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 ground Must 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 CELL WALL CYTOPLASM   1 Reception 2 Transduction 3 Response Receptor Relay molecules Activation of cellular responses Hormone or environmental stimulus Plasma membrane

Plant hormones help coordinate growth, development, and responses to stimuli 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 Shaded side of coleoptile Illuminated Tip removed Tip covered by opaque cap covered by trans- parent cap Base covered by opaque shield Tip separated by gelatin block by mica CONCLUSION 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.

In 1926, Frits Went 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 placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Control (agar block lacking chemical) has no effect Control Offset blocks cause 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 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.

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 Axillary buds Figure 39.9a

It is suggested that auxin and cytokinin counter act each other to control axillary bud inhibition 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 “Stump” after removal of apical bud Lateral branches Figure 39.9b

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 Gibberellins are used commercially 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 Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response. CONCLUSION Germinating pea seedlings were placed in the dark and exposed to varying ethylene concentrations. Their growth was compared with a control seedling not treated with ethylene. EXPERIMENT All the treated seedlings exhibited the triple response. 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 elongation 2. Thickening of the stem 3. Curvature causing stem to grow horizontally. Figure 39.13

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 0.5 mm Protective layer Abscission layer Stem Petiole Figure 39.16

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