Table 2.
Microscopy-based techniques used to monitor plant growth-promoting bacteria and root interaction.
| Strains | Experimental conditions | Methods | Plant substrate | Results | Reference | ||
|---|---|---|---|---|---|---|---|
| Burkholderia gladioli | Laboratory experiment on Panicum virgatum | Bright field microspy | Water agar plates | Bacterial cells adhered to surfaces of root hairs and root epidermal parenchyma | White et al., 2014 | ||
|
Azotobacter chroococcum W5 Trichoderma viride ITCC 2211 |
Pot (day/night temperature 22–24/18°C, humidity 60%) | SEM | Sterile sand and vermiculite (1:1) | Presence of Azotobacter cells, both individually both attached to the fungal mycelia, on root tissues | Velmourougane et al., 2017 | ||
|
Azotobacter chroococcum ATCC9043 Azotobacter chroococcum BCRC10599 Azotobacter chroococcum CCRC10599 Azotobacter chroococcum DSM2286 Azotobacter chroococcum IAM12666 |
In vitro assay on Arnebia hispidissima (25 ± 1°C, 60% relative humidity, 10 days) | TEM | MS culture medium | Endophytic interaction between bacterial strains and hairy roots | Singh and Sharma, 2016 | ||
|
Azospirillum spp. Azoarcus spp. Azorhizobium spp. |
Controlled conditions (22°C; 16-h/8-h light/dark; relative humidity 75%) | ESEM | MS agar medium | Colonization of root cavities, bacterial biofilm formation, colonization of inner root tissues | Dal Cortivo et al., 2017 | ||
| Azotobacter chroococcum Mac 27L | Phytotron chamber (12 h light, ca. 30,000 lux, 15–17°C/8–10°C day/night temperature, 28 days) | Immuno-fluorescence microscopy | Semisolid nutrient media | Bacteria were clearly detectable after 7 days of inoculation | Narula et al., 2007 | ||
|
Burkholderia sp. WPB Rhizobium tropici PTD1 Rahnella sp. WP5 |
Axenic conditions in growth chamber | GFP | N-free MS agar | Bacterial cells reside outside plant tissues in the apoplastic spaces and xylem tissue of rice plants | Kandel et al., 2015 | ||
| Pseudomonas sp. VM1449 Pseudomonas sp. VM1450 Pseudomonas sp. VM1453 | Pots (20–25°C, 16-h light/8-h dark) | GFP | Sterile compost/vermiculite substrate (3:1 ratio) | GFP-tagged cells were clearly visible in the rhizosphere and on different root tissues | Germaine et al., 2004 | ||
| Pseudomonas fluorescens SBW25 | Laboratory experiment on 5 days growth lettuce | GFP | Transparent soil of particles of Nafion (polymer with a low refractive index) | Colonization of root surfaces, rhizoplane, and surfaces of Nafion particles | Downie et al., 2014 | ||
| Azotobacter chroococcum Avi2 | In vitro assay on sterile rice seedlings (14-h light cycle, 30 ± 2°C, 7 days) | FRET-based technique | MS agar medium | Intracellular roots colonization (green fluorescence emitted by bacterial cells and blue fluorescence emitted by root tissues) | Banik et al., 2016 | ||
| Azotobacter chroococcum 67B Azotobacter chroococcum 76A | In vitro assay (sterile conditions) | Fluorescent Al3+-siderophore complex combined with CLSM | Pots containing a growth medium added of 2 mM of Al3+ | Ability of the two bacterial strains to colonize tomato roots | Viscardi et al., 2016 | ||
| Sphingomonas azotifigens DSMZ18530 | Gnotobiotic conditions in controlled-environment chamber (16-h light/8-h dark, 18–23°C) | GFP | Modified Evans medium supplemented with 8% agar | Visualization and localization of bacterial strain in different parts of annual ryegrass plants (preferentially localized along root hairs and in stem epidermis) | Castanheira et al., 2017 | ||
| Pseudomonas sp. G1Dc10 Paenibacillus sp. G3Ac9 | Gnotobiotic conditions in controlled-environment chamber (16-h light/8-h dark, 18–23°C) | FISH/Confocal laser-scanning microscopy | Modified Evans medium supplemented with 8% agar | Visualization and localization of bacterial strains in different parts of annual ryegrass plants (preferentially localized along root hairs and in stem epidermis) | Castanheira et al., 2017 | ||
FRET, fluorescence resonance energy transfer; SEM, scanning electron microscopy; TEM, transmission electron microscopy; ESEM, environmental scanning electron microscopy; GFP, green fluorescent protein; FISH, fluorescence in situ hybridization.