The Invisible Colours of Flowers

Determining flower structure.

How many of us have wondered at the variety of shapes and structures in a collection of beautiful blooms? The diversity is especially striking when we consider that flowers are typically made up of four basic parts - the outermost sepals surrounding the petals, then the stamens and finally one or more central carpels.

The diverse flower shapes and structures we see are determined by patterns that are created early in flower development. Zones are set up in the young flower bud while it is still less than one tenth of a millimetre in diameter (smaller than the full stop at the end of this sentence) and has no visible internal organisation. These zones determine where, and how many, different organ types develop in the maturing flower bud.

The zones are marked out by the activities of three different genes, called here A, B & C. Imagine the activity of each gene as “painting” the zone where it is active a particular “colour”: (A = Red, B = Yellow, C = Blue). In a normal flower bud, all three genes are active. Where gene A is active (zone A, red), sepals will form. Where gene A and gene B are both active (zone AB, red and yellow = orange), only petals are produced. Where genes B & C are active, that zone (zone BC, yellow and blue = green), produces stamens. The zone where only gene C is active (zone C, blue) produces carpels.

In reality, these three genes do not produce colours but chemicals that switch on other genes. The A gene produces a chemical trigger that switches on the many genes involved in making sepals and the C gene switches on those genes needed to make carpels. The combination of chemical triggers from the A and B genes switch on the genes involved in making petals, and B and C together switch on genes for stamen development.

If one of these genes is inactive (due to a mutation), then the pattern of zones, and the position and type of organs produced, will be different. In our example, gene B (yellow) is inactive, this means that only A & C (red and blue) zones are created, resulting in abnormal mature flowers that have only sepals and carpels.

There are many examples of plant varieties that have different numbers of flower organs. The most common are "double" flowers where, usually, stamens are replaced by extra petals. Rosa viridflora is an example where all the flower organs have been replaced by sepals (only the A gene is functioning correctly). Similar flowers are known in Primulas. Also in Primulas you can find “hose-in-hose” flowers, where sepals are replaced by petals.

Genes that control the activity of other genes in this way are extremely important in plant development and therefore of great interest to scientists. The snapdragon (Antirrhinum majus) has been used extensively by scientists in their studies of these genes, but the same genes have been identified in, and even extracted from, many other plant species.