1. Plant Growth Regulators
Lecture 13

2. Objectives Define hormones
Discuss the roles of plant growth regulators
Evaluate the mechanism of plant growth regulators.
Suggest how plant regulators can be used to increase efficiency and productivity in agriculture.

3. 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

4. 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

5. Auxin Production Shoot tips Developing seeds Some known actions
Establishment of polarity of root-shoot axis during embryogenesis
Cell elongation
Cell differentiation
Apical dominance
Lateral root formation and adventitious root formation
Fruit formation

6. Darwins’ (Charles and son) experiment
Under normal conditions, shoot tips bend towards the light
Without light on the tip, no bending
When not at tip, collar doesn’t prevent bending
Conclusion: Light is sensed at the tip, but response not at tip
New hypothesis: A substance or chemical is transported
Auxin later isolated from shoot tips and established to be involved in cell elongation
Drawings depicting seedlings of Zea (Gramineae family)

8. Evidence for the role of auxin in apical dominance
High auxin concentration
Low auxin concentration
Drawings depicting Coleus (Lamiaceae family)

9. Evidence for the role of auxin in adventitious root formation
With synthetic auxin
Without synthetic auxin
Adventitious roots growing from stem tissue
Saintpaulia (Gesneriaceae family)
Another example of misleading common name
The African violet is not in the violet family

10. Band of achenes removed
Evidence for the role of auxin in formation of fruit and structures of similar function (e.g. receptacle in strawberry)
Normal conditions
All achenes removed
Band of achenes removed
Without seed formation, fruits do not develop. Developing seeds are a source of auxin.
What do you expect?
Not shown: Auxin replacement restores normal fruit formation and can be used commercially to produce seedless fruits
However, too much auxin can kill the plant and thus synthetic auxins used commercially as herbicides
Fragaria (Rosaceae family)

11. 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

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

14. 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

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

17. How Auxin Works
Two families of proteins mediate auxin-induced changes in gene expression
Auxin response factors (ARFs)
Can enhance or suppress transcription
Aux/IAA proteins
Bind and repress proteins that activate the expression of ARF genes
TIR1 is the auxin receptor

19. How Auxin Works
Unlike with animal hormones, a specific signal is not sent to specific cells, eliciting a predictable response
Most likely, multiple auxin perception sites are present
Auxin is also unique among the plant hormones in that it is transported toward the base of the plant

20. How Auxin Works
One of the direct effects of auxin is an increase in the plasticity of the plant cell wall
Works only on young cell walls lacking extensive secondary cell wall formation
Acid growth hypothesis
Cells actively transport hydrogen ions from the cytoplasm into the cell wall space
Drop in pH activates enzymes that can break the bonds between cell wall fibers

22. 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

23. Cytokinins
Plant hormone that, in combination with auxin, stimulates cell division and differentiation
Synthetic cytokinins

24. 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

26. 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

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

28. 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

29. Adding gibberellins to certain dwarf mutants restores normal growth and development

30. Gibberellins
GA is used as a signal from the embryo that turns on transcription of genes encoding hydrolytic enzymes in the aleurone layer
When GA binds to its receptor, it frees GA-dependent transcription factors from a repressor
These transcription factors can now directly affect gene expression

32. Gibberellins Hasten seed germination
Used commercially to extend internode length in grapes
Result is larger grapes

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

34. 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

35. 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

36. 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

38. 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 senescence

39. Abscisic Acid Necessary for dormancy in seeds
Prevents precocious germination called vivipary
Important in the opening and closing of stomata