control Plant systems
Plants are complex and can be very large, and live in changing and often unpredictable environments. Control and coordination is essential: development and growth in appropriate ways (roots grow down, shoots grow up; there should be `balance' between root system and shoot system) response to predictable environmental changes (day-night, seasonal, etc: when to grow or open leaves, produce flowers, set fruit, etc.) response to unpredictable environmental changes (wind, rain, attack by herbivores or pathogens, etc.)
Plant control systems
Responses at the cellular level (signal transduction) stimulus reception transduction cellular response activation of cytoplasmic or membrane proteins; change in transcription or translation ! protein function
binding or other `second messenger' interaction of intracellular relay; a stimulus with signal amplification receptor protein system
The stimulus may be an environmental or internal factor A chemical signal produced in one body region that travels to and has an effect on target cells in another body region is a hormone.
Mechanisms of hormone function
Plant hormones influence their target cells in two general ways (same for animal hormones): change the activity of existing proteins, transporters, etc: Signal transduction pathway ("2nd messenger" system) in cytoplasm hormone binds to membrane receptor altered protein function Response
Mechanisms of hormone function
Plant hormones influence their target cells in two general ways (same for animal hormones): change the rate of synthesis of proteins: altered protein function hormone crosses membrane, binds to receptor in nucleus Response
changed translation... Changed transcription...
Mechanisms of hormone function
A classic hormonal-mediated response to an external stimulus: Phototropism -- bending towards light -- in growing seedlings
Phototropism in coleoptiles
Darwin & Darwin, 1880 These experiments show that the tip of the coleoptile is the site of the response to light.
One of the earliest examinations of hormone function; hormones first demonstrated about 100 years ago in plants and animals.
Control
no tip
opaque tip transparent cover tip cover; opaque base shield
Phototropism in coleoptiles
Darwins, 1880; Boysen-Jensen 1913
Phototropism in coleoptiles
Darwins, 1880; Boysen-Jensen 1913; Went 1926 These experiments suggest that the distribution of the chemical signal is responsible for bending.
These experiments indicate that a mobile chemical activated by light is the signal for bending and growth.
Control Control no tip opaque tip transparent gelatin cover tip cover; block opaque base shield mica block (impermeable) agar block agar block w/o auxin with auxin (no growth) (growth) offset blocks cause curving
Went named the unknown chemical auxin. Later it was identified as indoleacetic acid (IAA).
Phototropism in coleoptiles
Darwins, 1880; Boysen-Jensen 1913; Went 1926
IAA causes growth by inducing cell elongation; bending occurs because IAA concentrations are higher on the `dark' side of the plant.
Phototropism in coleoptiles
Darwins, 1880; Boysen-Jensen 1913; Went 1926 IAA causes growth by inducing cell elongation; bending occurs because IAA concentrations are higher on the `dark' side of the plant. Besides phototropic responses, IAA later discovered to have other effects in many parts of the plant: tropisms, elongation (low concentrations) root growth differentiation and branching fruit development apical dominance
Control
agar block agar block w/o auxin with auxin (no growth) (growth)
offset blocks cause curving
IAA moves by polar transport -- directly downward, not laterally in the stem
Differential cell elongation
Many plant hormones with diverse functions:
In addition to IAA: Cytokinins -- root growth and differentiation, germination, flowering, senescence. Synthesized in roots. Gibberellins (70+ types) -- seed and bud germination, stem elongation, leaf growth, flowering, fruit development, root growth and differentiation. Produced in apical meristems, roots, leaves, embryonic tissue in seeds, etc. Abscisic acid -- inhibits growth, sustains dormancy, and closes stomata during water stress. Produced in leaves, stems, and green fruit. Ethylene (C2H4) -- promotes fruit ripening; variable effects on growth and development of roots, leaves and flowers. Produced in ripening fruits, nodes, senescent leaves. Brassinosteroids -- inhibit root growth and leaf abcission; promote xylem differentiation. Produced in leaves, stems, fruit, seeds.
Many plant hormones with diverse functions:
In addition to IAA: Cytokinins -- root growth and differentiation, germination, flowering, molecule, the less easily it can move. senescence. Synthesized in roots and transported in xylem. Gibberellins (70+ types) -- seed and bud germination, stem elongation, Many move through the cell walls (apoplastic movement); leaf growth, flowering, fruit development, root growth and some move in xylem (e.g., cytokinins); ethylene embryonic differentiation. Produced in apical meristems, roots, leaves,is a gas that diffuses through the plant's gas spaces. tissue in seeds, etc. Abscisic acid -- inhibits growth, sustains dormancy, and closes stomata Generally, effects on growth and development are results during water stress. Produced in leaves, stems, and green fruit. Ethylene (C2H4) -- promotes fruit ripening; variable effects on growth differentiation. and development of roots, leaves and flowers. Produced in ripening fruits, nodes, senescent leaves. Brassinosteroids -- inhibit root growth and leaf abcission; promote dependent, and often influenced by other hormones. xylem differentiation. Produced in leaves, stems, fruit, seeds.
Most of these are fairly small molecules -- the bigger the
of changes in cell division, cell elongation, or cell
Effects are multiple, site-specific, concentration
Factors affecting hormonal responses
absolute concentrations: Auxin promotes stem growth at moderate concentrations (.00001 molar) inhibits stem growth at high concentrations (.01 molar) -- an effect used herbicides in like 2,4-D
Relative growth 4 2 0 -2 -4 0 10-6 10-5 10-4 10-3 10-2 IAA concentration (molar) 10-1
Factors affecting hormonal responses
absolute concentrations: Auxin promotes stem growth at moderate concentrations (.00001 molar) inhibits stem growth at high concentrations (.01 molar) -- an effect used in herbicides like 2,4-D relative concentrations: Apical dominance and lateral branching controlled by antagonistic effects of auxin and cytokinins: Large shoot (lots of auxin): branching suppressed (apical dominance) Large root system (lots of cytokinin): branching promoted
Responses to other external stimuli
Gravitropism: Plants detect gravity (root and shoot growth, etc.) -- positive (roots) and negative (shoots) -- detected via statoliths (plastids with dense starch grains)
Responses to other external stimuli
Gravitropism: Plants detect gravity (root and shoot growth, etc.) -- positive (roots) and negative (shoots) -- detected via statoliths (plastids with dense starch grains) Thigmotropism: Plants respond to mechanical stimuli: -- change in growth form (erect to prostrate in high winds) -- direction of growth (vine tendrils) -- short-term movements (leaf collapse in sensitive Mimosa)
Even without light cues (complete darkness or light from all directions), shoots will grow upwards and roots grow downwards.
Responses to other external stimuli
Gravitropism: Plants detect gravity (root and shoot growth, etc.) -- positive (roots) and negative (shoots) -- detected via statoliths (plastids with dense starch grains) Thigmotropism: Plants respond to mechanical stimuli: -- change in growth form (erect to prostrate in high winds) -- direction of growth (vine tendrils) -- short-term movements (leaf collapse in sensitive Mimosa)
Responses to other external stimuli
Gravitropism: Plants detect gravity (root and shoot growth, etc.) -- positive (roots) and negative (shoots) -- detected via statoliths (plastids with dense starch grains) Thigmotropism: Plants respond to mechanical stimuli: -- change in growth form (erect to prostrate in high winds) -- direction of growth (vine tendrils) -- short-term movements (leaf collapse in sensitive Mimosa)
Response to attack: -- release of toxins, repellants, etc. within attacked plant -- release of chemical signals to other plants -- release of chemical signals to attract animals that attack herbivores (typically, predators, parasites or parasitoids of herbivorous insects)
Response to attack: -- release of toxins, repellants, etc. within attacked plant -- release of chemical signals to other plants -- release of chemical signals to attract animals that attack herbivores (typically, predators, parasites or parasitoids of herbivorous insects)
Cyclic (predictable) environmental changes
Plants need to respond to (and anticipate) seasonal changes to appropriately time events such as: germination (seeds) leaf and stem growth phases flowering and fruit and seed production photosynthate production and storage (summer) and winter dormancy (in long-lived plants) Measurement and response to day length (duration of illumination, or photoperiod) is very common in plants.
Photoperiodic responses
Flowering in day-neutral plants isn't affected by photoperiod
Short-day plant
(responds to long nights) 24
Long-day plant
(responds to short nights)
24-h 12 day
0
Critical period
Time zero is sunset
Brief period of light
This shows that plants do not measure the length of the light period. Instead, they measure the duration of darkness.
Photoperiodic responses: wavelength sensitivity
Particular colors of light (administered during the dark period) have opposing effects on flowering: Both occur in sunlight (a Red light (" = 660 nm) blend of "), but can be applied Far-red light (" = 730 nm) as pure color in experiments Red Far-red
Photoperiodic responses: wavelength sensitivity
Short-day plant
(responds to long nights)
24
24-h 12 day
R FR R
R FR R
FR R FR R
0
Time zero is sunset Wavelength (") in nm
Long-day plant
(responds to short nights)
Plant responds to whatever it `saw' last
Photoperiodic responses
What transduces and transmits photoperiodic responses? For flowering, a hormone (`florigen') is suspected: if two plants are grafted together and ONE is given a photoperiod that induces flowering, BOTH plants will flower -- hormone moves between plants. photoperiodic detection seems localized in leaves (if one leaf remains, responses persist; if no leaves remain, there are no responses).
Photoperiodic responses: phytochrome system
Phytochrome (a dimer of two identical peptide chains) has a chromophore with two isomers that are photoreversible: synthesis
Red light (sunlight)
PrPr
Far-red light
Pfr fr P
Physiological responses (flowering, germination, etc.)
Slow conversion in dark (some plants)
Enzymatic breakdown
What mechanism detects night length? the phytochrome `switch'
Experiments only: pure far-red light does not occur in nature
Photoperiodism and intrinsic daily rhythms requires a biological clock
Plants probably `compare' their biological clock time with the appearance or persistence of Pfr to determine day length and induce responses.
MAKE SURE YOU UNDERSTAND:
why plants need coordinated responses basics of signal transduction; 2 modes of hormone action phototropism response: how the causal factor (auxin) was identified in terms of location and effects basic function of major plant hormone classes how plant hormones travel and how they affect growth and development effects of concentration and `balance' between hormones (auxin and cytokinins as example) responses to other environmental clues: gravity, wind, etc. responses to herbivory and pathogens photoperiodic control of flowering; critical periods, day length and phytochromes need for biological clocks and their characteristics
Biological clocks seem universal in eukaryotes
temperature compensated (unlike nearly all biochemical reactions) approximately (but not exactly) 24 hours: circadian entrainable to external time cues (photoperiod, temperature, etc.)