Responses to Light. Responses to Light

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Sensory Systems in Plants Chapter 41 Pigments other than those used in photosynthesis can detect light and mediate the plant s response to it Photomorphogenesis refers to

parts: -Chromophore which is light-receptive -Apoprotein which initiates a signaltransduction pathway 2 3 The phytochrome molecule exists in two interconvertible forms: -P r is the

forms of phytochromes have been characterized: PHYA to PHYE -Involved in several plant growth responses 1.

Shoot elongation -Etiolation occurs when shoot internodes elongate because red light and active P fr are not available 3.

1 Sensory Systems in Plants Chapter 41 Pigments other than those used in photosynthesis can detect light and mediate the plant s response to it Photomorphogenesis refers to nondirectional, light-triggered development Phototropisms are directional growth responses to light Both compensate for plants inability to move Phytochrome (P) consists of two parts: -Chromophore which is light-receptive -Apoprotein which initiates a signaltransduction pathway 2 3 The phytochrome molecule exists in two interconvertible forms: -P r is the inactive form -Absorbs red light at 660 nm -P fr is the active form -Absorbs far-red light at 730 nm -Tagged by ubiquitin for degradation in the proteasome 4 In Arabidopsis, five forms of phytochromes have been characterized: PHYA to PHYE -Involved in several plant growth responses 1. Seed germination -Inhibited by far-red light and stimulated by red light in many plants Shoot elongation -Etiolation occurs when shoot internodes elongate because red light and active P fr are not available 3. Detection of plant spacing -Crowded plants receive far-red light bounced from neighboring plants -This increases plant height in Phytochromes are involved in many signaling pathways that lead to gene expression -P r is found in the cytoplasm -When it is converted to P fr it enters the nucleus -P fr binds to transcription factors, leading to expression of light-regulated genes competition for sunlight

Phytochrome also works through proteinkinase signaling pathways -When P r is converted to P fr, its protein kinase domain causes autophosphorylation or phosphorylation of another protein -This

of light with blue wavelengths (460-nm range) 10 11 12 Phototropisms Phototropisms Circadian Clocks A blue-light receptor phototropin 1 (PHOT1) has been characterized -Has two regions -Blue-light

common among eukaryotes Have four characteristics: 1. Continue in absence of external inputs 2. Must be about 24 hours in duration 3. Cycle can be reset or entrained 4.

roots have a positive gravitropic response Responses to Gravity Four general steps lead to a gravitropic response: 1. Gravity is perceived by the cell 2.

2 Phytochrome also works through proteinkinase signaling pathways -When P r is converted to P fr, its protein kinase domain causes autophosphorylation or phosphorylation of another protein -This initiates a signaling cascade that activates transcription factors leading to expression of light-regulated genes Phototropisms Phototropic responses including the bending of growing stems to sources of light with blue wavelengths (460-nm range) Phototropisms Phototropisms Circadian Clocks A blue-light receptor phototropin 1 (PHOT1) has been characterized -Has two regions -Blue-light activates the light-sensing region of PHOT1 -Stimulates the kinase region of PHOT1 to autophosphorylate -Triggers a signal transduction Circadian rhythms ( around the day ) are particularly common among eukaryotes Have four characteristics: 1. Continue in absence of external inputs 2. Must be about 24 hours in duration 3. Cycle can be reset or entrained 4. Clock can compensate for differences in temperature 15 Responses to Gravity Gravitropism is the response of a plant to the gravitational field of the Earth -Shoots exhibit negative gravitotropism; roots have a positive gravitropic response Responses to Gravity Four general steps lead to a gravitropic response: 1. Gravity is perceived by the cell 2. A mechanical signal is transduced into a gravity-perceiving physiological signal 3. Physiological signal is transduced to other cells 4. Differential cell elongation occurs in the Responses to Gravity In shoots, gravity is sensed along the length of the stem in endodermal cells surrounding the vascular tissue -Signaling is in the outer epidermal cells In roots, the cap is the site of gravity perception -Signaling triggers differential cell elongation and division in the elongation zone 16 up and down sides of root and shoot

Stem Response to Gravity Auxin accumulates on lower side of the stem -Results in asymmetrical cell elongation and curvature of the stem upward Two

gravity-sensing amyloplasts 19 20 21 Root Response to Gravity Lower cells in horizontally oriented root cap are less elongated than those on upper side

elongation zone -However, it has an essential role in root gravitotropism -Curling of tendrils around objects 24 22 23 Thigmomorphogenesis is a permanent

occur in same direction independent of the stimulus Examples of touch responses: -Snapping of Venus flytrap leaves 25 Some touch-induced plant movements

becomes turgid and may also move 26 Mimosa pudica leaves have swollen structures called pulvini at the base of their leaflets -When leaves are stimulated,

3 Stem Response to Gravity Auxin accumulates on lower side of the stem -Results in asymmetrical cell elongation and curvature of the stem upward Two Arabidopsis mutants, scarecrow (scr) and short root (shr) do not show a normal gravitropic response -Due to lack of a functional endodermis and its gravity-sensing amyloplasts Root Response to Gravity Lower cells in horizontally oriented root cap are less elongated than those on upper side -Upper side cells grow more rapidly causing the root to ultimately grow downward Auxin may not be the long-distance signal between the root cap and elongation zone -However, it has an essential role in root gravitotropism -Curling of tendrils around objects Thigmomorphogenesis is a permanent form change in response to mechanical stresses Thigmotropism is directional growth of a plant or plant part in response to contact -Thigmonastic responses occur in same direction independent of the stimulus Examples of touch responses: -Snapping of Venus flytrap leaves 25 Some touch-induced plant movements involve reversible changes in turgor pressure -If water leaves turgid cells, they may collapse, causing plant movements -If water enters a limp cell, it becomes turgid and may also move 26 Mimosa pudica leaves have swollen structures called pulvini at the base of their leaflets -When leaves are stimulated, an electrical signal is generated -Triggers movement of ions to outer side of pulvini -Water follows by osmosis -Decreased interior turgor pressure causes the leaf to fold 27 3

Some turgor movements are triggered by light -This movement maximizes photosynthesis Bean leaves are horizontal during the day when their

Temperature Responses When water and temperature affect plants, responses can be short-term or long-term Dormancy results in the cessation

-One advantage is that nutrient sinks can Abscission involves changes that occur in an abscission zone at the petiole s base -Hormonal

Consists of 1-2 layers of swollen, gelatinous cells -As pectins break down, wind and rain separate the leaf from the stem be discarded,

often last decades and even longer without special care -Seeds that are thousands of years old have been successfully germinated Essential

Increasing number of unsaturated lipids in their plasma membranes 2. Limiting ice crystal formation to extracellular spaces 3.

4 Some turgor movements are triggered by light -This movement maximizes photosynthesis Bean leaves are horizontal during the day when their pulvini are rigid -But become more or less vertical at night as the pulvini lose turgor Water and Temperature Responses Water and Temperature Responses When water and temperature affect plants, responses can be short-term or long-term Dormancy results in the cessation of growth during unfavorable conditions -Often begins with dropping of leaves Abscission is the process by which leaves or petals are shed -One advantage is that nutrient sinks can Abscission involves changes that occur in an abscission zone at the petiole s base -Hormonal changes lead to differentiation of: -Protective layer = Consists of several layers of suberin-impregnated cells -Separation layer = Consists of 1-2 layers of swollen, gelatinous cells -As pectins break down, wind and rain separate the leaf from the stem be discarded, conserving resources Seed Dormancy Seeds allow plant offspring to wait until conditions for germination are optimal -Legume seeds often last decades and even longer without special care -Seeds that are thousands of years old have been successfully germinated Essential steps leading to dormancy include: -Accumulating food reserves, forming a Responses to Chilling Plants respond to cold temperatures by: 1. Increasing number of unsaturated lipids in their plasma membranes 2. Limiting ice crystal formation to extracellular spaces 3. Producing antifreeze proteins Some plants can undergo deep supercooling -Survive temperatures as low as 40 O C protective seed coat and dehydration

Responses to High Temperatures Plants produce heat shock proteins (HSPs) if exposed to rapid temperature increases -HSPs stabilize other proteins Plants can survive otherwise lethal temperatures if

part where they exert a response In plants, hormones are not produced by specialized tissues -Seven major kinds of plant hormones -Auxin, cytokinins, gibberellins, brassinosteroids, oligosaccharins,

covered with a lightproof cap -They do bend when a collar is placed below the tip 39 Auxin Auxin The Darwins hypothesized that shoots bend towards light in response to an influence transmitted

5 Responses to High Temperatures Plants produce heat shock proteins (HSPs) if exposed to rapid temperature increases -HSPs stabilize other proteins Plants can survive otherwise lethal temperatures if they are gradually exposed to increasing temperature -Acquired thermotolerance 37 Hormones and Sensory Systems Hormones are chemicals produced in one part of an organism and transported to another part where they exert a response In plants, hormones are not produced by specialized tissues -Seven major kinds of plant hormones -Auxin, cytokinins, gibberellins, brassinosteroids, oligosaccharins, ethylene, and abscisic acid 38 Auxin Discovered in 1881 by Charles and Francis Darwin -They reported experiments on the response of growing plants to light -Grass seedlings do not bend if the tip is covered with a lightproof cap -They do bend when a collar is placed below the tip 39 Auxin Auxin The Darwins hypothesized that shoots bend towards light in response to an influence transmitted downward from the tip Thirty years later, Peter Boysen-Jensen and Arpad Paal demonstrated that the influence was actually a chemical In 1926, Frits Went performed an experiment that explained all of the previous results -He named the chemical messenger auxin -It accumulated on the side of an oat seedling away from light -Promoted these cells to grow faster than those on the lighted side -Cell elongation causes the plant to bend towards light 42 Auxin Winslow Briggs later demonstrated that auxin molecules migrate away from the light into the shaded portion of the shoot -Barriers inserted in a shoot tip revealed equal amounts of auxin in both the light and dark sides of the barrier -However, different auxin concentrations produced different degrees of curvature

How Auxin Works Indoleacetic acid (IAA) is the most common natural auxin -Probably synthesized from

Two families of proteins mediate auxininduced changes in gene expression -Auxin responses factors

Auxin binds TIR1 in the SCF complex if Aux/IAA is present 2.

Transcriptional activators of ARF genes are released from repression by Aux/IAA 5.

in plasticity of the plant cell wall -The acid growth hypothesis provides a model linking auxin to

(IBA) have many uses in agriculture and horticulture -Prevent abscission in apples and berries

6 How Auxin Works Indoleacetic acid (IAA) is the most common natural auxin -Probably synthesized from tryptophan How Auxin Works The auxin receptor is the transport inhibitor response protein 1 (TIR1) Two families of proteins mediate auxininduced changes in gene expression -Auxin responses factors (ARFs) -Aux/IAA proteins How Auxin Works 1. Auxin binds TIR1 in the SCF complex if Aux/IAA is present 2. SCF complex tags Aux/IAA proteins with ubiquitin 3. These are degraded in the proteasome 4. Transcriptional activators of ARF genes are released from repression by Aux/IAA 5. Auxin-induced gene expression How Auxin Works One of the downstream effects of auxin is an increase in plasticity of the plant cell wall -The acid growth hypothesis provides a model linking auxin to cell wall expansion Synthetic Auxins Naphthalene acetic acid (NAA) and indolebutyric acid (IBA) have many uses in agriculture and horticulture -Prevent abscission in apples and berries -Promote flowering & fruiting in pineapples Cytokinins Are purines that appear to be derivatives of adenine Synthetic cytokinins 2,4-dichlorophenoxyacetic acid (2,4-D) is a herbicide commonly used to kill weeds

Cytokinins Cytokinins are produced in the root apical meristems and developing fruits -Stimulate cell division and differentiation, in

promotes their formation 55 Cytokinins Cytokinins promote the synthesis or activation of cytokinesis proteins -They also function as anti-aging

Cytokinins The plant pathogen Agrobacterium introduces genes into the plant genome that increase the production of cytokinin and auxin -Cause

grow very tall Gibberellins belong to a large class of over 100 naturally occurring plant hormones -All are acidic and abbreviated GA -Have

Gibberellins GA is used as a signal from the embryo that turns on transcription of genes encoding hydrolytic enzymes in the aleurone layer

7 Cytokinins Cytokinins are produced in the root apical meristems and developing fruits -Stimulate cell division and differentiation, in combination with auxin Cytokinins promote the growth of lateral buds into branches -They inhibit the formation of lateral roots, while auxin promotes their formation 55 Cytokinins Cytokinins promote the synthesis or activation of cytokinesis proteins -They also function as anti-aging hormones Plant tissue can form shoots, roots, or an undifferentiated mass depending on the relative amounts of auxin and cytokinin Cytokinins The plant pathogen Agrobacterium introduces genes into the plant genome that increase the production of cytokinin and auxin -Cause massive cell division and formation of a crown gall tumor Gibberellins Named after the fungus Gibberella fujikuroi which causes rice plants to grow very tall Gibberellins belong to a large class of over 100 naturally occurring plant hormones -All are acidic and abbreviated GA -Have important effects on stem elongation Gibberellins Adding gibberellins to certain dwarf mutants restores normal growth and development Gibberellins GA is used as a signal from the embryo that turns on transcription of genes encoding hydrolytic enzymes in the aleurone layer -When GA binds to its receptor, it frees GA-dependent transcription factors from a repressor -These transcription factors can now directly affect gene expression

Gibberellins GAs hasten seed germination -They also function as pheromones in ferns GAs are used commercially to extend internode length in grapes -The result is larger grapes 64 Brassinosteroids

-Are structurally similar to steroid hormones 65 Brassinosteroids Have a broad spectrum of physiological effects -Elongation, cell division, stem bending, vascular tissue development, delayed

-Can be released from the cell wall by enzymes secreted by pathogens -Signal the hypersensitive response (HR) In peas, oligosaccharins inhibit auxinstimulated elongation of stems -While in

suppresses stem and root elongation Ethylene Ethylene controls leaf, flower and fruit abscission It hastens fruit ripening -Indeed, an antisense copy of the gene has been used to create transgenic

8 Gibberellins GAs hasten seed germination -They also function as pheromones in ferns GAs are used commercially to extend internode length in grapes -The result is larger grapes 64 Brassinosteroids First discovered in the pollen of Brassica spp. -Are structurally similar to steroid hormones 65 Brassinosteroids Have a broad spectrum of physiological effects -Elongation, cell division, stem bending, vascular tissue development, delayed senescence and reproductive development Additive effects with auxins and gibberellins have been reported 66 Oligosaccharins Are complex plant cell wall carbohydrates that have a hormone-like function -Can be released from the cell wall by enzymes secreted by pathogens -Signal the hypersensitive response (HR) In peas, oligosaccharins inhibit auxinstimulated elongation of stems -While in regenerated tobacco tissue, they Ethylene A gaseous hydrocarbon (H 2 C CH 2 ) Auxin stimulates ethylene production in the tissues around the lateral bud and thus retards their growth Ethylene also suppresses stem and root elongation Ethylene Ethylene controls leaf, flower and fruit abscission It hastens fruit ripening -Indeed, an antisense copy of the gene has been used to create transgenic tomato -These stay fresh longer inhibit roots and stimulate flowers Abscisic Acid Abscisic acid is synthesized mainly in mature green leaves, fruits and root caps There is little evidence that this hormone plays a role in abscission Abscisic acid induces formation of dormant winter buds It counteracts gibberellins, by suppressing bud growth and elongation, and auxin, by promoting senescence Abscisic Acid Abscisic acid is also necessary for dormancy in seeds -Prevents precocious germination called vivipary Abscisic acid is important in the opening and closing of stomata -Triggers movement of K + out of guard cells 72 8

Sensory Systems in Plants

Sensory Systems in Plants 1. If temperatures suddenly rise 5 to 10º C, proteins are produced to help stabilize other proteins. 2. Rapid turgor pressure changes in specialized multicellular swellings called