Chloroplast magnesium transporters play essential but differential roles in maintaining magnesium homeostasis

Emilija Dukic; Kim A van Maldegem; Kashif Mohd Shaikh; Kento Fukuda; Mats Töpel; Katalin Solymosi; Jonna Hellsten; Thomas Hesselhøj Hansen; Søren Husted; John Higgins; Satoshi Sano; Sumio Ishijima; Cornelia Spetea

doi:10.3389/fpls.2023.1221436

Chloroplast magnesium transporters play essential but differential roles in maintaining magnesium homeostasis

Front Plant Sci. 2023 Aug 23:14:1221436. doi: 10.3389/fpls.2023.1221436. eCollection 2023.

Authors

Affiliations

¹ Department of Biological and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden.
² Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Kyoto, Japan.
³ Department of Marine Sciences, University of Gothenburg, Gothenburg, Sweden.
⁴ IVL Swedish Environmental Research Institute, Gothenburg, Sweden.
⁵ Department of Plant Anatomy, ELTE Eötvös Loránd University, Budapest, Hungary.
⁶ Copenhagen Plant Science Centre, Department of Plant and Environmental Sciences, University of Copenhagen, Copenhagen, Denmark.
⁷ Department of Geosciences, Princeton University, Princeton, NJ, United States.

Abstract

Magnesium (Mg²⁺) is essential for photosynthesis in the chloroplasts of land plants and algae. Being the central ion of chlorophyll, cofactor and activator of many photosynthetic enzymes including RuBisCO, magnesium-deficient plants may suffer from leaf chlorosis symptoms and retarded growth. Therefore, the chloroplast Mg²⁺ concentration is tightly controlled by magnesium transport proteins. Recently, three different transporters from two distinct families have been identified in the chloroplast inner envelope of the model plant Arabidopsis thaliana: MGT10, MGR8, and MGR9. Here, we assess the individual roles of these three proteins in maintaining chloroplast Mg²⁺ homeostasis and regulating photosynthesis, and if their role is conserved in the model green alga Chlamydomonas reinhardtii. Phylogenetic analysis and heterologous expression revealed that the CorC-like MGR8 and MGR9 transport Mg²⁺ by a different mechanism than the CorA-like MGT10. MGR8 and MGT10 genes are highest expressed in leaves, indicating a function in chloroplast Mg²⁺ transport. MGR9 is important for chloroplast function and plant adaptation in conditions of deficiency or excess of Mg²⁺. Transmission electron microscopy indicated that MGT10 plays a differential role in thylakoid stacking than MGR8 and MGR9. Furthermore, we report that MGR8, MGR9, and MGT10 are involved in building up the pH gradient across the thylakoid membrane and activating photoprotection in conditions of excess light, however the mechanism has not been resolved yet. While there are no chloroplast MGR-like transporters in Chlamydomonas, we show that MRS4 is a homolog of MGT10, that is required for photosynthesis and cell growth. Taken together, our findings reveal that the studied Mg²⁺ transporters play essential but differential roles in maintaining chloroplast Mg²⁺ homeostasis.

Keywords: Arabidopsis thaliana; Chlamydomonas reinhardtii; chlorophyll fluorescence; chloroplast; magnesium homeostasis; magnesium transporter; photosynthesis.

Grants and funding

This work was supported by grants from the Swedish Research Council to CS (VR 2016-03836 and 2021-03790) and JSPS KAKENHI to SI (JP15K07399). KMS was recipient of a postdoctoral fellowship from the Carl Tryggers Foundation (CTS 20:406). KS would like to acknowledge the Bolyai János Research Scholarship of the Hungarian Academy of Sciences and the National Research Development and Innovation Office of Hungary (grant OTKA FK 124748) for support.