Brassinosteroids induce tolerance to water deficit in soybean seedlings: contributions linked to root anatomy and antioxidant enzymes

Water deficit is a type of abiotic stress that often limits seed germination, causing oxidative stress, cellular damage and physiological quality reduction. However, brassinosteroids regulate several biochemical and physiological processes in plants, reducing the oxidative stress caused by abiotic stresses, such as water deficit. The present study aimed to evaluate the contributions of 24-epibrassinolide (EBR) on the root anatomy and antioxidant enzymes as well as the possible interference of EBR on the oxidative stress, germination and biomass of soybean seedlings subjected to water deficit. The experiment had a factorial design, with the factors completely randomized, and two osmotic potentials (0.0 and -0.3MPa, as the control and water deficit, respectively) and three levels of brassinosteroids (0, 50 and 100nM EBR). Soybean seedlings subjected to water deficit presented reductions of the root epidermis (RET), root endodermis (RDT), root cortex (RCT), vascular cylinder (VCD) and root metaxylem (RMD); however, the application of 100nM brassinosteroids promoted increases in these values of 21, 12, 15, 38 and 15%, respectively, compared with water deficit +0nM brassinosteroids. In relation to antioxidant enzymes, the treatment with 100nM brassinosteroids induced an increase in all of the enzymes evaluated. This study revealed that brassinosteroids promoted an increase in the germination, length and dry matter of soybean seedlings, and these results were explained by the increments in the root anatomy, particularly of the RET, RDT, RCT, VCD and RMD. Reductions in the concentrations of reactive oxygen species (superoxide and hydrogen peroxide) and membrane damage (malondialdehyde and electrolyte leakage) were intrinsically related to the higher activities of antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase and peroxidase), confirming the benefits of brassinosteroids on the antioxidant system. Moreover, 100nM brassinosteroids resulted in a better dose-response in soybean seedlings exposed to water deficit.