Lecture21 - 1 Mature plant cells cease dividing but many...

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Unformatted text preview: 1. Mature plant cells cease dividing, but many can be induced to begin dividing again by wounding or abscission. [1913, the Austrian plant physiologist G. Haberlandt demonstrated that vascular tissue contained a water-soluble substance(s) that stimulated the cell division of wounded potato tuber tissue] 2. Wounding-induced cell division is self-limiting The derivative cells stop dividing and re-differentiate 3. Agrobacteria cells invade a wound, cells keep dividing leading to a neoplastic (tumor forming) disease known as crown call A. Heat tumor to 42OC, bacteria die, callus continues to grow! B. Infected cells can produce cell-division stimulatory substance(s) C. Cells can grow on a simple nutrient medium as a mass of unorganized, undifferentiated cells called callus tissue. Plant tissue culture without hormones Plant 1. 1930s, Philip White demonstrated that tomato roots can be grown indefinitely roots in a simple nutrient medium containing only sucrose, mineral salts, and a few in vitamins, with no added hormones vitamins, 2. Isolated stem tissues exhibit very little growth in such medium 2. little limited growth may occur when auxin is added, when but such an auxin-stimulated growth is due to cell enlargement but 3. but, the cultured stem can grow into a whole new plant 3. the when adventitious roots form on an auxin-containing medium adventitious A. There is a difference in the regulation of cell division in root and shoot meristem B. Root may synthesize some factors that promote shoot growth B. Search for substances that promote shoot growth in culture Search 1. In 1941, Johannes van Overbeek 1. discovered that the milky endosperm from coconut also had this ability. 2. 1940-1950 Skoog and his coworkers at University of Wisconsin found that autoclaved herring sperm DNA had a powerful cell-division promoting effect 3. The first cytokinin was isolated from herring sperm by Miller in 1955 This compound was named kinetin because of its ability to promote cytokinesis. A by-product of the heat-induced the degradation of DNA degradation 4. The first naturally occurring cytokinin was isolated from corn in 1961 by Miller and Letham. It was later called zeatin. 5. In higher plants, zeatin occurs in both the cis and trans configurations. Trans-zeatin is much more active than cis-zeatin 6. Zeatin is the most prevalent cytokinin in higher plants. Cytokinin are defined as compounds that have Cytokinin biological activities similar to those of trans-zeatin biological 1. Induce cell division in callus cells 1. cell in the presence of auxin in 2. Promote root formation from callus cultures 2. root when in the appropriate molar ratios to auxin when 3. Delay senescence of leaves 3. Delay 4. Promote expansion of dicot cyteldons Nearly all compounds active as cytokinins are N6-substituted aminopurines. Biosynthesis of cytokinin Biosynthesis Adding a 5-C side chain to Adding a free adenine base free 1. 1st major precursor is AMP (adenosine monophosphate) 2. The side chains of the cytokinins are made by the terpene pathway (IPP---isoprene) and added to the AMP by isopentenyl transferase (IPT) 3. The plant and bacterial enzymes differ 3. in adenosine substrate and the side chain Donor 4. Addition of a -OH group by a 4. cytochrome P450 monooxygenase cytochrome 5. Removal of the ribose group to become zeatin zeatin 6. Cis-trans isomerase 7. Glucosidase/deglucosidase (conjugation/deconjugation) (conjugation/deconjugation) 8. Cytokinin oxidase (inactivation) 8. Cytokinins are synthesized primarily in the meristematic region of the roots. Cytokinins are also produced in other cells/tissues They enter the shoot organs via the xylem (transpiration stream). the zeatin ribosides are the main transport form; converted to the free base or glucosides in the leaves some cytokinin also moves in the phloem. Free cytokinins are readily converted to their respective nucleoside and nucleotide forms 1a. Cytokinins Regulate Cell Division in Shoots 1a. CKX-OE CKX-OE 1b. Cytokinin suppresses the size and cell division activity of roots 1b. CKX-OE CKX-OE Increasing cytokinin concentration 2. The auxin/cytokinin ratio regulates morphogenesis in cultured tissues Increasing auxin concentration Increasing Low auxin/cytokinin ratio: shoot development High auxin/cytokinin ratio: root growth Intermediate ratio: callus tissue (disorganized cell mass) 3. Cytokinin modify apical dominance and promote lateral bud growth 3. Auxin suppresses the growth pf axillary buds (via inhibiting AtIPT genes) cytokinin stimulates cell division/outgrowth cytokinin (cytokinin-overproducing mutants are bushy) (cytokinin-overproducing 5. Delay leaf senescence 5. 6. Promote chloroplast development 6. (even in the dark!) (even 7. Promote cell expansion in leaves and cotyledons 7. 8. Cytokinin promotes movement of nutrients DARK LIGHT Screen for cytokinin-independent mutants. Mutated Arabidopsis calli were cultured in the absence of phytohormones for 3 weeks. Note that a portion of one callus is green, whereas most of the transformed calli are a yellowish color. This green region proliferated rapidly and regenerated shoots in the absence of exogenous cytokinin. Identification of the 1st cytokinin receptor, CKI1 for cytokinin-independent Identification Identification of the second cytokinin receptor in Arabidopsis Screening for Arabidopsis mutants that were impaired in cytokinin responses, including rapid cell proliferation and shoot formation in tissue culture. cytokinin response 1-1 (cre1-1) CRE1 is a homologue of CKI1 CRE1 is identified by a loss-of-function approach CKI1 is discovered by a gain-of-function method A yeast genetic experiment proving that CRE1 is a cytokinin receptor yeast (In the absense of (In galactose) galactose) Cytokinin receptors are histidine kinase histidine which, when bound by hormone, can activate hormone, a phosphorelay phosphorelay ...
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