Brassinosteroids are involved in Fe homeostasis in rice (Oryza sativa L.)

Brassinosteroids (BRs) are steroid hormones that modulate numerous physiological processes in plants. However, few studies have focused on the involvement of BRs in sensing and responding to the stress of mineral nutrient deficiency. In the present study, we evaluated the roles of BRs in the response of rice (Oryza sativa) to iron (Fe) deficiency during Fe uptake, transport, and translocation. Exogenous application of 24-epibrassinolide (EBR) to wild-type (WT) plants exaggerated leaf symptoms of Fe deficiency and suppressed growth. EBR increased and decreased Fe concentrations in roots and shoots, respectively, under both Fe-deficient and Fe-sufficient conditions. Transcripts involved in Fe homeostasis, including OsIRT1, OsYSL15, OsYSL2, OsNAS1, and OsNAS2, were enhanced by EBR under Fe-deficient conditions. EBR depressed expression of OsNAS1, OsNAS2, and OsYSL2 in shoots, and inhibited Fe transport and translocation via the phloem. Rice mutant d2-1, which is defective in BR biosynthesis, was more tolerant to Fe deficiency than the WT, and accumulated greater amounts of Fe in roots than the WT under Fe-sufficient conditions. A greater upregulation of OsIRT1, OsYSL15, OsYSL2, OsNAS1, and OsNAS2 in the d2-1 mutant compared to the WT was found under Fe-sufficient conditions, while expression of these genes in the d2-1 mutant was lower than in the WT under Fe-deficient conditions. The greater tolerance of the d2-1 mutant could be partly mitigated by exogenous application of EBR. These novel findings highlight the important role of BR in mediating the response of strategy II plants to Fe deficiency by regulating Fe uptake and translocation in rice.