Rice HRZ ubiquitin ligases are crucial for the response to excess iron

Abstract

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.

How Does Rice Defend Against Excess Iron?: Physiological and Molecular Mechanisms

“…Kobayashi et al (2013) identified two Fe-binding ubiquitin ligases, OsHRZ1 and OsHRZ2, which negatively regulate Fe deficiency response in rice. HRZ-knockdown rice lines are hypersensitive to Fe toxicity, demonstrating severe growth defects and leaf bronzing since even under mild Fe excess conditions (Aung et al, 2018a). In addition, disruption of HRZ causes 5-to 10-fold hyperaccumulation of Fe in leaves, and the expression of Fe uptake-and transport-related genes including OsIRT1, OsYSL2, OsYSL15, TOM1, OsNAS1, OsNAS2 and OsIRO2 is severely elevated in roots of knockdown rice subjected to Fe excess stress (Aung et al, 2018a).…”

“…HRZ -knockdown rice lines are hypersensitive to Fe toxicity, demonstrating severe growth defects and leaf bronzing since even under mild Fe excess conditions ( Aung et al., 2018a ). In addition, disruption of HRZ causes 5- to 10-fold hyperaccumulation of Fe in leaves, and the expression of Fe uptake- and transport-related genes including OsIRT1 , OsYSL2 , OsYSL15 , TOM1 , OsNAS1 , OsNAS2 and OsIRO2 is severely elevated in roots of knockdown rice subjected to Fe excess stress ( Aung et al., 2018a ). Therefore, HRZ is an important protein to protect plant cells from Fe toxicity and maintain Fe homeostasis in plants under Fe excess by repressing the genes involved in Fe uptake and translocation ( Aung et al., 2018a , Figure 2 ).…”

Hemerythrin E3 Ubiquitin Ligases as Negative Regulators of Iron Homeostasis in Plants

“…For HRZ2 , RNAi lines and the hrz2-1 mutant also showed tolerance to Fe deficiency and accumulated Fe in leaves and seeds (Kobayashi et al, 2013). Interestingly, detailed phenotypic and gene expression studies of the mutants showed that the HRZ genes also have a function in Fe sufficiency and excess (Zhang et al, 2017; Aung et al, 2018).…”

The Molecular Mechanisms Underlying Iron Deficiency Responses in Rice

“…OsHRZ1 and OsHRZ2 are also regraded as candidates for Fe sensors because they directly bind Fe via hemerythrin domains. OsHRZ1 and OsHRZ2 also have RING (really interesting new gene) Zn-finger domains that act as E3 ubiquitin ligases [51,76]. OsHRZ1 and OsHRZ2 RNAi rice plants show enhanced tolerance to Fe deficiency treatment compared to wild-type rice plants, coupled with the increased contents of Fe in shoots and grains, as well as Fe deficiency responsive genes [51,76].…”

“…Additionally, the expression of both OsHRZ1 and OsHRZ2 is under the control of IDEF1 in rice. OsHORZ1 is also a protein that contains the hemerythrin domain and is suggested to repress OsHRZ functions [51,76].…”