Biostimulants from Hydrolyzed Proteins: Animal Versus Vegetal Sources
Protein hydrolysates (PHs) have emerged as a pivotal category of plant biostimulants in sustainable agriculture. They are derived from the enzymatic, chemical, or thermal hydrolysis of agro-industrial by-products of animal or plant origin. These complex mixtures of free amino acids, oligopeptides, and polypeptides enhance crop productivity, nutrient use efficiency, and abiotic stress resilience. This review examines and compares the production methods, chemical composition, agronomic performance, physiological mechanism, and safety profiles of animal-derived (A-PHs) and vegetal-derived (V-PHs) protein hydrolysates, with particular emphasis on hydrolyzed collagen (HC) as an emerging biostimulant. Furthermore, the specific physiological roles of proline in mediating plant stress tolerance and hydroxyproline-rich glycoproteins in maintaining cell wall integrity are evaluated. Animal-derived sources, including collagen, keratin, and fish by-products, are characterized by elevated glycine, proline, and hydroxyproline concentrations, amino acids with established roles in root architecture promotion, reactive oxygen species (ROS) scavenging, and osmotic adjustment under stress. Conversely, V-PH exhibit richer bioactive peptide profiles and superior environmental sustainability indices. Underlying mechanisms encompass hormone-like activities mimicking auxin and gibberellin signaling, transcriptional reprogramming of nitrogen assimilation pathways, antioxidant enzyme modulation, and rhizosphere microbiota stimulation.