Casting light on the pathway to betalain biosynthesis: A review

Abstract

The biosynthesis of pigments is crucial for plant life. Betalains are hydrosoluble pigments almost completely restricted to species of the core Caryophyllales and, within the fungal lineage, the Basidiomycetes. As important specialized metabolites, betalains have strong antioxidant properties, and may confer tolerance to a wide range of stress conditions. Despite having similar organ distribution and physiological functions as anthocyanins, both types of pigments have never been found in the same plant. Light is one of the most important environmental signals, triggering core physiological processes in plants, including the control of pigment biosynthesis. Due to the similarities between betalains and anthocyanins with respect to their function and organ distribution, many of the core mechanisms that govern their production in response to light and other environmental signals are expected to be evolutionarily conserved. Even so, the experimental evidence regarding the transduction pathways involved in the photocontrol of betalain biosynthesis is proportionally scarce, while some regulatory components clearly differ between both types of pigments. In the present work, we review the available information on the effect of the light environment on betalain production, as well as its interaction with key endogenous and exogenous factors such as phytohormones and temperature, given their marked influence on the responses to light cues. We also discuss the latest advances in the study of the underlying physiological and molecular mechanisms, comparing both betalainic and non-betalainic plant species. Finally, a conceptual framework summarizing the information discussed is presented, which is expected to help pave the way for future studies on the molecular regulation of the production of this important group of pigments.