Abstract
The application of biostimulants is considered a promising approach to promote crops growth and resilience against abiotic stresses. Biostimulants are materials which contain substance(s) and/or microorganisms applied to the plants with the aim to enhance nutrient uptake and efficiency, abiotic stress tolerance, and/or product quality of crops, independently of their nutrient content. Among the different categories of biostimulants, protein hydrolysates (PHs), i.e., the mixture of amino acids and small peptides obtained from the hydrolysis of protein matrixes, and plant growth-promoting rhizobacteria (PGPR) are receiving great interest. These plant biostimulants have been demonstrated to play key roles through direct and indirect effects that trigger growth, increase yield and alleviate the impact of abiotic stresses of open-field and greenhouse-cultivated crops. However, the mechanisms underpinning biostimulant effects induced by PHs in plants are still poorly understood under abiotic stress conditions. Moreover, the bioactivity of different PH molecular fractions has been scarcely investigated. Thus, the main objective of this research project is to assess the biostimulant action of either plant derived PHs or beneficial microorganism (i.e., Azospirillum brasilense) on tomato and lettuce as model crops with a focus on the investigation of molecular mode of action of PHs through a multi-omics approach. The biostimulants action has been evaluated under both optimal and adverse conditions (low nitrogen (N) availability or high salinity). Concerning salinity stress, a preliminary screening in terms of morpho-physiological and biochemical parameters enabled to identify the most effective PHs in coping with the stress for both tomato and lettuce crops. Then, a comparative study between tomato plants grown in no and high salt conditions treated with PHs or A. brasilense provided results in terms of morphological, biochemical and metabolomic effects. On the other hand, the molecular effects of the most-promising PH and its lighter molecular fraction on lettuce was evaluated in no and high salt conditions through the integration of transcriptomic and metabolomic data. Similarly, the same multi-omics approach was applied to investigate the impact of PHs and their molecular fractions on lettuce and tomato grown under optimal and N shortage conditions. This comprehensive study allowed confirming the effectiveness of these plant biostimulants in mitigating abiotic stresses in species-specific way and, simultaneously, giving novel insights on PHs and their bioactive fractions molecular mode of action. This progression in knowledge could be implemented in the designing of more efficient and tailored PHs according to crop and abiotic stress conditions.