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.
Significant past, present, and potential future research into the organellar (plastid and mitochondrial) genomes of gymnosperms that can provide insight into the unknown origin and evolution of plants is highlighted. Gymnosperms are vascular seed plants that predominated the ancient world before their sister clade, angiosperms, took over during the Late Cretaceous. The divergence of gymnosperms and angiosperms took place around 300 Mya, with the latter evolving into the diverse group of flowering plants that dominate the plant kingdom today. Although gymnosperms have reportedly made some evolutionary innovations, the literature on their genome advances, particularly their organellar (plastid and mitochondrial) genomes, is relatively scattered and fragmented. While organellar genomes can shed light on plant origin and evolution, they are frequently overlooked, due in part to their limited contribution to gene expression and lack of evolutionary dynamics when compared to nuclear genomes. A better understanding of gymnosperm organellar genomes is critical because they reveal genetic changes that have contributed to their unique adaptations and ecological success, potentially aiding in plant survival, enhancement, and biodiversity conservation in the face of climate change. This review reveals significant information and gaps in the existing knowledge base of organellar genomes in gymnosperms, as well as the challenges and research needed to unravel their complexity.
The maturation of green fleshy fruit to become colourful and flavoursome is an important strategy for plant reproduction and dispersal. In tomato (Solanum lycopersicum) and many other species, fruit ripening is intimately linked to the biogenesis of chromoplasts, the plastids that are abundant in ripe fruit and specialized for the accumulation of carotenoid pigments. Chromoplasts develop from pre-existing chloroplasts in the fruit, but the mechanisms underlying this transition are poorly understood. Here, we reveal a role for the chloroplast-associated protein degradation (CHLORAD) proteolytic pathway in chromoplast differentiation. Knockdown of the plastid ubiquitin E3 ligase SP1, or its homologue SPL2, delays tomato fruit ripening, whereas overexpression of SP1 accelerates ripening, as judged by colour changes. We demonstrate that SP1 triggers broader effects on fruit ripening, including fruit softening, and gene expression and metabolism changes, by promoting the chloroplast-to-chromoplast transition. Moreover, we show that tomato SP1 and SPL2 regulate leaf senescence, revealing conserved functions of CHLORAD in plants. We conclude that SP1 homologues control plastid transitions during fruit ripening and leaf senescence by enabling reconfiguration of the plastid protein import machinery to effect proteome reorganization. The work highlights the critical role of chromoplasts in fruit ripening, and provides a theoretical basis for engineering crop improvements.