Rice HRZ ubiquitin ligases are crucial for response to excess iron

Iron is essential for virtually all organisms but is toxic when present in excess. To acquire the proper amount of iron, plants induce expression of various genes involved in iron uptake and translocation in response to low iron availability. Two iron-binding ubiquitin ligases, OsHRZ1 and OsHRZ2, negatively regulate such iron deficiency responses in rice (Oryza sativa). Transgenic rice plants with repressed expression of OsHRZ1 and OsHRZ2 (HRZ knockdown lines) are tolerant to low iron availability and accumulate iron in shoots and seeds under both iron-sufficient and -deficient conditions without a growth penalty. Although the expression of OsHRZ1 and OsHRZ2 is transcriptionally upregulated under iron-deficient conditions, the physiological relevance of this induction is not known. In the present study, we analyzed the response of HRZ knockdown lines to excess iron. In the presence of severe excess iron, the HRZ knockdown lines grew worse than non-transformants. The HRZ knockdown lines showed stunted shoot and root growth and more severe leaf bronzing compared to non-transformants. Moreover, these lines accumulated more iron in shoots and exhibited severely elevated expression of various genes involved in iron uptake and translocation as well as jasmonate signaling compared to non-transformants. These results indicate that HRZ ubiquitin ligases are crucial for repressing iron deficiency responses and protecting cells from iron toxicity in the presence of excess iron. These results support the possibility that HRZs are intracellular Fe sensors and provide clues for developing plants tolerant of either iron deficiency or excess with higher iron contents in edible parts.