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) Scanning electron micrograph (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.



Ambrose BA, Lerner DR, Ciceri P et al. (2000) Molecular and genetic analyses of the silky1 gene reveal conservation in floral organ specification between eudicots and monocots. Molecular Cell 5: 569–579.

Bossinger G and Smyth DR (1996) Initiation patterns of flower and floral organ development in Arabidopsis thaliana. Development 122: 1093–1102.

Bowman JL, Alvarez J, Weigel D, Meyerowitz EM and Smyth DR (1993) Control of flower development in Arabidopsis thaliana by Apetala1 and interacting genes. Development 119: 721–743.

Bowman JL, Smyth DR and Meyerowitz EM (1989) Genes directing flower development in Arabidopsis. Plant Cell 1: 37–52.

Bowman JL, Smyth DR and Meyerowitz EM (1991) Genetic interactions among floral homeotic genes of Arabidopsis. Development 112: 1–20.

Chen X (2004) A microRNA as a translational repressor of APETALA2 in Arabidopsis flower development. Science 303: 2022–2025.

Coen ES and Meyerowitz EM (1991) The war of the whorls – genetic interactions controlling flower development. Nature 353: 31–37.

Dinneny JR, Yadegari R, Fischer RL, Yanofsky MF and Weigel D (2004) The role of JAGGED in shaping lateral organs. Development 131: 1101–1110.

Ditta G, Pinyopich A, Robles P, Pelaz S and Yanofsky MF (2004) The SEP4 gene of Arabidopsis thaliana functions in floral organ and meristem identity. Current Biology 14: 1935–1940.

Drews GN, Bowman JL and Meyerowitz EM (1991) Negative regulation of the Arabidopsis homeotic gene AGAMOUS by the APETALA2 product. Cell 65: 991–1002.

Gregis V, Sessa A, Colombo L and Kater MM (2006) AGL24, SHORT VEGETATIVE PHASE, and APETALA1 redundantly control AGAMOUS during early stages of flower development in Arabidopsis. Plant Cell 18: 1373–1382.

Honma T and Goto K (2001) Complexes of MADS‐box proteins are sufficient to convert leaves into floral organs. Nature 409: 525–529.

Irish VF and Sussex IM (1990) Function of the apetala‐1 gene during Arabidopsis floral development. Plant Cell 2: 741–753.

Jofuku KD, den Boer BG, Van Montagu M and Okamuro JK (1994) Control of Arabidopsis flower and seed development by the homeotic gene APETALA2. Plant Cell 6: 1211–1225.

Keck E, McSteen P, Carpenter R and Coen E (2003) Separation of genetic functions controlling organ identity in flowers. EMBO Journal 22: 1058–1066.

Lewis MW, Leslie ME and Liljegren SJ (2006) Plant separation: 50 ways to leave your mother. Current Opinion in Plant Biology 9: 59–65.

Litt A (2007) An evaluation of A‐function: evidence from the APETALA1 and APETALA2 gene lineages. International Journal of Plant Sciences 168: 73–91.

Lohmann JU, Hong RL, Hobe M et al. (2001) A molecular link between stem cell regulation and floral patterning in Arabidopsis. Cell 105: 793–803.

Mandel MA, Bowman JL, Kempin SA et al. (1992a) Manipulation of flower structure in transgenic tobacco. Cell 71: 133–143.

Mandel MA, Gustafson‐Brown C, Savidge B and Yanofsky MF (1992b) Molecular characterization of the Arabidopsis floral homeotic gene APETALA1. Nature 360: 273–277.

Melaragno JE, Mehrotra B and Coleman AW (1993) Relationship between endopolyploidy and cell size in epidermal tissue of Arabidopsis. Plant Cell 5: 1661–1668.

Mizukami Y and Ma H (1992) Ectopic expression of the floral homeotic gene AGAMOUS in transgenic Arabidopsis plants alters floral organ identity. Cell 71: 119–131.

Nagasawa N, Miyoshi M, Sano Y et al. (2003) SUPERWOMAN1 and DROOPING LEAF genes control floral organ identity in rice. Development 130: 705–718.

Ohno CK, Reddy GV, Heisler MG and Meyerowitz EM (2004) The Arabidopsis JAGGED gene encodes a zinc finger protein that promotes leaf tissue development. Development 131: 1111–1122.

Pelaz S, Ditta GS, Baumann E, Wisman E and Yanofsky MF (2000) B and C floral organ identity functions require SEPALLATA MADS‐box genes. Nature 405: 200–203.

Pelaz S, Gustafson‐Brown C, Kohalmi SE, Crosby WL and Yanofsky MF (2001a) APETALA1 and SEPALLATA3 interact to promote flower development. Plant Journal 26: 385–394.

Pelaz S, Tapia‐Lopez R, Alvarez‐Buylla ER and Yanofsky MF (2001b) Conversion of leaves into petals in Arabidopsis. Current Biology 11: 182–184.

Pinyopich A, Ditta GS, Savidge B et al. (2003) Assessing the redundancy of MADS‐box genes during carpel and ovule development. Nature 424: 85–88.

Pnueli L, Hareven D, Rounsley SD, Yanofsky MF and Lifschitz E (1994) Isolation of the tomato AGAMOUS gene TAG1 and analysis of its homeotic role in transgenic plants. Plant Cell 6: 163–173.

Smyth DR, Bowman JL and Meyerowitz EM (1990) Early flower development in Arabidopsis. Plant Cell 2: 755–767.

Sridhar VV, Surendrarao A and Liu Z (2006) APETALA1 and SEPALLATA3 interact with SEUSS to mediate transcription repression during flower development. Development 133: 3159–3166.

Theissen G (2001) Development of floral organ identity: stories from the MADS house. Current Opinion in Plant Biology 4: 75–85.

Vrebalov J, Pan IL, Arroyo AJ et al. (2009) Fleshy fruit expansion and ripening are regulated by the tomato SHATTERPROOF gene TAGL1. Plant Cell 21: 3041–3062.

Warner KA, Rudall PJ and Frohlich MW (2009) Environmental control of sepalness and petalness in perianth organs of waterlilies: a new Mosaic theory for the evolutionary origin of a differentiated perianth. Journal of Experimental Botany 60: 3559–3574.

Wellmer F, Alves‐Ferreira M, Dubois A, Riechmann JL and Meyerowitz EM (2006) Genome‐wide analysis of gene expression during early Arabidopsis flower development. PLoS Genetics 2: e117.

Wellmer F, Riechmann JL, Alves‐Ferreira M and Meyerowitz EM (2004) Genome‐wide analysis of spatial gene expression in Arabidopsis flowers. Plant Cell 16: 1314–1326.

Whipple CJ, Ciceri P, Padilla CM et al. (2004) Conservation of B‐class floral homeotic gene function between maize and Arabidopsis. Development 131: 6083–6091.

Whipple CJ and Schmidt RJ (2006) Genetics of grass flower development. Advances in Botanical Research Incorporating Advances in Plant Pathology 44: 385–424.

Whipple CJ, Zanis MJ, Kellogg EA and Schmidt RJ (2007) Conservation of B class gene expression in the second whorl of a basal grass and outgroups links the origin of lodicules and petals. Proceedings of the National Academy of Sciences of the USA 104: 1081–1086.

Xu B, Li Z, Zhu Y et al. (2008) Arabidopsis genes AS1, AS2, and JAG negatively regulate boundary‐specifying genes to promote sepal and petal development. Plant Physiology 146: 566–575.

Further Reading

Endress PK (1994) Diversity and Evolutionary Biology of Tropical Flowers. Cambridge: Cambridge University Press.

Heywood VH, Brummitt RK, Culham A and Seberg O (2007) Flowering Plants Families of the World. Buffalo: Firefly Books Ltd.

Meyerowitz EM (1994) The genetics of flower development. Scientific American 271: 56–65.

Soltis DE, Leebens‐Mack JH and Soltis PS (eds) (2006) Developmental genetics of the flower. In: Callow JA (Series ed.) Advances in Botanical Research Incorporating Advances in Plant Pathology, vol. 44. San Diego: Academic Press.

Stebbins GL (1974) Flowering Plants: Evolution Above the Species Level. Cambridge, MA: The Belknap Press of Harvard University Press.

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