Abstract
Acclimation is an essential process in plants on many levels, but especially in chloroplasts under changing light conditions. It is partially known how the photosynthetic machinery reacts upon exposure to high light intensities, including rearrangement of numerous protein complexes. Since the majority of proteins residing within chloroplasts needs to be posttranslationally imported into the organelles, we endeavored to study how this important process is regulated upon subjecting plants from pea and Arabidopsis to high light. Our results reveal that acclimation takes place on the one hand in the cytosol by differential phosphorylation of preproteins and resulting from the altered expression of the responsible kinases, and on the other hand at the level of the translocation machineries in the outer (TOC) and inner (TIC) envelope membranes. Intriguingly, while phosphorylation is more pronounced under high light, import itself shows a lower efficiency, along with a reduced accumulation of the Toc receptor proteins Toc34 and Toc159.
Highlights
Plants as sessile organisms strongly rely on their abilities to adapt to fluctuating environmental conditions
Preproteins need to be protected from misfolding or degradation and are translocated in an unfolded state into the chloroplast across the two surrounding membranes through two translocon complexes, i.e., the translocon at the outer membrane of chloroplasts (Toc) and the translocon at the inner membrane of chloroplasts (Tic) [2]
A plethora of metabolic pathways as well as photosynthetic activity need to be adapted to changing light conditions, which includes timely provision of enzymes and other proteins
Summary
Plants as sessile organisms strongly rely on their abilities to adapt to fluctuating environmental conditions. The mandatory protein targeting and import mechanisms represent sophisticated checkpoints to control and modulate chloroplast function [1] After their synthesis, preproteins need to be protected from misfolding or degradation and are translocated in an unfolded state into the chloroplast across the two surrounding membranes through two translocon complexes, i.e., the translocon at the outer membrane of chloroplasts (Toc) and the translocon at the inner membrane of chloroplasts (Tic) [2]. Preproteins are bound by various chaperones, including HSP70, HSP90, and 14-3-3 proteins [3,4,5], where 14-3-3 binding is preceded by phosphorylation of a subset of preproteins [4] The formation of these preprotein complexes is thought to ensure the maintenance of a high import competence of the freshly synthesized preproteins and recruitment to the chloroplast membrane.
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