The chloroplast-associated protein degradation pathway controls chromoplast development and fruit ripening in tomato

Fruit ripening is a fine-tuned process involving wholesale changes to both the structure and metabolism of the fruit. Now, the CHLORAD proteolytic pathway is shown to regulate chromoplast development, thus altering the ripening process of tomato fruits. 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.

Automated summary

The research shows that the CHLORAD proteolytic pathway regulates chromoplast development, affecting the ripening process of tomato fruits. The SP1 homologue accelerates fruit ripening by promoting chloroplast-to-chromoplast transition, with broader effects on fruit softening, gene expression, and metabolism changes. This study highlights the critical role of chromoplasts in fruit ripening and provides a theoretical basis for engineering crop improvements.