Biostimulant action of a plant-derived protein hydrolysate produced through enzymatic hydrolysis
The growing demand for food, feed, fuel, fiber, and raw materials and the increasing resource depletion and ecosystem degradation impose the use of more sustainable methods in the agriculture production systems. Several organic products called “biostimulants” are now available in the market to make agriculture more sustainable. As defined by European Biostimulant Industry Council (www.biostimulants.eu), “plant biostimulants contain substance(s) and/or micro-organisms whose function when applied to plants or the rhizosphere is to stimulate natural processes to enhance/benefit nutrient uptake, nutrient efficiency, tolerance to abiotic stress, and crop quality.” Kauffman et al. (2007) classified organic biostimulant compounds, into three major groups on the basis of their source and content: humic substances, seaweed extracts, and amino acids containing products. The last group consists of free amino acids and polypeptides obtained through chemical and/or enzymatic hydrolysis of agroindustrial by-products from animal or plant origins or from dedicated biomass crops (Cavani et al., 2006). Many studies (Morales-Payan and Stall, 2003; Parrado et al., 2007; Kowalczyk et al., 2008; Ertani et al., 2009; Gurav and Jadhav, 2013) reported beneficial effects of soil and foliar protein hydrolysates applications on growth, yield and fruit quality of agricultural crops (e.g., corn, banana, papaya, strawberry, red grape). Cerdán et al. (2009) and Ertani et al. (2009) observed that applications of plant-derived protein hydrolysates on corn and tomato plants increased nutrient uptake in particular nitrogen and iron as a result of increased nitrate reductase and glutamine synthetase activities, and Fe(III)-chelate reductase activity, respectively. Other authors have highlighted the positive effect of amino acid-derived biostimulants in plant nutrition as chelating agents (Ashmead et al., 1986). Protein hydrolysates can improve crop tolerance to abiotic stresses as reported by Ertani et al. (2013) who observed that root applications of a plant derived-protein hydrolysate improved salinity tolerance of corn due to a better nitrogen metabolism, and an higher K/Na ratio and proline accumulation in leaves.
Protein hydrolysate could also act as plant growth regulators due to the presence of peptides. Several bioactive peptides produced in a variety of plants have been found to have phytohormone-like activities (Ito et al., 2006; Kondo et al., 2006). Phytosulfokine, systemin, SCR/SP11, and CLE are endogenous plants peptides involved in cell differentiation, protease inhibitor induction, cell division, and the pollen self-incompatibility response (Ryan et al., 2002). Recently, Matsumiya and Kubo (2011) isolated from degraded soybean meal products a peptide having root hair promoting activity in Brassica rapa and tomato cuttings. Moreover, Ertani et al. (2009) observed that two protein hydrolysates elicited gibberellin-like activity and a weak auxin-like activity.
Besides the plant biostimulant effects of protein hydrolysates, there are also several studies (Ruiz et al., 2000; Cerdán et al., 2009; Lisiecka et al., 2011) reporting that foliar applications of commercial protein hydrolysate products from animal origin can cause phytotoxicity and plant growth depression. On the contrary, no phytotoxicity and growth depression was observed in tomato plants after foliar applications of plant-derived amino acid (Cerdán et al., 2009). Foliar applications of a commercial animal derived-protein hydrolysate caused necrotic spots on basil leaves while no phytotoxic symptoms and growth depression were observed in basil plants after foliar applications of the commercial plant-derived protein hydrolysate “Trainer” up to 10 times the recommended rate (unpublished data). Growth depression caused by animal derived-protein hydrolysates seems to be related to their higher content in free amino acids (especially small size amino acids like glycine, and proline), and salts (e.g., NaCl) than in plant-derived protein hydrolysates.
Recently, there is a growing concern on the use of animal-derived protein hydrolysates in terms of food safety as demonstrated by the ban of animal-derived protein hydrolysate application on the edible parts of crops in organic farming (European Regulation no. 354/2014). Additional limitations may be imposed on animal derived-protein hydrolysate application in the production of food for vegetarians or people with religious dietary restrictions on the consumption of meat due to the need to exclude any contamination of food with animal derived products.
The development of new plant derived-protein hydrolysates with high plant biostimulant activity has become the focus of much research interest. An enzymatic hydrolysis system (LISIVEG®;) was recently developed by Italpollina S.p.A. (Rivoli Veronese, Italy) to produce protein hydrolysate (“Trainer”) containing a high concentration of amino acids and soluble peptides. The aim of this study was to investigate the biostimulant action (hormone like activity, nitrogen uptake and growth enhancement) of the plant-derived protein hydrolysate “Trainer” by means of two laboratory bioassays (a corn coleoptile elongation rate test, a rooting test on tomato cuttings) and two greenhouse experiments (a dwarf pea growth test, and a tomato nitrogen uptake trial).