Plant Growth and Development Chapter 31

Overview: The Race to Live Young seedlings must outcompete their neighbors in the race for resources in order to survive Unlike animals, which respond through movement, plants must respond to environmental challenges by altering their growth and development 2

Hormones and Development Chemical signals that modify or control one or more specific physiological processes within a plant Help to coordinate growth, development and responses to environmental stimuli Produced in very low concentrations a minute amount can greatly affect growth and development of a plant organ Synthesized or stored in one part of the plant Travel within phloem, xylem or cell to cell

Plant Hormones Auxins Cytokinins Gibberellins Brassinosteroids Abscisic Acid Ethylene

Auxins Auxin’s effects

Auxins Promote stem lengthening (elongation) Play a role in responses to gravity and light Indoleacetic acid (IAA) is the most common auxin in nature Produced in shoot tips and is transported down the stem Certain synthetic auxins are used as herbicides

Auxins The role of auxin in cell elongation Polar transport of auxin stimulates proton pumps in the plasma membrane Proton pumps lower the pH in the cell wall, activating expansins enzymes that loosen the wall’s fabric with the cellulose loosened, the cell can elongate 7

4 3 H 2 H H H H H H H 1 ATP H 5 CELL WALL 4 Cell wall-loosening enzymes cleave cross-linking polysaccharides. 3 Low pH activates expansins. H2O Cell wall H Plasma membrane 2 Acidity increases. H H H H H H H Figure 31.5 Cell elongation in response to auxin: the acid growth hypothesis 1 Proton pump activity increases. Nucleus Cytoplasm Plasma membrane Vacuole ATP H 5 Sliding cellulose microfibrils allow cell to elongate. CYTOPLASM 8

Plant Tropisms Adjustment of plant growth toward or away from an environmental stimulus Gravitropism Phototropism Thigmotropism

Gravity and statolith distribution Gravitropism Gravity and statolith distribution

Gravitropism Roots tend to grow toward pull of gravity; shoots grow against it Gravitational field is sensed via position of statoliths (a type of amyloplast) Auxin is involved in response; causes asymmetric cell elongation

Gravitropism

Gravitropism

Phototropism Phototropism

Phototropism Change in growth in response to light Controlled by the flow of auxin produced in the plant tip

Phototropism The coleoptile bends after the auxin diffuses from tip to cells on shaded side Rays of sunlight strike one side of a coleoptile

Thigmotropism Growth in response to contact with a solid object Allows vines and tendrils to wrap around supports Cells not on the contact side elongate, causing stem to curl Auxin and ethylene may be involved

Thigmotropism

Cytokinins Promote cell division Work together with auxin to control cell division and differentiation Most abundant in roots shoot meristems maturing fruits In mature plants, produced in roots and transported to shoots

Cytokinins Anti-aging effects inhibits protein breakdown stimulates RNA and protein synthesis Used to artificially extend the shelf life of cut flowers; delays leaf death

Gibberellins In nature, gibberellin: helps seeds and buds break dormancy makes stems lengthen Enhances cell elongation and cell division influences flowering fruit formation Works with auxin Applied by growers to enhance stem length, control ripening

Seed Germination Process by which the plant embryo resumes growth after seed dispersal Depends upon environmental factors temperature soil moisture oxygen levels

Germination Gibberellin acts in the endosperm’s aleurone, a protein-storing layer Transcription of the gene for amylase Sugar monomers released are transported to fuel plant growth

Splitting the Seed Coat Imbibition water molecules move into a seed As water moves in, the seed swells and the coat ruptures

Aleurone Endosperm Water Radicle Scutellum (cotyledon) GA GA -amylase Sugar GA GA Water Figure 31.7 Mobilization of nutrients by gibberellins during the germination of grain seeds such as barley Radicle Scutellum (cotyledon) 26

Plant Development Plant development

Brassinosteroids Chemically similar to cholesterol and sex hormones of animals Induce cell elongation division in stem segments and seedlings They slow leave abscission and promote xylem differentiation

Abscisic Acid (ABA) Causes the suspension of growth Two of the many effects of ABA: Plays a role in drought response ABA accumulation causes stomata to close Promotes dormancy of buds and seeds a predictable period of metabolic inactivity short days; long, cool nights trigger dormancy used to induce dormancy in plants to be shipped

Daylength and dormancy

Breaking Dormancy Seeds and buds respond to environmental cues by resuming growth ABA is removed heavy rains, light, prolong cold

Ethylene Produced in response to stress drought flooding mechanical pressure injury infection The effects of ethylene include response to: mechanical stress senescence leaf abscission fruit ripening

Mechanical Stress Stem elongation and plant growth patterns can be affected by winds or other mechanical stress trees in windswept areas tend to be stubbier shaking inhibited growth of tomato plants The triple response to mechanical stress slowing of stem elongation a thickening of the stem horizontal growth

Senescence Senescence Sum total of processes that lead to death of a plant or some of its parts Factors that influence senescence: decrease in daylight is recurring factor wounds, drought, or nutritional deficiencies can also bring it about

control (pods not removed) experimental plant (pods removed)

Control of Abscission Abscission dropping of flowers, fruits, or leaves What brings it about? auxin production declines cells in abscission zone produce ethylene enzymes digest cell walls that attach leaf or fruit to plant

Abscission

Table 31.1 Overview of plant hormones 38

Signal Transduction Pathway of cell communication in plants 3 steps Reception Transduction Response

cytoplasm of target cell Signal Transduction signal reception signal transduction cellular response Activated receptor activates enzymes and other molecules, which enter reactions that induce or inhibit some cellular activity Hormone binds to and activates cell receptor cell wall membrane receptor cytoplasm of target cell

Biological Clocks Internal timing mechanisms trigger shifts in daily activity help induce seasonal adjustments Phytochrome is part of the switching mechanism pigments that regulate many of a plant’s responses to light throughout its life responses include seed germination and shade avoidance blue-green plant pigment

Phytochrome conversions

Phytochrome Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses Red light triggers the conversion of Pr to Pfr Far-red light triggers the conversion of Pfr to Pr The conversion to Pfr is faster than the conversion to Pr Sunlight increases the ratio of Pfr to Pr and triggers germination 43

Interconversion of Phytochrome Flowering response experiments

Vernalization Low temperature stimulation of flowering Seasonal responses