Chapter Summary

• As sessile organisms, plants maximize their ability to grow by using meristems, forming new organs, and growing throughout life.

• The environment, receptors, hormones, and the plant’s genome all regulate plant development.

• Seed dormancy, which has adaptive advantages, is maintained by a variety of mechanisms. In nature, dormancy is broken by, for example, abrasion, fire, leaching, and low temperatures. When dormancy ends and the seed imbibes water, it germinates and develops into a seedling. Review Figure 36.1, Activities 36.1, 36.2

• Plant hormones differ in structure and physiology from animal hormones. Review Table 36.1

• Plants have several hormones, each of which regulates multiple aspects of development. Interactions among these hormones are often complex. Review Table 36.2

• Genetic screens using the model organism Arabidopsis thaliana have contributed greatly to our understanding of signaling in plants. Review Figure 36.2

• Both gibberellins and auxin can induce growth in plants otherwise genetically destined to be dwarfs. Review Figure 36.3

• Gibberellins have many effects that vary among different plants, including cell elongation, fruit ripening, and mobilization of seed storage polymers. Review Figures 36.3–36.5, Activity 36.3

• Auxin was discovered in the context of stem and coleoptile growth, in particular phototropism. In the shoot, it is made in the growing tip and transported down to stimulate cell elongation. Review Investigating Life: The Darwins’ Phototropism Experiment, Figures 36.6, 36.7, Animations 36.1, 36.2

• According to the acid growth hypothesis, auxin stimulates cell elongation through the release of protons into the cell wall (acidification of the cell wall). Review Figure 36.9, Animation 36.3

• Both auxin and gibberellins act through the breakdown of transcriptional repressors. Review Focus: Key Figure 36.10

• Cytokinins are adenine derivatives that promote plant cell division, promote seed germination in some species, inhibit stem elongation, promote lateral swelling of stems and roots, stimulate the growth of axillary buds, promote the expansion of leaf tissue, and delay leaf senescence.

• A balance between auxin and ethylene controls leaf abscission. Ethylene promotes senescence and fruit ripening. It indirectly causes the formation of a protective apical hook in eudicot seedlings. In stems, it inhibits elongation, promotes lateral swelling, and causes a loss of gravitropic sensitivity.

• Dozens of different brassinosteroids affect cell elongation, pollen tube elongation, vascular tissue differentiation, and root elongation.

• Phototropins are blue-light photoreceptors for phototropism and chloroplast movements. Zeaxanthin acts in conjunction with the phototropins to mediate the light-induced opening of stomata. Cryptochromes are blue-light photoreceptors that control seedling development, stem elongation, and floral initiation.

• Phytochrome exists in the cytoplasm in two interconvertible forms, P_r and P_fr. The relative amounts of these two forms are a function of the ratio of red to far-red light. Phytochrome affects seedling growth, flowering, and etiolation. Review Figures 36.12, 36.13

• The phytochrome signal transduction pathway affects transcription in two different ways; the P_fr form interacts directly with some transcription factors, and influences transcription indirectly through interactions with protein kinases. Review Figure 36.14

• Circadian rhythms are activities that occur on a near-24-hour cycle. Light can entrain these activities through photoreceptors such as phytochrome.