The bHLH Transcription Factor PhbHLH121 Regulates Response to Iron Deficiency in .

Iron (Fe) is an essential micronutrient for plants. Due to the low Fe bioavailability in cultivated soils, Fe deficiency is a widespread agricultural problem. In this study, we present the functional characterization of a petunia () basic-helix-loop-helix transcription factor PhbHLH121 in response to Fe shortage. Real-time PCR revealed that the expression of in petunia roots and shoots was downregulated under Fe-limited conditions. CRISPR/Cas9-edited mutant plants were generated to investigate the functions of PhbHLH121 in petunia. Loss-of-function of PhbHLH121 enhanced petunia tolerance to Fe deficiency. Further investigations revealed that the expression level of several structural genes involved in Fe uptake in petunia, such as and , was higher in mutants compared to that in wild-type under Fe-limited conditions, and the expression level of several genes involved in Fe storage and Fe transport, such as , and , was lower in mutants compared to that in wild-type under Fe-deficient conditions. Yeast one-hybrid assays revealed that PhbHLH121 binds to the G-box element in the promoter of genes involved in Fe homeostasis. Yeast two-hybrid assays revealed that PhbHLH121 interacts with petunia bHLH IVc proteins. Taken together, PhbHLH121 plays an important role in the Fe deficiency response in petunia.