24-Epibrassinolide induces protection against nickel excess in soybean plants: anatomical evidences

Nickel (Ni) excess delays plant growth due to damage to root and leaf structures, reducing nutrient uptake, and carbon fixation, respectively. 24-Epibrassinolide (EBR) is a biodegradable plant growth regulator extracted from plant tissues and is highly efficient against oxidative stress. The objective of this research is to determine whether EBR can improve tolerance to toxic metals and the possible mechanism involved by evaluating the root and leaf structures of soybean plants under high Ni concentrations. The experiment was randomized with four treatments, including two Ni concentrations (0 and 200 µM Ni, described as – Ni2+ and + Ni2+, respectively) and two concentrations of 24-epibrassinolide (0 and 100 nM EBR, described as – EBR and + EBR, respectively). Ni2+ excess provoked damage to root and leaf structures, causing anatomical disorders in these tissues. In roots, EBR increased the epidermis (27%), protecting the root against Ni2+ ions. For leaf tissue, significant increases in palisade (11%) and spongy parenchyma (29%) were detected in plants sprayed with EBR and exposed to Ni2+, which were intrinsically related to stomatal density and stomatal functionality. Our results confirm that pretreatment with 100 nM EBR clearly mitigated the anatomical disorders occasioned by excess Ni on the leaf and root structures of soybean plants.