Plants Basic Structures Apical meristems enable the plant

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

Plant Hormones • Auxin • Cytokins • Gibberelins • Abscisic Acid • Ethylene • Plant hormones help coordinate growth, development, and responses to stimuli • Hormones – Are chemical signals that coordinate the different parts of an organism

• 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 Discovery of Plant Hormones • Any growth response – Is often caused by hormones • EX: results in curvatures of whole plant organs toward or away from a stimulus =tropism – Shoots growing towards the light is positive phototropism – Shoots growing up is negative geotropism – Roots growing down is positive geotropism

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 – Inhibiting lateral bud growth

Other Effects of Auxin • Auxin affects secondary growth – 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

• Charles Darwin and his son Francis – Conducted some of the earliest experiments on phototropism, a plant’s response to light, in the late 19 th century EXPERIMENT 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. RESULTS Control Boysen-Jensen (1913) Darwin and Darwin (1880) Shaded side of coleoptile Light Illuminated side of coleoptile CONCLUSION Tip removed Tip covered by opaque cap Tip covered by transparent cap Base covered by opaque shield Tip separated by gelatin block Tip separated by 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.

• 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. EXPERIMENT 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. RESULTS 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 Control (agar block lacking chemical) has no effect Agar block with chemical stimulates growth Offset blocks cause curvature CONCLUSION 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.

Cytokinins • Cytokinins (like cytokinesis) – Stimulate cell division – Are produced in actively growing tissues such as roots, embryos, and fruits – Makes the plant branch out.

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. 9 a

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 • Control the plant’s yearly cycle – Tells plant when to go dormant, when to flower… • 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 (opposite of gibberelins) • In the fall, when leaves change color. ABA basically cuts the leaves off the tree. • Suppresses fruit formation • Stops transpiration • Basically causes plants to go into dormancy – Preparing for winter – Drought tolerance

Ethylene • It’s a gas and travels outside the plant • Is an example of positive feedback loop

The Triple Response to Mechanical Stress • Ethylene induces the triple response – Which allows a growing shoot to avoid obstacles 1. Slowing of stem elongation 2. Thickening of the stem 3. Curvature causing stem to grow horizontally. EXPERIMENT 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. RESULTS All the treated seedlings exhibited the triple response. Response was greater with increased concentration. 0. 00 0. 10 0. 20 0. 40 0. 80 Ethylene concentration (parts per million) Figure 39. 13 CONCLUSION Ethylene induces the triple response in pea seedlings, with increased ethylene concentration causing increased response.

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

In animals, internal & external signals regulate a variety of physiological responses that synchronize with environmental cycles and cues.

Circadian rhythms Resets every day; is influence by internal & external signals such as light and dark; meal times; stress; exercise…

Plants are influenced by external factors as well. Such as night length. This is called photoperiodism. Phytochrome is an important macromolecule.

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.

During the day, PR is converted to PFR During the night the PFR is converted back to PR PR PFR IN SHORT DAY PLANTS PR triggers flowering and PFR inhibits flowering So the long nights give the plant plenty of time to get rid of the PFR that built up during the day. IN LONG DAY PLANTS PR inhibits flowering and PFR triggers flowering So the long days give the plant plenty of time to build up the PFR & the nights are too short to get rid of all the PFR