CHAPTER 2: Carotenoid Assembly in Fruits and Vegetables

Carotenoids are well-known for their biological activities in plants, animals and humans. For instance, they serve as light-harvesting and photoprotective accessory pigments in plants and they exert several health benefits, such as improving vitamin A status and supporting eye health when being consumed by humans. Among numerous factors, both their molecular structure and deposition form in planta represent inherent traits determining their bioavailability from plant foods. Therein, carotenoids are biosynthesised and accumulated in various types of plastids such as proplastids, etioplasts, chloroplasts, amyloplasts and chromoplasts. The aforementioned plastids may be interconverted into each other, particularly during fruit ripening or tissue maturation in general. Carotenoids may be deposited within the plastids in protein-complexed, solid-crystalline, liquid-crystalline, amorphous and oil-dissolved states or in combinations of the aforementioned forms. Their deposition is highly dependent on specific molecular features, such as the presence of polar groups, the geometric configuration of their double bonds or the esterification with fatty acids. Carotenoid esters have consistently been observed to accumulate in exceptional tubular plastidal ultrastructural elements, presumably as liquid-crystalline J-aggregates. In the present chapter, an overview of the full array of deposition forms found in plants, focusing on the exceptional tubular and presumably liquid-crystalline forms found in carotenoid ester-rich plants, is provided. Exemplary transmission electron micrographs of the plastids from mamey sapote and mango fruits are presented.