Flower Development and the ABC Model

The flower is built from four concentric whorls of organs, and the elegant ABC model explains how a small set of overlapping genes assigns each whorl its identity — one of the founding results of plant developmental genetics.

Definition

Flower development is the process by which a vegetative shoot apex is converted into a floral meristem and patterned into sepals, petals, stamens, and carpels, and the ABC model is the genetic framework explaining how organ identity is specified.

Scope

This topic covers the transition to flowering, the organization of the floral meristem into whorls, and the genetic ABC(DE) model of floral organ identity, including the homeotic mutants and MADS-box transcription factors that established it.

Core questions

• How does a shoot meristem become a floral meristem with distinct whorls?
• How do the A, B, and C classes of genes combine to specify floral organ identity?
• What do homeotic floral mutants reveal about the logic of flower patterning?

Key theories

ABC combinatorial model

Three classes of homeotic gene activity act in overlapping domains across the four whorls — A alone specifies sepals, A plus B petals, B plus C stamens, and C alone carpels — so that loss of a class transforms organs predictably.

MADS-box transcription factors

Most floral identity genes encode MADS-box transcription factors that combine into multimeric complexes, providing the molecular basis for the combinatorial ABC code and its later D and E extensions.

Mechanisms

Floral organ identity genes are expressed in defined, overlapping territories of the young floral meristem. A-class activity in the outer two whorls, B-class in the middle two, and C-class in the inner two combine so each whorl receives a unique code that directs sepal, petal, stamen, or carpel development; A and C functions are mutually antagonistic. The genes encode MADS-domain proteins that assemble into tetrameric complexes, an interaction captured in the later quartet model, and loss-of-function mutations cause homeotic conversions of one organ type into another.

Clinical relevance

The ABC framework guides ornamental breeding — for example, the doubling of flowers in roses and many garden plants reflects altered organ-identity gene activity — and informs efforts to manipulate flower and fruit structure in crops.

History

Parallel genetic screens in Arabidopsis and Antirrhinum in the late 1980s identified homeotic floral mutants, which Coen and Meyerowitz synthesized into the ABC model in 1991; molecular work later identified MADS-box genes and extended the model with D and E functions.

Key figures

• Enrico Coen
• Elliot Meyerowitz
• Günter Theißen

Seminal works

• coen1991
• taiz2015

Frequently asked questions

What are the four whorls of a flower?

From outside to inside, a typical flower has four whorls: sepals, petals, stamens, and carpels, and the ABC model explains how each whorl's identity is genetically specified.

What happens in an ABC mutant?

Loss of a gene class causes homeotic transformations: for example, losing C-class activity converts stamens to petals and carpels to sepals, producing the showy double flowers familiar in many cultivated plants.