† Background and Aims Brassinosteroids (BR) are a class of plant polyhydroxysteroids with diverse functions in

plant growth and development. However, there is little information on the role of BRs played in the response to

nutrient deficiency.

† Methods To evaluate the role of BR in the response of plants to iron (Fe) deficiency, the effect of 24-epibrassinolide  (EBR) on ferric reductase (FRO) activity, acidification of the rhizosphere and Fe content in cucumber  (Cucumis sativus) seedlings under Fe-deficient (1 mM FeEDTA) and Fe-sufficient (50 mM FeEDTA) conditions were investigated.

† Key Results There was a significant increase in FRO activity upon exposure of cucumber seedlings to an Fedeficient medium, and the Fe deficiency-induced increase in FRO activity was substantially suppressed by EBR. In contrast, application of EBR to Fe-sufficient seedlings stimulated FRO activity. Ethylene production evoked by Fe deficiency was suppressed by EBR, while EBR induced ethylene production from Fe-sufficient seedlings. Fe contents in shoots were reduced by treatment with EBR, while Fe contents in roots were markedly increased under both Fe-deficient and Fe-sufficient conditions. The reductions in Fe contents of shoots were independent of chlorophyll (CHL) contents under Fe-sufficient conditions, but they were positively correlated with CHL contents under Fe-deficient conditions. At the transcriptional level, transcripts encoding FRO (CsFRO1) and Fe transporter (CsIRT1) were increased upon exposure to the Fe-deficient medium, and the increases in transcripts were reversed by EBR.

†Conclusions The results demonstrate that BRs are likely to play a negative role in regulating Fe-deficiencyinduced FRO, expressions of CsFRO1 and CsIRT1, as well as Fe translocation from roots to shoots.