植物激素

植物生理

植物荷爾蒙

李澤民

海洋植物研究室

A Survey of Plant Hormones

 The major plant hormones include

 Auxin  Cytokinins  Gibberellins  Abscisic acid  Ethylene  Brassinosteroids  Jasmonates  Strigolactones

Auxin

 Any response resulting in curvature of organs toward or away from a stimulus is called a

tropism

 In the late 1800s, Charles Darwin and his son

Francis conducted experiments on phototropism, a plant’s response to light

 They observed that a grass seedling could bend toward light only if the tip of the coleoptile was present

 They postulated that a signal was transmitted from the tip to the elongating region

 In 1913, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance

Figure 39.5 Control Light Shaded side Illuminated side Boysen-Jensen Light Light

Darwin and Darwin

Results Tip removed Opaque cap Trans-parent cap Opaque shield over curvature Gelatin (permeable) Mica (impermeable)

Figure 39.5a Control Light Shaded side Illuminated side

Figure 39.5b

Light

Darwin and Darwin

Tip removed Opaque cap Trans-parent cap Opaque shield over curvature

Figure 39.5c Boysen-Jensen Light Gelatin (permeable) Mica (impermeable)

 The term auxin refers to any chemical that promotes elongation of coleoptiles

 Indoleacetic acid (IAA) is a common auxin in plants; in this lecture the term auxin refers specifically to IAA

 Auxin is produced in shoot tips and is transported down the stem

 Auxin transporter proteins move the hormone from the basal end of one cell into the apical end of the neighboring cell

Figure 39.6 Cell 1 Cell 2 Basal end of cell Results 100 µm Epidermis Cortex Phloem Xylem Pith 25 µm

Figure 39.6a 100 µm Epidermis Cortex Phloem Xylem Pith

Figure 39.6b Cell 1 Cell 2 Basal end of cell 25 µm

The Role of Auxin in Cell Elongation

 According to the acid growth hypothesis, auxin

stimulates proton pumps in the plasma membrane

 The proton pumps lower the pH in the cell wall, activating expansins, enzymes that loosen the wall’s fabric

Figure 39.7 CELL WALL CYTOPLASM Plasma membrane H+ H+ H+ H+ H+ H+ H+ H+ H+ ATP Low pH activates expansins. Acidity increases. Proton pump activity increases. Cell wall-loosening enzymes cleave cross-linking polysaccharides. Plasma membrane Cell wall H₂O Nucleus Cytoplasm Vacuole Sliding cellulose microfibrils allow cell to elongate. 1 2 3 4 5

Figure 39.7a Low pH activates expansins. Acidity increases. Proton pump activity increases. 1 2 3 4 CELL WALL CYTOPLASM Plasma membrane

Cell wall-loosening enzymes cleave cross-linking polysaccharides. H+ H+ H+ H+ H+ H+ H+ H+ H+ ATP

Figure 39.7b Plasma membrane Cell wall H₂O Nucleus Cytoplasm Vacuole

Sliding cellulose microfibrils allow cell to elongate.

 Auxin also alters gene expression and stimulates a sustained growth response

Auxin’s Role in Plant Development

 Polar transport of auxin plays a role in pattern formation of the developing plant

 Reduced auxin flow from the shoot of a branch stimulates growth in lower branches

 Auxin transport plays a role in phyllotaxy, the arrangement of leaves on the stem

 Polar transport of auxin from leaf margins directs leaf venation pattern

 The activity of the vascular cambium is under control of auxin transport

Practical Uses for Auxins

 The auxin indolbutyric acid (IBA) stimulates adventitious roots and is used in vegetative propagation of plants by cuttings

 An overdose of synthetic auxins can kill plants

 For example 2,4-D is used as an herbicide on eudicots

Cytokinins

 Cytokinins are so named because they stimulate cytokinesis (cell division)

Control of Cell Division and Differentiation

 Cytokinins are produced in actively growing tissues such as roots, embryos, and fruits

 Cytokinins work together with auxin to control cell division and differentiation

Control of Apical Dominance

 Cytokinins, auxin, and strigolactone interact in the control of apical dominance, a terminal bud’s

ability to suppress development of axillary buds

 If the terminal bud is removed, plants become bushier

Figure 39.8

(a) Apical bud intact (not shown in photo)

(b) Apical bud removed

(c) Auxin added to decapitated stem

Lateral branches

“Stump” after removal of apical bud

Figure 39.8a

Figure 39.8b

(b) Apical bud removed

Lateral branches

“Stump” after removal of apical bud

Figure 39.8c

Anti-Aging Effects

 Cytokinins slow the aging of some plant organs by inhibiting protein breakdown, stimulating RNA and protein synthesis, and mobilizing nutrients from

Gibberellins

 Gibberellins have a variety of effects, such as stem elongation, fruit growth, and seed

Stem Elongation

 Gibberellins are produced in young roots and leaves

 Gibberellins stimulate growth of leaves and stems

 In stems, they stimulate cell elongation and cell division

Figure 39.9

(a) Rosette form (left) and

gibberellin-induced bolting (right)

(b) Grapes from control vine (left) and gibberellin-treated vine (right)

Figure 39.9a

(a) Rosette form (left) and

gibberellin-induced bolting (right)

Figure 39.9b

(b) Grapes from control vine (left) and gibberellin-treated vine (right)

Fruit Growth

 In many plants, both auxin and gibberellins must be present for fruit to develop

 Gibberellins are used in spraying of Thompson seedless grapes

Germination

 After water is imbibed, release of gibberellins from the embryo signals seeds to germinate

α-amylase Figure 39.10 Aleurone Endosperm Water Scutellum (cotyledon) Radicle 1 2 3 GA GA α-amylase Sugar

Abscisic Acid

 Abscisic acid (ABA) slows growth

 Two of the many effects of ABA

 Seed dormancy

Abscisic acid

Seed Dormancy

 Seed dormancy ensures that the seed will germinate only in optimal conditions

 In some seeds, dormancy is broken when ABA is removed by heavy rain, light, or prolonged cold

 Precocious (early) germination can be caused by inactive or low levels of ABA

Figure 39.11 Red mangrove (Rhizophora mangle) seeds Maize mutant Coleoptile

Figure 39.11a

Red mangrove

(Rhizophora mangle) seeds

Figure 39.11b

Maize mutant

Drought Tolerance

 ABA is the primary internal signal that enables plants to withstand drought

Ethylene

 Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection

 The effects of ethylene include response to

mechanical stress, senescence, leaf abscission, and fruit ripening

The Triple Response to Mechanical Stress

 Ethylene induces the triple response, which allows a growing shoot to avoid obstacles

 The triple response consists of a slowing of stem elongation, a thickening of the stem, and

Figure 39.12

0.00 0.10 0.20 0.40 0.80

 Ethylene-insensitive mutants fail to undergo the triple response after exposure to ethylene

 Other mutants undergo the triple response in air but do not respond to inhibitors of ethylene

Rice coleoptile elongation – anoxia and

ethylene

Figure 39.13

ein mutant

ctr mutant

(b) ctr mutant (a) ein mutant

Figure 39.13a

ein mutant

Figure 39.13b

ctr mutant

Senescence

 Senescence is the programmed death of cells or organs

 A burst of ethylene is associated with apoptosis, the programmed destruction of cells, organs, or whole plants

Leaf Abscission

 A change in the balance of auxin and ethylene

controls leaf abscission, the process that occurs in autumn when a leaf falls

Figure 39.14

Stem Petiole

Protective layer Abscission layer 0.5 mm

Figure 39.14a

Stem Petiole

Protective layer Abscission layer 0.5 mm

Fruit Ripening

 A burst of ethylene production in a fruit triggers the ripening process

 Ethylene triggers ripening, and ripening triggers release of more ethylene

 Fruit producers can control ripening by picking green fruit and controlling ethylene levels

More Recently Discovered Plant Hormones

 Brassinosteroids are chemically similar to the sex hormones of animals

 They induce cell elongation and division in stem segments

 They slow leaf abscission and promote xylem differentiation

 Jasmonates, including jasmonate (JA) and methyl jasmonate (MeJA) play important roles in plant

defense and development

 They are produced in response to wounding and involved in controlling plant defenses

 Jasmonates also regulate many other physiological processes, including  Nectar secretion  Fruit ripening  Pollen production  Flowering time  Seed germination  Root growth  Tuber formation  Mycorrhizal symbiosis  Tendril coiling

 Strigolactones are xylem-mobile chemicals that

 Stimulate seed germination

 Suppress adventitious root formation

 Help establish mycorrhizal associations

 Help control apical dominance

 Strigolactones are named for parasitic Striga plants

 Striga seeds germinate when host plants exude