Protein hydrolysates derived from pig blood increase growth performances of tomato by regulating phenolic biosynthesis and antioxidant enzymes under drought stress

Drought has been recognized as a major threat to crop yields. Protein hydrolysates, a group of plant biostimulants, appear to be a promising eco-friendly strategy for enhancing plant tolerance to various environmental stresses. The current study aimed to evaluate the effects of a novel protein hydrolysates derived from pig blood (PP) on growth performances of tomato and its underlying mechanisms under drought stress. The results showed that PEG-induced drought stress significantly increased the levels of hydrogen peroxide (H2O2), superoxide (O2·−), and malondialdehyde (MDA), resulting in the obvious reduction of plant height, stem diameter, and shoot or root fresh weight. Exogenous PP stimulated the rapid accumulation of total phenolic, flavonoids, and individual phenolic compounds in tomato plants by regulating the relative expression of genes involved in phenolic biosynthesis, including phenylalanine ammonia-lyase (PAL), cinnamic acid 4-hydroxylase (C4H), 4-coumarate-CoA ligase (4CL), chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase(F3H), and flavonol synthase (FLS). Meanwhile, PP application also significantly reduced the content of H2O2, O2·−, and MDA, and improved the antioxidant capacity and growth performances of tomato plants under drought stress. However, exogenous phenolic biosynthesis inhibitors, 2-aminoindan-2-phosphonic acid (AIP), significantly decreased the contents of total phenolic and flavonoids, which partially abolished the positive impacts of PP in reducing ROS accumulation, oxidative damage and increasing drought tolerance under drought stress. In addition, PP treatment also simultaneously enhanced the enzyme activities of superoxide (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), and total antioxidant activity of tomatoes under drought stress. In conclusion, PP application might be a useful method in reducing ROS accumulation and oxidative damage by regulating the accumulation of phenolic compounds and activities of antioxidant enzymes, thus enhancing the drought tolerance of tomato.