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 11 essential amino acids- can not be synthesized by the human body

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

PLANT HORMONES 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 (late 1800s) 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- (Frits Went 1926) 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

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

Auxin 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

Hypothesis 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

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

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

Gibberellins Gibberellins affect stem elongation and have numerous other effects Figure 33.5A

Gibberellins Stimulate the development of fruit Function in embryos in some of the early events of seed germination When sprayed at a certain time can produce seedless fruits Figure 33.5B

Abscisic Acid 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

Ethylene 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

CONNECTION 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