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Molecular switches that regulate fruit colour and flavonol accumulation in grapes

Flowering plants originated about 125 million years ago and became the dominant species to rule the natural world. They seemed to have two advantages over other plants; they had an exuberance of flowers, to help attract animals for pollination, partly through colour, and they also had fruits as a new and safe means of dispersal. Many plants promote seed dispersal by attracting animals to consume their fruit and to disperse seeds. Fruits provide nutritive rewards to animals, often with some fleshy material rich in sugars and starches, and sometimes with proteins. To  advertise for the services of specific, fruits evolved specific patterns of size, texture, fleshiness, taste, nutrition, and most strikingly, colour.

Colour became an important part of the attractiveness of fruits to animals, and a range of colour and colour combinations evolved. Colour also became an important signal of the readiness of seeds for dispersal, and the ripeness of the reward especially when combined with flavour.  These fruits were visually attractive to appeal to the tastes of target animals. Unlike some terrestrial animals, birds (and some primates, including us) are sensitive to red wavelengths and can distinguish among blues, greens, yellows, oranges, black, and reds.  Humans also depend on the nutrition offered by fruits and we enhanced this colour selection by manipulating plants over thousands of years. A part of the process of fruit domestication was selecting for colours that were attractive, culturally important, or had nutritional value. These days, there is an increased interest in the principal colour pigments of fruits, grains, and vegetables, such as carotenoids, anthocyanins, and flavonols, largely due to their antioxidant activities and alleged value in protecting against cancer, cardiovascular diseases and aging.

The Chemistry of Fruit Pigments

The carbon atoms present as double bonds on the benzene ring, absorb radiation in the ultraviolet region very effectively, and modifications to the ring can shift the absorbance toward longer wavelengths. As immovable organisms, plants have evolved selected changes and modifications in chemical pathways to produce an enormous variety of ringed compounds for specific functions. These modifications have produced miraculous molecules such as cements (lignins) that make plant cell walls stronger than steel, proportionate to their weight, and pigments that intercept the harsh ultraviolet radiation on land. During the evolution of flowering plants, other complex ringed compounds were produced using simple phenylpropane (similar to our disinfectant phenol) as a building block. The molecules produced via this pathway are frequently called phenolics. This phenolic pathway starts with phenylalanine and is responsible for the production of an enormous variety of plant pigments producing most of the pinks, reds, and purples that we observe among plants. Phenylalanine is an essential amino acid that we incorporate into our proteins, alas! we do not use it to make such colourful pigments. The most common pigments resulting from phenylalanine in plants are flavonols and anthocyanins. Anthocyanins are responsible for most of the orange-red to violet colours produced in flowers, grains, vegetables, and fruits, and in the reds of developing and dying leaves.  The flavonols usually produce dull ivory, cream or yellow shades, but are medicinally important.

New molecular players in the production of colours and flavonols

The basic details of how these pigments were formed was elucidated by many scientists half a century ago, using radioactive carbon labels to determine the intermediates in their synthesis. Today, with advances in molecular genetics, we have started to understand the roles of genes, and their control, for enzymes that catalyze the synthesis of anthocyanins and flavonols. 

Varsha Tirumalai from our laboratory has found a new regulatory gene that is in turn regulated by non-protein-coding small RNAs, that accounts for the production of specific anthocyanins (and specific colours) and flavonols. She used local grape varieties rich in colour (such as Bangalore Blue and Red Globe) or Dilkhush (a green grape rich in flavonols) to look at the differences in DNA, RNA, protein, and metabolites. During the analysis she has identified a new transcription factor (a molecular accelerator) required for flavonols to be made. The RNA coding for the same regulator must be degraded by tiny non coding RNAs called microRNAs, if the fruit has to produce colourful anthocyanins. She has engineered plants with diverse leaf and flower colours. With the identification of such regulators, the research team is confident that they can produce medicinally important anthocyanins or flavonols at large quantities. We anticipate that this research will enhance the scope for bio-fortification of anthocyanins and flavonols.

Varsha’s paper published in Journal of Experimental Botany can be accessed here: