Revealing strategies of quorum sensing in Azospirillum brasilense strains Ab-V5 and Ab-V6

Abstract

Azospirillum brasilense is an important plant-growth promoting bacterium (PGPB) that requires several critical steps for root colonization, including biofilm and exopolysaccharide (EPS) synthesis and cell motility. In several bacteria these mechanisms are mediated by quorum sensing (QS) systems that regulate the expression of specific genes mediated by the autoinducers N-acyl-homoserine lactones (AHLs). We investigated QS mechanisms in strains Ab-V5 and Ab-V6 of A. brasilense, which are broadly used in commercial inoculants in Brazil. Neither of these strains carries a luxI gene, but there are several luxR solos that might perceive AHL molecules. By adding external AHLs we verified that biofilm and EPS production and cell motility (swimming and swarming) were regulated via QS in Ab-V5, but not in Ab-V6. Differences were observed not only between strains, but also in the specificity of LuxR-type receptors to AHL molecules. However, Ab-V6 was outstanding in indole acetic acid (IAA) synthesis and this molecule might mimic AHL signals. We also applied the quorum quenching (QQ) strategy, obtaining transconjugants of Ab-V5 and Ab-V6 carrying a plasmid with acyl-homoserine lactonase. When maize (Zea mays L.) was inoculated with the wild-type and transconjugant strains, plant growth was decreased with the transconjugant of Ab-V5-confirming the importance of an AHL-mediated QS system-but did not affect plant growth promotion by Ab-V6.

Quorum sensing communication: Bradyrhizobium‐Azospirillum interaction via N‐acyl‐homoserine lactones in the promotion of soybean symbiosis

“…In relation to the PGPB A. brasilense strains Ab‐V5 and Ab‐V6, we have shown that their genomes encompass 14 and 15 putative lux genes, respectively, representing eight different putative luxR ‐type transcriptional regulatory genes, duplicated throughout the genomes; no luxI gene was detected in any of the strains, confirming our previous report . This observation is in agreement with the reports that luxI genes are rare in Azospirillum , although they have been detected in A. lipoferum , but not in A. brasilense .…”

Production of polyhydroxybutyrate (PHB) and biofilm by Azospirillum brasilense aiming at the development of liquid inoculants with high performance

“…Although the genomes of the two strains show high identity of nucleotides (unpublished genomes, but with average nucleotide identity of 99.99%), differences in the CDW and growth in different formulations point out to probable differences in their metabolism. Other mechanisms controlling cell growth, such as quorum sensing (Boyer and Wisniewski-Dyé, 2009), deserve to be investigated, and indeed, Fukami et al (2017a) have recently shown differences between Ab-V5 and Ab-V6 in quorum sensing mechanisms. Differences were also observed between the strains and formulations in the production of PHB, overall being higher in Ab-V6 and in FORM2+P3.…”

“…Watnick and Kolter (2000) suggested that biofilm-forming microorganisms can return to mobile lifestyles when nutrient availability becomes scarce, a likely response to the search for new sources of nutrients; the change in lifestyle may result in the colonization of new environments (Costerton et al, 1995). However, intriguingly, biofilm formation in Ab-V5 was not affected by the medium composition and as biofilm is also related to quorum sensing (Boyer and Wisniewski-Dyé, 2009;Fukami et al, 2017a), this is an additional evidence pointing out that the differences between the two strains might be related to this mechanism.…”

“…Furthermore, many of these bacteria have additional mechanisms such as the synthesis of phytohormones, induction of plant-stress tolerance and defense genes, among others (Bashan and de-Bashan, 2010;Fukami et al, 2017a, b), which may help to promote plant growth. Due to their multiple beneficial mechanisms, these bacteria have been classified as plant-growth-promoting bacteria (PGPB), and among them, those belonging to the Azospirillum brasilense species are the most studied and employed as inoculants worldwide, with consistent responses to inoculation in all continents, highly contributing to the economy of chemical fertilizers (Okon and Labandera-Gonzalez, 1994;Dobbelaere et al, 2001;Hartmann and Bashan, 2009;Bashan and de-Bashan, 2010;Hungria et al, 2010;Okon et al, 2015;Cassán and Diaz-Zorita, 2016;Pereg et al, 2016;Fukami et al, 2016Fukami et al, , 2017a.…”

“…Therefore, it is expected that the accumulation of PHB may favor cell longevity in inoculants. In addition to PHB, biofilm formation provides several benefits to the bacterial community, such as the improvement in bacterial cell-to-cell communication, tolerance of stressful environmental conditions, and host plant colonization (Morris and Monier, 2003;Kreft, 2004;Fukami et al, 2017a). Although studies about the application of biofilms for the benefit of agriculture are scarce, they could contribute to the success of inoculation, protecting bacterial cells from the competition with other soil microbial communities and improving BNF (Jayasinghearachchi and Seneviratne, 2004;Karivaradharajan et al, 2013;Fukami et al, 2017a).…”