Taxonomic breakdown of 16S rRNA gene sequences of the cultivable endophytic community composition as isolated from hybrid poplar H11-11 and willow trees. Taxonomic classifications were determined according to Wang et al. (44). The central pie shows percentages by phyla; each outer annulus progressively breaks these down to finer taxonomic levels, with families, genera, and species in the outermost annuli. Numbers indicate the relative abundance, expressed as a percentage, of the different taxonomic groups.

Growth indexes for poplar cuttings inoculated with different endophytic bacteria. Growth indexes were determined 10 weeks after the inoculating and planting of the cuttings in sandy soil. Per condition, seven plants were used. Plants were grown in the greenhouse. Noninoculated plants were used as references. Bars indicate standard errors. Growth indexes were calculated as (Mt − M0)/M0 after 10 weeks of growth in the presence or absence of endophytic inoculum. M0, plant's weight (g) at week 0; Mt, plant's weight (g) after 10 weeks. The statistical significance of the increased biomass production of inoculated plants, compared to that of noninoculated control plants, was confirmed at the 5% level (**) using the Dunnett test.

Effects of S. proteamaculans 568 on the rooting and shoot formation of poplar DN-34. Plants were incubated hydroponically in half-strength Hoagland's solution in the absence (control) or presence (568) of strain 568. Root and shoot development are presented after 1 week (A) and 10 weeks (B).

Endophytic colonization of the poplar DN-34 roots by gfp-labeled derivatives of S. proteamaculans 568, Enterobacter sp. strain 638, and P. putida W619. (A) Colonization of the surface of a poplar root by a gfp-labeled derivative of P. putida W619. The picture was taken by fluorescence microscopy. Arrows indicate the positions of microcolonies on the root surface. (B) Interior view of a translateral section of a poplar root colonized by a gfp-labeled derivative of P. putida W619. The picture was taken with the help of the apotome feature of the fluorescence microscope. Arrows indicate the zones of dense interior colonization. (C and D) Interior views of a section of a poplar root colonized by a gfp-labeled derivative of S. proteamaculans 568 (C) and Enterobacter sp. strain 638 (D). The root tissue was stained with 0.05% methyl violet to decrease autofluorescence.

Metabolic pathways involved in the production of plant growth hormones (IAA, diacetyl, acetoin, and 2,3-butanediol) found on the genomes of selected endophytic bacteria. The metabolic pathways were constructed using PRIAM predictions mapped on the KEGG database (http://www.genome.ad.jp). For each organism, differently colored arrows are used to indicate the presence of the putative pathways: S. maltophilia R551-3 (red), P. putida W619 (green), B. vietnamiensis G4 (orange), Enterobacter sp. strain 638 (dark blue), and S. proteamaculans 568 (light blue). Black arrows indicate known pathway steps that could not be identified. Dashed arrows correspond to the presence of putative enzymes (PRIAM E value below 10⁻³⁰). In the tryptophan-dependent IAA synthesis, the enzymes involved are tryptophan transaminase Lao1 (1), indolepyruvate decarboxylase IpdC (2), indole-3-acetaldehyde dehydrogenase DhaS (3), tryptophan decarboxylase Dcd1 (4), amine oxidase (5), tryptophan 2-monooxygenase IaaM (6), deaminase IaaH (7), nitrile hydratase (8), nitrilase YhcX (9), and indole-3-acetaldehyde reductase AdhC (10). Besides the production of IAA, the main pathway for tryptophan metabolism is via tryptophan 2,3-dioxygenase KynA (11). In butanoate metabolism, the production of acetoin from pyruvate is catalyzed by the acetolactate synthase AlsS (12) and the acetolactate decarboxylase AlsD (13). The genome of Enterobacter sp. strain 638 encodes the acetoin dehydrogenase ButA (14) that is able to catalyze the conversion of acetoin into diacetyl. It is unknown if this compound has plant growth stimulatory effects. The 2,3-butanediol dehydrogenase ButB (15) involved in the conversion of acetoin into 2,3-butanediol was not found encoded on the genomes of the endophytic bacteria but is present on the poplar genome.