Abstract
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.
Highlights
Iron (Fe) is one of the essential mineral nutrients required for plant growth and development
To test whether BR is involved in Fe deficiency-induced changes in physiological processes in rice plants, the effect of exogenous EBR at varying concentrations (0–500 nM) on cholorophyll concentrations of rice seedlings grown in Fe-sufficient and Fe-deficient media was studied
The magnitude of upregulation of OsYSL2, OsNAS1 and OsNAS2 expression was much greater in WT and d2-1 mutants challenged by Fe deficiency and EBR together than by treatment with Fe deficiency and EBR alone (Fig. 6A–I), suggesting that Fe deficiency and EBR may have additive effects on the expression of these genes
Summary
Iron (Fe) is one of the essential mineral nutrients required for plant growth and development. Similar to Arabidopsis, rice possesses an FRD-like gene (OsFRDL1), which encodes a citrate transporter localized at the rice root pericycle cells and mediates the translocation of Fe to the shoot in the form of Fe-citrate complex (Yokosho et al, 2009). Our previous studies showed that brassinosteroids are involved in response of cucumber (Cucumis sativus) to Fe deficiency by regulating Fe deficiency-induced FRO and Fe translocation from roots to shoots (Wang et al, 2012). Our results demonstrate that BR played an important role in the response of strategy II plant to Fe deficiency by regulating long-distance transport and translocation of Fe via the phloem
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