Biochemical bases and molecular regulation of pigmentation in the peel of Citrus fruit

• Citrus fruits exhibit a high diversity of colouration genetically determined.
• Chlorophylls and carotenoids are the main pigments in the peel of citrus fruit.
• Peel colour is determined by the content and composition of carotenoids, which are tightly transcriptionally regulated.
• Changes in pigmentation are controlled by environmental, nutritional and hormonal signals.

External colour of Citrus fruits is one of the most important quality traits and a decisive factor for consumer acceptance. Pigmentation of fruit peel is highly diverse among the different species and cultivars of the genus Citrus, ranging from the green of limes to the yellow of lemons, orange in mandarins and sweet oranges, and pink in red grapefruits. Colouration of the peel is due to the presence of two main pigments: chlorophylls which provide green colour, and carotenoids, which are responsible for the characteristic colouration of mature fruits of most species and cultivars. Anthocyanins are a third group of pigments, providing a red to purple tint, in a specific group, blood oranges, and mainly restricted to the flesh. Chlorophylls and carotenoids are isoprenoid-derived pigments, synthesized and accumulated in plastids and, therefore, changes in these compounds during natural ripening are driven by the transformation of chloroplasts into chromoplasts. Most of the structural genes involved in chlorophylls and carotenoids metabolism have been characterized in Citrus, concluding that content and composition of these pigments are mostly genetically determined, and highly regulated at the transcriptional level. However, other mechanisms such as post-transcriptional regulation, the formation of specific suborganellar structures or stabilizing-complexes may also operate. Environmental factors, such as light and temperature, are known to play critical influence in the development of colouration and that biochemical and molecular bases of their action are being elucidated. Moreover, nutritional status (mainly nitrogen and sugars) is a key determinant of the rate and intensity of peel colouration. The consensus hypothesis establishes that peel colouration is governed by environmental and nutritional factors acting throughout the action of different hormonal signals. In this review we summarize content and composition of main pigments in the peel of fruits of relevant Citrus species and varieties. A comprehensive overview of metabolic pathways implicated in the metabolism of the main pigments, with emphasis on the key regulatory steps, gene expression and their regulation during fruit ripening and in response to environmental, nutritional and hormonal signals is critically revised and discussed.