Plants

Basic Structures

Apical meristems enable the plant to grow in length. -located in tips of roots and in the buds.

Plants, being rooted to the groundMust respond to whatever

environmental change comes their way

• Plants have cellular receptors– That they use to detect important changes in their

environment• For a stimulus to elicit a response

– Certain cells must have an appropriate receptor

• The potato’s response to light– Is an example of cell-signal processing

Figure 39.3

CELLWALL

CYTOPLASM

1 Reception 2 Transduction 3 Response

Receptor

Relay molecules

Activationof cellularresponses

Hormone orenvironmentalstimulus

Plasma membrane

• Plant hormones help coordinate growth, development, and responses to stimuli

• Hormones– Are chemical signals that coordinate the different

parts of an organism

The Discovery of Plant Hormones

• Any growth response– That results in curvatures of whole plant organs

toward or away from a stimulus is called a tropism– Is often caused by hormones– Shoots growing towards the light is positive

phototropism

• Charles Darwin and his son Francis– Conducted some of the earliest experiments on phototropism, a plant’s

response to light, in the late 19th century

In 1880, Charles Darwin and his son Francis designed an experiment to determine what part of the coleoptile senses light. In 1913, Peter Boysen-Jensen conducted an experiment to determine how the signal for phototropism is transmitted.

EXPERIMENT

RESULTS

Control Darwin and Darwin (1880) Boysen-Jensen (1913)

Light

Shadedside ofcoleoptile

Illuminatedside ofcoleoptile

Light

Tipremoved

Tip coveredby opaquecap

Tipcoveredby trans-parentcap

Base coveredby opaqueshield

Light

Tip separatedby gelatinblock

Tip separatedby mica

In the Darwins’ experiment, a phototropic response occurred only when light could reach the tip of coleoptile. Therefore, they concluded that only the tip senses light. Boysen-Jensen observed that a phototropic response occurred if the tip was separated by a permeable barrier (gelatin) but not if separated by an impermeable solid barrier (a mineral called mica). These results suggested that the signal is a light-activated mobile chemical.

CONCLUSION

• In 1926, Frits Went– Extracted the

chemical messenger for phototropism, auxin, by removing the coleoptile tip & placed it on a block of agar. This allowed the chemical to travel through. Went concluded that a coleoptile curved toward light because its dark

side had a higher concentration of the growth-promoting chemical, which he named auxin.

The coleoptile grew straight if the chemical was distributed evenly. If the chemical was distributed unevenly, the coleoptile curved away from the side with the block, as if growing toward light, even though it was grown in the dark.

Excised tip placedon agar block

Growth-promotingchemical diffusesinto agar block

Agar blockwith chemicalstimulates growth

Control(agar blocklackingchemical)has noeffectControl

Offset blockscause curvature

RESULTS

CONCLUSION

In 1926, Frits Went’s experiment identified how a growth-promoting chemical causes a coleoptile to grow toward light. He placed coleoptiles in the dark and removed their tips, putting some tips on agar blocks that he predicted would absorb the chemical. On a control coleoptile, he placed a block that lacked the chemical. On others,he placed blocks containing the chemical, either centered on top of the coleoptile to distribute the chemical evenly or offset to increase the concentration on one side.

EXPERIMENT

A Survey of Plant Hormones

• In general, hormones control plant growth and development– By affecting the division, elongation, and

differentiation of cells• Plant hormones are produced in very low

concentrations– But a minute amount can have a profound effect on

the growth and development of a plant organ

Auxin– Is used for any chemical substance that promotes

cell elongation in different target tissues

• Auxin transporters– Move the hormone from

shoot tip to base and never in the reverse even if the plant is turned upside down.

• Auxin– Is involved in the formation and branching of roots

Other Effects of Auxin

• Auxin affects secondary growth ms & roots to thicken)– By inducing cell division in the vascular cambium and

influencing differentiation of secondary xylem

• Developing seeds synthesize auxin• tomatoes grown in greenhouse conditions sprayed with auxin

induce fruit development without a need for pollination• This allows for seedless tomatoes

Cytokinins• Cytokinins

– Stimulate cell division

– Are produced in actively growing tissues such as roots, embryos, and fruits

– Work together with auxin

Control of Apical Dominance• Cytokinins, auxin, and other factors interact in

the control of apical dominance– The ability of a terminal bud to suppress

development of axillary buds

Figure 39.9a

Axillary buds

• If the terminal (apical) bud is removed (which is the primary source of auxin– Plants become bushier

• If a plant is sprayed with cytokinins the plant becomes bushier

Figure 39.9b

“Stump” afterremoval ofapical bud

Lateral branches

It is suggested that auxin and cytokinin counter act each other to control axillary bud inhibition

Anti-Aging Effects

• Cytokinins retard the aging of some plant organs– By inhibiting protein breakdown, stimulating RNA

and protein synthesis, and mobilizing nutrients from surrounding tissues

Florists use cytokinins on their cut flowers to keep them fresh.

Gibberellins• Gibberellins have a variety of effects

– Such as stem elongation, fruit growth, and seed germination

• Gibberellins stimulate growth of both leaves and stems

• In stems– Gibberellins stimulate cell elongation and cell

division

The release of gibberellins from the embryo of a seed signals the seed to break dormancy and germinate.

Fruit Growth• In many plants

– Both auxin and gibberellins must be present for fruit to set

• Gibberellins are used commercially– In the spraying of

Thompson seedless grapes making them grow larger.

Abscisic Acid• Two of the many effects of abscisic acid (ABA)

are– Seed dormancy (presence of ABA)

• Seed dormancy has great survival value– Because it ensures that the seed will germinate only when there

are optimal conditions– Prepares it for winter

– 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, infection, fruit ripening and programmed cell death (apoptosis).

The Triple Response to Mechanical Stress• Ethylene induces the triple response

– Which allows a growing shoot to avoid obstacles

Figure 39.13 Ethylene induces the triple response in pea seedlings,with increased ethylene concentration causing increased response.CONCLUSION

Germinating pea seedlings were placed in thedark and exposed to varying ethylene concentrations. Their growthwas compared with a control seedling not treated with ethylene.

EXPERIMENT

All the treated seedlings exhibited the tripleresponse. Response was greater with increased concentration.RESULTS

0.00 0.10 0.20 0.40 0.80

Ethylene concentration (parts per million)

1. Slowing of stem elongation2. Thickening of the stem3. Curvature causing stem to grow horizontally.

Apoptosis: Programmed Cell Death• A burst of ethylene

– Is associated with the programmed destruction of cells, organs, or whole plants

Fruit Ripening• A burst of ethylene production in the fruit

– Triggers the ripening process

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 39.16

0.5 mm

Protective layer Abscission layer

Stem Petiole

In animals, internal & external signals regulate a variety of physiological responses that

synchronize with environmental cycles and cues.

Circadian rhythms

PHOTOPERIODISM

From the results above these plants can be more appropriately called “long night plants” and “short night plants”. Photoperiodism is controlled by the length of the night= critical night length

It is the leaves that give the cue that the night is long enough or short enough and signals the buds to flower.

Photoperiodism

• Day-neutral plants: light has no effect on their growth

Tomatoes

Rice

Dandelions

Photoperiodism

• Vernalization:– Some plants (winter wheat) need to be exposed to

cold temperatures for several weeks

Plants respond to a wide variety of stimuli other than light

Gravitropism

• Auxin plays a key role

Underground how does a seedling know which way to grow?

Thigmomorphogenesis

• Plants growing on a windy range– Have thicker trunks

• Touching leaves can alter plant growth– Climbing vines