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The Significance of Organellar Membrane Contact Sites in Photosynthetic Metabolism of Plant Cells

Subject Area Plant Physiology
Plant Cell and Developmental Biology
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 387162851
 
Final Report Year 2022

Final Report Abstract

Plant metabolism is flexibly and rapidly adapting to changes in environmental conditions and requires the cooperation of several membrane-bounded organelles. Membranes constitute boundaries for many molecules while they are selectively permeable to others. For example, local redox environments depend on the distinct permeabilities of organellar membranes to ROS or the oxidised form of the redox buffer glutathione, glutathione disulfide. In addition, each cell compartment contains a unique set of redox-active proteins resulting in a compartment-specific redox balance including local metabolic control via thiol switches. While membrane boundaries limit exchanges between compartments, local changes in one compartment can lead to changes in other compartments, e.g. via metabolite levels, protein (re-)localisation or second messengers. For example, redox-state of subcellular compartments can be inter-linked. Thus, chloroplasts can constitute a ROS-source for the cytosol. On the one hand, interaction between compartments can occur globally, i.e. on the whole membrane surface. On the other hand, specific membrane contacts sites between organelle pairs are emerging specific interaction areas with functions in organelle dynamics and positioning as well as molecule transfer. In plants, the identity and functional role of MCS is still largely unknown. This project investigates the question of how the spatial organisation of plant cells, i.e. positioning and specific membrane contact sites, underpins metabolism and plant responses to stress with a focus on the cell-biological basis of efficient photosynthesis. Physical tethering between chloroplasts and peroxisomes has been previously evidenced and proximity of photorespiratory organelles is often observable in confocal microscopy of fluorescently labelled organelles. We aim at detecting MCS between chloroplasts, mitochondria and peroxisomes using different types of fluorescent sensors. In our hands, split- and self-assembly-based fluorescent proteins homogenously targeted to the (outer) surface of chloroplasts, mitochondria and peroxisomes can indicate proximity of organelles but lead to synthetic organelle tethering. However, we did not observe specific sensor signal at MCS, testing different linker lengths. A FRET/FLIM-based system was not suitable to dynamically trace MCS between chloroplasts and peroxisomes. Notably, synthetic tethers can be used as tool to perturb interactions of different organelles. We designed synthetic tethers for chloroplast/mitochondria/peroxisome in plants and found that changes in relative organelle positioning impacted growth, inflorescence development as well as stress responses to abiotic stresses in stable transgenic Arabidopsis thaliana lines. The future challenge is to systematically map physiological effects of synthetic tethering between organelle pairs and to fully characterise proximity sensor systems for MCS detection.

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