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Abstract Background The synthesis of cellulose is among the most important but poorly understood biochemical processes, especially in bacteria, due to its complexity and high degree of regulation. In this study, we analyzed both the production of cellulose by all known members of the Rhizobiaceae and the diversity of Rhizobium celABC operon predicted to be involved in cellulose biosynthesis. We also investigated the involvement in cellulose production and biofilm formation of celC gene encoding an endoglucanase (CelC2) that is required for canonical symbiotic root hair infection by Rhizobium leguminosarum bv. trifolii. Results ANU843 celC mutants lacking (ANU843ΔC2) or overproducing cellulase (ANU843C2+) produced greatly increased or reduced amounts of external cellulose micro fibrils, respectively. Calcofluor-stained cellulose micro fibrils were considerably longer when formed by ANU843ΔC2 bacteria rather than by the wild-type strain, in correlation with a significant increase in their flocculation in batch culture. In contrast, neither calcofluor-stained extracellular micro fibrils nor flocculation was detectable in ANU843C2+ cells. To clarify the role of cellulose synthesis in Rhizobium cell aggregation and attachment, we analyzed the ability of these mutants to produce biofilms on different surfaces. Alteration of wild-type CelC2 levels resulted in a reduced ability of bacteria to form biofilms both in abiotic surfaces and in planta. Conclusions Our results support a key role of the CelC2 cellulase in cellulose biosynthesis by modulating the length of the cellulose fibrils that mediate firm adhesion among Rhizobium bacteria leading to biofilm formation. Rhizobium cellulose is an essential component of the biofilm polysaccharidic matrix architecture and either an excess or a defect of this "building material" seem to collapse the biofilm structure. These results position cellulose hydrolytic enzymes as excellent anti-biofilm candidates. ; We thank Nancy Fujishige and Peter De Hoff for help in the biofilm assays. This work was supported by Junta de Castilla y León Grant GR49 and MICINN Grants AGL2008-03360 and AGL2011-29227. M.R. and L.R. were supported by a PhD fellowship from the Spanish government ; Peer Reviewed

After a forest wildfire, the microbial communities have a transient alteration in their composition. The role of the soil microbial community in the recovery of an ecosystem following such an event remains poorly understood. Thus, it is necessary to understand the plant-microbe interactions that occur in burned soils. By high-throughput sequencing, we identified the main bacterial taxa of burnt holm-oak rhizosphere, then we obtained an isolate collection of the most abundant genus and its growth promoting activities were characterised. 16S rRNA amplicon sequencing showed that the genus Arthrobacter comprised more than 21% of the total community. 55 Arthrobacter strains were isolated and characterized using RAPDs and sequencing of the almost complete 16S rRNA gene. Our results indicate that isolated Arthrobacter strains present a very high genetic diversity, and they could play an important ecological role in interaction with the host plant by enhancing aerial growth. Most of the selected strains exhibited a great ability to degrade organic polymers in vitro as well as possibly presenting a direct mechanism for plant growth promotion. All the above data suggests that Arthrobacter can be considered as an excellent PGP rhizobacterium that may play an important role in the recovery of burned holm-oak forests. ; This work was funded by the following grants: P08-CVI-03549 from The Department of Innovation, Science and Enterprise of the Autonomous Government of Andalusia; OAPN 021/2007 from The National Parks Autonomous Body (Ministry of the Environment) and 20134R069, RECUPERA 2020 from the Spanish Ministry of Economy and Competitiveness and CSIC, including ERDF (European Regional Development Fund). The work conducted by the U.S. Department of Energy Joint Genome Institute, a DOE Office of Science User Facility, is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. AJFG was awarded a pre-doctoral fellowship (FPU) from the Spanish Ministry of Education; JFCD was awarded a pre-doctoral fellowship from the Junta de Andalucía, and both with a postdoctoral contract from RECUPERA 2020. PMH was awarded a postdoctoral fellowship from Ramón Areces foundation ; Peer Reviewed