Pyrenoid loss in Chlamydomonas reinhardtii causes limitations in CO2 supply, but not thylakoid operating efficiency.

Oliver D Caspari,Tracy Lawson,Moritz T Meyer,Nik J Cunniffe,Dimitri Tolleter,Tyler M Wittkopp,Howard Griffiths,Arthur R Grossman

doi:10.1093/jxb/erx197

Abstract

The pyrenoid of the unicellular green alga Chlamydomonas reinhardtii is a microcompartment situated in the centre of the cup-shaped chloroplast, containing up to 90% of cellular Rubisco. Traversed by a network of dense, knotted thylakoid tubules, the pyrenoid has been proposed to influence thylakoid biogenesis and ultrastructure. Mutants that are unable to assemble a pyrenoid matrix, due to expressing a vascular plant version of the Rubisco small subunit, exhibit severe growth and photosynthetic defects and have an ineffective carbon-concentrating mechanism (CCM). The present study set out to determine the cause of photosynthetic limitation in these pyrenoid-less lines. We tested whether electron transport and light use were compromised as a direct structural consequence of pyrenoid loss or as a metabolic effect downstream of lower CCM activity and resulting CO2 limitation. Thylakoid organization was unchanged in the mutants, including the retention of intrapyrenoid-type thylakoid tubules, and photosynthetic limitations associated with the absence of the pyrenoid were rescued by exposing cells to elevated CO2 levels. These results demonstrate that Rubisco aggregation in the pyrenoid functions as an essential element for CO2 delivery as part of the CCM, and does not play other roles in maintenance of photosynthetic membrane energetics.

Highlights

The pyrenoid of the model green alga Chlamydomonas reinhardtii has long been implicated in a number of functions, most notably the establishment of a carbon-concentrating mechanism (CCM; Badger et al, 1980; Wang et al, 2015)
Thylakoids appear as single lamellae or appressed in stacks that often run parallel to the chloroplast envelope and occupy most of the stroma
Absence of the pyrenoid matrix has no effect on thylakoid ultrastructure assembly of photosynthetic protein complexes, which occurs in pyrenoid-associated translation zones in the WT (Uniacke and Zerges, 2007, 2009), probably functions normally in pyr– cells [unless a growth arrest (Fig. 5H) is masking a defect confined to low CO2]

Summary

Introduction

The pyrenoid of the model green alga Chlamydomonas reinhardtii has long been implicated in a number of functions, most notably the establishment of a carbon-concentrating mechanism (CCM; Badger et al, 1980; Wang et al, 2015). The pyrenoid may be needed to maintain structural aspects of the chloroplast, including thylakoid membrane organization and translation of proteins and their assembly into complexes, which was found to be localized to specialized zones at the pyrenoid periphery (Uniacke and Zerges, 2007, 2009). Disruption of these pyrenoid-associated processes could potentially impact chloroplast energetics and photosynthetic efficiency. In Chlamydomonas RBCS substitution strains expressing vascular plant RBCS, Rubisco fails to assemble into pyrenoids, rendering these pyrenoid-less (pyr–) cells unable to establish a functional CCM (Genkov et al, 2010; Meyer et al, 2012)

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