Carotenoid responses to environmental stimuli: integrating redox and carbon controls into a fruit model

Carotenoids play an important role in plant adaptation to fluctuating environments as well as in the human diet by contributing to the prevention of chronic diseases. Insights have been gained recently into the way individual factors, genetic, environmental or developmental, control the carotenoid biosynthetic pathway at the molecular level. The identification of the rate-limiting steps of carotenogenesis has paved the way for programmes of breeding, and metabolic engineering, aimed at increasing the concentration of carotenoids in different crop species. However, the complexity that arises from the interactions between the different factors as well as from the coordination between organs remains poorly understood. This review focuses on recent advances in carotenoid responses to environmental stimuli and discusses how the interactions between the modulation factors and between organs affect carotenoid build-up. We develop the idea that reactive oxygen species/redox status and sugars/carbon status can be considered as integrated factors that account for most effects of the major environmental factors influencing carotenoid biosynthesis. The discussion highlights the concept of carotenoids or carotenoid-derivatives as stress signals that may be involved in feedback controls. We propose a conceptual model of the effects of environmental and developmental factors on carotenoid build-up in fruits.