LCI9 is required for normal pyrenoid starch sheath formation and efficiency of the CO ₂ -concentrating mechanism in Chlamydomonas reinhardtii

Pyrenoid -based CO ₂ -concentrating mechanisms (pCCMs) boost photosynthesis by delivering elevated levels of CO ₂ to Rubisco. Pyrenoids are often surrounded by a starch sheath thought to enhance pCCM efficiency, but little is known about how the sheath is formed. Here we have assessed the role of the pyrenoid-associate protein LCI9 in starch sheath formation in Chlamydomonas reinhardtii .

Using an lci9 mutant we assessed the role of LCI9 in starch sheath morphology by fluorescence and electron microscopy, and its impact on pCCM function by CO ₂ uptake and growth assays. We determined potential interactors of LCI9 via TurboID proximity labeling.

In the absence of LCI9, cells fail to correctly assemble the starch sheath, have lower affinity for inorganic carbon and grow slower under CO ₂ -limiting conditions. LCI9 localises to the starch sheath and is physically close to a range of starch metabolism enzymes and other potential scaffolding proteins. Finally, we show that LCI9 associates with starch in the land plant Arabidopsis thaliana .

LCI9 likely acts as a pyrenoid specific scaffold for starch metabolism enzymes. LCI9 could contribute to ongoing efforts to build a functional pyrenoid in a land plant.