Regulation of iron homeostasis by IMA1 and bHLH104 under phosphate starvation in Arabidopsis

Phosphate (Pi) starvation disrupts iron (Fe) nutrition at phenotypic, physiological, and transcriptional levels. The alteration of Fe homeostasis plays an important role in the adaptive response to Pi starvation. However, utilizing the antagonistic mechanism between P and Fe nutrition to improve adaptation to Pi deficiency in plants still needs to be explored. Here, we constructed inducible and constitutive expression of Fe regulators IMA1 and bHLH104, driven by the CaMV 35S promoter and the promoters of Pi-starvation responsive genes (proIPS1 and proPHT1;4), respectively. The Fe regulators bHLH104 and IMA1 were successfully upregulated in a constitutive and inducible manner under Pi deficiency in these transgenic plants. Regardless of Pi condition, upregulation of bHLH104 and IMA1 had no significant influence on primary root length or root Fe distribution. Nevertheless, the upregulation of bHLH104 and IMA1 induced Fe accumulation in the shoots of transgenic plants, particularly under Pi deficiency. Correspondingly, shoot chlorophyll content increased under Fe deficiency in the transgenic plants. In addition, in situ Fe^III distribution revealed that bHLH104 and IMA1 likely interfere with Fe distribution through different pathways. The inducible upregulation of IMA1 significantly led to shoot zinc (Zn) accumulation under Pi deficiency, while the inducible upregulation of bHLH104 resulted in a decrease in shoot Zn and manganese (Mn) contents. The enhancement of Fe and Zn accumulation under the inducible expression of IMA1 under Pi deficiency was attributed to the induction of high expression of key Fe-uptake genes FRO2 and IRT1. The expression of the Zn and Mn uptake genes was also affected in these transgenic plants, which correlated with the changes in Zn and Mn contents. Overall, IMA1 is an excellent candidate for enhancing plant Fe and Zn accumulation and can be specifically induced under conditions of Pi deficiency.