1. Sensory Systems in Plants
Chapter 41

2. 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

3. Phototropisms Directional growth responses
Connect environmental signal with cellular perception of the signal, transduction into biochemical pathways, and ultimately an altered growth response

4. Gravitropism
Response of a plant to the gravitational field of the Earth
Shoots exhibit negative gravitropism
Roots have a positive gravitropic response

5. 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

7. 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

8. 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

10. Responses to Mechanical Stimuli
Some turgor movements are triggered by light
This movement maximizes photosynthesis

11. 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

12. 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

13. 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

15. 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

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

18. 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

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

21. 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

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

23. 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

25. 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

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

27. 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

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

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

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

31. 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

32. 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

33. 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

35. 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

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