Perturbation of protein homeostasis brings plastids at the crossroad between repair and dismantling

Abstract

The chloroplast proteome is a dynamic mosaic of plastid- and nuclear-encoded proteins. Plastid protein homeostasis is maintained through the balance between de novo synthesis and proteolysis. Intracellular communication pathways, including the plastid-to-nucleus signalling and the protein homeostasis machinery, made of stromal chaperones and proteases, shape chloroplast proteome based on developmental and physiological needs. However, the maintenance of fully functional chloroplasts is costly and under specific stress conditions the degradation of damaged chloroplasts is essential to the maintenance of a healthy population of photosynthesising organelles while promoting nutrient redistribution to sink tissues. In this work, we have addressed this complex regulatory chloroplast- quality-control pathway by modulating the expression of two nuclear genes encoding plastid ribosomal proteins PRPS1 and PRPL4. By transcriptomics, proteomics and transmission electron microscopy analyses, we show that the increased expression of PRPS1 gene leads to chloroplast degradation and early flowering, as an escape strategy from stress. On the contrary, the overaccumulation of PRPL4 protein is kept under control by increasing the amount of plastid chaperones and components of the unfolded protein response (cpUPR) regulatory mechanism. This study advances our understanding of molecular mechanisms underlying chloroplast retrograde communication and provides new insight into cellular responses to impaired plastid protein homeostasis.

A system biology-oriented investigation of Arabidopsis proteomes altered in chloroplast biogenesis and retrograde signaling reveals adaptive responses at whole cell level

“…Furthermore, budding vesicles observed in PsbO1-GFP chloroplasts strongly resemble micro-and macro-autophagy processes (Jeran et al, 2021;Lemke & Woodson, 2022;Lemke et al, 2021), while no vesicle was detected upon CV downregulation. PsbO1-GFP accumulation also resulted in vacuolated structures and disrupted vacuolar membrane resembling those observed in cells subjected to programmed cell death (Woodson et al, 2015;Tadini et al, 2023). This well correlates with the identification of vacuolar-related hubs, including RD21A (Koh et al, 2016), among gun1+Lin co-expression hubs.…”

“…Immunoblots were performed on total protein extracts. Fresh plant material was homogenized in sample buffer containing 20% (v/v) glycerol, 4% (w/v) SDS, 160 mm Tris–HCl pH 6.8, 10% (v/v) 2-mercaptoethanol (10 µL of buffer every mg of plant material; (Tadini et al , 2023). Samples were incubated at 65 °C for 15 min and centrifuged for 10 min at 16 000 g to pellet debris.…”

“…The Chl-a fluorescence was measured in vivo employing the imaging Chl fluorometer (Walz Imaging PAM; walz.com), as previously described (Tadini et al , 2023). Dark-adapted seedlings allowed maximum quantum yield of PSII ( Fv / Fm ) measurements.…”