Sensory Systems in Plants

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