2-Cys peroxiredoxins and the chaperone cpHSP70 act in concert in chloroplast biogenesis in Arabidopsis seedlings.

The redox balance between NADPH-dependent thioredoxin reductase C (NTRC) and 2-Cys peroxiredoxins (PRXs) help chloroplast photosynthetic performance acclimate rapidly to environmental cues. The Arabidopsis (Arabidopsis thaliana) 2cpab mutant, which lacks chloroplast 2-Cys PRXs A and B, shows impaired embryogenesis and cotyledon development. This phenotype indicates that these enzymes have a relevant function at early stages of plant development, although this function is poorly understood. Here, we show that the albino cotyledons of the 2cpab mutant have unstructured chloroplasts and decreased chloroplast lipid contents, revealing a key function of 2-Cys PRXs in cotyledon chloroplast differentiation. These phenotypes are mimicked by NTRC overexpression, whereas loss of NTRC function has no effect. RNA-Seq analyses showed transcriptomic changes resembling the response to proteotoxic stress in 2cpab seedlings, which was confirmed by the high sensitivity of 2cpab seedlings to heat stress. In de-etiolating seedlings 2-Cys PRXs interact with chloroplast-localized chaperone heat shock protein 70 (cpHSP70); moreover, Arabidopsis seedlings simultaneously lacking 2-Cys PRXs and cpHSP70-1 exhibit a dramatic alteration of seedling development. Based on these results, we propose that 2-Cys PRXs contribute to cotyledon chloroplast differentiation by affecting organellar proteostasis. This function is exerted in concert with chaperone cpHSP70 and is essential for seedling establishment.