No abstract

20Secondary metabolites have an important impact on the biocontrol potential of soil-21 derived microbes. In addition, various microbe-produced chemicals have been 22 suggested to impact the development and phenotypic differentiation of bacteria, 23including biofilms. The non-ribosomal synthesized lipopeptide of Bacillus subtilis, 24 surfactin, has been described to impact the plant promoting capacity of the bacterium. 25Here, we investigated the impact of surfactin production on biofilm formation of B. 26 subtilis using the laboratory model systems; pellicle formation at the air-medium 27 interface and architecturally complex colony development, in addition to plant root-28 associated biofilms. We found that the production of surfactin by B. subtilis is not 29 essential for pellicle biofilm formation neither in the well-studied strain, NCIB 3610, nor 30 in the newly isolated environmental strains, but lack of surfactin reduces colony 31 expansion. Further, plant root colonization was comparable both in the presence or 32 absence of surfactin synthesis. Our results suggest that surfactin-related biocontrol and 33 plant promotion in B. subtilis strains are independent of biofilm formation. 34 35 Keywords: Bacillus subtilis, biofilm, surfactin, plant root colonization, pellicle 36 39 Several species from the "Bacillus subtilis complex" are well-characterized plant growth-40 promoting rhizobacteria (PGPRs), providing various beneficial activities for plants and 41 inhibiting fungal and bacterial pathogens [1]. Many strains of Bacillus subtilis, Bacillus 42 amyloliquefaciens and Bacillus velezensis are currently used in organic and traditional 43 agriculture to prevent infection and/or increase yields of various crops [2-4]. These 44 species are of particular interest because they can form stress-resistant endospores, a 45 cell-type ideal for product formulation. Most PGPR Bacillus spp. also produce a wide 46 range of bioactive molecules, such as lipopeptides, which directly influences plant growth 47 and defence [5]. 48 49 Many of these molecules are synthesized by multienzyme-complexes called non-50 ribosomal peptide synthetases (NRPS) [6]. B. subtilis NCIB3610 possesses 3 NRPS 51 clusters and one NRPS/polyketide synthetase (PKS) cluster, which is few compared to 52 the bioactive molecule synthesis capacity of B. velezensis strains [1]. Bacillaene, a broad 53 spectrum antibiotic, is synthesized by proteins encoded in 80 kB pksA-S cluster [7]. The 54ppsA-E encodes for the peptide synthetase responsible for the synthesis of plipastatin 55 (fengycin family), a strong antifungal molecule [5,8], while the siderophore bacillibactin is 56 synthesized by the product of the dhbA-F operon [9]. Finally, SrfAA-AD produces versatile 57 molecules from the surfactin family [10]. 58 59Surfactin molecules are composed of a heptapeptide, i.e. two acidic and five nonpolar 60 amino acids, interlinked with a β-hydroxy fatty acid, and condensed in a cyclic lactone 61 right structure [10,11]. The amino acid sequence, the length, and the ...

Bacillus subtilis and Bacillus velezensis are used in organic agriculture as an alternative to chemical pesticides to fight against phytopathogen organisms. These Gram-positive soil-dwelling bacteria are able to resist harsh conditions and survive by differentiating into endospores. Few studies have examined how bacterial populations change on plants over time, and whether they remain active or enter a dormant state. Nonetheless, these characteristics are strikingly important to determine the usage of B. subtilis and B. velezensis and their efficacy in environmental conditions. Here, we investigated the population dynamics of B. subtilis NCIB3610 and B. velezensis QST713 when applied as spores on different ornamental plants. We report that on all the plants studied (Echinacea purpurea ‘Salsa red’, Echinacea purpurea ‘Fatal attraction’, and Lavandula angustifolia ‘Hidecote blue’), spores rapidly germinated and colonized the rhizoplane, maintaining a relatively low proportion of spores in the population over time, whereas the bacterial population on the leaves rapidly declined. Bacteria in the surrounding soil did not germinate and persisted as spores. Taken together, these results suggest that only cells found at the rhizosphere remain metabolically active to allow the formation of a lasting relationship with the plant, making possible beneficial effects from the inoculated bacteria.