Fig. 9.

Proposed model for the role of FIT, CAT2, and H₂O₂ in the regulation of Fe acquisition and homeostasis under prolonged Fe deficiency. Subgroup IVc proteins ILR3 and bHLH104, together with other transcription factors (not depicted), form a complex with PYE and regulate the expression of downstream Fe homeostasis genes (dark dotted arrows), including PYE itself and subgroup Ib BHLHgenes (BHLH038, BHLH039, BHLH100, and BHLH101). Subgroup Ib bHLHs interact with FIT, a complex which positively regulates FIT, Fe acquisition genes, such as FRO2 and IRT1, and CAT2. CAT2 protein is a heme-containing H₂O₂ dismutating enzyme, which requires Fe, imported with the help of FRO2 ferric reductase and the transporter IRT1. Under prolonged Fe deficiency, H₂O₂ production is increased and acts in two directions (red dotted lines). First, high H₂O₂ levels negatively influence the positive regulation by PYE–ILR3/bHLH104. Thus, transcription of PYE and subgroup Ib BHLH genes is down-regulated. This depletion of subgroup Ib bHLH proteins is the first factor that negatively influences the formation of the positive Fe acquisition transcriptional complex FIT–bHLH Ib and results in attenuation of FRO2 and IRT1 expression, and therefore Fe import. The second effect of H₂O₂ accumulation is the activation of the ROS-responsive transcription factor ZAT12. ZAT12 can interact with FIT, forming an inactive FIT complex. This is the second factor that prevents the formation of the FIT–bHLH Ib positive regulatory complex. The role of CAT2 would be to limit the H₂O₂ levels by dismutating the excess H₂O₂ to balance the efficiency at which the transcriptional regulatory cascade functions.