Native bacterial endophytes promote host growth in a species-specific manner; phytohormone manipulations do not result in common growth responses

Abstract

Background: All plants in nature harbor a diverse community of endophytic bacteria which can positively affect host plant growth. Changes in plant growth frequently reflect alterations in phytohormone homoeostasis by plant-growth-promoting (PGP) rhizobacteria which can decrease ethylene (ET) levels enzymatically by 1-aminocyclopropane-1-carboxylate (ACC) deaminase or produce indole acetic acid (IAA). Whether these common PGP mechanisms work similarly for different plant species has not been rigorously tested.

Methodology/principal findings: We isolated bacterial endophytes from field-grown Solanum nigrum; characterized PGP traits (ACC deaminase activity, IAA production, phosphate solubilization and seedling colonization); and determined their effects on their host, S. nigrum, as well as on another Solanaceous native plant, Nicotiana attenuata. In S. nigrum, a majority of isolates that promoted root growth were associated with ACC deaminase activity and IAA production. However, in N. attenuata, IAA but not ACC deaminase activity was associated with root growth. Inoculating N. attenuata and S. nigrum with known PGP bacteria from a culture collection (DSMZ) reinforced the conclusion that the PGP effects are not highly conserved.

Conclusions/significance: We conclude that natural endophytic bacteria with PGP traits do not have general and predictable effects on the growth and fitness of all host plants, although the underlying mechanisms are conserved.

Figure 1. Effects of endophytic bacteria on seedling vigor.

Mean (±SE; n  =  30–40) percentage germination, root length (cm) and hypocotyl length (cm). Seeds treated with sterile distilled water served as controls (white bars). The different shadings of the bars indicate the origin of the isolate (roots/stem leaves from S. nigrum plants collected from 2 field plots of Max Planck Institute for Biogeochemistry (BGCR/SL), roots/stem leaves from S. nigrum plants collected in the Dornburg field (DR/SL), roots from S. nigrum plants grown in Dornburg field soil in the glasshouse (DSR) and roots from plants grown in the glasshouse in soil from the Saale valley (SSR)). Arrows identify the sixteen isolates that were selected for further study.

Figure 2. Regression of bacterial traits that influence ethylene and auxin signaling against S. nigrum root growth as measured in the 16 isolates identified in Figure 1.

(A) Regression of bacterial ACC deaminase activity and root lengths of seedlings inoculated with bacterial isolates. (B) Regression of bacterial ACC deaminase activity and ethylene emission from seedlings inoculated with bacterial isolates. (C) Regression of bacterial IAA and root lengths of seedlings inoculated with bacterial isolates.

Figure 3. Confocal laser scanning microscopy of roots colonized by the GFP-tagged endophytic bacterial isolates.

(A and B) Root colonization by GFP-tagged BGCR2-8(1) isolate at magnification of 100x (A) and 200x (B) and (C and D) root colonization by GFP-tagged DR5 isolate at magnification of 100x (C) and 200x (D).

Figure 4. Comparison of of S. nigrum and N. attenuata seedling growth to bacterial colonization (A and B) of endophytic bacteria isolated from S. nigrum.

Six isolates were selected based on their effects on S. nigrum seedling growth. (A) Mean root length (±SE) and (B) mean hypocotyl length (±SE) of S. nigrum and N. attenuata seedlings. Asterisks indicate significant differences in promotion of root and hypocotyl growth in S. nigrum and N. attenuata seedlings by the bacterial isolates compared to the control at P<0.05 (*); P<0.001 (**); and P<0.0001 (***).

Figure 5. Reproductive growth and fitness responses of S. nigrum and N. attenuata plants to known mutualistic bacterial strains (Pseudomonas brassicacearum D13227, Bacillus pumilis D1794, Pseudomonas putida D50194, Pseudomonas marginalis D50276, Methylobacterium fujisawaense D5686 and Pseudomonas fluorescens D8568).

(A) Mean stalk length (±SE) of S. nigrum and N. attenuata, (B) Mean (±SE) fruit number per plant (S. nigrum) and capsule number per plant (N. attenuata). Asterisks indicate significant differences (Fisher's PLSD test; P<0.05 (*); P<0.001 (**); and P<0.0001 (***)).