several species of legume trees. LMG 14919(T) was isolated in 1987 from a nodule recovered from the roots of the tree Prosopis chilensis growing in Kosti, Sudan. LMG 14919(T) is highly effective at fixing nitrogen with P. chilensis (Chilean mesquite) and Acacia senegal (gum Arabic tree or gum acacia). LMG 14919(T) does not nodulate the tree Leucena leucocephala, nor the herbaceous species Macroptilium atropurpureum, Trifolium pratense, Medicago sativa, Lotus corniculatus and Galega orientalis. Here we describe the features of E. arboris LMG 14919(T), together with genome sequence information and its annotation. The 6,850,303 bp high-quality-draft genome is arranged into 7 scaffolds of 12 contigs containing 6,461 protein-coding genes and 84 RNA-only encoding genes, and is one of 100 rhizobial genomes sequenced as part of the DOE Joint Genome Institute 2010 Genomic Encyclopedia for Bacteria and Archaea-Root Nodule Bacteria (GEBA-RNB) project.