MM19-GENOMIC FACTORS ALTERING BRADYRHIZOBIA GROWTH

IAN MEDICI; LEILA BARTROLÍ; ALEXIS ROMANI; DIEGO COMERCI; MONGIARDINI, ELIAS; ALFONSO SOLER BISTUE

los cocos Cordoba

Congreso; Congreso SAMIGE 2022; 2022

SAMIGE

Bradyrhizobia are among the slowest growing cultivable bacteria known. The study of thisbacterial genus is of great economic importance for Argentina, due to its use as an inoculant insoybean cultivation. However, its low growth rate (GR) diffucults its study and biotechnologicalutility. The genetic factors shaping GR are still unknown. However, the growing genomicdatabase and comparative genomics studies, offer some clues. For instance, the number ofribosomal RNA operons (rrn) and their proximity to the origin of replication of the chromosome(oriC) correlates to GR. Bacteria bearing a high number of (rrn), display higher GRs reflected inshorter generation times (GT). The rrn number varies from 1 to 16 copies in Bacterial genomes,with an average of 6 copies per genome. Examination of complete Bradyrhizobium genomes (GTbetween 10 and 18hs), shows that these clade bear only 1 or 2 rrn. Previous results of our group,indicate that strains of B. diazoefficiens, which possess only one rrn, grow slower than B.japonicum, which possesses two copies of the operon. Moreover, strains bearing 2 rrn, displayeda shorter lag phase and outcompeted strains with 1 rrn when co-cultured. To further test thelink between the observed effects and ploidy of rrn, we delete one of the rrn copies in B.japonicum E109 and its growth was analyzed. We observed that the elimination of one of thecopies of the rrn in B. japonicum E109, caused a 20% decrease in GR and a 25% increase in theduration of the lag phase. As a parallel approach to obtain mutants with faster GR, we sat up along-term evolution experiment (LTEE). This methodology consists of making populations evolveby continuous culturing strains while avoiding populations bottlenecks. Thus, within evolvedpopulations, one can search for clones with modified GR, and determine which mutationsgenerate the phenotype by genome sequencing. Here, we carried out an LTEE of 5 populationsof B. japonicum E109. At 500 generations, we observed a reproducible increase in GR, in allpopulations. The faster evolved populations (GT= 6hs), showed a reduction of 30% of GTcompared to the non-evolved same population (GT = 8,5hs). Fast growing clones were isolatedfrom these populations and were fully sequenced. They also showed an improved growth,consistent with the observed at population levels. In the future, these two approaches will leadus to a better understanding of the genomic factors that shape GR, and will make it possible toreprogram the bacterial GR.