Characterization of MTP8 as a tonoplast Fe/Mn transporter essential for Fe efficiency and for Fe and Mn localization in the subepidermis of Arabidopsis embryos
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- Tez No: 402161
- Danışmanlar: PROF. DR. NICOLAUS VON WIREN
- Tez Türü: Doktora
- Konular: Biyomühendislik, Moleküler Tıp, Bioengineering, Molecular Medicine
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2015
- Dil: İngilizce
- Üniversite: Martın Luther Unıversıty Of Halle-Wıttenberg
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
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Özet (Çeviri)
Although iron (Fe) is a highly abundant element in the earth's crust, plants often suffer from Fe deficiency when grown on alkaline soils, in which Fe availability is low. To increase Fe acquisition from soils, plants have developed different strategies, which are not yet fully understood at the molecular level. Therefore, the aim of the present thesis was to identify genes involved in the Fe deficiency response employing a forward-genetics approach in Arabidopsis. In a first step, a high-pH, low-Fe agar medium was developed, which mimics the pH-dependent Fe limitation observed in alkaline soils. Using this medium, a collection of Arabidopsis T-DNA insertion lines was screened allowing to identify severely chlorotic plants, which regreened by the supplementation of Fe. These lines carried an insertion in MTP8, encoding a member of the MTP family of Mn transporters which belong to the superfamily of Cation Diffusion Facilitators. Since a MTP8-EYFP fusion protein localized to the tonoplast, MTP8 complemented growth of a Mn-sensitive yeast mutant, and mtp8 knockout lines were hypersensitive to low Fe availability only in the presence of Mn, MTP8 was attributed a role in Mn detoxification. Expression of MTP8 was confined to outer root cell layers and strongly induced by low Fe and high Mn supplies in the medium, which was strictly dependent on the transcription factor FIT, a master regulator of Fe-deficiency responses. Fe deficiency-induced chlorosis observed in mtp8 mutants depended on the presence of Mn in the medium and was correlated with low Fe but high Mn concentrations in shoots. Investigating other components of the Fe deficiency response revealed that the mtp8 mutant was defective in enhancing ferric chelate reduction, although the corresponding FRO2 gene was upregulated at the transcript level. These findings indicate that sequestering Mn to the root vacuoles by MTP8 is an important component of the Fe deficiency response, which is essential to increase Fe reduction rate in the presence of Mn. Mn and Fe have previously been reported to take in a distinct cell type-specific localization in mature seeds, whereby Mn is primarily localized in the subepidermis of the abaxial side of cotyledons. In the present work, the question was addressed whether this localization pattern is dependent on MTP8. In cooperation with a Japanese partner laboratory, mtp8 mutant seeds were analyzed by synchrotron X-ray fluorescence, showing that Mn localization in the subepidermis of the abaxial side of the cotyledons and hypocotyl was abolished. Instead, Mn co-localized with Fe around the vascular strands of the embryo. Consistent with a previous study showing that Fe localizes to the abaxial side of the cotyledons in the embryo when tonoplast Fe transporter VIT1 is lacking, it was assumed that Fe localization to the abaxial side of the cotyledons in vit1 also depended on MTP8. To verify this hypothesis, a mtp8 vit1 double knock-out line was generated and Fe was stained by Perls/DAB. As expected, stained Fe was detected in the abaxial side of the cotyledons in vit1 and this distribution was abolished in the double knock-out line. A yeast complementation approach further showed that MTP8 is able to transport Fe besides Mn. However, germination experiments conducted with mtp8 and mtp8 vit1 mutants under Fe or Mn limitation failed to detect any phenotype related to the mislocalization of Fe and Mn. These findings indicate that the interplay between VIT1 and MTP8 determines the cell type-specific localization of Fe and Mn in the embryo of mature seeds.
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