Although the crosstalk between iron (Fe) and copper (Cu) homeostasis signalling networks exists in plants, the underlined molecular mechanism remains unclear. FIT (FER‐LIKE IRON DEFICIENCY‐INDUCED TRANSCRIPTION FACTOR) and four bHLH Ib members (bHLH38, bHLH39, bHLH100 and bHLH101) are the key regulators of Fe homeostasis. Here, we reveal that FIT and bHLH Ib control the up‐regulation of Cu‐uptake genes (COPT2, FRO4 and FRO5) by Fe deficiency, and Cu is required for improving plant growth under Fe‐deficiency conditions. The induction of Cu‐uptake gene expression and the elevation of Cu concentration are inhibited in the fit‐2 or bhlh4x (the quadruple mutant of four bHLH Ib genes) under Fe‐deficiency conditions. The dual overexpression of both bHLH38 (or bHLH39) and FIT activates the expression of COPT2, FRO4 and FRO5 and increases Cu accumulation. Furthermore, bHLH Ib proteins directly bind to the promoters of COPT2, FRO4 and FRO5. Either Cu supplement or overexpression of COPT2 or FRO4 improves the growth of fit‐2 under Fe‐deficiency conditions. Moreover, the induction of COPT2, FRO4 and FRO5 by Fe deficiency is independent of SPL7, a central regulator of Cu‐deficiency responses. This work through the link between bHLH Ib/FIT and COPT2/FRO4/FRO5 under Fe‐deficiency conditions establishes a new relationship between Cu and Fe homeostasis.

中文翻译：

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FIT 和 bHLH Ib 转录因子调节拟南芥中的铁和铜串扰

尽管植物中存在铁 (Fe) 和铜 (Cu) 稳态信号网络之间的串扰，但下划线的分子机制仍不清楚。FIT（FER-LIKE IRON DEFICIENCY-INDUCTION FACTOR）和四个 bHLH Ib 成员（bHLH38、bHLH39、bHLH100 和 bHLH101）是 Fe 稳态的关键调节因子。在这里，我们揭示了 FIT 和 bHLH Ib通过缺铁控制 Cu 吸收基因（COPT2、FRO4和FRO5）的上调，并且在缺铁条件下，Cu 是改善植物生长所必需的。在fit-2或bhlh4x中抑制Cu摄取基因表达的诱导和Cu浓度的升高（四个 bHLH Ib 基因的四重突变体）在缺铁条件下。bHLH38（或bHLH39）和FIT的双重过表达激活COPT2、FRO4和FRO5的表达并增加Cu积累。此外，bHLH Ib 蛋白直接与COPT2、FRO4和FRO5的启动子结合。铜补充或COPT2或FRO4 的过表达可改善缺铁条件下fit-2的生长。此外，COPT2、FRO4和FRO5的诱导铁缺乏与 SPL7 无关，SPL7 是铜缺乏反应的中心调节剂。这项工作通过在缺铁条件下bHLH Ib/FIT 和COPT2/FRO4/FRO5之间的联系建立了 Cu 和 Fe 稳态之间的新关系。