Volatile Organic Compounds (VOCs) are involved into phenomena disturbing the atmospheric balance and global change (Varin, 2009, 2017). The disturbances are more or less important depending on nature of VOCs. VOCs contribute to global change including greenhouse effect by simultaneously performing: i) an ozone production via solar and cosmic radiations; ii) and increase free radicals producing of low-altitude clouds. VOCs emissions generate two types of impact: a direct impact on human health and an indirect impact on the environment.

The ability of plants to take up water and mineral nutrients from the soil depends on their capacity to develop an extensive root system and on the interactions of roots with their soil environment, the rhizosphere. Studies of the mechanisms involved in the communication between plant growth-promoting soil micro-organisms and the root system is expected to lead to improved management strategies for fertility of soils, control of growth and development of organisms for example plants. Up to now, the characterization of such interactions has been mainly focused on liquid diffusates, but it has been recently reported that volatile organic compounds (VOC) also play a role as chemical messengers in positive or negative interactions occurring in the rhizosphere and involving plants, bacteria, fungi and insects (Ryu et al., 2003; Kai et al., 2007).

Despite the demonstrated capacity of certain soil bacteria to stimulate plant growth and stress response, much has still to be discovered on the mechanisms underlying these effects.

In this context and in order to better understand how roots and bacteria may interact in the rhizosphere without physical contact , my project was to better understand the functional ecology and ecophysiology of the rhizosphere of Arabidopsis thaliana and Brachypodium distachyon for best production of biofuels and bioplastics (Figure. 1).

For this purpose, a collection of 18 rhizobacterial strains belonging to nine genera were selected for their potential growth promotion ability and two collections of contrasting ecotypes for Arabidopsis thaliana et Brachypodium distachyon was formed. Thus, Two plant species were used, Arabidopsis thaliana as a dicotyledonous model, and Brachypodium distachyon as a monocotyledonous (graminaceous) model. Regarding the bacterial partners, 19 strains belonging to 8 genera were selected: Azospirillum, Azotobacter, Bacillus, Burkholderia, Paenibacillus, Pseudomonas, Raoultella, Serratia.

The VOC-emission capacity of each strain was measured after 24 and 48 hours of growth on agar media (Farag et al., 2006) using solid-phase microextraction followed by gas chromatography coupled to a mass spectrometer.

Finaly, an experimental system was designed, allowing the co-cultivation of seedlings with PGPR bacteria on compartmented Petri dishes in a shared atmosphere, with no physical contact between both organisms and with no diffusion of water-soluble compounds between compartments for 10 days within a shared atmosphere.

In both dicot and monocot species, the results point to the capacity of bacterial volatile compounds to strongly bio-stimulate plants and roots in vitro. For mono and dicotyledonous, the bio-stimulating VOCs were identified by one multivariate statistical analysis. The observed biostimulating effects (toxic or promotions) include significant changes in total biomass, root / shoot ratios, and in the number of secondary and adventitious roots per plant. Experiments were used to compare the response of both model species to perception of rhizobacterial volatiles (Delaplace et al., 2015; Varin, 2017) .

References

Delaplace P., Delory B., Baudson C., Mendaluk-Saunier de Cazenave M., Spaepen S., Varin S., du Jardin P., 2015. Influence of rhizobacterial volatiles on the root system architecture and the production and allocation of biomass in the model grass Brachypodium distachyon (L.) P. Beauv. BMC Plant Biology 15, 195-210.

Farag MA, Ryu CM, Sumner LW, Paré PW. 2006. GC–MS SPME profiling of rhizobacterial volatiles reveals prospective inducers of growth promotion and induced systemic resistance in plants. Phytochemistry 67: 2262-2268.

Kai M, Effmert U, Berg G, Piechulla B. 2007. Volatiles of bacterial antagonists inhibit mycelial growth of the plant pathogen Rhizoctonia solani. Arch Microbiol 187: 351-360.

Ryu CM, Farag MA, Hu CH, Reddy MS, Wei HX, Paré PW, Kloepper JW. 2003. Bacterial volatiles promote growth in Arabidopsis. Proc Natl Acad Sci USA 8: 4927-4932.