The Invisible Colours of Flowers

Gene switches.

A mature plant is a remarkably complex structure compared with the single fertilised egg cell from which it develops. The single egg cell, and the plant embryo it soon becomes, must gradually establish a set of complex patterns, such as those required to form leaves and flowers, for example. These patterns must be established at the correct time and place in the plant, for the flower to develop properly.

Essentially all cells in a plant contain all the genetic information needed to reproduce every developmental stage in that plant's life cycle. So plants, and all living organisms, must be able to control and coordinate the use of the genetic information in their cells. To do this they use gene switches.

Genes are made up of several parts. The “coding" part is the piece of the gene that is "read" and interpreted by the cell to produce the chemical product (usually a protein) whose structure is uniquely encoded in the gene. There are start and stop signals at either end of the coding part that are the "punctuation marks" at the ends of this "sentence" of code. Genes also include a part, or parts, that acts as "switches". It is through these switches that plants are able to regulate and coordinate the activity of their genes and so control their growth and development.

The switches are a part of the DNA that can bind, very specifically, particular proteins and it is this binding that switches the gene on or off. The proteins that activate the switches are produced by a special class of "master" genes. These master genes can control large numbers of other genes through the proteins that they produce.

In the early stages of flower bud growth, master genes are active in cells in particular zones of the bud. The proteins they produce switch on genes whose products are needed to produce particular flower organs. (They may also switch off other genes that are not required, such as genes that produce signals to make roots or leaves).

The master genes may themselves be switched on and off by proteins produced by other genes. If these master genes are activated according to their position in a piece of tissue then it is possible for the plant to establish a pattern of gene activity related to position. For example, a master gene that is switched on in cells on, and close to, the surface of a piece of tissue, will switch on its subordinate genes in the cells in the surface layers of the tissue but not elsewhere.