Brassinosteroids trigger tolerance to iron toxicity in rice.

Iron (Fe) toxicity is one of the most frequent abiotic stresses in rice, as it affects from 15% to 30% of the total production. Brassinosteroids (BRs), including 24-epibrassinolide (EBR), regulate ion homeostasis and improve the antioxidant system. The aim of this research was to determine whether EBR can contribute to the tolerance of rice plants exposed to Fe toxicity and to evaluate the possible effect on anatomical characteristics, nutrient concentrations, the antioxidant system, and gas exchange. The experiment was randomized with four treatments, two with different concentrations of Fe (250 and 6250 μM, control and toxicity, respectively) and these were either supplied with EBR or not (0 and 10 nM EBR, described as -EBR and +EBR, respectively). Treating plants grown under Fe toxic conditions with EBR caused an 70% increase in root aerenchyma area, compared to plants without steroid treatment. Our results revealed that EBR treatment could mitigate the deleterious effects of Fe toxicity in rice plants, by modulating the aerenchyma area, which contributes to the formation of an oxidative barrier and reduce the Fe mobilization at the root surface. Plants that were exposed to Fe toxic concentrations and treated with EBR showed (1) an increase in the enzyme activities of superoxide dismutase, catalase, ascorbate peroxidase and peroxidase, (2) mitigation of oxidative damage and (3) increased scavenging of reactive oxygen species. Finally, EBR alleviated the negative impacts induced by excess Fe on the net photosynthetic rate and the instantaneous carboxylation efficiency. These benefits were directly related to higher electron transport and stomatal density and indirectly linked to the protection mechanism exercised by the antioxidant enzymes on photosynthetic machinery. We conclude that EBR is able to confer tolerance to Fe toxicity in rice plants.