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

Plant biostimulants (PBs) are an attractive and environmental friendly strategy to mitigate the continuous application of chemical fertilisers which disrupt the environment by degrading soil fertility and contaminating ground water. The research described in this PhD thesis explored strategies to select, characterise and design a new generation of PBs formulations. Numerous parameters are involved in the multistep process of formulating inoculants and add up to an infinite amount of possible approaches. This requires an elaborate and high-throughput screening to narrow down all the possible criteria. Several authors suggest that this is best achieved by using a stepwise screening approach. However, the scientific literature lack of a suitable screening strategy. We addressed this facet by designing a stepwise screening procedure to select the best microbial candidates as promising new active ingredients of PBs products. Furthermore, this screening was validated by using a case study: PGPR combined with humic acids (HA) to be applied on tomato plants. This validation led us to 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 achieve prominent multifactorial effects on crop productivity. To develop a PGPR consortium, we evaluated how P. agglomerans MVC 21 and Ps. putida MVC 17 interact together and the effect of this interaction on tomato seedlings. Compatibility studies revealed that both strains may be combined in a biostimulant product. Experiments testing the effect of the PGPR interaction indicated that both PGPR interact together and with tomato seedlings mainly by volatile organic compounds (VOCs) produced by P. agglomerans MVC 21. Metabolomic studies pinpointed VOCs belonging to the family of alcohols, ketones and sulfide as the main VOCs released by P. agglomerans MVC 21. Moreover, the identification of the main VOC responsible of the effects of P. agglomerans MVC 21 VOCs deciphered new ecological roles of VOCs as chemical signals able to modulate behaviour of PGPR as well as the interaction between plants and PGPR. Another promising area of PBs formulations is the use of HA combined with PGPR which have shown to better benefit plant growth. Our study, conducted under in vitro conditions, showed that HA synergistically modulate plant growth-promoting activities of P. agglomerans MVC 21. Whole genome sequencing analysis of P. agglomerans MVC 21 will be subject of future studies. Moreover, transcriptomic analysis will be carried out to better understand the effect of HA on P. agglomerans MVC 21 genes related to plant growth-promoting activities.