37 regulation of plant growth. 37: regulation of plant growth: read ib hormones only!! 37.1 how does...

37Regulation of Plant Growth

37: Regulation of Plant Growth: Read IB Hormones ONLY!!

• 37.1 How Does Plant Development Proceed?

• 37.2 What Do Gibberellins Do?

• 37.3 What Does Auxin Do?

• 37.4 What Do Cytokinins, Ethylene, Abscisic Acid, and Brassinosteroids Do?

• 37.5 How Do Photoreceptors Participate in Plant Growth Regulation?

37.1 How Does Plant Development Proceed?

Plant development is regulated in complex ways. Four factors regulate growth:

• Environmental cues

• Receptors such as photoreceptors

• Hormones

• The plant’s genome


Hormones: regulatory chemicals that act at low concentrations at sites often quite distant from where they were produced.

Each plant hormone is produced in many cells, and has multiple roles. Interactions can be complex.

Table 37.1 Plant Growth Hormones


Photoreceptors are involved in many developmental processes.

They are pigments (molecules that absorb light) associated with proteins.

Light acts directly on photoreceptors, which regulate processes of development.


Plants make use of signal transduction pathways—sequences of biochemical reactions by which a cell responds to a stimulus.

Protein kinase cascades amplify responses to signals just as they do in other organisms.


Seeds are dormant—the cells do not divide, expand, or differentiate.

As the seed begins to germinate, it takes up water. The growing embryo obtains chemical building blocks by digesting the food stored in the seed.

Germination is completed when the radicle (embryonic root) emerges. Now called a seedling.


If the seedling germinates underground, it must elongate rapidly, and cope with darkness for a time.

A series of photoreceptors directs this stage of development.

Early seedling development varies in monocots and eudicots.

Figure 37.1 Patterns of Early Shoot Development (Part 1)

Photo 37.2 Nyssa aquatica growing in a millpond.

Acorn germination & growth


Formation of flowers may be initiated when plant reaches a certain size or age.

Some plants flower at certain times of the year; plant must be able to distinguish seasons.

Light absorption by photoreceptors is the first step in measuring time. Hormone signals then trigger flowering.

Flower & Fruit Formation


Hormones also control growth of pollen tube, fertilization, and fruit and seed development.


Some plants are perennials; they continue to grow year after year.

Annuals complete life cycle in a single year, then senesce (deteriorate due to aging) and die.

Senescence is controlled by hormones such as ethylene.


In some perennials, leaves senesce and fall at the end of the growing season.

Leaf fall is regulated by interplay of ethylene and auxin.


Seed dormancy may last weeks, months, or years. Mechanisms that maintain dormancy include:

• Exclusion of water or oxygen by impermeable seed coat.

• Mechanical restraint of embryo by tough seed coat.

• Chemical inhibition of embryo development.


Seed dormancy must be broken for germination to begin.

Seed coats may be abraded by physical processes, or in the digestive tract of an animal.

Soil microorganisms may soften seed coats.

Photo 37.3 A developing runner bean. Seeds of Phaseolus collineus, showing seed coats.


Fire ends dormancy for many seeds.

Leaching of chemical inhibitors by water can also end dormancy.

Figure 37.2 Fire and Seed Germination

Photo 37.1 Fireweed (Arnica sp.), growing after a fire that fostered seed germination.


Advantages of seed dormancy:

• Survival through unfavorable conditions.

• Prevent germination while still attached to parent plant.

• Seeds that must be scorched by fire avoid competition by germinating only in fire-scarred areas.

• Long-distance dispersal of seeds.


Dormancy of some seeds is broken by exposure to light.

They germinate at or near soil surface; are tiny—small food reserves. They would not survive if they germinated deep in the ground.

Other seeds germinate only when buried deeply, and in darkness. These are large seeds with large food reserves.


Annuals must cope with year to year variation in rainfall and other factors. Seeds may remain dormant in unfavorable years.

Other seeds germinate at specific times of year to ensure correct conditions.

Dormancy can also ensure that seeds only germinate in specific environments.

Figure 37.3 Leaching of Germination Inhibitors


Imbibition, or uptake of water, is the first step in seed germination.

A seed’s water potential is very negative; water will enter if the seed coat is permeable. Expanding seeds exert tremendous force.

Enzymes are activated with hydration, RNA and proteins are synthesized and respiration increases. Initial growth is by expansion of pre-formed cells.


Until the seedling can photosynthesize, it depends on food reserves in the cotyledons or endosperm.

Starch, lipids, and proteins must be broken down by enzymes into monomers that can enter the embryo’s cells.

37.2 What Do Gibberellins Do?

Gibberellins are a class of plant hormone.

In germinating cereal seeds, gibberellins diffuse through the endosperm to surrounding tissue called the aleurone layer underneath the seed coat.

Gibberellins trigger a cascade in this layer, causing it to secrete enzymes to digest the endosperm.

Figure 37.4 Embryos Mobilize Their Reserves

Photo 37.4 Dissected seed showing ungerminated embryo attached to cotyledon.

Photo 37.5 Germinating seeds; primary and secondary roots and root hairs.

Figure 37.5 The Effect of Gibberellins on Dwarf Plants


Hypothesis: the dwarf plants do not produce gibberellins, but the wild types do.

Dwarf plants have normal roots, leaves, and flowers, but stems are much shorter.

Gibberellins are required for normal stem elongation.


Gibberellins also affect fruit growth.

Seedless grape varieties have smaller fruit than seeded varieties. Experimental removal of seeds from maturing fruit resulted in small fruits, suggesting seeds were the source of a growth regulator.

Spraying young seedless grapes with gibberellins caused them to grow as large as seeded varieties.


Gibberellins also cause fruit to grow from unfertilized flowers, promote seed germination, and help bring spring buds out of winter dormancy.

37.3 What Does Auxin Do?

Auxins are a group of plant hormones; the most important is indoleacetic acid (IAA).

Discovery of auxin can be traced to Charles Darwin and his son Francis, who were studying plant movements.

Phototropism is the growth of plant organs towards light (or away from light, as roots do).


The Darwins worked with canary grass.

Young grass seedlings have a coleoptile—a sheath that protects it as it pushes through the soil. Coleoptiles are phototropic.

If the coleoptile tip was covered, there was no phototropic response. A signal travels from the tip to the growing region.

Figure 37.7 The Darwins’ Phototropism Experiment (Part 1)

Figure 37.7 The Darwins’ Phototropism Experiment (Part 2)


In the 1920s, Fritz Went removed coleoptile tips and placed the cut surfaces on agar.

When the agar was placed on the cut plants, they showed the phototropic response.

A hormone had diffused into the agar block. It was IAA.

Figure 37.8 Went’s Experiment


Movement of auxin through certain plants is polar—unidirectional from apex to base, but it is not due to gravity.

Transport depends on the location of auxin anion efflux carriers, membrane proteins that are confined to the basal ends of cells.

In the cytoplasm at neutral pH, auxins have a negative charge (anions).

Figure 37.9 Polar Transport of Auxin


Proton pumps in the plasma membrane pump H+ out, making the cell walls acidic.

As anions, auxins can only leave the cell at the base by efflux carriers.

A gradient of auxin is established, so auxin acts as a morphogen, instructing cells as to their orientation.


Polar auxin transport establishes orientation of growth.

Lateral redistribution of auxin causes plant movements. Carrier proteins move to sides of cell rather than base.

When light strikes a coleoptile on one side, auxin moves laterally to the other side, cell growth increases on that side.

Figure 37.10 Plants Respond to Light and Gravity


If a shoot is tipped over, even in the dark, auxin will move to the lower side—cell growth results in bending of the shoot so that it grows up—gravitropism.

The upward gravitropic response of shoots is negative gravitropism; the downward response of roots is positive gravitropism.

Photo 37.9 Phototropism: Plants grow toward light.

Photo 37.10 Corn seedling; negative gravitropism of coleoptile and positive gravitropism of root.

Photo 37.6 Germination of Phaseolus collineus in soil.

Gravitropism in a root

Vine Thigmotropism

Thigmotropism of Mimosa


Cuttings from shoots of some plants can form roots and develop into a new plant.

Undifferentiated cells in the stem must become organized into a meristem for a new root. This can be stimulated by dipping the shoot into an auxin solution.


Abscission is the detachment of old leaves from the stem.

Auxin inhibits abscission, which results from the breakdown of cells in the abscission zone of the petiole.

Timing of leaf fall is determined in part by a decrease in the movement of auxin from the blade through the petiole.

Figure 37.11 Changes Occur when a Leaf Is About to Fall


Apical dominance: apical buds inhibit the growth of axillary buds—results in one main stem and little branching.

Apical buds are a major site of auxin production. Removal of apical bud results in growth of axillary buds.

Figure 37.12 Auxin and Apical Dominance


Auxin promotes stem elongation but inhibits elongation of roots.

Why the different organs respond differently is unknown, but is the subject of current research.


Expansion of cells results in plant growth; cell walls play key roles in controlling rate and direction of growth.

Auxin acts on cell walls.

Expansion of plant cells is driven primarily by water uptake. The central vacuole presses the cytoplasm against the cell wall, and the wall resists this force.

Figure 37.13 Cellulose in the Cell Wall

Figure 37.14 Plant Cells Expand

Figure 37.15 Auxin Acts on Cell Walls

Figure 37.16 Similar Signal Transduction Pathways Are Used by Auxin and Gibberellins

37.4 What Do Cytokinins, Ethylene, Abscisic Acid, and Brassinosteroids Do?

The gas ethylene is produced by all parts of a plant; promotes senescence.

Ethylene promotes leaf abscission, and speeds ripening of fruit.

Ethylene also causes an increase in its own production. Once ripening begins, more and more ethylene is produced.


Commercial fruit growers use ethylene gas to hasten fruit ripening.

Ripening can be delayed by using “scrubbers” to remove ethylene gas from storage chambers.

Cut flowers are sometimes put into silver thiosulfate solution to inhibit ethylene (probably by combining with ethylene receptors).

Figure 37.18 The Signal Transduction Pathway for Ethylene


Abscisic acid also has multiple effects.

During seed formation, it allows expression of genes that encode for storage proteins.

Initiates and maintains dormancy in seeds.

Inhibits stem elongation.


Called the stress hormone —accumulates when plants are deprived of water.

May play a role in maintaining bud dormancy in winter.


Abscisic acid also affects the guard cells of stomata; causes stomata to close.

In response to abscisic acid,

guard cells open Ca channels;

increase in Ca concentration in cell leads to opening of K channels,

loss of K+ and water,

the cells sag together and close the stomata.

37.5 How Do Photoreceptors Participate in Plant Growth Regulation?

Plants use night length as an accurate environmental cue to time several aspects of growth and development.

Other environmental cues include presence or absence of light, light intensity, and wavelength.

Several photoreceptors are involved in plant responses to light.


Five phytochromes (photoreceptor proteins) mediate effects of red and dim blue light.

Three or more blue-light receptors mediate effects of high intensity blue light.


Blue and red light promote seed germination in some species.

Far-red light reverses the effects of a previous exposure to red light.

If exposed to alternating periods of red and far-red light, lettuce seeds respond only to the final exposure.

Figure 37.20 Sensitivity of Seeds to Red and Far-Red Light


Two interconvertible forms of phytochromes exist in the cytosol.

One form absorbs red light (Pr) and is converted to the form that absorbs far-red (Pfr).


When seedlings germinate in the dark in the soil, they are pale and spindly with undeveloped leaves—etiolated.

Etiolated plants do not form chlorophyll, but use food reserves to elongate quickly to grow out of soil to the light.

Eudicots also form the apical hook.


The etiolation-associated phenomena are regulated by phytochromes.

In darkness, all the phytochrome is Pr, exposure to light converts it to Pfr, which initiates reversal of etiolation;

leaves expand, hook unfolds, photosynthesis starts—photomorphogenesis.


Different phytochromes have differing roles during plant development.

A seedling growing in shade receives much more far-red than red light. Interplay among the signal transduction pathways leads to stem elongation to get leaves to a higher light intensity.

37 regulation of plant growth. 37: regulation of plant growth: read ib hormones only!! 37.1 how does...

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