Sepals are the outermost organs of a flower. They are sterile and generally green leaf‐like organs that surround and protect the developing reproductive structures inside the bud before the flower blooms. The sepals are the first organs initiated from the floral meristem and quickly grow to cover the floral meristem and initiating organ primordia. Sepal organ identity is traditionally thought to be specified by the A function in the ABC model. Sepals often contain specialised cell types such as hair cells or giant cells, which are present on the outer surface of Arabidopsis sepals. The sepals and petals in the flower interact with the environment by both attracting pollinators as well as defending against predators and abiotic factors such as the weather. Current and future research on the development of sepals will lead to a better understanding of floral organ formation as well as underlying principles of cell division and growth.

Key Concepts:

  • The perianth is composed of the sterile organs of the flower, which are the set of sepals (calyx) and the set of petals (corolla).

  • Although generally sepals are green photosynthetic protective organs, some sepals are showy organs similar to petals and designed for attracting pollinators.

  • Sepals serve their protective function by overlapping such that they completely cover the developing bud.

  • Sepals generally contain defensive cell types including hair cells and produce toxic chemicals to defend the developing reproductive organs from predators.

  • Arabidopsis sepals have a characteristic pattern of diverse cell sizes including giant cells on the outer surface.

  • The four Arabidopsis sepals are the first organs initiated from the floral meristem and are formed in a whorled phyllotaxy.

  • APETALA1 (AP1), APETALA2 (AP2) and SEPALLATA1–4 (SEP1–4) specify sepal organ identity.

  • After fertilisation, sepals often senesce and abscise.

Keywords: flowers; reproduction; angiosperms; bracts; buds; giant cells

Figure 1.

Arabidopsis sepals. (a) Mature Arabidopsis flower (stage 14) showing the outer whorl of four sepals, followed by four petals, six stamens and two carpels at the centre of the flower. (b) (SEM) of the outer (abaxial) side of the sepal showing the characteristic cell size pattern including giant cells (false coloured red). (c) High magnification view of the abaxial sepal epidermis showing the diversity of cell sizes (giant cells are false coloured red) and the presence of stomata (false coloured green) for gas exchange. (d) High magnification view of the inner (adaxial) side of the sepal showing that the cells are of a more uniform size and interspersed with a few stomata (false coloured green). Two ball‐shaped pollen grains are present.

Figure 2.

Development of the Arabidopsis sepal. (a) Scanning electron micrograph (SEM) of the inflorescence meristem (m) from an Arabidopsis thaliana Landsberg erecta plant. The surrounding flower primordia are visible. The sepal primordia emerge as ridges on the flanks of the late stage 3 floral meristem. The sepal primordia grow to cover the floral meristem, whereas the stamen primordia initiate at stage 5. The sepals completely cover the floral meristem at stage 6. (b) A stage 9 bud shows the development of the specialised cell size pattern including giant cells (g) and the differentiation of guard cells (gc) proceeding basipetally (from top to bottom). Note the trichome (t) at the top of the sepal. (c) At stage 12 bud, the sepals remain closed. (d) In stage 13, the petals and stamens push the sepals open as the flower blooms. (e) At stage 14, the flower is completely open and the sepal is fully mature. (f) At stage 16, the sepals senesce, turn yellow and are abscised.

Figure 3.

ABC model of floral organ identity. The ABC model postulates that there are three activities that act alone or concert to specify the identity of the floral organs. A function specifies sepals, A+B function specifies petals or lodicules, B+C function specifies stamens and C function alone specifies carpels. Whether the grass lemma or palea can be considered first whorl oranges and whether they are specified by an A function is a current subject of investigation.



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Roeder, Adrienne HK(May 2010) Sepals. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0002064.pub2]