In this article we will discuss about the genetic engineering of flower colour.
Ornamental plants having aesthetic value of flower colour is significant from the consumer’s point of view. Certain ornamental plants like roses, tulips, carnations and chrysanthemum occupy more than 60% of the cut flower industry.
Flower colour has been understood biochemically and genetically. Synthesis of colour pigments is generally due to attraction of pollinators. Pigments can protect the cells from ultraviolet radiation.
Carotenoids and flavanoids are the two most important types of pigments which impart colouring to the flowers. Carotenoids alone or in association with flavanoids are responsible for the production of yellow and orange flowers. Other flowers colours like pinks, red, and violet, blue are mainly due to anthocyanins which are coloured class of flavanoids Fig. 22.9.
These pigments generally accumulate in vacuoles or anthocyanoplast. Changes in various pH in petals can produce different colours. In addition, certain complex process like glycosylation, acylation to non-coloured flavanoids results in colouration to flowers. Pathway involved in biosynthesis of flavanoids is well established.
One of the most significant and first enzyme participate in flavour production is chalcone synthase. This biosynthetic pathway begins with stepwise condensation of three acetate molecule into malonyl CoA. Chalcone synthase converts malonyl CoA into pale yellow coloured tetrahydroxy chalcone.
This important step is followed by isomerization of tetrahydroxy chalcone through chalcone isomerase in the colourless naringinin. Colourless naringinin undergoes hydroxylation process by flavanon-3-hydroxylase leads to the formation of dihydrokaemferol. Further hydroxylation of this compound to yield dihydroxyquorcetin and the dihydromyricetin with the aid of flavanoid-3-hydroxylase and flavanoid hydroxylase respectively.
In the subsequent reaction dihydro flavanons is reduced to leucoanthocyanides and this is converted to anthocyanidins by anthocyanidine synthase (ANS) and then to anthocyanide-3-glucoside by flavanoid glucosyltransferase. Depending upon number of hydroxyl group anthocyanide-3-glucoside imparts brick-red (pelargonidine-3-glucoside), red (cyanidine-3-glucoside) and purple (delphinidine-3-glucoside) colour.
Biosynthesis of Anthocyanins:
CHS – Chalcone synthase
CHI – Chalcone isomerase
F3H – Flavanone-3-hydroxylase
F3’H – Flavanoid-3′-hydroxylase
F3’5’H – Flavanoid 3’5′-hydroxylase
DFR – Dihydro flavonol 4-reductase
3GT – UDP-glucose, flavonoid 3-O-glucosyl transferase
Production of Acyanic Flower:
Although, white is not a novel color due to its wide occurence, white variety is still preferable because it is not easily obtained due to the retention of other characters. Since chalcone synthase is the key starting enzyme for the synthesis of varieties of color pigments, antisense mediated inhibition of chalcone synthase result in white colour flower.
Chalcone synthase gene when expressed in petunia, demonstrated white colour. Antisense strategy was extended in petunia hydrid cultivars syrfinia. These are white colour with long flowering period petunias and is in greatest demands for upto 60 million pots a year globally. Obtaining pure white variety was shown to be technically difficult.
Therefore, antisense suppression of chalcone synthase gene in petunia produce paler colour is quite stable in transformed plants. Similarly, introduction of antisense CHS cDNA under the control of CaMV35 promoter, blocked anthocyanin pigments production resulted in altered flower pigmentation.
Co-suppression and Flower Colour:
It was realized that transgene often induce rapid turnout of homologous endogenous transcripts. This phenomenon is known as co-suppression i.e., transgene silencing of its endogenous gene. This co-suppression of homologous gene is responsible for small increase in gene expression for example, when US team led by Rice Jorgenson was manipulating the plant genetically to produce more deeply coloured flowers.
This idea was to produce more deep coloured flowers by adding extra copies chalcone synthase genes, a key enzyme in the pigment synthesis. Introduction of a chimeric CHS transgene (sense orientation) resulted in white flowers that exhibit co-suppression of endogenous gene and increased CHS transcripts.
Co-suppression is sometimes called sense suppression. Such suppression of CHS in petunia produces several colour patterns in addition to white colour flower. White and pink flower varieties have been obtained by introducing sense and antisense chalcone synthase transgene into several ornamental plants.
Introduction of transcription activator on several times is indispensable in colour change of the flowers. Introduction of sense and antisense gene construct of the chrysanthemum chalcone synthase suppress or block endogenous chalcone synthase gene in chrysanthemum plant.
As a consequence anthocyanin synthase had been suppressed. This co-suppression and alone or along with antisense suppression methods are also useful in the production of white colour flower.
Mechanism of Suppression:
Several possible mechanisms have been implicated in the molecular mechanism of co- suppression and antisense suppression. Some of the possible mechanisms are first, transient pairing of homologous DNA sequence. This would result in the alteration of chromatid structure. This may ultimately affect some aspect of gene transcription.
Second is Specific base pairing between transgene transcripts RNA with a target gene transcript leads to degradation? Third hypothesis include both sense and antisense mediated suppression when RNA-DNA interaction in which either sense or antisense transgene transcript base pairs with target gene and consequently disrupt transcription.
Engineering Blue Colour of the Flower:
In ornamental flower, delphinidine is a key pigment for blue flower. Most blue flower contains acylated anthocyanins. Some plant species do not produce blue flower because they are unable to produce delphinidine derivatives. Lack of the key enzyme F3’5H is responsible for lack of blue colour. Engineering blue flower was accomplished by cloning F3‘5H gene from petunia.
Transgenic petunia flower contains high level of malvidin and depending upon the pH, colour changed from pale pink to red purple in low pH and from pink to blue purple in a higher pH. Similarly, transgenic violet carnation have been produced by transferring of petunia F3‘5H gene. Once expression of this transgene takes place in petals of petunia produced more of delphinidine instead of cyanide and pelargonidin.
Production of deeper colour of flower is another goal in ornamental industry. Engineering of blue flower is accomplished by the introduction of aromatic acyl transferase to the roses, carnations. Genetically, lack of this enzyme produce deep bluish colour.
The enzyme aromatic acyltransferase add aromatic acyl group to anthocyanins—Rose anthocyanins are modified to 3-glucose or 5 glucose. Simultaneous expression of F3’5H and one of the acyl transferase on petals may impart deep blue than the expression of F3’5H alone.