Strategies to design a new generation of biofertilisers for a more sustainable agriculture

Plant biostimulants (PBs) are an attractive and environmental friendlystrategy to mitigate the continuous application of chemical fertilisers whichdisrupt the environment by degrading soil fertility and contaminating groundwater. The research described in this PhD thesis explored strategies to select,characterise and design a new generation of PBs formulations. Numerousparameters are involved in the multistep process of formulating inoculantsand add up to an infinite amount of possible approaches. This requires anelaborate and high-throughput screening to narrow down all the possiblecriteria. Several authors suggest that this is best achieved by using a stepwisescreening approach. However, the scientific literature lack of a suitablescreening strategy. We addressed this facet by designing a stepwise screeningprocedure to select the best microbial candidates as promising new activeingredients of PBs products. Furthermore, this screening was validated byusing a case study: plant growth-promoting rhizobacteria (PGPR) combinedwith humic acids (HA) to be applied on tomato plants. This validation led usto select two PGPR, Pantoea agglomerans MVC 21 and Pseudomonas (Ps.)putida MVC 17 as potential candidates for PBs formulations.Recently, co-inoculation of two or multiple PGPR is used to achieveprominent multifactorial effects on crop productivity. To develop a PGPR consortium, we evaluated how P. agglomerans MVC 21 and Ps. putida MVC17 interact together and the effect of this interaction on tomato seedlings.Compatibility studies revealed that both strains may be combined in abiostimulant product. Experiments testing the effect of the PGPR interactionindicated that both PGPR interact together and with tomato seedlings mainlyby volatile organic compounds (VOCs) produced by P. agglomerans MVC21. Metabolomic studies pinpointed VOCs belonging to the family ofalcohols, ketones and sulfide as the main VOCs released by P. agglomeransMVC 21. Moreover, the identification of the main VOC responsible of theeffects of P. agglomerans MVC 21 VOCs deciphered new ecological roles ofVOCs as chemical signals able to modulate behaviour of PGPR as well as theinteraction between plants and PGPR.Another promising area of PBs formulations is the use of HA combined withPGPR which have shown to better benefit plant growth. Our study, conductedunder in vitro conditions, showed that HA synergistically modulate plantgrowth-promoting activities of P. agglomerans MVC 21. Whole genomesequencing analysis of P. agglomerans MVC 21 will be subject of futurestudies. Moreover, transcriptomic analysis will be carried out to betterunderstand the effect of HA on P. agglomerans MVC 21 genes related toplant growth-promoting activities