Flowering

• In "annuals", like the snapdragon, the vegetative phase begins with germination of the seed. Flowering follows and ends with the senescence and death of the plant.
• In biennials, the vegetative phase takes up the first year; flowering followed by death occurs the second year.
• In perennials, flowering typically occurs year after year when conditions are appropriate.

Vegetative growth of the above-ground part of the plant - the shoot - occurs at the apical meristem. This is a mass of undifferentiated cells at the tip of the stem. Mitosis of these cells produces cells that differentiate to form

• more stem
• leaves
• secondary meristems. Also called lateral buds, these form in the axils of the leaves and will form branches.

The signal to flower

• availability of water (common among desert annuals)
• maturity of the plant
• temperature
• and, for many plants, photoperiod - the relative length of day and night.

Photoperiod is detected in the leaves. The cocklebur, drawn here, needs at least 8.5 hours of darkness in order to flower. Even if only a part of one leaf is exposed to the correct photoperiod, the entire plant will bloom (middle figure).

The leaves produce a chemical signal - called florigen - that is transmitted to the apical meristems to start their conversion into floral meristems. The right-hand drawing shows that grafting a cocklebur (B) that receives the required period of darkness to one (A) that does not causes flowering in both. Evidently the florigen signal passes from B to A through their connected vascular systems.

The chemical nature of florigen has been sought for decades. The most recent evidence suggests that it is a zinc-finger transcription factor.

Converting the apical meristem to a floral meristem

Structure of the Flower

1. The cells in whorl 1 develop into a whorl of sepals. These form at the lowest level. Collectively they make up the calyx.
2. Whorl 2 forms above the calyx, forming the petals. Collectively these make up the corolla of the flower (the part that most ornamentals are grown for).
3. Whorl 3 develops into the stamens, the male reproductive organs.
4. The innermost whorl, 4, forms carpels, the female reproductive organs. Carpels often fuse to form a single structure, which some botanists call the pistil.

What triggers the various parts of the floral meristem to enter one or another of these four developmental pathways?

The ABC Model of Flower Development

• Cells in which only A genes are expressed develop into sepals. This occurs at the lowest level of the floral meristem.
• Cells in which both A and B genes are expressed develop into petals. This occurs at the next higher level.
• Expression of B and C genes turns on the developmental program to form stamens.
• Expression of C genes alone turns on the development of carpels in the innermost band of cells.

The LEAFY protein, in addition to its role in making the floral meristem, plays a major role in turning on the A, B, and C group genes in the appropriate locations.

• The LEAFY protein alone turns on AP1 in whorls 1 and 2.
• LEAFY plus a protein encoded by the gene UFO (for "unusual floral organs") turn on AP3 in whorls 2 and 3.
• LEAFY and a second, still unidentified, protein turn on AG in whorls 3 and 4.

If LEAFY alone is sufficient to turn on AP1, why isn't AP1 expressed in all four whorls?

The answer: AGAMOUS blocks the expression of AP1, so any cell expressing AGAMOUS cannot express AP1.

In fact, the antagonism is reciprocal: AP2 in whorls 1 and 2 (A group) inhibits AGAMOUS so the gene expression in whorls 3 and 4 remains distinct from that in whorls 1 and 2.

The proteins encoded by APETALA3 and PISTILLATA (Group B) form a heterodimer that binds to

• the APETALA1 protein to form petals
• the AGAMOUS protein to form stamens.

Aided by a fourth transcription factor encoded by the gene SEPALLATA3, these quaternary complexes bind to specific sequences of DNA turning on the expression of the various genes needed to form whorls 2 and 3. Further research may reveal similar behavior for the other genes.

So formation of a flower requires a cascade of sequential gene activity that gradually converts a mass of undifferentiated cells (the apical meristem) into the parts of a flower. The genes encode transcription factors that act as master switches, turning on (or off) downstream genes that ultimately make each part of the flower in its appropriate location.

This same strategy of genetic control of developmental pathways is seen in animal development. Try this link to see some examples in Drosophila.

Recommended reading: The Genetics of Flower Development by Elliot M. Meyerowitz (in whose lab many of these discoveries were made). It was published in the November 1994 issue of Scientific American.