Plant Responses to Internal & External Signals

Signal transduction pathways link signal reception to response A potato left growing in darkness produces shoots that look unhealthy, and it lacks elongated roots These are morphological adaptations for growing in darkness, collectively called etiolation After exposure to light, a potato undergoes changes called de-etiolation, in which shoots and roots grow normally © 2011 Pearson Education, Inc.

(a) Before exposure to light (b) Figure 39.2 Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes. (a) Before exposure to light (b) After a week’s exposure to natural daylight

(a) Before exposure to light Figure 39.2a Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes. (a) Before exposure to light

After a week’s exposure to natural daylight Figure 39.2b Figure 39.2 Light-induced de-etiolation (greening) of dark-grown potatoes. (b) After a week’s exposure to natural daylight

A potato’s response to light is an example of cell-signal processing The stages are reception, transduction, and response © 2011 Pearson Education, Inc.

Activation of cellular responses Figure 39.3 CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and Activation of cellular responses second messengers Receptor Figure 39.3 Review of a general model for signal transduction pathways. Hormone or environmental stimulus Plasma membrane

Reception Internal and external signals are detected by receptors, proteins that change in response to specific stimuli In de-etiolation, the receptor is a phytochrome capable of detecting light © 2011 Pearson Education, Inc.

Transduction Second messengers transfer and amplify signals from receptors to proteins that cause responses Two types of second messengers play an important role in de-etiolation: Ca2+ ions and cyclic GMP (cGMP) The phytochrome receptor responds to light by Opening Ca2+ channels, which increases Ca2+ levels in the cytosol Activating an enzyme that produces cGMP © 2011 Pearson Education, Inc.

1 Reception CYTOPLASM Plasma membrane Phytochrome Cell wall Light Figure 39.4-1 1 Reception CYTOPLASM Plasma membrane Phytochrome Cell wall Light Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response.

Protein kinase 1 Protein kinase 2 Figure 39.4-2 1 Reception 2 Transduction CYTOPLASM Plasma membrane cGMP Protein kinase 1 Second messenger Phytochrome Cell wall Protein kinase 2 Light Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response. Ca2 channel Ca2

De-etiolation (greening) response proteins Figure 39.4-3 1 Reception 2 Transduction 3 Response Transcription factor 1 CYTOPLASM NUCLEUS Plasma membrane cGMP Protein kinase 1 P Second messenger Transcription factor 2 Phytochrome P Cell wall Protein kinase 2 Transcription Light Translation Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response. Ca2 channel De-etiolation (greening) response proteins Ca2

Response A signal transduction pathway leads to regulation of one or more cellular activities In most cases, these responses to stimulation involve increased activity of enzymes This can occur by transcriptional regulation or post-translational modification © 2011 Pearson Education, Inc.

Plant Hormones Hormones are chemical signals that coordinate the various parts of an organism A hormone is a compound produced in one part of the body which is then transported to other parts of the body, where it triggers responses in target cells and tissues Examples of human hormones: Adrenaline, testosterone, estrogen, epinephrine…

Hormones: chemical messengers that coordinate different parts of a multicellular organism Important plant hormones: Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide) Cytokinins – cell division (cytokinesis) & differentiation Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

Auxin Stimulates stem elongation Stimulates development of fruit Involved in phototropism and gravitropism

Cytokinins Stimulate cell division and growth Stimulate cytokinesis Stimulate germination and flowering

Gibberellins Trigger seed and bud germination Promote stem elongation and leaf growth Important in the growth of fruit

Ethylene Gas: Fruit Ripening Canister of ethylene gas to ripen bananas in shipping container Untreated tomatoes vs. Ethylene treatment

Abscisic Acid Induces seed dormancy Inhibits cell growth Anti-gibberellin Inhibits cell growth Anti-cytokinin Inhibits fruit ripening Anti-ethylene Closes stomata during water stress, allowing many plants to survive droughts

Tropisms Tropisms are growth responses that result in curvatures of whole plant organs toward or away from a stimuli There are three major stimuli that induce tropisms Light (Phototropism) Gravity (Gravitropism/Geotropism) Touch (Thigmotropism)

Phototropism Phototropism is the growth of a shoot towards light This is primarily due to the action of auxin Auxin elongates the cells on the non-light side

Positive gravitropism in roots: the statolith hypothesis.

Thigmotropism: Toward or away from a stimulus; rapid turgor movements by Mimosa plant  action potentials

Biological Clocks/Circadian Rhythms A physiological cycle with a frequency of about 24 hours is called a circadian rhythm Even without external, environmental cues, circadian rhythms persist in humans and in all eukaryotes Example: jet lag in humans

Night length is a critical factor! Photoperiodism: physiological response to the relative length of night & day (i.e. flowering) Short-day plants: flower when nights are long (mums, poinsettia) Long-day plant: flower when nights are short (spinach, iris, veggies) Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions) Night length is a critical factor!

Plant Defenses Plants defend themselves against herbivores in several ways Physical defenses, such as thorns Chemical defenses, such as producing distasteful/toxic compounds

Plant responses to stress

Flooding (O2 deprivation): Drought (H2O deficit): close stoma release abscisic acid to keep stoma closed Inhibit growth roll leaves  reduce SA & transpiration deeper roots Flooding (O2 deprivation): release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

Excess Salt: Heat: Cold: cell membrane – impede salt uptake produce solutes to ↓ψ - retain H2O Heat: evap. cooling via transpiration heat shock proteins – prevent denaturation Cold: alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity) increase cytoplasmic solutes antifreeze proteins

Herbivores: Pathogens: physical (thorns) chemicals (garlic, mint) recruit predatory animals (parasitoid wasps) Pathogens: 1st line of defense = epidermis 2nd line = pathogen recognition, host-specific