Environmental cues govern plant growth Zivuku M

Objectives Discuss environmental cues governing plant growth Outline the role of phytochrome in phototropism. Describe geotropism, thigmotropism Explain how plants response to water and temperature response Hormones and sensory response 2

How Do Seeds Germinate? Germination is the resumption of growth after a time of arrested embryonic development Environmental factors influence germination –Spring rains provide the water amounts necessary to swell and rupture the seed coat (taking in water is imbibition) –Oxygen moves in and allows the embryo to switch to aerobic metabolism –Increase temperatures and number of daylight hours

Genetic Programs, Environmental Cues Patterns of germination and development have a heritable basis dictated by a plant’s genes Early cell divisions may result in unequal distribution of cytoplasm –Cytoplasmic differences trigger variable gene expression, which may results in variations in hormone synthesis –Even though all cells have the same genes, it is the selective expression of those genes that results in cell differentiation.

Growth and Development Growth and development are necessary for plants to survive –Growth is defined as an increase in the number, size, and volume of cells –Development is the emergence of specialized, morphologically different body parts

6 Responses to Light Pigments not used in photosynthesis Detect light and mediate the plant’s response to it Photomorphogenesis –Nondirectional, light-triggered development Phototropisms –Directional growth responses to light Both compensate for inability to move

Responses to Light Phytochrome molecule exists in 2 forms –P r – absorbs red light at 660 nm Biologically inactive Converted to P fr when red photons available –P fr – absorbs far-red light at 730 nm Active form Converted back to P r when far-red photons available 7

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9 Responses to Light Phytochrome (P) consists of two parts: –Chromophore which is light-receptive –Apoprotein which facilitates expression of light- response genes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. COOH Chromophore Apoprotein SerineProtein-binding siteProtein kinase Ubiquitin-binding site H2NH2N

Responses to Light 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 with other proteins that form a transcription complex, leading to the expression of light-regulated genes 10

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Responses to Light Phytochrome also works through protein kinase-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 12

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Responses to Light Amount of P fr is also regulated by degradation Ubiquitin tags P fr for transport to the proteasome Process of tagging and recycling P fr is precisely regulated to maintain needed amounts of phytochrome in the cell 14

Responses to Light Phytochrome is involved in –Seed germination Inhibited by far-red light and stimulated by red light in many plants Only germinate when exposed to direct sunlight –Shoot elongation Etiolation is caused by a lack of red light –Detection of plant spacing Plants measure the amount of far-red light bounced back from neighboring plants 15

Phototropisms Directional growth responses Connect environmental signal with cellular perception of the signal, transduction into biochemical pathways, and ultimately an altered growth response 16

Phototropisms Blue-light receptor phototropin 1 (PHOT1) –2 light-sensing regions – change conformation in response to blue light –Stimulates the kinase region of PHOT1 to autophosphorylate –Triggers signal transduction 17

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Circadian Clocks ~ 24-hour rhythms 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 Phytochrome action 4.Clock can compensate for differences in temperature 19

Gravitropism Response of a plant to the gravitational field of the Earth Shoots exhibit negative gravitropism Roots have a positive gravitropic response 20

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 physiological signal 3.Physiological signal is transduced inside the cell and to other cells 4.Differential cell elongation occurs in the “up” and “down” sides of root and shoot 21

Responses to Gravity In shoots, gravity is sensed along the length of the stem in endodermal cells surrounding the vascular tissue –Signaling toward 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 22

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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 25

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Root Response to Gravity Gravity-sensing cells are located in the root cap Cells that actually undergo asymmetrical growth are in the distal elongation zone (closest to root cap) Auxin may be involved –Still occurs if auxin transport is suppressed 27

Thigmomorphogenesis 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 –Curling of tendrils around objects 28

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Responses to Mechanical Stimuli Some touch-induced plant movements involve reversible changes in turgor pressure If water leaves turgid cells, they may collapse, causing plant movement If water enters a limp cell, it becomes turgid and may also move 30

Responses to Mechanical Stimuli 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 31

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Responses to Mechanical Stimuli Some turgor movements are triggered by light This movement maximizes photosynthesis 33

Responses to Mechanical Stimuli Bean leaves –Pulvini are rigid during the day –But lose turgor at night –Reduce water loss from transpiration during the night –Maximize photosynthetic surface area during the day 34

Water and Temperature Responses Responses can be short-term or long-term Dormancy results in the cessation of growth during unfavorable conditions –Often begins with abscission – dropping of leaves –Advantage is that nutrient sinks can be discarded, conserving resources 35

Water and Temperature Responses Abscission involves changes that occur in an abscission zone at the petiole’s base Hormonal changes lead to differentiation –Protective layer – consists of several layers of suberin-impregnated cells –Separation layer – consists of 1–2 layers of swollen, gelatinous cells Pectins will break down middle lamellae of these cells 36

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Seed Dormancy Seeds allow plant offspring to wait until conditions for germination are optimal Triggers to break seed dormancy –Water leaching away inhibitor; cracking seed coat osmotically Favorable temperatures, day length, and amounts of water can release buds, underground stems and roots, and seeds from a dormant state 38

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Responses to Chilling Lipid composition of a plant’s membranes can help predict whether the plant will be sensitive or resistant to chilling –The more unsaturated the membrane lipids are, the more resistant the plant is to chilling Supercooling – survive as low as –40 o C –Limits ice crystal formation to extracellular spaces Antifreeze proteins 40

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 41

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 42