Functional and genomic analysis of Aminobacter sp. BA135, a noncanonical rhizobium isolated from Lotus tenuis nodules

ESTRELLA, MARÍA J.; SPARHOLT JORGENSEN, TUE; FIGUEROA GALVIS, INGRID PAOLA; CUMPA-VELÁSQUEZ, LIZ MARJORY; STALDER, SANTIAGO; PISTORIO, MARIANO; SANJUAN PINILLA, JUAN; SANNAZZARO, ANALIA INÉS

Congreso; XVIII Congreso de la Sociedad Argentina de Microbiología General; 2023

Prospecting studies of L. tenuis nodule symbionts in sodic alkaline soils revealed the presence of non-canonical rhizobia belonging to the genus Aminobacter. Species of this genus are soil saprophytes with a methylotrophic metabolism, which enables them to degrade a variety of xenobiotic compounds. There are no studies that allow us to infer how this saprophytic bacterium acquired the ability to nodulate a legume. Since L. tenuis forms very specific mutualistic symbioses with bacteria of the genus Mesorhizobium, we assume that symbiotic Aminobacter must have acquired the symbiotic functions present in bacteria of this genus.Nodulation assays were carried out to compare the symbiotic performance of two strains of Aminobacter and Mesorhizobium. Both genera were found to be efficient in N fixation and growth promotion of L. tenuis. Histological sections of the nodules were performed and similar morphologies and infection patterns were observed. The presence of symbiotic genes (such as nodC and nifH) and a region indicative of the Mesorhizobium symbiotic island insertion site were confirmed. These results provide evidence for the acquisition of a symbiosis genomic island with L. tenuis specificity.The genome of the strain Aminobacter BA135 was sequenced to gain insight into the functions that allowed this bacterium to colonize a new niche such as the nodule, and to deepen our knowledge of other adaptive functions to stressful environments. The BA135 genome consists of a chromosome of 5,782,940 bp with a 63.3% G+C content. Two megaplasmids of 584,697 bp and 362,065 bp -with 62.6 and 60.2% GC content, respectively- were found and their presence was confirmed by Eckhardt gel electrophoresis. In silico comparative genomic and phylogenetic analyses placed Aminobacter BA135 in the species A. ciceronei.A complete 489,742 bp symbiosis island was identified, with nif, fix, and nod genes with high similarity to those found in Mesorhizobium islands. BA135 contains genes related to plant-microbe interaction traits, such as 1-aminocyclopropane-1-carboxylate deaminase to degrade the ethylene precursor ACC to 2-oxobutanoate and ammonia, auxin (indolacetic acid) synthesis, siderophore production and transport, etc. Type II, IV, and VI secretion systems (SS) have been found in the chromosome, while p1 contains genes for type I and IV SS. Genes associated with Co, Zn, and Cd resistance and xenobiotic degradation were also found. The above-mentioned genes suggest that BA135 holds a plethora of adaptive functions that enable this strain to cope with challenging environmental conditions. These characteristics, together with its ability to colonize a new niche such as the nodules of leguminous plants, distinguish Aminobacter ciceronei BA135 as a microorganism with great potential to be studied for its role as a biofertiliser in stressed soils. It also represents an interesting model for studies on the mechanisms of horizontal gene transfer between different genera.