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Flashfrozen resin globule6/8/2023 Although the lipid and protein composition of PGs is distinct from the associated thylakoids, their reversible shrinking and swelling indicates rapid and reversible exchange of components between the two structures. Purification of PGs revealed a small proteome of ∼30 strongly enriched proteins ( Vidi et al., 2006 Ytterberg et al., 2006 Lundquist et al., 2012a), as well as various prenyl-lipids, especially plastoquinone-9 and α-tocopherol, and nonpolar lipids, such as triacylglycerols ( TAGs) and fatty acid phytyl esters ( Gaude et al., 2007 Zbierzak et al., 2009 Eugeni Piller et al., 2012 Lippold et al., 2012). In chloroplasts, PGs are formed by blebbing of the outer leaflet of the thylakoid membrane and remain attached to the thylakoid membrane system, providing a conduit for metabolite exchange between the two structures ( Austin et al., 2006). Plastoglobules ( PGs) are plastid-localized lipid-protein particles with dynamic morphologies, and they are responsive to changing developmental states and environmental conditions ( Bréhélin et al., 2007). Thus, the ABC1K1/3 complex contributes to PG function in prenyl-lipid metabolism, stress response, and thylakoid remodeling. Plastid jasmonate biosynthesis enzymes were recruited to the k1 k3 PGs but not wild-type PGs, while pheophytinase, which is involved in chlorophyll degradation, was induced in k1 k3 and not wild-type plants and was localized to PGs. Isolated PGs from k1 k3 showed a modified prenyl-lipid composition, suggesting reduced activity of PG-localized tocopherol cyclase (VTE1), and was consistent with loss of carotenoid cleavage dioxygenase 4. Total PG volume increased during light stress in wild type and k1 k3 plants, but with different size distributions. The senescence-like phenotype was independent of the EXECUTER pathway that mediates genetically controlled cell death from the chloroplast and correlated with increased levels of the singlet oxygen–derived carotenoid β-cyclocitral, a retrograde plastid signal. This senescence-like phenotype involved degradation of the photosystem II core and upregulation of chlorophyll degradation. Also, k1 k3 showed a slower, but irreversible, senescence-like phenotype during moderate light stress that was phenocopied by drought and nitrogen limitation, but not cold stress. Null mutants for both genes ( abc1k1 and abc1k3) and the double mutant ( k1 k3) displayed rapid chlorosis upon high light stress. Several lines of evidence suggested that ABC1K1 and ABC1K3 form a protein complex. This study examines the function of PG-localized kinases ABC1K1 and ABC1K3 in Arabidopsis thaliana. Plastoglobules ( PGs) are plastid lipid-protein particles.
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