Aung May Sann, Kobayashi Takanori, Masuda Hiroshi, Nishizawa Naoko K
Research Institute for Bioresources and Biotechnology, Ishikawa Prefectural University, Nonoichi, 921-8836, Japan.
Physiol Plant. 2018 Apr 14. doi: 10.1111/ppl.12698.
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.
铁对几乎所有生物体来说都是必不可少的，但过量存在时具有毒性。为了获取适量的铁，植物会在铁供应不足时诱导参与铁吸收和转运的各种基因的表达。两种铁结合泛素连接酶OsHRZ1和OsHRZ2对水稻（Oryza sativa）的这种缺铁反应起负调控作用。OsHRZ1和OsHRZ2表达受抑制的转基因水稻植株（HRZ基因敲低系）对铁供应不足具有耐受性，并且在铁充足和缺铁条件下，其地上部和种子中都能积累铁，且不会对生长造成不利影响。尽管在缺铁条件下OsHRZ1和OsHRZ2的表达在转录水平上上调，但其诱导作用的生理相关性尚不清楚。在本研究中，我们分析了HRZ基因敲低系对过量铁的反应。在严重铁过量的情况下，HRZ基因敲低系的生长比未转化植株差。与未转化植株相比，HRZ基因敲低系的地上部和根系生长受阻，叶片黄化更严重。此外，与未转化植株相比，这些株系地上部积累了更多的铁，并且参与铁吸收和转运以及茉莉酸信号传导的各种基因的表达显著升高。这些结果表明，HRZ泛素连接酶对于抑制缺铁反应以及在铁过量时保护细胞免受铁毒性至关重要。这些结果支持了HRZs是细胞内铁传感器的可能性，并为培育在可食用部分中铁含量更高、耐缺铁或耐铁过量的植物提供了线索。
