Summary

Concept 26.1: Plants Develop in Response to the Environment

• Plant development is regulated by environmental cues, receptors, hormones, and the plant’s genome.
• Seed dormancy, which has adaptive advantages, is maintained by a variety of mechanisms. When dormancy ends, the seed germinates and develops into a seedling. Review Figure 26.1 and ACTIVITIES 26.1 and 26.2
• Hormones and photoreceptors act through signal transduction pathways to regulate plant growth and development.
• Genetic screens using the model organism Arabidopsis thaliana have contributed greatly to our understanding of signal transduction pathways in plants. Review Figure 26.2

Concept 26.2: Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action

• Gibberellins stimulate growth of stems and fruits as well as mobilization of seed reserves in cereal crops. Review Figure 26.4 and ACTIVITY 26.3
• Auxin is made in cells at the shoot apex and moves down to the growing region in a polar manner. Review Figure 26.5
• Lateral movement of auxin, mediated by auxin efflux carriers, is responsible for phototropism and gravitropism. Review Figure 26.6 and ANIMATED TUTORIAL 26.1
• Auxin plays roles in lateral root formation, leaf abscission, and apical dominance.
• The acid growth hypothesis explains how auxin promotes cell expansion by increasing proton pumps in the cell membrane, which loosens the cell wall. Review Figure 26.7 and ANIMATED TUTORIAL 26.2
• Both auxin and gibberellins act by binding to their respective receptors, which then bind to a transcriptional repressor, leading to the repressor’s breakdown in the proteasome. Review Figure 26.8

Concept 26.3: Other Plant Hormones Have Diverse Effects on Plant Development

• Cytokinins, most of which are adenine derivatives, often interact with auxin. They promote plant cell division, promote seed germination in some species, and inhibit stem elongation, among other activities.
• Cytokinins act on plant cells through a two-component signal transduction pathway. Review Figure 26.9
• The ratio of auxin to ethylene controls leaf abscission. Ethylene promotes senescence and fruit ripening. It causes the stems of eudicot seedlings to form a protective apical hook. In stems, it inhibits elongation, promotes lateral swelling, and decreases sensitivity to gravitropic stimulation.
• Brassinosteroids promote cell expansion, pollen tube elongation, and vascular tissue differentiation but inhibit root elongation. Unlike animal steroids, these hormones act at a cell membrane receptor.
• Abscisic acid inhibits seed germination, promotes dormancy, and stimulates stomatal closing in response to dry conditions in the environment.

Concept 26.4: Photoreceptors Initiate Developmental Responses to Light

• Phototropin is a blue-light receptor protein involved in phototropism. Zeaxanthin acts in conjunction with phototropin to mediate the light-induced opening of stomata. Cryptochromes are blue-light receptors that affect seedling development and flowering and inhibit cell expansion. Review Figure 26.10
• Phytochrome is a photoreceptor that exists in the cytosol in two interconvertible isoforms, P_r and P_fr. The relative amounts of these two isoforms are a function of the ratio of red to far-red light. Phytochrome plays a number of roles in photomorphogenesis. Review Figure 26.11
• The phytochrome signal transduction pathway affects transcription in two ways: the P_fr isoform interacts directly with some transcription factors and influences transcription indirectly by phosphorylating other proteins. Review Figure 26.12
• Circadian rhythms are changes that occur on a daily cycle. Light can entrain circadian rhythms through photoreceptors such as phytochrome.