Plastoglobules Research Articles

Protein Profiling of Plastoglobules in Chloroplasts and Chromoplasts. A Surprising Site for Differential Accumulation of Metabolic Enzymes

Plastoglobules (PGs) are oval or tubular lipid-rich structures present in all plastid types, but their specific functions are unclear. PGs contain quinones, alpha-tocopherol, and lipids and, in chromoplasts, carotenoids as well. It is not known whether PGs contain any enzymes or regulatory proteins. Here, we determined the proteome of PGs from chloroplasts of stressed and unstressed leaves of Arabidopsis (Arabidopsis thaliana) as well as from pepper (Capsicum annuum) fruit chromoplasts using mass spectrometry. Together, this showed that the proteome of chloroplast PGs consists of seven fibrillins, providing a protein coat and preventing coalescence of the PGs, and an additional 25 proteins likely involved in metabolism of isoprenoid-derived molecules (quinines and tocochromanols), lipids, and carotenoid cleavage. Four unknown ABC1 kinases were identified, possibly involved in regulation of quinone monooxygenases. Most proteins have not been observed earlier but have predicted N-terminal chloroplast transit peptides and lack transmembrane domains, consistent with localization in the PG lipid monolayer particles. Quantitative differences in PG composition in response to high light stress and degreening were determined by differential stable-isotope labeling using formaldehyde. More than 20 proteins were identified in the PG proteome of pepper chromoplasts, including four enzymes of carotenoid biosynthesis and several homologs of proteins observed in the chloroplast PGs. Our data strongly suggest that PGs in chloroplasts form a functional metabolic link between the inner envelope and thylakoid membranes and play a role in breakdown of carotenoids and oxidative stress defense, whereas PGs in chromoplasts are also an active site for carotenoid conversions.

Ultrastructural studies on differentiating chloroplasts in the ‘forma fuscoviridis’ of Ceratozamia mexicana Brongn.*

summaryChloroplast ontogenesis has been examined ultrastructurally in leaflets of ‘forma fuscorviridis’ of Ceratozamia mexicana Brongn., a cycad displaying a dark red‐brown colour in young developing leaves. In very young leaves the chloroplasts contain large paracrystalline prolamellar bodies (PLBs), widely extending thylakoid strips, starch, membrane‐bounded paracrystals (MbPs) and conspicuous electron‐dense plastoglobules (PGBs) densely packed around the PLBs. All these features progressively disappear as the development of the thylakoid system proceeds. In mature leaves the chloroplasts are characterized by randomly orientated grana stacks containing large numbers of thylakoids. The conspicuous presence of different membrane precursors during leaf ontogeny may be associated with the very low rate of leaf development in Ceratozamia.

Chromoplast Ultrastructure in Fruit of Solanum pseudocapsicum and Fruit and Sepals of Physalis alkekengi

The chromoplasts of ripe fruit of Solanum pseudocapsicum contain mostly β-carotene, with lesser amounts of lutein, antheraxanthin, zeaxanthin, mutatochrome and violaxanthin. The ultrastructure of these plastids reveals that the carotenoids appear to crystallize out from the plastoglobules into many small slabs. In addition, fibrils are also present and develop from plastoglobules. In fruit from wild plants, they consist of a single microfibril with a diameter of 21.3 ± 1.4 nm ( ± SD), while those from an ornamental cultivar, Solanum Christmas Cherry Jubilee, contain fibrils consisting of several small microfibrils, with an average centre-to-centre distance of 4.9 ± 0.5 nm (±SD). The chromoplasts of ripe fruit of Physalis alkekengi are remarkably similar to those of S. pseudocapsicum with respect to their ultrastructure, although they contain mainly zeaxanthin dipalmitate and β-carotene. Fruit from one plant contain the electron-transparent crystalloids described for S. pseudocapsicum, as well as fibrils consisting of one or more large microfibrils. Fruit from a second plant contain crystalloids and the short, small fibrils composed of several microfibrils with a centre- to-centre distance of 4.1 ± 0.2 nm (± SD ) . Treatment of mature green fruit of S. pseudocapsicum with CPTA, which inhibits β-carotene accumulation, prevents the formation of crystalloids and both types of fibrils. It is suggested that the crystalloids are the site of β-carotene deposition in S. pseudocapsicum, and of zeaxanthin di- palmitate in P. alkekengi. Furthermore, it is postulated that there are different types of plasto- globules, one type generating crystalloids and the other fibrils.