Iron (Fe) is an essential micronutrient for plants. However, plants often suffer from Fe deficiency, which reduces crop yield and nutritional quality. Plants have evolved sophisticated mechanisms that maintain Fe homeostasis under Fe deficiency conditions by transcriptome reprogramming. The regulatory network modulating Fe homeostasis is complex. 

In Arabidopsis FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) is a crucial regulator of Fe deficiency response. FIT is activated indirectly by bHLH IVc transcription factors (TFs) under Fe deficiency; however, it remains unclear which protein(s) act as a linker to mediate the activation of FIT by bHLH IVc TFs. 

In a study published in Molecular Plant, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) revealed that bHLH121 is the missing link between bHLH IVc proteins and FIT.  

The researchers employed three different methods (yeast-one-hybrid, ChIP-qPCR and EMSA) and provided compelling evidence showing that bHLH121 directly binds the FIT promoter. 

They found that loss-of-function mutations of bHLH121 cause severe Fe deficiency symptoms, reduced Fe accumulation, and disrupted expression of genes associated with Fe homeostasis. 

They also found that bHLH121 functions downstream of and is a direct target of bHLH IVc TFs, and its expression is induced by Fe deficiency in a bHLH IVc-dependent manner. 

Genetic analysis showed that FIT is epistatic to bHLH121 and FIT overexpression partially rescues the bHLH121 mutant.  

"Our results establish a direct link that bHLH121 functions together with bHLH IVc TFs to mediate the activation of FIT and thus plays a pivotal role in maintaining Fe homeostasis in Arabidopsis", said Dr. LIANG Gang, principal investigator of the study.