Towards the understanding of action mechanisms of plant biostimulants through chemical, biochemical, and biotechnological approaches

To feed the world crop performances must be improved, however, it is imperative to do so sustainably. Plant biostimulants are effectively employed to reach this goal. The observed beneficial effects of PBs on plants are clearly presented in literature, however their action mechanisms are yet to be elucidated. With an interdisciplinary approach this thesis will present new findings on a particular class of PBs, protein hydrolysates (PHs), mixtures of peptides and free amino acids (AAs), produced by the hydrolysis of protein-enriched solutions obtained from industrial wastes. When possible, connections between their chemical composition and their biostimulatory activity are drawn. Firstly, it is presented a new thermo-chemical process to simultaneously obtain protein hydrolysates and bioplastic precursors, namely, polyhydroxyalkanoates from a microbial biomass. A second chapter characterizes the effects of a PH from corn gluten (an industrial by-waste) employed to hasten grape berry ripening. The product application induces physiological, biochemical and transcriptomic responses. Moreover, a third chapter highlights the beneficial effects of a PH derived from the tanning of bovine hides. This product is enriched in collagen, and is an excellent multi-stress mitigator. A thourough molecular characterization indicates that plant might perceive it though specific receptors. A conclusive section underlines the difficulties regarding the study of PBs, which often arise from their heterogeneous composition. Finally, future perspectives are presented