Priming of plant growth promotion by volatile compounds of root-associated Microbacterium

Volatile compounds produced by plant-associated microorganisms represent a diverse resource to promote plant growth and health. Here we investigated the effect of volatiles from root-associated Microbacterium species on plant growth and development. Volatiles of eight strains induced significant increases in shoot and root biomass of Arabidopsis, but differed in their effects on root architecture. Microbacterium strain EC8 also enhanced root and shoot biomass of lettuce and tomato. Biomass increases were also observed for plants exposed only shortly to volatiles from EC8 prior to transplantation of the seedlings to soil. These results indicate that volatiles from EC8 can prime plants for growth promotion without direct and prolonged contact. We further showed that the induction of plant growth promotion is tissue specific: exposure of roots to volatiles from EC8 led to an increase in plant biomass whereas shoot exposure resulted in no or less growth promotion. GC-QTOF analysis revealed that EC8 produces a wide array of sulfur containing compounds as well as ketones. Bioassays with synthetic sulfur volatile compounds revealed that the plant growth response to dimethyl trisulfide was concentration-dependent with a significant increase in shoot weight at 1 μM and negative effects on plant biomass at concentrations higher than 1 mM. Genome-wide transcriptome analysis of volatile-exposed Arabidopsis seedlings showed up-regulation of genes involved in assimilation and transport of sulfate and nitrate. Collectively, these results show that root-associated Microbacterium primes plants, via the roots, for growth promotion most likely via modulation of sulfur and nitrogen metabolism. Importance In the past decade, various studies have described the effects of microbial volatiles on other (micro)organisms in vitro, but their broad-spectrum activity in vivo and the mechanisms underlying volatile-mediated plant growth promotion have not been addressed in detail. Here, we revealed that volatiles from root-associated bacteria of the genus Microbacterium can enhance growth of different plant species and can prime plants for growth promotion without direct and prolonged contact between the bacterium and the plant. Collectively, these results provide new opportunities for sustainable agriculture and horticulture by exposing roots of plants only briefly to a specific blend of microbial volatile compounds prior to transplantation of the seedlings to the greenhouse or field. This strategy has no need for large-scale introduction, root colonization and survival of the microbial inoculant

Use of mycorrhizal fungi for the phytostabilisation of radio-contaminated environment (European project MYRRH) : Overview on the scientific achievements

Because plants significantly affect radionuclides (RN) cycling and further dispersion into the biosphere, it is important to understand the biological factors influencing RN plant uptake, accumulation and redistribution. In this respect, mycorrhizal fungi are of particular interest. The effects of ecto-mycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungi on the transport of uranium (U) or radiocaesium (Cs) were investigated both under pot and in vitro culture conditions. Results obtained in vitro demonstrated that AM hyphae can take up and translocate U and Cs towards roots, while this uptake and translocation were not perceptible using pot culture systems with soil. These contrasting results could be due to different experimental conditions, including the K level in the external solution and the bio-availability of Cs. The in vitro studies also indicated that root colonisation by AM fungi might limit U and Cs root transport. Under pot culture conditions, they appeared to significantly reduce root to shoot translocation of U. Under the same conditions, ECM transport of Cs was demonstrated, and appeared to be dependent on fungal species. A better estimation of the potential use of mycorrhizal fungi for the phytoremediation of RN-contaminated areas is now available and will be further discussed

Arbuscular Mycorrhizal Fungi May Mitigate the Influence of a Joint Rise of Temperature and Atmospheric CO2 on Soil Respiration in Grasslands

We investigated the effects of mycorrhizal colonization and future climate on roots and soil respiration (Rsoil) in model grassland ecosystems. We exposed artificial grassland communities on pasteurized soil (no living arbuscular mycorrhizal fungi (AMF) present) and on pasteurized soil subsequently inoculated with AMF to ambient conditions and to a combination of elevated CO2 and temperature (future climate scenario). After one growing season, the inoculated soil revealed a positive climate effect on AMF root colonization and this elicited a significant AMF x climate scenario interaction on root biomass. Whereas the future climate scenario tended to increase root biomass in the noninoculated soil, the inoculated soil revealed a 30% reduction of root biomass under warming at elevated CO2 (albeit not significant). This resulted in a diminished response of Rsoil to simulated climatic change, suggesting that AMF may contribute to an attenuated stimulation of Rsoil in a warmer, high CO2 world

Role and influence of mycorrhizal fungi on radiocesium accumulation by plants

This review summarizes current knowledge on the contribution of mycorrhizal fungi to radiocesium immobilization and plant accumulation. These root symbionts develop extended hyphae in soils and readily contribute to the soil-to-plant transfer of some nutrients. Available data show that ecto-mycorrhizal (ECM) fungi can accumulate high concentration of radiocesium in their extraradical phase while radiocesium uptake and accumulation by arbuscular mycorrhizal (AM) fungi is limited. Yet, both ECM and AM fungi can transport radiocesium to their host plants, but this transport is low. In addition, mycorrhizal fungi could thus either store radiocesium in their intraradical phase or limit its root-to-shoot translocation. The review discusses the impact of soil characteristics, and fungal and plant transporters on radiocesium uptake and accumulation in plants, as well as the potential role of mycorrhizal fungi in phytoremediation strategies