Biology Main points/Questions 1.How can plants respond to stimuli? 2.What stimuli will they respond to? 3.What are some of the chemicals that they use to communicate?

A season of change at a Rhode Island Stream...

Signal pathways link signals to response Plants have cellular receptors that detect changes in their environment

An example in potatoes A potato left growing in darkness produces shoots that look unhealthy and lacks elongated roots

This helps the potato grow out of the soil into the light, called etiolation But after exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally

(a) Before exposure to light (b) After a week’s exposure to natural daylight

A potato’s response to light is an example of cell-signal processing Stimulus detected (increased light exposure) A signal is sent out and then… Some response occurs – in this case changes in growth patterns and greening Response needs to be coordinated across entire organism

Plant hormones Hormones are chemical signals that coordinate different parts of an organism In plants many different hormones coordinate a plants response to its environment

This plant is growing in a window and is exhibiting a growth pattern called positive phototropism! Light!

The Discovery of Plant Hormones Any response resulting in curvature of organs toward or away from a stimulus is called a tropism Early experiments in tropisms led to the discovery of the first plant hormones

In the late 1800s, Charles Darwin and his son Francis conducted experiments on phototropism, a plant’s response to light They observed that a growing grass seedling could bend toward light

Phototropism – growth in response to light

Opaque cap over tip. How could you tell where is light sensed? Darwin and his son discovered that you could block phototropism if you covered the tip of the plant

Opaque sleeve over bending region. Clear cap over tip. They performed this control experiment - Why do you think they did this? (what does it tell you?)

Porous gelatin placed between tip and shoot. Other biologists showed that the signal could pass through gelatin from the tip to the lower part of the plant.

Porous gelatin placed between tip and shoot. Impenetrable barrier between tip and shoot. Light But that the signal didn’t pass through an impenetrable barrier

Porous gelatin placed between tip and shoot. Impenetrable barrier between tip and shoot. What does this tell you? Light

Porous gelatin placed between tip and shoot. Impenetrable barrier between tip and shoot. Scientists decided this meant there was a chemical signal that diffused through the gelatin! Light

Later, in the 1920’s, a biologist used a similar experiment to investigate the chemical signal He removed the tip of several growing plants then placed them on agar (a substance a little like gelatin)

Tips placed on agar.

After some time he placed these agar blocks, now infused with the signal, on the plants that had had their tips removed What do you think will happen to these plants? Why?

Figure How Went demonstrated the effects of auxin on plant growth Agar without treatment has no effect on plants but agar that has been under tips…

Figure How Went demonstrated the effects of auxin on plant growth Agar that has been treated causes cells to elongate

Figure How Went demonstrated the effects of auxin on plant growth If these treated agar blocks are placed on the edge the shoot curves just like in phototropism!

A chemical signal, Auxin, is produced in the tip of the growing shoot. This signal causes cells below the tip to elongate If there is more light on one side the auxin moves to the shaded side of the stem How phototropism works

Figure Auxin causes cells to elongate Excess auxin on the shaded side causes the curving response

Other Plant Hormones While auxin was the first hormone discovered there are many other plant hormones including: Gibberellins are synthesized in the apical portions of roots and shoots and affect stem elongation

Figure The effect of a gibberellin The plant on the right was treated with giberellin the one on the left was not

Other Plant Hormones Gibberellins Cytokinins stimulate cell division in plants and help determine the course of differentiation Cytokinins work with Auxin to determine what cells differentiate into

Fig a What are Axillary buds? What keeps them dormant (not growing)? Apical dominance! Auxin from the structures above – so remove the structures…

Fig b The axillary buds grow! Only with cytokinin around though…

Fig b This shoot development occurs because there is excess cytokinin and no auxin. What do you think would happen if you cut the tip but added auxin?

(a) Apical bud intact (not shown in photo) Auxin added to decapitated stem Apical bud removed Axillary buds Lateral branches “Stump” after removal of apical bud Excess auxin on the tip blocks branch growth Without auxin the lateral branches form – what if you add auxin?

Other Plant Hormones Gibberellins Cytokinins – important for root shoot balance. Ethylene, when applied to fruit, hastens ripening and can cause leaf senescence

Ripening tomatoes Depends on ethylene gas Tomatoes picked green are ripened after shipping!

Figure The effects of ethylene

Other Plant Hormones Gibberellins Cytokinins Ethylene One more…

Just before dawn guard cells are closed – but light causes them to pump potassium into the cells. What will this do? Potassium draws water & cells swell open!

Figure But what if plants need to conserve water?

Other Plant Hormones Gibberellins Cytokinins Ethylene Abscisic acid can cause plants close guard cells in response to drought stress

Photoperiodism and Dormancy Photoperiodism plants sense seasonal changes in day and night length three categories of plants –long-day plants flower as days get longer –short-day plants flower as days get shorter –day-neutral plants use other cues to control flowering

How photoperiodism works Long day plants flower as nights get shorter and shorter Short day are opposite

How photoperiodism works If you interrupt the night though plants think it is two short nights… so who flowers? Really it is night length that is key!

Photoperiodism Plants contain a pigment called phytocrome that influences flowering this pigment exists in two interconvertible forms P r (inactive) and P fr (active) in short-day plants, the presence of P fr suppresses flowering

(a) Before exposure to light (b) After a week’s exposure to natural daylight Remember de-etiolation? Light is detected by phytochrome!

CYTOPLASM Reception Plasma membrane Cell wall Phytochrome activated by light Light Signal transduction Signal molecules inside The cells of the potato NUCLEUS 1 2 When phytochrome absorbes light… It triggers changes in the cell that alter gene expression!

Signals in Animals Animals also need to coordinate activities in a lot of different places As you know they can use the nervous system to do this but… Animals use a large number of different chemical signals

Signals in Animals Neurons use electrical changes for high speed communication Diffusion of signal molecules important for local communication Hormones are signal molecules that are used over long distances

Local and long-distance cell communication in animals

Hormone Signals in Animals Used for longer term signals than neurons Different cells respond to different hormones Hormones often key for homeostasis

33.02 The Timescale over Which Chemical Messengers Work CD33020.GIF

Hormone signaling is a series of simple steps 1.issuing the command 2.transporting the signal –most are transported through body by the blood 3.hitting the target –the hormone binds to a receptor on the target cell 4.having an effect –when the hormone binds, the protein changes shape and triggers a change in cell activity

Issuing the command “hit the target” Transport

Water vs. Lipid based Which is which? –Steroids are lipids –Peptide hormones are water soluble

NUCLEUS Signal receptor (a)(b) TARGET CELL Signal receptor Transport protein Water- soluble hormone Fat-soluble hormone Peptide based –Bind to receptor on cell membrane Steroid –Transported attached to a protein –Bind to receptor inside the cell

Signal receptor TARGET CELL Signal receptor Transport protein Water- soluble hormone Fat-soluble hormone Gene regulation Cytoplasmic response Gene regulation Cytoplasmic response OR (a) NUCLEUS (b) Peptide based –Signals are often more transient (just in the cytoplasm) –May alter gene expression Steroid –Mostly alter gene expression –Tend to be long lasting effects

Hormones are produced in glands throughout your body

Hormones are key players in maintaining homeostasis Commonly used as signals in negative feedback loops Remember Insulin & Glucagon? Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis using negative feedback