How genes paint flowers and seeds

Colour is one of the most attractive features of flowers determining its capacity to attract insect pollinators. It also plays a major role in its market acceptance. In nature, flowers and seeds of different hues and shades are observed. In the beginning of 19th century, scientists proposed the Blending hypothesis to explain the inheritance of flower and seed colour. During 1866, the Austrian monk Gregor Johann Mendel proved this theory false. Subsequently, the Particulate hypothesis of inheritance accrued wide acceptance (Holton and Cornish, 1995). Pigments contribute to flower and seed colour. However, other factors like co-pigmentation, vacuolar pH and cell shape also influence colour development. The pigments belonging to classes viz., chlorophyll, flavonoid, carotenoid and betalain are responsible for colour development. Of these, flavonoid (mainly anthocyanins) is the most common pigment group contributing to the development of range of colours from red to purple. These are found in vacuoles (Grotewold, 2006). Pigments are the end-products of various biosynthetic pathways. The different intermediate steps in these pathways are catalysed by enzymes; the production of which is governed by genes. Any alteration in the genes encoding the enzymes or regulation of gene expression will result in modification of pigment development leading to various shades and hues of flowers and seeds. For example, genes encoding key enzymes in the branch of the flavonoid biosynthetic pathway produce pigmentation in flower (Mol et al., 1998). Flavonoids are derived from a general phenylpropanoid pathway with aromatic amino acid phenylalanine as the basic substrate. Phenylalanine is catalyzed by the enzyme chalcone synthase (CHS) into chalcone which is a key intermediate in the formation of flavonoids. Chalcone imparts yellow pigmentation and any alteration in the CHS coding gene will affect chalcone production and thereby inhibit yellow colour development. Regulatory genes MYB, bHLH and WD40 are also involved in controlling the expression of the flavonoid biosynthesis genes. In addition, variegated flowers are said to result from insertion or excision of transposons in flavonoid biosynthetic genes or regulatory genes. Such variegated flowers have been observed in petunia, snapdragon, morning glory, azalea and others (Iida et al., 2004). Conventional breeding methods have been extensively used to develop cultivars with flowers varying in both colour and intensity. The cultivated roses were developed by extensive inter-specific hybridization involving yellow-flowered (producing carotenoids) and orange-flowered (producing pelargonidins) wild species. Mutation breeding has played a major role in the development of variable flower and seed coat colour. Introduction of novel genes encoding enzyme activities or transposable elements and inactivation of endogenous genes to modify flower and seed colour have been attempted through genetic engineering. Blue roses were produced by introduction of pansy F3'5'H genes into rose. This resulted in a significant amount of delphinidin derived anthocyanin production and accumulation in petals of the transgenic rose plants. Suppression of CHS gene in petunia through gene silencing approaches resulted in production of white flowers (Tanaka et al., 2009).