Abstract
Rhizophagus irregularis, an arbuscular mycorrhizal fungus, and Bacillus amyloliquefaciens, a bacterium, are microorganisms that promote plant growth. They associate with plant roots and facilitate nutrient absorption by their hosts, increase resistance against pathogens and pests, and regulate plant growth through phytohormones. In this study, eight local plant species in Finland (Antennaria dioica, Campanula rotundifolia, Fragaria vesca, Geranium sanguineum, Lotus corniculatus, Thymus serpyllum, Trifolium repens, and Viola tricolor) were inoculated with R. irregularis and/or B. amyloliquefaciens in autoclaved substrates to evaluate the plant growth−promoting effects of different plant/microbe combinations under controlled conditions. The eight plant species were inoculated with R. irregularis, B. amyloliquefaciens, or both microbes or were not inoculated as a control. The impact of the microbes on the plants was evaluated by measuring dry shoot weight, colonization rate by the arbuscular mycorrhizal fungus, bacterial population density, and chlorophyll fluorescence using a plant phenotyping facility. Under dual inoculation conditions, B. amyloliquefaciens acted as a “mycorrhiza helper bacterium” to facilitate arbuscular mycorrhizal fungus colonization in all tested plants. In contrast, R. irregularis did not demonstrate reciprocal facilitation of the population density of B. amyloliquefaciens. Dual inoculation with B. amyloliquefaciens and R. irregularis resulted in the greatest increase in shoot weight and photosynthetic efficiency in T. repens and F. vesca.
Highlights
Countless microorganisms reside and propagate in the rhizosphere where plant roots and soil meet
In the R+B treatment, arbuscular mycorrhizal fungus (AMF) structures such as arbuscules and hyphae were observed in all eight plant species (Fig 1) and vesicles were observed in all tested plants except C. rotundifolia
Mycorrhiza helper bacterium (MHB) is a generic name for bacteria that can stimulate the formation of a mycorrhizal association with host plants [63]
Summary
Countless microorganisms reside and propagate in the rhizosphere where plant roots and soil meet. R. irregularis is an arbuscular mycorrhizal fungus (AMF) that is found in most soil ecosystems and types [2,3] It colonizes plants by forming intraradical hyphae and arbuscules as well as vesicles inside roots. The use of plant growth−promoting microorganisms has increased at a rate of 10% annually in crop production worldwide over the last decade [29] They have the potential to be used in green roofs (rooftops covered with vegetation), which are especially desirable in cities where they can provide multiple ecosystem services to urban residents, including mitigating air pollution, relieving the urban heat island effect, saving energy, and retaining stormwater [30,31,32,33,34,35]. Shoot biomass and the photosynthetic efficiency were chosen as indicators of physiological performance, as they directly reflect plant physiology and are relatively easy to measure
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