Arguably one of the greatest accomplishments of evolution is the invention by higher plants of the flower. The diversity of flowers, from the deceptive simplicity of a daisy to the baroque complexity of an orchid, almost confounds the suggestion that they can be built in the same way. Now the spotlight of modern molecular biology is beginning to reveal what natural historians have suspected for centuries: that the architectures of flowers are variations on the same basic theme.

The outstanding feature of Arabidopsis is its tiny genome which, being among the smallest of any plant, makes the identification and study of its genes considerably easier than in other plants. The needle is hidden in only a single straw bale, rather than a whole haystack.

So what causes a plant to flower? Once the appropriate switch has been thrown the growing shoot quickly organizes itself to produce one or any number of flower buds. Each of these is arranged like an onion, with concentric layers of cells, known as 'whorls'. Lilies have three whorls and roses five, but many plants, including the snapdragon, Antirrhinum and Arabidopsis, have four. Each of these whorls produces one of the four components of most flowers. Outermost are the sepals: these surround the petals, which encircle the pollen-producing male stamen. In the centre is the female carpel, at the base of which lie the ovaries destined to form seeds and a seed pod.

The mechanism defining which whorl produces which organ is simple and elegant. It requires the interaction of a group of homeotic genes of the MADS-box family [see Aliens in my soup]. A number of genes are involved and they fall neatly into three groups -- A, B and C.

A-genes are expressed in the outer two whorls and are mutually antagonistic to the C-genes, which thus can only be expressed in the inner two whorls. B-genes, on the other hand, overlap the whorls in which A and C occur but are not present in either the innermost or outermost whorl. A-genes alone or C-genes alone produce sepals and carpels, respectively; when expressed with B-genes, As and Cs produce petals and stamens. Or so researchers thought.

Now it seems that the appealing simplicity of this ABC model was as much a product of experimental serendipity as anything else. The model was based on studies, from the early 1990s, of mutations in genes of each class. For example, plants with a defect in a B-gene have flowers with two whorls of sepals and two whorls of carpels.

But Dr Martin Yanofsky at the University of California in San Diego has identified three MADS-box genes with very similar effects to the ABC genes but which do not fit easily into the model, as he reports in Nature1. If anything, they have a combined B/C function. Disruptions of these genes produce plants with flowers made entirely of sepals (they look something like miniature artichokes). In line with the convention for naming genes after what happens when they are disrupted, Yanofsky has called his discoveries 'SEPALLATA'.

These genes have taken so long to be discovered because until recently plant genetics relied on the far-from-ideal random production of mutations, whether naturally or artificially, which produced interesting defects. Yanofsky and colleagues instead used a new and powerful technique called reverse genetics.

The SEPELLATA genes were first identified during the sequencing of the complete genome of Arabidopsis. Researchers then produced the triple mutant plants, with all three SEPELLATA genes disrupted, to identify their crucial role in both B and C gene function.

Whatever the specific pattern, the formation of flowers using an alphabet of MADS-box genes is more ancient than flowers themselves. ABC-like genes dictate the development of flowers in all known flowering plants, including grasses and even ancient plants such as pepper and betel nut which have neither petals nor sepals. The genes also control the development of cones in pine trees; and they are even present in ferns.

Clearly without the 'invention' of this ABC scheme for building reproductive organs by some highly primitive plant some 400 or more million years ago this world would be a much less colourful and fragrant place to live.