Patterning Plant Appendages

In this chapter, we discuss the formation of flowers and leaves, which are secondary structures in plants that are comparable in several respects to appendages in animals. In Chapter 7, we discussed formation of the apical shoot meristem during embryonic development of mustard plants. The central region of the shoot meristem continues to grow and give rise to sections of stem throughout the life of the plant. Once the plant begins to mature, secondary meristems arise along the periphery of the shoot meristem, which give rise to branches, leaves, and flowers. These secondary meristems share several properties with the primary shoot meristem, including the function of a common set of genes. Unlike the primary shoot meristem, which continues to grow and produce secondary meristems, however, secondary floral meristems produce only a single flower. It is worth distinguishing between "determinate" and "indeterminate" forms of development at this point, as these are the two primary modes of adult plant development. The primary shoot meristem is an example of indeterminate development in that it continues to generate branches, leaves, or flowers throughout the life of the plant. In contrast, under normal circumstances, floral meristems follow a determinate form of development in that they generate the primordium for a flower and then, after a short time, cells stop dividing and differentiate into floral organs (e.g., sepals, petals, stamens, and carpels). As we will see, there are mutants in which the floral meristems fail to undergo determinate development and instead continue to generate floral organ primordia. Unlike most mutants, which are often freakish and unappealing, indeterminate floral mutants such as roses can be very beautiful and have been selected by intensive breeding. There also are mutants in which the shoot meristem fails to maintain indeterminacy and instead terminates in a flower. Let us now consider the first step in flower development, the formation of a floral meristem.

Terms

ABC model A model of flower development to explain the behavior of three different classes of floral patterning mutants (A, B, and C mutants) which proposes that A, B, and C genes function in a pairwise fashion to specify the four floral organ identities.

Apical shoot meristem A small group of self-renewing cells located at the apical tip of a plant from which all above-ground structures of the plant derive.

Apical-basal axis (A/B) The vertical axis of a plant extending from the shoot (apical end) to the root (basal end).

Carpel The female reproductive organ onto which the pollen is deposited to begin the life cycle of the flowering plant.

Cotyledon Seed leaves of a plant embryo. Embryos of dicot plants have two cotyledons whereas those of monocots have only one.

Determinate development A mode of meristem development in which cells proliferate for a limited period to generate the primordium for a structure (e.g., a flower) and then stop dividing to differentiate.

Dicot plants Flowering plants that have embryos with two seed leaves or cotyledons.

Dorsal surface of leaf The surface of the leaf forming nearest the shoot.

Floral meristem A small group of cells forming at the flank of the apical shoot meristem that generate the primordium for a flower and differentiate into floral organs.

Floral organs The concentrically organized sepals, petals, stamens, and carpels of a flower.

Fruit The seed-containing structure that develops from the basal portion of the carpel following fertilization of the eggs.

Globular embryo A spherical morphologically undifferentiated mass of embryonic cells that forms after several divisions of the fertilized plant egg.

Heart-stage embryos The first stage of plant embryonic development when the embryo becomes visibly polarized and in which the primordia of cotyledons can be distinguished as the lobes of a heart.

Homeotic genes Genes that determine regional cellular identities, such as homeotic/Hox genes in animal embryos and floral organ-identity genes in plants.

Indeterminate development A self-regenerating mode of meristem development (e.g., the apical shoot meristem) in which cells proliferate continuously during the life of the plant to provide new cells for growth of the plant (e.g., the central stem) and for formation of secondary meristems (e.g., primordia giving rise to branches and leaves or to flowers).

Leaf primordium The default state of a secondary apical meristem.

MADS-box genes Genes encoding a class of transcription factors present in plants and animals. For example, in plants, MADS-box genes define the identities of floral organs and direct fruit development.

Margin of leaf The edge of the leaf that forms at the junction between the dorsal and ventral surfaces.

Medial-lateral axis of leaf (M/L) The axis of the leaf running perpendicular to the proximal-distal (P/D) axis, which is marked by structures such as veins that branch in particular locations.

Monocot plants Flowering plants that have embryos with a single cotyledon.

Ovary (floral) The portion of the female organ (carpel) in which the egg-containing ovules develop.

Petals The second whorl of floral organs that form just inside the sepals and are the most prominent structure of the flower (e.g., the red petals of a rose).

Proximal-distal axis of leaf (P/D) The axis running from the stem (proximal) to the tip (distal) of the leaf.

Secondary meristem A small group of cells that arise during maturation of the plant along the periphery of the apical shoot meristem and give rise to branches, leaves, or flowers.

Sepals The outer whorl of floral organs, resembling leaves, that encloses the flower.

Shattering The process by which pod-type fruits break open and release their seeds.

Stamen The male reproductive organ in a flowering plant that produces pollen.

Valve The fleshy sectors of a fruit that get eaten in edible fruits.

Valve border The narrow stripes of cells running along the edges of the sectors of valves.

Ventral surface of leaf The surface of the leaf forming farthest from the shoot.

Whorls The four concentric rings of floral organs (e.g., sepals, petals, stamens, and carpels).

Genes

Agamous (AG) A C-function floral homeotic gene expressed in the center of the floral meristem in cells giving rise to stamens and carpels.

Apetalal (AP1) An A-function floral homeotic gene expressed in an outer ring of floral meristem in cells giving rise to sepals and petals.

Apetala3 (AP3) A B-function floral homeotic gene expressed in a central ring of floral meristem cells that overlaps the domains of cells expressing AG and AP1 and gives rise to petals and stamens.

Apterous A fly homeobox gene that is required for formation of dorsal wing cells.

Cauliflower (CAL) A gene closely related to API that functions together with API to prevent floral meristems from developing as primary indeterminate shoot meristems.

Cup-shaped2 (CUC2) A gene expressed in a stripe of cells bisecting the apex of the globular embryo which is required to split the cotyledon primordium into two separated parts.

Cup-shapedl (CUC1) A gene required to split the cotyledon primordium into two separated parts.

Fruitful (FUL) A gene expressed in the region giving rise to the primordium of the fleshy fruit valve that is required for formation of the valve.

FT A gene encoding a protein related in structure to that of TFL that promotes initiation of floral development by opposing the activity of TFL.

Leafy (LFY) A gene expressed in early developing floral meristems that is required to initiate development of the floral meristem.

Notch A gene encoding a receptor that is required for outgrowth of the wing and formation of the wing margin in flies and for formation of a specialized group of cells (the AER) at the junction between the dorsal and ventral surfaces of vertebrate limb buds.

Phantastica A snapdragon gene required for outgrowth of the leaf and for formation of the leaf margin at the junction between the dorsal and ventral surfaces of the leaf.

Shatterproof genes A pair of highly related genes expressed specifically in valve border cells that are required for formation of valve borders.

Terminal flower (TFL) A gene encoding a likely inhibitory signal that suppresses floral development in secondary meristems by repressing expression of API and LFF.Flowering is initiated by genes such as FT, which oppose the action of TFL.

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