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 imbibes water, germinates, and develops into a seedling. Review Figure 26.1 and WEB 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 WEB 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 plasma 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 are adenine derivatives that 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
   
• A balance between auxin and 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 elongation, pollen tube elongation, and vascular tissue differentiation but inhibit root elongation. Unlike animal steroids, these hormones act at a plasma 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 elongation. 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 Pfr 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.