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Plant Responses to Signals A weed is a plant that has mastered every survival skill except for learning how to grow in rows. - anonymous 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. Internal Signals • Signals can be processed by growing and non-growing cells, • Signals are transduced into biologically meaningful results through numerous and coordinated pathways, – changes in ion flux, – regulation of metabolic pathways, – regulation of gene expression, – changes in the cytoskeleton. Signal Transduction models Signal Signal Signal Signals Signal Response Response Response Linear Model Network Model 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. Plant Receptors i.e. light responses • 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 (+) De-Etiolation (italics) De-etiolation greening • 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 Phytochrome plant photoreceptor • 1920’s, – researchers observed chlorophyll deficient mutants (albinos) that underwent deetiolation when given physiologically active light, • 1950’s, – phytochrome discovered, • Molecular switch, signal transducer. N Phytochrome or chromophore • Proteins with tetra-pyrrole chromophores, and transmission kinase domains, kinase domains • Phytochrome gene family contains at least five members, C • Gene family members serve different functions. The Experiments Phytochrome Signal Transduction hypothesis Phytochrome is a greening receptor, cGMP is a second messenger, Signal red light Response Calcium is a second messenger. See Fig. 39.2 for general signal transduction. greening phytochrome (Pfr) …enhances the expression of MYB, Phytochrome Signal Transduction for 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, red light activates phytocrome, active phytochrome (Pfr) is transported to the nucleus, … CCA1 enhances the expression of CAB (chlorophyll A/B) proteins. CCA1 expression, in turn, enhances greening proteins. ...thousands of genes, phytochromes …hundreds of transcription factors, ...response. Nature Reviews Molecular Cell Biology 3; 85-93 Plant Responses to Signals II Phytohormones 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. 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. Phototropism Tropism: a growth response in plants that results in curvature toward, or away from a stimulus. Charles Darwin and his son Francis localized the location of perception for blue light phototropism. Positive Phototropism: growth toward a light stimulus Peter Boysen-Jensen demonstrated that a diffusable substance was involved . IAA Natural auxins... Peter Went demonstrated that the diffusible substance resulted in cell expansion. Went isolated the active compound. Bioassays auxin Oat Coleoptile (2 cm) • Oat Coleoptile (> 2 cm) Bioassay: identification (or quantitation) of a biologically active substance by measuring the effect the substance has on living material. Auxin Transport polar Synthesized in the SAM Auxin moves basipetally (from apex to base). Auxin …induces apical dominance, SAM intact SAM removed “Leader” basipetal movement of auxin inhibits axillary buds, axillary buds are released. Apical Dominance SAM intact SAM removed basipetal movement of axillary buds are released, auxin inhibits axillary buds, Auxins …induce vascular differentiation hydathode …xylem differentiation occurs around the wound, …xylem differentiates between hydathodes and leaf vasculature ...following the path of auxin diffusion. ...following the path of basipetal auxin transport. Auxins …induces lateral and adventitious root formation, Honeysuckle cut stems Rooting compounds. [ auxin ] Auxins …promotes fruit development, seeds removed seed Normal …embryo produces auxin that stimulates fruit development. seeds removed + auxin Strawberry See Fig. 39.11 Auxin …promotes cell expansion, Auxin activates the H+-ATPase. Acidification of the cell wall activates expansin (cell wall proteins). See Fig. 39.11a Acid Growth Hypothesis ABP + Auxin AUXIN BINDING PROTEIN Time course of action. …activates the pump. Mode of action (hypothesis) Auxin Binding Protein auxin receptor? over-express ABP1 gene • Auxin receptor has not conclusively been identified, – a protein that is found to bind auxin, (Auxin Binding Protein, ABP) has been proposed as a candidate receptor, add auxin • A common test for receptors, is to overexpress (express more than the normal amount of protein), and test to see if the signal is amplified, • ABP passes this test. Auxin Binding Protein dead auxin receptor? wild type abp mutant ABP controls cell division and elongation in embryogenesis. http://genesdev.cshlp.org/content/15/7/902 TIR1 To know: TIR1 represents a new signal transduction paradigm. auxin receptor U IV III U E2 I II IAA TIR1 IV U IV IAA III IAA III …affects transcriptional control RBX ASK CSN CUL1 TIR1, F-box portion of SCFTIR1 ubiquitin ligase complex, represents an auxin receptor (Dharmasiri et al. (2005) Nature 435: 441-445; Kepinski and Leyser (2005) Nature 435: 446-451) AuxRE GENE “X” (Harper et al.(2002) (2000)Plant PlantMol. Cell Biol. 12: 757-770; (Liscum & Reed 49: 387-400; Tatematsu al. (2004) Plant 16:49: 379-393) Esmon et al.et(2005) Int. J. Dev.Cell Biol. 665-674 Auxins …promote gene expression, 5 - 50 minutes Genes Products Concept Map Receptor Discovery Auxin Function(s) Transport