Apical meristems can become inflorescence meristems

The first visible sign of the transition to the flowering state may be a change in one or more apical meristems in the shoot system. During vegetative growth, an apical meristem continually produces leaves, lateral buds, and internodes (Figure 39.9a). This unrestricted growth is indeterminate (see Chapter 35).

Flowers may appear singly or in an orderly cluster that constitutes an inflorescence. If a vegetative apical meristem becomes an inflorescence meristem, it ceases production of leaves, lateral buds, and internodes and produces other structures: smaller leafy structures called bracts, as well as new meristems in the angles between the bracts and the intern-odes (Figure 39.9b). These new meristems may also be inflorescence meristems, or they may be floral meristems, each of which gives rise to a flower.

Each floral meristem typically produces four consecutive whorls or spirals of organs—the sepals, petals, stamens, and carpels—separated by very short internodes, keeping the flower compact (Figure 39.9c). In contrast to vegetative apical meristems and some inflorescence meristems, floral meristems are responsible for determinate growth—the limited growth of the flower to a particular size and form.

Apical Meristem Inflorescence Meristem

Carpel

A floral meristem gives rise to a flower.

A floral meristem gives rise to a flower.

Stamen

Petal

Sepal

39.9 Flowering and the Apical Meristem A vegetative apical meristem (a) grows without producing flowers. Once the transition to the flowering state is made, inflorescence meristems (b) give rise to bracts and to floral meristems (c),which become the flowers.

Carpel

Stamen

Petal

Sepal

Modified leaflike structures of the flower

39.9 Flowering and the Apical Meristem A vegetative apical meristem (a) grows without producing flowers. Once the transition to the flowering state is made, inflorescence meristems (b) give rise to bracts and to floral meristems (c),which become the flowers.

A cascade of gene expression leads to flowering

How do apical meristems become inflorescence meristems, and how do inflorescence meristems give rise to floral meristems? How does a floral meristem give rise, in short order, to four different floral organs? How does each flower come to have the correct number of each of the floral organs? Numerous genes collaborate to produce these results. We'll refer here to some of the genes whose actions have been most thoroughly studied in Arabidopsis and snapdragons.

► Expression of a group of meristem identity genes initiates a cascade of further gene expression.

► This cascade begins with cadastral genes, which participate in pattern formation—the spatial organization of the whorls of organs.

► Cadastral genes trigger the expression of floral organ identity genes, which work in concert to specify the successive whorls (see Figure 19.12)

Floral organ identity genes are homeotic genes, and their products are transcription factors that mediate the expression of still other genes.

Having seen how flowering occurs, we will now consider how the transition from the vegetative to the flowering state is initiated.

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