Genetic Modification of Flower Pigmentation

Novelty is a key driver of the commercial success of ornamental crops. There is much scope to introduce novel flower colours, as some of the leading ornamental crops show only a narrow colour spectrum, whilst in others specific colours, like blue or yellow, are lacking. GM of pigment biosynthesis offers a route to colours outside the existing range, and in the long term is likely to have a major influence on ornamental breeding. Indeed, the first ornamental products from GM of flavonoid biosynthesis are already in the marketplace. Carnations (Dianthus) with novel mauve and violet flower colours are being grown in South America and Australia for sale in Australia, Japan and the USA (Tanaka et al., 2005). Further products are ready for general release, including roses (Rosa hybrida) that accumulate delphinidin-based anthocyanins in the flowers and have mauve colours (Suntory Ltd, press release 8826, 6 June 2004). The potential exists to use GM approaches to introduce novelty into other commercially important species, as genetic transformation systems are available for most of the other major cut flower crops as well as some leading bedding plants, bulbs, pot plants and ground-covers (Deroles et al., 2002).

Successful GM approaches to modify flower colour have primarily targeted the flavonoid pathway. Here we present an overview of the major GM approaches that have been taken to modify flower colour for the different pigment pathways, with a few representative examples discussed in detail. A comprehensive listing of individual experiments is given in Tables 8.2 to 8.5. Additional discussion of GM of flavonoid-based flower colour can be found in Schwinn and Davies (2004).

Non-plant genes that code for coloured proteins or biosynthetic activities for novel pigments have also been used to alter plant/flower colour. Production of green fluorescent protein from the jellyfish Aequorea victoria has been introduced into lisianthus (Eustoma grandiflorum) flowers (Mercuri et al., 2001). Also, algal or bacterial genes have been used to modify carotenoid biosynthesis (Table 8.5). However, as the focus of this volume is on the molecular biology of flowering, we do not present the non-plant gene approaches in detail.

Flavonoids

There are numerous examples of modification of flavonoid biosynthesis in flowers of transgenic plants. There are also several examples for grains, fruit and vegetable crops, recently reviewed in Schijlen et al. (2004). The major methods are based on manipulation of pathway flux. The approaches to increasing, preventing or redirecting flux into or within the pathway have used up- or downregulation of the pathway using regulatory factors; introducing new biosynthetic activities; increasing specific endogenous biosynthetic activities; and abolishing branches of the pathway. The latter may cause substrate to accumulate or be directed into alternative biosynthetic branches. These approaches may target the coloured flavonoids directly, or change the

Table 8.2. Examples of genetic modification of flower colour using inhibition of flavonoid biosynthetic gene activity by sense or antisense RNA.

Transgene

Species modified

Effect on flower coloura

Reference

CHS Sense

Sense and antisense

Antisense

Antisense

Antisense

Antisense

Sense

Sense

Sense and antisense

Sense or RNA interference (RNAi)

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