Regulation of Plant Growth

1. 37 Regulation of Plant Growth

2. Features that maximize plants’ ability to obtain resources for growth and reproduction: • Meristemsallow growth throughout the plant’s life • Post-embryonic organ formation — new organs can develop throughout life • Differential growth — they can grow organs most needed, e.g., more leaves http://www.ncsec.org/team8/fp.gif

3. Plants must monitor their environment and redirect growth as appropriate A plant’s environment is never completely stable light changes day to night, and with seasons neighbor plants compete for light, nutrients, etc. http://www.howplantswork.net/wp-content/uploads/2009/10/winding_road.jpg

4. Signals (environmental cues, photoreceptors, and hormones) affect three fundamental processes: • Cell division • Cell expansion • Cell differentiation http://aggie-horticulture.tamu.edu/faculty/davies/students/ngo

5. Plant development is regulated in complex ways. Four factors regulate growth: Presence of environmental cues Receptors, e.g. photoreceptors, to sense environmental cues Hormones mediate effects of cues The plant’s genome www.ryanphotographic.com/images/Scenes/

6. Seeds are dormant — cells do not divide, expand, or differentiate As seed begins to germinate, it takes up (imbibes) water Growing embryo obtains chemical building blocks by digesting food stored in seed Germination is completed when radicle (embryonic root) emerges Now called a seedling http://imagessvt.free.fr/physioV/germination

7. If seedling germinates underground, it must elongate rapidly, and cope with darkness for a time Series of photoreceptors directs this stage of development Early seedling development varies in monocots and eudicots

8. Seed dormancy may last weeks, months, or years. Mechanisms that maintain dormancy include: Exclusion of water or oxygen by impermeable seed coat Mechanical restraint of embryo by tough seed coat Chemical inhibition of embryo development Iris seeds www.aphotoflora.com

9. Seed dormancy must be broken for germination to begin Seed coats may be abraded by physical processes, or chemically in the digestive tract of an animal Soil microorganisms or freeze-thaw cyclesmay soften seed coats Fire ends dormancy for many seeds by melting waterproof wax in seed, or by cracking the seed coat Leaching of chemical inhibitors by soaking in water can also end dormancy

10. Advantages of seed dormancy: Survival through unfavorable conditions Prevent germination while still attached to parent plant Seeds that must be scorched by fire avoid competition by germinating only in fire-scarred areas Long-distance dispersal of seeds www.biol.canterbury.ac.nz/mistletoes/images Mistletoe seedling Jack pine seedling sprouting following a fire in Wisconsin http://nature.org/initiatives/fire/work

11. Dormancy of some seeds is broken by exposure to light Germinate at or near soil surface Tiny with little food reserves and would not survive if they germinated deep in the ground Large seeds with large food reserves, germinate only when buried deeply, and in darkness (light inhibited) Photo 37.19 Corn, squash, and Arabadopsis (small brown) seeds.

12. Process of germination Imbibition, or uptake of water, is first step Seed’s water potential is very negative  water will enter if seed coat is permeable Expanding seeds exert tremendous force Enzymes activated with hydration RNA and proteins are synthesized and respiration increases Initial growth is by expansion of pre-formed cells, not cell division Comparison of non-imbibed and imbibed (swollen) pea seeds www.cropsci.uiuc.edu/classes/cpsc112/images/SeedsGerm

13. During early stages of plant development, plants respond to internal and external cues Responses are initiated and maintained by two types of regulators Hormones Photoreceptors

14. Hormones Regulatory chemicals that act at low concentrations at sites distant from where they were produced Each plant hormone is produced in many cells, and has multiple roles – interactions can be complex

15. Photoreceptors involved in many developmental processes They are pigments (molecules that absorb light) associated with proteins Light acts directly on photoreceptors  regulate processes of development http://www.scielo.br/img/fbpe/gmb/v24n1-4/9424f1.gif

16. Plants use signal transduction pathways — series of biochemical reactions by which a cell responds to a stimulus Protein kinase cascades amplify responses to signals as in other organisms  regulates genes expression http://www.bio.miami.edu/dana/pix/de-etiolation_pathway.jpg

17. Plant’s genome ultimately determines the limits of plant development The genome encodes plant’s “master plan”, but its interpretation depends on environmental conditions Environmental effects on plant growth can be tested in the lab using genetically identical plants to sort out genomic vs. environmental causation http://www.odec.ca/projects/2005/ster5b0/public_html/homepa1.jpg

18. Much recent progress in understanding plant growth and development has come from studies of Arabidopsis thaliana Used as model organism — it is small, matures quickly, it’s genome is small and has been fully sequenced Mutants provide insights into mechanisms of hormones and receptors http://aggie-horticulture.tamu.edu/faculty/davies/students/ngo

19. One technique for identifying genes involved in a plant signal transduction pathway is called a genetic screen: Mutants are created by insertion of transposons or point mutations by a chemical mutagen, usually ethyl methane sulfonate A large number of mutated plants are then screened for a specific phenotype, usually something easy to see or measure Once mutant plants have been selected, their genotypes and phenotypes are compared to those of wild-type plants http://www.cepceb.ucr.edu/images/members/raikhel/Fig9_031504.gif

20. Figure 37.3 A Genetic Screen Test tube has mutagen Exposed seeds are then grown and exposed to ethylene, one grows taller (shows that it has a gene that has mutated to make it resistant to methylene

21. Gibberellins In Asia, “foolish seedling disease” in rice causes plants to grow rapidly  tall and spindly, and dies before producing seeds It is caused by an ascomycete fungus Gibberella fujikuroi The fungus releases a molecule that stimulates plant growth (first isolated in 1925) Asci of Gibberella fujikuroi G. fujikuroi on maize www.rbgsyd.gov.au/__data/page/2288/

22. Gibberellins The action of gibberellin was studied in dwarf strains of corn and tomatoes. Gibberellin applied to seedlings of the dwarf strains caused them to grow as tall as wild type plants. Wild-type plants were shown to have much more gibberellin than dwarf strains.

23. Gibberellins Gibberellins are a class of plant hormone that stimulate stem elongation. They belong to a family of common plant metabolites called diterpenoids. They have multiple roles in regulating plant growth, as shown by experiments in which gibberellins are blocked at various stages of plant development.

24. Gibberellins Gibberellins regulate fruit growth. Seedless grape varieties have smaller fruit than seeded varieties. Experimental removal of seeds resulted in small fruits, suggesting seeds were the source of a growth regulator. Spraying young seedless grapes with gibberellins caused them to grow as large as seeded varieties.

25. Gibberellins In germinating cereal seeds, gibberellins diffuse through the endosperm to surrounding tissue called the aleurone layer underneath the seed coat Gibberellins trigger a cascade in this layer, causing it to secrete enzymes to digest the endosperm.

26. Gibberellins In the beer brewing industry, gibberellins are used to enhance “malting” (germination) of barley. Breakdown of the endosperm produces sugar that is fermented to alcohol. http://4e.plantphys.net/images/ch20/wt2002c_s.jpg

27. Gibberellins Inhibitors of gibberellin synthesis cause reduction in stem elongation in wild-type plants. These inhibitors are used in greenhouses to prevent plants from becoming tall and spindly. Also used to prevent “bolting” (producing a tall stem that flowers) in plants such as cabbage. Bolting

28. Auxin Auxins are a group of plant hormones Most important is indoleacetic acid (IAA) Discovery of auxin traced to Charles Darwin and his son Francis, who were studying plant movements Phototropism is growth of plant organs towards light (or away from light, as roots do) Photo 37.9 Phototropism: Plants grow toward light.

29. Auxin Darwins worked with canary grass Young grass seedlings have a coleoptile — a sheath that protects it as it pushes through soil Coleoptiles are phototropic If coleoptile tip was covered, there was no phototropic response. A signal travels from tip to growing region Light Source

30. Auxin In 1920s, Fritz Went removed coleoptile tips and placed cut surfaces on agar When agar was then placed on cut plants, they showed phototropic response A hormone had diffused into agar block…it was IAA

31. Auxin Lateral distribution of auxin causes plant movements Carrier proteins move to one side of cell rather than to the base When light strikes coleoptile on one side, auxin moves to other side, and elongation increases on that side. Coleoptile bends toward light (phototropism)

32. Auxin If shoot is tipped over, even in dark, auxin will move to lower side Cell growth results in bending of shoot so that it grows up — gravitropism. Upward gravitropic response of shoots is negative gravitropism; downward response of roots is positive gravitropism

33. Auxin How does a plant cell sense light and gravity? Phototropism—membrane receptor (phototropin) absorbs blue light When activated, a signal transduction pathway results in redistribution of auxin transport carriers Gravitropism some plant cells have large plastids called amyloplasts that store starch These plastids tend to settle on downward side of a cell in response to gravity This may disturb ER membranes and trigger auxin transport

34. Auxin Abscission – detachment of old leaves from stem Auxin inhibits abscission, which results from breakdown of cells in abscission zone of petiole Timing of leaf fall is determined in part by decrease in movement of auxin from blade through petiole

35. Auxin Fruit development normally depends on fertilization of the egg If unfertilized ovaries are treated with auxin or gibberellins, fruit will form — parthenocarpy Some plants spontaneously form parthenocarpic fruits (e.g., grapes, bananas, some cucumbers).

36. Auxin Auxin is essential for plant survival No mutants without auxin have ever been found. Some synthetic auxins are used as herbicides 2,4-D is lethal to eudicots at concentrations harmless to monocots Eudicots can’t break down the 2,4-D, and “grow themselves to death.” 2,4-D is a selective herbicide that can be used on lawns and cereal crops to kill eudicot weeds

37. What Are the Effects of Cytokinins, Ethylene, and Brassinosteroids? Plant cells such as parenchyma cells can be grown in a medium containing sugars and salts The cells will divide continuously until they run out of nutrients. Early work on cell culturing showed that coconut milk was the best growth supplement. A molecule in the milk likely stimulated cell division.

38. Several experiments identified adenine derivatives called cytokinins as the factor that stimulates cell division Over 150 different cytokinins have been isolated http://4e.plantphys.net/images/ch21/wt2102a_s.png

39. Cytokinins have many effects: With auxin, they stimulate rapid cell division in tissue cultures Cause light-requiring seeds to germinate in darkness In cell cultures, high cytokinin-to-auxin ratio promotes formation of shoots; a low ratio promotes formation of roots http://www2.ulg.ac.be/cedevit/image/hormones/utilis-horm_e.gif

40. Inhibit stem elongation but cause lateral swelling of stems and roots Stimulate axillary buds to grow. Auxin-to-cytokinin ratio controls extent of branching Delay senescence of leaves http://www2.ulg.ac.be/cedevit/image/hormones/utilis-horm_e.gif

41. Ethylene gas is produced by all parts of a plant promotes senescence promotes leaf abscission Balance of ethylene and auxin control leaf abscission Speeds ripening of fruit Ripening fruit loses chlorophyll and break down cell walls once ripening begins, more and more ethylene is produced Ripening apple gives off ethylene gas, which then causes leaf abscission in holly www.cropsci.uiuc.edu/classes/cpsc112/images/PGR

42. Commercial fruit growers use ethylene gas to speed up fruit ripening Ripening can be delayed by using “scrubbers” to remove ethylene gas from storage chambers Cut flowers are sometimes put into silver thiosulfate solution to inhibit ethylene (probably by combining with ethylene receptors) Effect of using ethylene on green tomatoes (on right) www.cropsci.uiuc.edu/classes/cpsc112/images/PGR

43. Plant steroid hormones were not discovered until the 1970s. Brassinosteroids were first isolated from mustard family plants Stimulated cell elongation, pollen tube elongation, and vascular tissue differentiation But inhibited root elongation. Mutant plants that don’t make brassinosteroids or have defects in signal transduction pathway are usually dwarf, infertile, and slow to develop. These effects can be reversed by adding small amounts of brassinosteroi