Plant Responses to Signals

1. Plant Responses to Signals A weed is a plant that has mastered every survival skill except for learning how to grow in rows. - anonymous

2. External Signals • External signals are used by plant cells to alter their physiology, morphology and development, • physical environment, • chemical environment, • biological environment, • sometimes other plants, • Plants receive signals at the cell level, and have no well defined sensory organs, • Except for gravity, all other signals are constantly variable.

3. Internal Signals • Signals can be processed by growing and non-growing cells, • Signals are transduced into biologically meaningful results through numerous and co-ordinated pathways, • changes in ion flux, • regulation of metabolic pathways, • regulation of gene expression, • changes in the cytoskeleton.

4. Network Model Signal Transductionmodels Signal Signal Signal Signals Signal Response Response Response Linear Model

5. Reception • Cell surface receptors, • hydrophilic molecules such as peptides and carbohydrates don’t readily cross the membrane, • are perceived on the cell surface, • Amphiphilic and hydrophobic molecules may pass through the membrane to receptors, • steroid hormones for example, • Light may be perceived at the cell surface, or in the cell.

6. De-Etiolation (italics) Plant Receptorsi.e. lightresponses • Where does one look? • no clearly differentiated organs (i.e., eyes, ears, etc.), • sensitive tissues, however, no clearly differentiated cells, • lots of responses. • Germination (+/-) • Stem length (-) • Leaf expansion (+) • Flowering (+/-) • Phototropism (+/-) • Stomatal opening (+) • Chloroplast development (+) • Pigment synthesis (+)

7. De-etiolationgreening Etiolated De-etiolated • Etiolated growth habit, • long stem, • unexpanded closed leaves, • etioplasts vs chloroplasts, • lack of chlorophyll, • apical hook, • short root, • Photomorphogenesis, • Inhibited stem growth, • Expanded leaves, • Pigmentation. • Root development. Nature Reviews Molecular Cell Biology 3; 85-93

8. Phytochromeplant photoreceptor • 1920’s, • researchers observed chlorophyll deficient mutants (albinos) that underwent de-etiolation when given physiologically active light, • 1950’s, • phytochrome discovered, • Molecular switch, signal transducer.

9. C N Phytochrome • Proteins with tetra-pyrrole chromophores, and transmission kinase domains, • Phytochrome gene family contains at least five members, • Gene family members serve different functions. or chromophore kinase domains

10. The Experiments

11. Phytochrome is a greening receptor, cGMP is a second messenger, Calcium is a second messenger. Phytochrome Signal Transductionhypothesis Signal red light Response greening See Fig. 39.2 for general signal transduction.

12. phytochrome (Pfr) …enhances the expression of MYB, Phytochrome Signal Transductionfor real • Active phytochrome (Pfr) is transported to the nucleus, … binds and inactivates transcriptional repressor (PIF3), • …MYB transcription factor is expressed, in turn activates CCA1 transcription, • … CCA1 enhances the expression of CAB (chlorophyll A/B) proteins. red light activates phytocrome, active phytochrome (Pfr) is transported to the nucleus, CCA1 expression, in turn, enhances greening proteins.

13. ...response. ...thousands of genes, phytochromes …hundreds of transcription factors, Nature Reviews Molecular Cell Biology 3; 85-93

14. Plant Responses to Signals IIPhytohormones No one can look at the plants growing on a bank or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated...they are extremely heliotropic; and this probably serves...as a guide (for) the buried seeds through fissures in the ground or through overlying masses of vegetation, into the light and air. - Charles Darwin “The Power of Movement in Plants” (1880) • Charles Darwin was a Plant Physiologist, • Phototropism, • Introduction to the plant hormone Auxin.

15. Phytohormones …a plant product that is able to stimulate physiological responses at very low concentrations, • either in the tissue in which it is synthesised, • or in other regions of the plant to which it is transported, ...do not operate in isolation from one another, but often act in co-ordination to produce subtle responses, …affect gene expression, enzyme activity and membrane function.

16. Tropism: a growth response in plants that results in curvature toward, or away from a stimulus. Phototropism Charles Darwin and his son Francis localized the location of perception for blue light phototropism.

17. Positive Phototropism: growth toward a light stimulus Peter Boysen-Jensen demonstrated that a diffusable substance was involved .

18. Natural auxins... Peter Went demonstrated that the diffusible substance resulted in cell expansion. Went isolated the active compound. IAA

19. Oat Coleoptile (2 cm) Oat Coleoptile (> 2 cm) Bioassays • Bioassay: identification (or quantitation) of a biologically active substance by measuring the effect the substance has on living material. auxin

20. Synthesized in the SAM Auxin Transportpolar Auxin moves basipetally (from apex to base).

21. “Leader” Auxin…induces apical dominance, SAM intact SAM removed basipetal movement of auxin inhibits axillary buds, axillary buds are released.

22. SAM intact SAM removed basipetal movement of auxin inhibits axillary buds, axillary buds are released, Apical Dominance

23. Auxins …induce vascular differentiation hydathode …xylem differentiates between hydathodes and leaf vasculature ...following the path of basipetal auxin transport. …xylem differentiation occurs around the wound, ...following the path of auxin diffusion.

24. Auxins …induces lateral and adventitious root formation, Honeysuckle cut stems Rooting compounds. [ auxin ]

25. Auxins …promotes fruit development, seeds removed seeds removed + auxin seed Normal …embryo produces auxin that stimulates fruit development. Strawberry

26. See Fig. 39.11 Auxin…promotes cell expansion, Auxin activates the H+-ATPase. Acidification of the cell wall activates expansin (cell wall proteins).

27. See Fig. 39.11a Mode of action (hypothesis) Acid Growth Hypothesis ABP + Auxin …activates the pump. AUXIN BINDING PROTEIN Time course of action.

28. Auxin Binding Proteinauxin receptor? over-express ABP1 gene • It’s a protein that is found to bind auxin, (Auxin Binding Protein, ABP) has been proposed as a candidate receptor, • A common test for receptors, is to over-express (express more than the normal amount of protein), and test to see if the signal is amplified, • ABP passes this test. • However, it’s not the Auxin Receptor add auxin

29. wild type abp mutant Auxin Binding Proteinauxin receptor? dead ABP controls cell division and elongation in embryogenesis. http://genesdev.cshlp.org/content/15/7/902

30. IV IV U U U TIR1 III III IAA IAA IAA E2 RBX ASK IV CSN III CUL1 II TIR1, F-box portion of SCFTIR1 ubiquitin ligase complex, represents an auxin receptor I AuxRE GENE “X” (Harper et al. (2000) Plant Cell 12: 757-770; Tatematsu et al. (2004) Plant Cell 16: 379-393) Liscum & Reed (2002) Plant Mol. Biol. 49: 387-400; Esmon et al. (2005) Int. J. Dev. Biol. 49: 665-674 (Dharmasiri et al. (2005) Nature 435: 441-445; Kepinski and Leyser (2005) Nature 435: 446-451) To know: TIR1 represents a new signal transduction paradigm. TIR1auxin receptor …affects transcriptional control

31. Auxins …promote gene expression, 5 - 50 minutes Genes Products

32. Concept Map Receptor Discovery Auxin Function(s) Transport