Fe is an essential micronutrient for plant growth and development; plants have developed sophisticated strategies to acquire ferric Fe from the soil. Nongraminaceous plants acquire Fe by a reduction-based mechanism, and graminaceous plants use a chelation-based mechanism. In Arabidopsis thaliana, which uses the reduction-based method, IRON-REGULATED TRANSPORTER1 (IRT1) functions as the most important transporter for ferrous Fe uptake. Rapid and constitutive degradation of IRT1 allows plants to quickly respond to changing conditions to maintain Fe homeostasis. IRT1 degradation involves ubiquitination. To identify the specific E3 ubiquitin ligases involved in IRT1 degradation, we screened a set of insertional mutants in RING-type E3 ligases and identified a mutant that showed delayed degradation of IRT1 and loss of IRT1-ubiquitin complexes. The corresponding gene was designated IRT1 DEGRADATION FACTOR1 (IDF1). Evidence of direct interaction between IDF1 and IRT1 in the plasma membrane supported the role of IDF1 in IRT1 degradation. IRT1 accumulation was reduced when coexpressed with IDF1 in yeast or Xenopus laevis oocytes. IDF1 function was RING domain dependent. The idf1 mutants showed increased tolerance to Fe deficiency, resulting from increased IRT1 levels. This evidence indicates that IDF1 directly regulates IRT1 degradation through its RING-type E3 ligase activity.
To survive in variable soil conditions, plants possess homeostatic mechanisms to maintain a suitable concentration of essential heavy metal ions. Certain plants, inhabiting heavy metal-enriched or -contaminated soil, thus are named hyperaccumulators. Studying hyperaccumulators has great potential to provide information for phytoremediation. To better understand the hyperaccumulating mechanism, we used an Arabidopsis cDNA microarray to compare the gene expression of the Zn/Cd hyperaccumulator Arabidopsis halleri and a nonhyperaccumulator, Arabidopsis thaliana. By analyzing the expression of metal-chelators, antioxidation-related genes, and transporters, we revealed a few novel molecular features. We found that metallothionein 2b and 3, APX and MDAR4 in the ascorbate-glutathione pathway, and certain metal transporters in P(1B)-type ATPase, ZIP, Nramp, and CDF families, are expressed at higher levels in A. halleri than in A. thaliana. We further validated that the enzymatic activity of ascorbate peroxidase and class III peroxidases are highly elevated in A. halleri. This observation positively correlates with the higher ability of A. halleri to detoxify H2O2 produced by cadmium and paraquat treatments. We thus suggest that higher peroxidase activities contribute to the heavy metal tolerance in A. halleri by alleviating the ROS damage. We have revealed genes that could be candidates for the future engineering of plants with large biomass for use in phytoremediation.
Copper (Cu) is essential for plant growth but toxic in excess. Specific molecular mechanisms maintain Cu homeostasis to facilitate its use and avoid the toxicity. Cu chaperones, proteins containing a Cu-binding domain(s), are thought to assist Cu intracellular homeostasis by their Cu-chelating ability. In Arabidopsis (Arabidopsis thaliana), two Cu chaperones, Antioxidant Protein1 (ATX1) and ATX1-Like Copper Chaperone (CCH), share high sequence homology. Previously, their Cu-binding capabilities were demonstrated and interacting molecules were identified. To understand the physiological functions of these two chaperones, we characterized the phenotype of atx1 and cch mutants and the cchatx1 double mutant in Arabidopsis. The shoot and root growth of atx1 and cchatx1 but not cch was specifically hypersensitive to excess Cu but not excess iron, zinc, or cadmium. The activities of antioxidant enzymes in atx1 and cchatx1 were markedly regulated in response to excess Cu, which confirms the phenotype of Cu hypersensitivity. Interestingly, atx1 and cchatx1 were sensitive to Cu deficiency. Overexpression of ATX1 not only enhanced Cu tolerance and accumulation in excess Cu conditions but also tolerance to Cu deficiency. In addition, the Cu-binding motif MXCXXC of ATX1 was required for these physiological functions. ATX1 was previously proposed to be involved in Cu homeostasis by its Cu-binding activity and interaction with the Cu transporter Heavy metal-transporting P-type ATPase5. In this study, we demonstrate that ATX1 plays an essential role in Cu homeostasis in conferring tolerance to excess Cu and Cu deficiency. The possible mechanism is discussed.
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