Accessory fruit

Abstract: Luscious cherries, sweet peaches, creamy avocados, and tropical papayas are just a few of the tasty treats that come to mind when we think of fruit. Indeed, fruit come in all shapes and sizes, from gigantic pumpkins to the tiny fruit of the duckweed Wolffia angusta, which are as small as a grain of salt. Fruit range in texture from soft and fleshy to dry and papery with each design optimized for a different seed dispersal strategy. Fleshy fruit are often sweet, brightly colored, and are generally adapted to be eaten by vertebrates, which carry the seeds to a new location before depositing them in a pile of fertilizer. In contrast, wind, water, and the force generated by the opening of the seedpod commonly distribute the seeds of dry fruit. Of course there are many exceptions, such as the spiked, barbed, dry fruit that snag a ride by adhering to the fur of passing animals. Dry fruit are classified as either dehiscent, in which the walls of the ovary open to release the seeds into the environment, or indehiscent, in which the seeds remain enclosed in the fruit and the fruit is shed from the plant. Many important crops including peas, beans, lentils, soybeans and canola have dehiscent fruit. Both crops with fleshy fruit and with dehiscent fruit are of such importance to agriculture and the human diet that fruit have been the focus of extensive research in recent years. Research on fleshy fruit has focused primarily on tomato and great progress has been made in understanding the genes that control the size and ripening of tomato fruit (for reviews see Giovannoni, 2004; Tanksley, 2004; Adams-Phillips, et al., 2004). Research on dehiscent fruit has focused on Arabidopsis thaliana, which will be the focus of this chapter (for additional reviews see Dinneny and Yanofsky, 2004; Ferrandiz, et al., 1999; Bowman et al., 1999). In this chapter, we will first discuss wild-type fruit development and then turn to the genes and hormones that are known to regulate fruit formation in Arabidopsis. Specifically, we will examine the genes that are involved in specifying the development of the different tissue types within the fruit, the genes that control the formation of axes within the fruit, and the processes that regulate fruit development after fertilization (see Table 1 for a list of genes involved in fruit development). The fruit is arguably the most complex plant organ and its development is just beginning to be understood, making fruit development a ripe field for many years to come. Table 1. Genes involved in fruit development 1.1 Wild-type Fruit Structure The fruit is defined as the mature ovary (and, in some types of fruit, additional floral tissues) that forms a specialized structure designed to protect the seeds while they develop and disperse them at maturity. The fruit develops from the gynoecium after fertilization. The gynoecium is the female reproductive structure including the ovary and is usually formed from one or more fused carpels at the center of the flower. A carpel is a single ovule bearing structural unit of the gynoecium and is thought to have originated from a modified bract or leaf (Bowman et al., 1999). The Arabidopsis gynoecium is composed of two fused carpels, each of which consists of a seedpod wall and surrounding tissues. The fusion of the carpels is congenital, meaning that the gynoecium arises as a single primordium. The Arabidopsis fruit develops from the fertilized gynoecium to form a silique, or seedpod, which dries and dehisces at maturity, releasing the seeds. The Arabidopsis fruit consists of many distinct cell types, which are derived from the gynoecium. From top to bottom there are four different regions of the gynoecium and subsequently the fruit: the stigma, style, ovary, and gynophore (Figure 1; see Figure 2 for definitions of axes in the fruit and other terms). Figure 1. Structure of the wild-type fruit. (A) Scanning electron micrograph (SEM) of a Landsberg erecta (Ler) fruit at stage 17. The fruit has been false colored to distinguish the different parts and this color code has been used throughout the review. At the ... Figure 2. Terminology. (A) Picture of a stage 17 fruit with axes labeled. (B) Cross section of a stage 17 fruit with axes labeled. The scale bar in A represents 1 mm and the scale bar in B represents 100 µm. Stigma At the top of the gynoecium, the stigma is comprised of a single layer of elongated papillar cells specialized for the germination of pollen (Figure 1B). The stigma is the first component of the transmitting tract, a set of cells that secrete a polysaccharide-rich extracellular matrix, which-forms a pathway for the growth and guidance of pollen tubes (Sessions and Zambryski, 1995; for reviews of pollen tube guidance see Lord and Russell, 2002 and Palanivelu and Preuss, 2000).