Control Systems in Plants Chapter 33 Control Systems in Plants

What Are the Health Benefits of Soy? Soy protein Is one of the few plant proteins that contains all the essential amino acids

Phytoestrogens, a class of plant hormones Are found in soy CH3 OH HO O Estrogen (Estradiol) Phytoestrogen (Genistein) Chemical structures of a human estrogen and a plant phytoestrogen

Soy products contain isoflavones A type of phytoestrogen that may provide human health benefits Soybeans

PLANT HORMONES 33.1 Experiments on how plants turn toward light led to the discovery of a plant hormone Plants exhibit phototropism The growth of shoots in response to light Figure 33.1A

Microscopic observations of plants Indicate that a cellular mechanism underlies phototropism Shaded side of shoot Illuminated side of shoot Light Figure 33.1B

Showing That Light Is Detected by the Shoot Tip Charles Darwin showed that the tip of a grass seedling detects light And transmits a signal down to the growing region of a shoot Light Control Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block by mica Darwin and Darwin (1880) Boysen-Jensen (1913) Figure 33.1C

Isolating the Chemical Signal The hormone auxin Was determined to affect phototropism Promotes faster cell elongation on the shaded site of the shoot Agar Shoot tip placed on agar block. Chemical (later called auxin) diffuses from shoot tip into agar. Other controls: Blocks with no chemical have no effect. Offset blocks with chemical stimulate curved growth. Control Block with chemical stimulates growth. No light Figure 33.1D

33.2 Five major types of hormones regulate plant growth and development Even in small amounts, plant hormones Trigger signal transduction pathways Regulate plant growth and development

The major types of plant hormones Table 33.2

33.3 Auxin stimulates the elongation of cells in young shoots Plants produce auxin (IAA) In the apical meristems at the tips of shoots

At different concentrations, auxin Stimulates or inhibits the elongation of shoots and roots Roots Stems 0.9 g/L   10–8 10–6 10–4 10–2 1 102 Increasing auxin concentration (g/L) Inhibition Promotion Elongation Figure 33.3A, B

Auxin may act by weakening cell walls Allowing them to stretch when cells take up water 3 H2O Cell wall 1 Plasma membrane Cellulose molecule Cell wall H+ H+ 2 Cell elongation H+ pump (protein) Vacuole Enzyme Cytoplasm Cellulose loosens; cell can elongate Cellulose molecule Cross-linking molecule Figure 33.3C

Auxin promotes growth in stem diameter By stimulating the development of vascular tissues and cell division in vascular cambium

33.4 Cytokinins stimulated cell division Are produced by growing roots, embryos, and fruits Promote cell division

Causing lower buds to develop into branches Cytokinins from roots may balance the effects of auxin from apical meristems Causing lower buds to develop into branches Terminal bud No terminal bud Figure 33.4

33.5 Gibberellins affect stem elongation and have numerous other effects Stimulate the elongation of stems Figure 33.5A

Stimulate the development of fruit Gibberellins Stimulate the development of fruit Function in embryos in some of the early events of seed germination Figure 33.5B

33.6 Abscisic acid inhibits many plant processes Abscisic acid (ABA) Inhibits the germination of seeds The ratio of ABA to gibberellins Often determines whether a seed will remain dormant or germinate

Seeds of many plants remain dormant Until their ABA is inactivated or washed away Figure 33.6

ABA also acts as a “stress hormone” Causing stomata to close when a plant is dehydrated

33.7 Ethylene triggers fruit ripening and other aging processes As fruit cells age They give off ethylene, which triggers a variety of aging processes

Fruit Ripening Ethylene Triggers fruit ripening 1 2 3 Figure 33.7A

The Falling of Leaves A changing ratio of auxin to ethylene Is triggered by shorter days Probably causes autumn color changes and the loss of leaves from deciduous trees Leaf stalk Stem (twig) Abscission layer Protective Leaf stalk LM 20 Figure 33.7B

33.8 Plant hormones have many agricultural uses CONNECTION 33.8 Plant hormones have many agricultural uses Farmers use auxin To delay or promote fruit drop Figure 33.8

Auxins and gibberellins Are used to produce seedless fruits A synthetic auxin called 2,4-D Is used to kill weeds Has safety questions associated with its use

GROWTH RESPONSES AND BIOLOGICAL RHYTHMS IN PLANTS 33.9 Tropisms orient plant growth toward or away from environmental stimuli Plants sense and respond to environmental changes In a variety of ways

Tropisms are growth responses That change the shape of a plant or make it grow toward or away from a stimulus

Response to Light Phototropism, bending in response to light May result from auxin moving from the light side to the dark side of a stem

Response to Gravity A response to gravity, or gravitropism May be caused by the settling of special organelles on the low sides of shoots and roots This settling of organelles May trigger a change in the distribution of hormones Figure 33.9A

Response to Touch Thigmotropism, a response to touch Is responsible for the coiling of tendrils and vines around objects Figure 33.9B

33.10 Plants have internal clocks An internal biological clock Controls sleep movements and other daily cycles in plants

Innate Biological Rhythms and Their Fine-Tuning by Environmental Cues Circadian rhythms Persist with periods of about 24 hours even in the absence of environmental cues Need environmental cues to keep them synchronized with day and night Noon Midnight Figure 33.10

The Nature of Biological Clocks In plants, biological clocks May depend on the synthesis of a protein that regulates its own production through feedback control

33.11 Plants mark the seasons by measuring photoperiod Is the relative lengths of night and day The timing of flowering Is one of the seasonal responses to photoperiod

Short-day (long-night) plants Long-day (short-night) plants Plants whose flowering is triggered by photoperiod fall into two groups Short-day (long-night) plants Long-day (short-night) plants Short-day (long-night) plants Long-day (short-night) plants Darkness Flash of light Light Time (hr) Critical night length 24 Figure 33.11

33.12 Phytochrome is a light detector that may help set the biological clock Phytochromes are proteins with a light-absorbing component That may help plants set their biological clock and monitor photoperiod

Phytochromes were discovered During studies on how different wavelengths of light affect seed germination 1 2 3 4 Long-day (short-night) plant Critical night length R FR Short-day (long-night) plant 24 20 16 12 8 Time (hr) Figure 33.12A

Phytochrome conversion in daylight and darkness Affects the biological clock in plants Red light Far-red Rapid conversion in daylight Slow conversion in darkness Pr Pfr Figure 33.12B

TALKING ABOUT SCIENCE 33.13 Joanne Chory studies the effects of light and hormones in the model plant Arabidopsis A small, wild mustard called Arabidopsis Is a popular model organism for plant molecular biologists Figure 33.13

PLANT DEFENSES 33.14 Defenses against herbivores and infectious microbes have evolved in plants Plants use chemicals To defend themselves from both herbivores and pathogens

Defenses Against Herbivores Some plants recruit predatory animals To help defend them against certain herbivores 4 Recruitment of wasp 5 Wasp lays eggs Adapted from Edward Farmer, “Plant Biology: New Fatty Acid–Based Signals: A Lesson from the Plant World” Science 276 (1997), p. 912. 1997 American Association for the Advancement of Science. 3 Synthesis and release of chemical attractants 1 Damage to plant and chemical in caterpillar saliva Plant cell 2 Signal transduction pathway Figure 33.14A

Defenses Against Pathogens So-called avirulent pathogens interact with host plants in a specific way That stimulates both local and systemic defenses in the plant 3 Enhanced local response 5 Signal transduction pathway 1 Binding of pathogen’s signal molecule to plant’s receptor molecule 6 Additional defensive chemicals 4 Hormones Avirulent pathogen 2 Signal transduction pathway Systemic acquired resistance R-Avr recognition leading to a strong local response Figure 33.14B

Local defenses include Microbe-killing chemicals and sealing off the infected area Hormones Trigger generalized defense responses in other organs (systemic acquired resistance)

TALKING ABOUT SCIENCE 33.15 Plant biochemist Eloy Rodriguez studies how animals use defensive chemicals made by plants Some animals may medicate themselves By eating plants containing certain defensive chemicals