Sepals

Abstract 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.