Iron tolerance modulated by brassinosteroids is triggered enhancing aerenchyma area and metal homeostasis, coupled to benefits on ros scavenging and CO2 assimilation in rice plants.

Abstract

Iron (Fe) toxicity is one of the most frequent abiotic stresses in rice, affecting from 15% to 30% of total production. Brassinosteroids (BRs), including the 24-epibrassinolide (EBR) regulate ion homeostasis and improve antioxidant system. The aim of this research was to answer if the EBR can contribute to tolerance of rice plants exposed to Fe toxicity, evaluating the possible repercussions on anatomical characteristics, nutrient concentrations, antioxidant system, and gas exchange. The experiment was randomized with four treatments, including two Fe supply (250 and 6250 Mm Fe described as control and toxicity, respectively) and two concentrations of brassinosteroids (0 and 10 nM EBR, described as - EBR and + EBR, respectively). Plants exposed to Fe toxicity and treated with EBR presented increases in Mg2+/Fe2+, Mn2+/Fe2+, Cu2+/Fe2+ and Zn2+/Fe2+ratios in shoot of 83%, 89%, 103% and 120%, comparing with same treatment without EBR. This research revealed that the EBR mitigated the deleterious effects of the Fe toxicity on rice plants, modulating the aerenchyma area, in which this structure contribute to the formation of an oxidative barrier, Fe immobilization in root surface. Concomitantly, this steroid increased the contents of other metais, such as magnesium, manganese, copper and zinc, proving the influence related to metal homeostasis, confirmed by increases in metal ratios. Plants under Fe toxicity and treated with EBR presented increases in ali enzyme activities evaluated, superoxide dismutase, catalase, ascorbate peroxidase and peroxidase, mitigating the oxidative damages and favoring the scavenging of reactive oxygen species. Finally, the EBR actions alleviated the negative impacts induced by Fe excess on net photosynthetic rate and carboxylation instantaneous efficiency, being these benefits directly related to higher electron transport and stomatal density and indirectly linked to protection mechanism exercised by the antioxidant enzymes on photosynthetic machinery. Therefore, our results clearly indicate that the EBR is able to confer tolerance in rice plants under Fe toxicity.