Manganese homeostasis in oxygenic photosynthetic organisms

The metal manganese (Mn) is an essential micronutrient for all organisms. Mn2+ ions function as redox active cofactors in active sites of enzymes and are thus involved in various physiological reactions. The best-studied Mn-dependent enzymes are the Mn-superoxide dismutase and Mn-catalase, both functioning in detoxification of reactive oxygen species. In oxygenic photosynthetic organisms, such as plants, algae and their evolutionary ancestors, the cyanobacteria, Mn plays an essential role, as it constitutes the catalytically active part of the oxygen-evolving complex in photosystem II.

Despite the basic need for Mn, elevated levels of this micronutrient are detrimental to the organism. Mn2+ ions compete with other bivalent ions, such as Mg2+, for incorporation into enzymes or other biomolecules. Furthermore, free Mn2+ ions act as oxidizing agent and may hence promote oxidative stress and generation of reactive oxygen species. Accordingly, cellular Mn homeostasis needs to be carefully maintained. Mn toxicity is of special significance for plants growing on acid soil, since the bioavailability of Mn is increased at low pH. To tolerate high intracellular Mn concentrations plants employ diverse mechanisms. For example, excess Mn is complexed by organic acids, is sequestered in the vacuole, or gets secreted via vesicles.

We are studying Mn homeostasis in the cyanobacterium Synechocystis sp. PCC 6803. Working with Synechocystis as model for oxygenic photosynthesis is advantageous. Its generation time is short, a complete molecular toolkit is available, and target genes can be specifically manipulated by homologous recombination.

In cyanobacteria Mn uptake is coordinated via inducible and constitutive Mn import systems, respectively. In contrast to the well-studied uptake, it is largely unknown, how Mn that is released inside the cell, e.g., during breakdown of photosystem II, is transferred back into the periplasmic storage. Recently, we identified a protein with so far unknown function as hypothetical Mn export protein in Synechocystis. We named this protein MNX as abbreviation for MANGANESE EXPORTER. The knockout mutant Dmnx shows sensitivity towards externally supplied Mn and accumulates the metal in the cytoplasm.

Knockout of the hypothetical Mn export protein MNX leads to Mn sensitivity in the cyanobacterium Synechocystis.

Future Projects and Goals

In our project we aim at unraveling the function and biological significance of MNX, a so far unknown type of Mn transporter in the cyanobacterium Synechocystis. We will focus on:

  • Verifying the function of MNX as Mn exporter.
  • Elucidating the role of MNX in maintaining Mn homeostasis in Synechocystis.
  • Understanding the global Mn homeostasis in cyanobacteria.

Finally, we will transfer the gained knowledge to higher plant plastid metabolism. Our results will help to understand plant Mn homeostasis and improve plant performance under Mn excess conditions

 

Selected Publications

Mustila H, Allahverdiyeva Y, Isojärvi J, Aro EM, Eisenhut M (2014) The bacterial-type [4Fe-4S] ferredoxin 7 has a regulatory function under photooxidative stress conditions in the cyanobacterium Synechocystis sp. PCC 6803. Biochim Biophys Acta 1837: 1293-1304

Eisenhut M, Planchais S, Cabassa C, Guivarc'h A, Justin AM, Taconnat L, Renou JP, Linka M, Gagneul D, Timm S, Bauwe H, Carol P, Weber AP (2012) Arabidopsis A BOUT DE SOUFFLE is a putative mitochondrial transporter involved in photorespiratory metabolism and is required for meristem growth at ambient CO2 levels. Plant J 73: 836-84

Eisenhut M, Ruth W, Haimovich M, Bauwe H, Kaplan A, Hagemann M (2008) The photorespiratory glycolate metabolism is essential for cyanobacteria and might have been conveyed endosymbiontically to plants. Proc Natl Acad Sci USA (PNAS) 105: 17199-1720

Photo of Marion  Eisenhut

Dr. Marion Eisenhut

Curriculum vitae

Ph.D. work with Martin Hagemann, University of Rostock (2004-2008)

Postdoctoral work with Eva-Mari Aro, University of Turku, Finland (2008-2010)

Research Associate, Institute for Plant Biochemistry, Heinrich Heine University Düsseldorf (since 2010)

Verantwortlich für den Inhalt: E-Mail sendenDr. Petra Fackendahl