Condensation of Rubisco into a proto-pyrenoid in higher plant chloroplasts

Nicky Atkinson,Alistair J McCormick,Kher Xing Chan,Yuwei Mao

doi:10.1038/s41467-020-20132-0

Nicky Atkinson, Alistair J McCormick + Show 2 more

Open Access

https://doi.org/10.1038/s41467-020-20132-0

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Abstract

Photosynthetic CO2 fixation in plants is limited by the inefficiency of the CO2-assimilating enzyme Rubisco. In most eukaryotic algae, Rubisco aggregates within a microcompartment known as the pyrenoid, in association with a CO2-concentrating mechanism that improves photosynthetic operating efficiency under conditions of low inorganic carbon. Recent work has shown that the pyrenoid matrix is a phase-separated, liquid-like condensate. In the alga Chlamydomonas reinhardtii, condensation is mediated by two components: Rubisco and the linker protein EPYC1 (Essential Pyrenoid Component 1). Here, we show that expression of mature EPYC1 and a plant-algal hybrid Rubisco leads to spontaneous condensation of Rubisco into a single phase-separated compartment in Arabidopsis chloroplasts, with liquid-like properties similar to a pyrenoid matrix. This work represents a significant initial step towards enhancing photosynthesis in higher plants by introducing an algal CO2-concentrating mechanism, which is predicted to significantly increase the efficiency of photosynthetic CO2 uptake.

Highlights

Photosynthetic CO2 fixation in plants is limited by the inefficiency of the CO2-assimilating enzyme Rubisco
We initially sought to test whether high levels of expression of a mature form of EPYC1 could lead to phase separation in a higher plant chloroplast
A dual GFP expression system was developed to achieve high levels of EPYC1 expression and a favourable stoichiometry with Rubisco. This consisted of a binary vector containing two gene expression cassettes, each encoding mature EPYC1 with an Arabidopsis chloroplastic signal peptide and fused to a different version of GFP (turboGFP or enhanced GFP) to avoid possible recombination events (Fig. 1a and Supplementary Fig. 1)

Summary

Introduction

Photosynthetic CO2 fixation in plants is limited by the inefficiency of the CO2-assimilating enzyme Rubisco. We show that expression of mature EPYC1 and a plant-algal hybrid Rubisco leads to spontaneous condensation of Rubisco into a single phase-separated compartment in Arabidopsis chloroplasts, with liquid-like properties similar to a pyrenoid matrix. Most unicellular eukaryotic photosynthetic organisms and some non-vascular land plants have evolved highly efficient CO2-concentrating mechanisms (CCMs) that condense Rubisco into a microcompartment within the chloroplast called the pyrenoid[9,10]. The CCM functions to enrich the pyrenoid with high concentrations of CO2 to enhance the performance of Rubisco carboxylation and suppress oxygenation[11] Introduction of such CCMs into C3 plants is predicted to lead to significant increases in CO2-fixation efficiency and biomass yield[8,12,13]. Photosynthetic CO2 fixation and growth is not impaired in stable transformants compared to azygous segregants

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