17 Arabidopsis Flower Development

The Arabidopsis flower develops sixteen organs in four concentric rings, or whorls (Fig. 1). In the outermost whorl, whorl 1, four green sepals develop. Interior and alternate to the sepals are four white petals which occupy whorl 2. Whorl 3 contains six stamens: two pairs of medial stamens and two lateral stamens, which are shorter than the medial stamens. The center of the flower, whorl 4, consists of two fused carpels that comprise the gynoecium. The development of wild-type flowers has been well characterized by Smyth et al. (1990), and readers are referred to that article for an in-depth analysis. All stages of development as described in this chapter are according to those outlined by Smyth et al. (1990). Flowers initially arise as undifferentiated bulges on the flank of the apical meristem (stage 1) (Fig. 1). Soon afterward, four sepal primordia develop along the edges of the floral meristem, establishing the first whorl (stage 3). The sepals grow to overlie the floral primordium (stage 4). Four petal and six stamen primordia initiate in whorls 2 and 3, respectively (stage 5), followed by carpel primordium development in the center of the floral meristem (stage 6). Each floral organ has distinctive features that can be used to distinguish it from other organs. These characteristics are often used to identify mosaic or intermediate organ types seen in mutant flowers. Sepals are leaf-like, but display distinct differences from leaves (Fig. 2A, C, E). Sepals lack flanking stipules at their bases and develop simple, as...