Responses to Light

1. Responses to Light Photomorphogenesisrefers to non-directional, light-triggered development Phototropismsare directional growth responses to light Both compensate for plants’ inability to move The phytochromemolecule exists in two interconvertible forms: -Pris the inactive form -Absorbs red light at 660 nm -Pfris the active form -Absorbs far-red light at 730 nm

2. Responses to Light 1. Seed germination -Inhibited by far-red light and stimulated by red light in many plants 2. Shoot elongation -Etiolation occurs when shoot internodes elongate because red light and active Pfr are not available 3. Detection of plant spacing -Crowded plants receive far-red light bounced from neighboring plants -This increases plant height in competition for sunlight

3. Responses to Light Phytochromes are involved in many signaling pathways that lead to gene expression -Pr is found in the cytoplasm -When it is converted to Pfr it enters the nucleus -Pfr binds to transcription factors, leading to expression of light-regulated genes

4. Phototropisms Phototropic responses including the bending of growing stems to sources of light with blue wavelengths (460-nm range)

5. Circadian Clocks 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

6. Responses to Gravity Gravitropism is the response of a plant to the gravitational field of the Earth Four general steps of 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 “up” and “down” sides of root and shoot

7. Responses to Gravity Shoots exhibit negative gravitotropismIn shoots, gravity is sensed in endodermal cells surrounding the vascular tissue -Signaling is in the outer epidermal cells Roots have a positive gravitropic response In roots, the cap is the site of gravity perception -Signaling triggers differential cell elongation and division in the elongation zone

8. Response to Gravity Auxin accumulates on lower side of the stem -Results in asymmetrical cell elongation and curvature of the stem upward Lower cells in root cap are less elongated than those on upper side -Upper side cells grow more rapidly causing the root to ultimately grow downward

9. Responses to Mechanical Stimuli Thigmomorphogenesis permanent form change in response to mechanical stresses Thigmotropism directional growth of a plant 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

10. Responses to Mechanical Stimuli Some touch-induced plant movements involve reversible changes in turgor pressure -Touch triggers movement of ions to outer side of pulvini -Water follows by osmosis -Decreased interior turgor pressure causes the leaf to fold

11. Responses to Mechanical Stimuli 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

12. Water and Temperature Responses Abscission involves changes at the petiole’s base Protective layer = Consists of several layers of suberin-impregnated cells Separation layer = Consists of 1-2 layers of swollen, gelatinous cells ind and rainseparate the leaf from the stem -Nutrient sinks are discarded, conserving resources

13. Seed Dormancy Accumulating food reserves, forming a protective seed coat and dehydration Offspring to wait until conditions for germination are optimal -Legume seeds last decades or longer -Seeds that are thousands of years old have been successfully germinated

14. Auxin Discovered in 1881 by Charles and Francis Darwin

15. Auxin In 1926, Frits Went performed an experiment that explained all of the previous results -He named the chemical messenger auxin -Cell elongation causes the plantto bend towards light

16. Auxin Winslow Briggs later demonstrated that auxin molecules migrate away from the light into the shaded portion of the shoot -Barriers in a shoot tip revealed equal amounts of auxin in both the light and dark sides -However, different auxin concentrations produced different degrees of curvature

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

18. 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 2,4-dichlorophenoxyacetic acid (2,4-D) is a herbicide commonly used to kill weeds

19. Cytokinins Cytokinins are produced in the root apical meristems and fruits -Stimulate cell division and differentiation & promote the growth of lateral buds into branches Are purines that appear to be derivatives of adenine Synthetic cytokinins

20. 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 -Have important effects on stem elongation

21. Brassinosteroids First discovered in the pollen of Brassica spp. -Are structurally similar to steroid hormones-Elongation, cell division, stem bending, vascular tissue development, delayed senescence and reproductive development

22. 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 auxin-stimulated elongation of stems -While in regenerated tobacco tissue, they inhibit roots and stimulate flowers

23. Ethylene A gaseous hydrocarbon (H2C–CH2) -thus retards their growth in lateral bud -suppresses stem and root elongation 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

24. Abscisic Acid Abscisic acid is synthesized mainly in mature green leaves, fruits and root caps -induces formation of dormant winter buds -necessary for dormancy in seeds -important in the opening and closing of stomata -Triggers movement of K+ out of guard cells