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Review
. 2019 Jul;38(7):803-818.
doi: 10.1007/s00299-019-02420-2. Epub 2019 May 11.

Differentiation of Chromoplasts and Other Plastids in Plants

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Free PMC article
Review

Differentiation of Chromoplasts and Other Plastids in Plants

Najiah M Sadali et al. Plant Cell Rep. .
Free PMC article

Abstract

Plant cells are characterized by a unique group of interconvertible organelles called plastids, which are descended from prokaryotic endosymbionts. The most studied plastid type is the chloroplast, which carries out the ancestral plastid function of photosynthesis. During the course of evolution, plastid activities were increasingly integrated with cellular metabolism and functions, and plant developmental processes, and this led to the creation of new types of non-photosynthetic plastids. These include the chromoplast, a carotenoid-rich organelle typically found in flowers and fruits. Here, we provide an introduction to non-photosynthetic plastids, and then review the structures and functions of chromoplasts in detail. The role of chromoplast differentiation in fruit ripening in particular is explored, and the factors that govern plastid development are examined, including hormonal regulation, gene expression, and plastid protein import. In the latter process, nucleus-encoded preproteins must pass through two successive protein translocons in the outer and inner envelope membranes of the plastid; these are known as TOC and TIC (translocon at the outer/inner chloroplast envelope), respectively. The discovery of SP1 (suppressor of ppi1 locus1), which encodes a RING-type ubiquitin E3 ligase localized in the plastid outer envelope membrane, revealed that plastid protein import is regulated through the selective targeting of TOC complexes for degradation by the ubiquitin-proteasome system. This suggests the possibility of engineering plastid protein import in novel crop improvement strategies.

Keywords: Chloroplast; Chromoplast; Organelle; Plastid; Plastid biogenesis; Plastid protein import; SP1.

Conflict of interest statement

The authors declare no conflicts of interest. Application of the authors’ research is covered by patent applications GB1803833.1, GB1803834.9, GB1815206.6 and US 16/643507.

Figures

Fig. 1
Fig. 1
Hypothetical model showing a possible mechanism for the regulation of the chloroplast-to-chromoplast transition during fruit ripening. The chloroplasts in young green fruit are equipped with TOC complexes with specificity for precursor proteins of the photosynthetic apparatus (PS). For fruit ripening to proceed normally, metabolic shifts coupled with the down-regulation of the thylakoid biogenesis machinery and the up-regulation of carotenoid biosynthesis must occur, as part of the transitioning of chloroplasts into chromoplasts. Developing chromoplasts must import a range of different ripening-related precursor proteins, and to do this, they may require different TOC complexes with specificity for non-photosynthetic precursor proteins (non-PS). Thus, remodelling of the protein import machinery occurs during the chloroplast-to-chromoplast transition, and this may be mediated by the outer membrane E3 ligase, SP1, which targets unwanted TOC components for degradation by the 26S proteasome (26SP) in the cytosol. RNF, RING-finger domain; TMD, transmembrane domain; Ub, ubiquitin; E2, E2 conjugase

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