MOST PEOPLE LOOK FORWARD to the spring season and the profusion of flowers it brings. Many vacationers carefully time their travels to coincide with specific blooming seasons: Citrus along Blossom Trail in southern California, tulips in Holland. In Washington, D.C., and throughout Japan, the cherry blossoms are received with spirited ceremonies. As spring progresses into summer, summer into fall, and fall into winter, wildflowers bloom at their appointed times.
Although the strong correlation between flowering and seasons is common knowledge, the phenomenon poses fundamental questions that will be addressed in this chapter:
• How do plants keep track of the seasons of the year and the time of day?
• Which environmental signals control flowering, and how are those signals perceived?
• How are environmental signals transduced to bring about the developmental changes associated with flowering?
In Chapter 16 we discussed the role of the root and shoot apical meristems in vegetative growth and development. The transition to flowering involves major changes in the pattern of morphogenesis and cell differentiation at the shoot apical meristem. Ultimately this process leads to the production of the floral organs—sepals, petals, stamens, and carpels (see Figure 1.2.A in Web Topic 1.2).
Specialized cells in the anther undergo meiosis to produce four hap-loid microspores that develop into pollen grains. Similarly, a cell within the ovule divides meiotically to produce four haploid megaspores, one of which survives and undergoes three mitotic divisions to produce the cells of the embryo sac (see Figure 1.2.B in Web Topic 1.2). The embryo sac represents the mature female gametophyte. The pollen grain, with its germinating pollen tube, is the mature male gametophyte generation. The two gametophytic structures produce the gametes (egg and sperm cells), which fuse to form the diploid zygote, the first stage of the new sporophyte generation.
Clearly, flowers represent a complex array of functionally specialized structures that differ substantially from the vegetative plant body in form and cell types. The transition to flowering therefore entails radical changes in cell fate within the shoot apical meristem. In the first part of this chapter we will discuss these changes, which are manifested as floral development. Recently genes have been identified that play crucial roles in the formation of the floral organs. Such studies have shed new light on the genetic control of plant reproductive development.
The events occurring in the shoot apex that specifically commit the apical meristem to produce flowers are collectively referred to as floral evocation. In the second part of this chapter we will discuss the events leading to floral evocation. The developmental signals that bring about floral evocation include endogenous factors, such as circadian rhythms, phase change, and hormones, and external factors, such as day length (photoperiod) and temperature (vernalization). In the case of photoperiodism, transmissible signals from the leaves, collectively referred to as the floral stimulus, are translocated to the shoot apical meristem. The interactions of these endogenous and external factors enable plants to synchronize their reproductive development with the environment.
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