Table 2.

Some applications of metabolomics in elucidating mechanisms and modes of action of plant biostimulants.

Study	Biostimulant	Effects	Plant	Mechanism of Action—Metabolic Changes	References
Capsicum chinensis L. growth and nutraceutical properties are enhanced by biostimulants in a long-term period: Chemical and metabolomic approaches	Plant-based biostimulants: one derived from AH and RG	Growth promotion	Pepper	−Production of secondary metabolites, such as phenols.	[138]
The effect of a plant-derived biostimulant on metabolic profiling and crop performance of lettuce grown under saline conditions	Plant-derived protein hydrolysates	Growth promotion and resistance to salt stress	Lettuce	−Improved plant nitrogen metabolism −Oxidative stress mitigation via the increase in osmolytes, changes in sterols, glucosinolates and terpenes composition	[94]
Dunaliella salina exopolysaccharides: a promising biostimulant for salt stress tolerance in tomato (Solanum lycopersicum)	Microalgal exopolysaccharides	Resistance to salt stress	Tomato	−Activation of jasmonic pathway −Accumulation of VLCFAs to strengthens the plant’s cuticles −Inactivation of SA pathway −Increase in phytosterols to regulate membrane fluidity −Induction of ROS scavenging enzymes and other antioxidant molecules	[137]
Effects of humic substances and indole-3-acetic acidon Arabidopsis sugar and amino acid metabolic profile	Humic substances	Growth promotion	Arabidopsis	−Possible increased activity of glycolysis −Reduced free amino acids, suggesting increased protein and/or secondary metabolites production	[139]
A vegetal biopolymer-basedbiostimulant promoted root growth in melon while triggeringbrassinosteroids and stress-related compounds	Vegetal biopolymer-basedbiostimulant	Growth promotion	Melon	−Changes of brassinosteroid content was linked to increased root development, as well as the modulation of photosynthetic activity −Interference with hormonal crosstalk in leaves	[140]
Understanding the biostimulant action of vegetal-derived protein hydrolysates by high-throughput plant phenotyping and metabolomics: A Case Study on Tomato	Vegetal-derived protein hydrolysates	Growth promotion	Tomato	−Multilayer regulation of ethylene and polyamines metabolism −ROS-mediated signaling pathways	[141]
A combined phenotypic and metabolomic approach for elucidating the biostimulant action of a plant-derived protein hydrolysate on tomato grown under limited water availability	Plant-derived protein hydrolysate	Resistance against drought	Tomato	−Phytohormonal changes (reduced cytokinins and increased salicylates) −Changes in lipids and terpenes −Oxidative stress mitigation via hydroxycinnamic amides, carotenoids, prenyl quinones, and reduced biosynthesis of tetrapyrrole coproporphyrins.	[11]
Metabolomic analysis of the effects of a commercial complex biostimulant on pepper crops	Commercial biostimulant, Actium	Growth promotion	Pepper	−Increased phenylalanine and total monosaccharides, suggesting a further stage in ripening. −Increase in carotenoids concomitant with an increase in some digalactosyl diacylglycerols	[142]
Effect of microalgae polysaccharides on biochemical and metabolomics pathways related to plant defense in Solanum lycopersicum	Microalgae polysaccharides	Plant defense	Tomato	−Lipid remodeling −Increased lipids such as VLCFAs associated with thickened cuticular wax	[143]
Biostimulants from food processing by-products: agronomic, quality and metabolic impacts on organic tomato (Solanum lycopersicum L.)	Biostimulants from food processing by-products	Growth promotion	Tomato	−Increase in citric acid content and the decrease in β-glucose content	[144]
Inoculation of Rhizoglomus irregulare or Trichoderma atroviride differentially modulates metabolite profiling of wheat root exudates	T. atroviride and R. irregulare	Growth promotion	Wheat	−Differential changes in lipids, phenolic compounds, terpenoids, siderophores, chelating acids, derivatives of amino acids and phytohormones	[145]
Vegetal-derived biostimulant enhances adventitious rooting in cuttings of basil, tomato, and Chrysanthemum via brassinosteroid-mediated processes	Vegetal-derived biostimulant	Growth promotion	Basil, Tomato, and Chrysanthemum	−Biostimulant-derived BRs and auxin were suggested to modulate endogenous BR pool, −Induces morphological and metabolic changes during adventitious rooting of cuttings in plants	[146]
A biostimulant obtained from the seaweed Ascophyllum nodosum protects Arabidopsis thaliana from severe oxidative stress	Seaweed extracts	Oxidative stress	Arabidopsis	−Accumulation of maltose and fumarate and malate −Reduction in lipids such as TAGs which induces cell death and chloroplast degradation	[147]

Abbreviations: SA = salicylic acid, ROS = reactive oxygen species, VLCFAs = very-long-chain fatty acids, BRs = brassinosteroids, AH = alfalfa plants, RG = red grape, TAGs = triacylglycerols.