The wonders of mixing and matching pigments and colourful flowers

Colourful plants have long fascinated and inspired people, from Monet’s and Van Gogh’s paintings to Mendel’s pea petal colours. Pigments are not only aesthetically pleasing but have numerous roles, from attracting the right pollinator to help plants deal with environmental stresses. 

Dr Eduardo Narbona and colleagues review the ecological biochemistry of pigments affecting petal colour and highlight their importance in non-pollinator functions (e.g. UV-absorption). The three authors all research evolutionary and ecological aspects of flower colours. Dr Eduardo Narbona and José Carlos del Valle mainly research light-induced flavonoids in plants, whilst Dr Justen B. Whittall also investigates flower signals for pollinators.

The review focuses first on the four major groups of pigments: chlorophylls, carotenoids, flavonoids and betalains. The presence of these pigments define the flower’s colour, whilst different concentrations lead to hues. Colour mixing is not straightforward, though. For example, red petals could be due to a combination of purple anthocyanins (a type of flavonoid) with orange carotenoids or the sole presence of red anthocyanins, betalains or carotenoids.

Flavonoids are water-soluble and provide the greatest variety of floral hues (blue, purple, pink and red colours), and some form a group called “UV-absorbing flavonoids”. Protection against UV radiation leads to darker plants closer to the Equator. It is expected to be a key selective trait in the future for both flowering plants and their pollinators. 

Narbona and colleagues suggest that anthocyanins provide an excellent opportunity to understand pigment production and regulation. Plants that produce flavonoids cannot produce betalains that make yellow, pink and red colours in, for example, cacti. The anthocyanin biosynthetic pathway has been thoroughly studied, and six core enzymes serve as branch points for other flavonoid production. 

Flower colours are not fixed throughout a plant’s life. For example, the flowers of Yesterday-today-and-tomorrow, Brunfelsia pauciflora, change from purple to white as the anthocyanins degrade. Another plant, Moricandia arvensis, produces UV-reflecting purple flowers in the spring and then switches to UV-absorbing white in the summer. 

Some pigment groups are controlled independently and lead to dramatic differences. The presence and absence of anthocyanins and carotenoids lead to four distinct colours in Raphanus sativus.

“Although the propriety of flower pigments to paint the green canvas has unquestionably dazzled both pollinators and humans, it is only recently that we are beginning to understand some of the overlooked effects of pigments to cope with environmental stresses,” Narbona and colleagues wrote.

This review highlights the many mechanisms that underlie the mixing and matching of pigments to protect the plants against different stresses and attract suitable pollinators.

“With the help of new molecular, biochemical and data analysis techniques, we are beginning to unravel processes that have long concerned early plant biochemists.”