Exogenous Serotonin and 24-Epibrassinolide Boost Root Protection and Suppress Oxidative Damages Occasioned by Severe Water Deficit in Soybean Seedlings

Water restriction negatively affects seed germination by delaying the imbibition stage, seedling development, and causing oxidative damages. Serotonin (SER) is a neurotransmitter that was first found in mammals and plays a role in stress physiology and circadian modulation. In plants, SER acts on the development of roots and shoots, embryogenesis, germination, and flowering. 24-epibrassinolide (EBR) is the most bioactive form of brassinosteroids, it is also organic and biodegradable, being a molecule that has multiple benefits on plant metabolism. This research aims to investigate whether SER and EBR can mitigate the damage caused by water stress in soybean seedlings, evaluating the repercussions on seedling root anatomy, germination rate, antioxidant enzymes, and biomass. The experiment was randomized with eight treatments. The control and water deficit used 0.0 and - 0.5 MPa, respectively. To neutransmitter, -SER and + SER were applied to 0 and 50 mu M SER, respectively. For brassinosteroids, -EBR and + EBR were tested at 0 and 100 nM EBR, in this order. Treatment with SER and EBR minimized the deleterious effects of severe water deficit in seedlings. These growth regulators protected the radicle by stimulating the epidermis, resulting in higher root development. SER and EBR promoted increases in the metaxylem and vascular cylinder, favoring water absorption and germination. These molecules (SER and EBR) stimulated the antioxidant machinery in order to combat the accumulation of hydrogen peroxide and oxidative damages. In biomass, SER and EBR increased the length and dry matter of the seedlings, confirming the beneficial effects on root anatomy, antioxidant enzymes, and reduction of reactive oxygen species.