Sensory Systems in Plants

Chapter 41

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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

Phototropisms

• Directional growth responses• Connect environmental signal with cellular perception of the

signal, transduction into biochemical pathways, and ultimately an altered growth response

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Gravitropism

• Response of a plant to the gravitational field of the Earth

• Shoots exhibit negative gravitropism

• Roots have a positive gravitropic response

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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

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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

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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 8

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Responses to Mechanical Stimuli

• Some turgor movements are triggered by light• This movement maximizes photosynthesis

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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

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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

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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

• Thirty years later, Peter Boysen-Jensen and Arpad Paal demonstrated that the “influence” was actually a chemical

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Auxin

• 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

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• Chemical enhanced rather than retarded cell elongation• Frits Went named the substance that he had discovered

auxin

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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

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How Auxin Works

• Indoleacetic acid (IAA) is the most common natural auxin

• Probably synthesized from tryptophan

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 and fruiting in pineapples

• 2,4-dichlorophenoxyacetic acid (2,4-D) is a herbicide commonly used to kill weeds

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Cytokinins

• Plant hormone that, in combination with auxin, stimulates cell division and differentiation

22Synthetic cytokinins

Cytokinins

• Produced in the root apical meristems and developing fruits

• In all plants, cytokinins, working with other hormones, seem to regulate growth patterns

• Promote the growth of lateral buds into branches

• Inhibit the formation of lateral roots– Auxin promotes their formation

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Cytokinins

• Promote the synthesis or activation of cytokinesis proteins

• Also function as antiaging hormones

• Agrobacterium inserts genes that increase rate of cytokinin and auxin production– Causes massive cell division– Formation of crown gall tumor

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• Plant tissue can form shoots, roots, or an undifferentiated mass depending on the relative amounts of auxin and cytokinin

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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

• Enhanced if auxin present

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• Adding gibberellins to certain dwarf mutants restores normal growth and development

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Gibberellins

• Hasten seed germination

• Used commercially to extend internode length in grapes– Result is larger grapes

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Brassinosteroids

• First discovered in the pollen of Brassica spp.• Are structurally similar to steroid hormones

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Brassinosteroids

• Functional overlap with other plant hormones, especially auxins and gibberellins

• Broad spectrum of physiological effects– Elongation, cell division, stem bending,

vascular tissue development, delayed senescence, membrane polarization and reproductive development

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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)-causes cells that surround the pathogen to die

• In peas, oligosaccharins inhibit auxin-stimulated elongation of stems

• While in regenerated tobacco tissue, they inhibit roots and stimulate flowers

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Ethylene

• Gaseous hydrocarbon (H2C―CH2)

• Auxin stimulates ethylene production in the tissues around the lateral bud and thus retards their growth

• Ethylene also suppresses stem and root elongation

• Major role in fruit development – hastens ripening– Transgenic tomato plant can’t make ethylene– Shipped without ripening and rotting

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Abscisic Acid

• Synthesized mainly in mature green leaves, fruits, and root caps

• Little evidence that this hormone plays a role in abscission

• Induces formation of dormant winter buds• Counteracts gibberellins by suppressing bud

growth and elongation• Counteracts auxin by promoting

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Abscisic Acid

• Necessary for dormancy in seeds– Prevents precocious germination called

vivipary

• Important in the opening and closing of stomata

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