Flower Color and Its Engineering by Genetic Modification

Abstract

Flower color is mainly determined by the constituent profile of the chemicals flavonoids and the colored subclass of those compounds, the anthocyanins. Flowers often contain specific flavonoids, and thus limited flower colors are available within a species due to genetic constraints. Engineering the flavonoid biosynthetic pathway by expressing a heterologous gene has made it possible to obtain color varieties that cannot be achieved within a species by hybridization or mutational breeding. General tactics for successful engineering flower color have been established on the basis of engineering results obtained in model species such as petunia and torenia. Highly efficient expression of a heterologous gene(s) can be achieved by an optimal combination of promoter, translational enhancer, coding region sequence, and terminator. In addition to expression of heterologous gene, downregulation of competing pathways and/or using color biosynthesis mutant hosts is necessary. As well as a suitable genetic background, it is also important to select hosts with a high market position and value. An efficient transformation system for each target species has to be established. Technical skills and enough finance are also necessary to obtain permits to commercialize genetically modified plants. Violet carnations, roses, and chrysanthemums have been developed by expressing a petunia, pansy, or campanula flavonoid 3{\textasciiacutex},5{\textasciiacutex}-hydroxylase gene, and genetically modified carnation and rose varieties have been commercialized. Expression of the anthocyanin 3{\textasciiacutex},5{\textasciiacutex}-glucosyltransferase gene in chrysanthemum in addition to flavonoid 3{\textasciiacutex},5{\textasciiacutex}-hydroxylase resulted in production of pure blue flower color due to a copigmentation effect with endogenous flavones. Orange petunia expressing maize dihydroflavonol 4-reductase gene and accumulating non-native pelargonidin have been grown worldwide. Though this has been from a non-intentional release of a genetically modified organism, the case provides a good example to show that a combination of genetic engineering and hybridization breeding can produce commercially highly sought after cultivars.