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. 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. Sepal organ identity is specified by the A function in the ABC model. Arabidopsis sepals are a model system for investigating morphogenesis (development of organ size and shape) and patterning (differentiation of specialised cell types such as giant cells). Current and future research on the development of sepals is leading to a better understanding of floral organ formation as well as underlying principles of cell division and growth. Key Concepts Sepals are the outermost floral organs, which are typically green leaf‐like organs that enclose and protect the developing flower bud before blooming. Sepals are thought to have evolved from leaves. Sepal organ identity is specified by A and E function in the ABC model of floral organ identity, and A function is specified by AP1/CAL/FUL clade MADS domain transcription factors, AP2 domain family transcription factors and microRNAs. The sepal is a commonly studied in Arabidopsis to discover fundamental principles of morphogenesis, such as how plant organs grow to reach the right size and shape. Arabidopsis sepals are the first floral organs to initiate on the flanks of the floral meristem where they quickly grow to cover and protect the developing reproductive organs, keep the bud closed until blooming and senesce and abscise after the fruit is fertilised. A scattered pattern of giant cells forms in between smaller cells in the outer epidermis of Arabidopsis sepals. The Arabidopsis thaliana MERISTEM LAYER1 (ATML1) transcription factor concentration fluctuates in epidermal cells of the developing sepal; if ATML1 reaches a high concentration during G2 phase of the cell cycle, the cell becomes giant. Giant cells become enlarged through endoreplication (also known as endoreduplication), during which a cell replicates its DNA and grows without division, becoming highly enlarged and polyploid. Sepal size and shape are highly reproducible; synchronous timing of both the initiation of sepal primordia and their maturation and cessation of growth are required for all four sepals in the Arabidopsis flower to maintain the same size to enclose the flower bud. The growth and division of sepal cells are highly variable, and gene expression is stochastic; however, sepals maintain robust size and shape through spatiotemporal averaging of growth variability and suppression of gene expression noise by the Paf1C complex.