Presentation is loading. Please wait.

Presentation is loading. Please wait.

Management of Reserves Alison Smith CO 2 fixation produces triose P conversion to sucrose for translocation to sink tissues nutrient.

Similar presentations

Presentation on theme: "Management of Reserves Alison Smith CO 2 fixation produces triose P conversion to sucrose for translocation to sink tissues nutrient."— Presentation transcript:


2 Management of Reserves Alison Smith CO 2 fixation produces triose P conversion to sucrose for translocation to sink tissues nutrient assimilation - N, S, P – and synthesis of all cellular components synthesis of reserves and their mobilisation regulation is essential

3 Management of Reserves Lectures 9. Carbohydrate synthesis, transport and storage 10. Lipid synthesis, mobilisation of reserves 11. Regulation of metabolism

4 Lecture 9 - Carbohydrate Synthesis, Transport and Storage Sugars produced by plant Pathway of sugar synthesis from photosynthate How do you work out whats happening?!

5 chloroplast Photosynthetic cell Synthesis of Starch and Sucrose sucrose starch 1,3 bisPGA PGA CO 2 RuBP Triose P Sucrose is principal photosynthetic product –accounts for most of CO 2 absorbed important storage sugar –tap root of carrots and sugar beet (up to 20% dry weight) –and in leaves, eg 25% leaf dry weight in ivy major form for translocation of carbon –from photosynthetic leaves (source leaves) –in germinating seedlings after starch or lipid breakdown RuBP = ribulose 1,5-bis-phosphate (pentose) 3-PGA = 3-phosphoglycerate 1,3 bisPGA = 1,3 bis-phosphoglycerate

6 Sugar Translocation is Essential Sugars required for metabolism –all the time, in all tissues Sugars produced only –by source tissues –in light period Translocation occurs –source to sink over short term –from storage tissues to young tissues over long term

7 Sugar translocation Analysis of phloem sap –shallow incision produces little sap CH 2 OH | HCOH HOCH | HOCH HOCH | HCOH HCOH | HCOH HCOH | CH 2 OH CH 2 OH Sorbitol (Rosaceae) Mannitol (Combretaceae) Sugar alcohols (Polyols) Sugars Sucroseglucose-fructose (G-F) RaffinoseG-G-F StachyoseGalactose-G-G-F - severed aphid stylets most effective! stylet bundle cut by laser or radiofrequency microcautery

8 Sugar composition of phloem sap > 500 different species (100 families) of dicots (Zimmermann & Ziegler, 1975) most families transport sucrose concentration in phloem sap can reach 1 M SucroseRaffinose Stachyose Sugar alcohols Most families Aceraceae (maple)++++ Tr Tr - Anacardiaceae (cashew)+++ Tr Tr - Asteraceae (aster)+ Tr Tr - Betulaceae (birch) Buddleiaceae (butterfly bush) Caprifoliaceae (honeysuckle) Tr - Combretaceae (white mangrove) Fabaceae (legume)++++ Tr Tr - Fagaceae (beech & oak)++++ Tr Tr - Moraceae (fig) Oleaceae (olive) Rosaceae (rose)+++ Tr Tr ++++ Verbenaceae (verbena)

9 Photosynthetic cell transitory starch storage green leaves Starch is made in photosynthetic and non- photosynthetic cells amyloplast Non-photosynthetic cell long-term starch storage roots, tubers, seeds starch chloroplast sucrose Triose P starch sucrose

10 Importance of Starch Sunflower after 47 min photosynthesis Carbon absorbed (mg) 7.87 Hexose accumulated 1.17 Sucrose 4.20 Starch 1.84 Starch is the dominant storage polysaccharide in most plants In leaves - transitory starch - in chloroplasts –high percentage of CO 2 assimilated goes directly into starch In nonphotosynthetic cells - storage starch in amyloplasts –storage organs bananas, tubers (up to 80% dry weight), cereal grains (75% dry weight) –herbaceous roots, underground stems, bulbs perennials –trees young twigs, roots, parenchyma of bark xylem & phloem

11 Composition of Starch Amylose – -1,4-glucan –~1000 glucose units Starch grain –Water insoluble, –size & shape is species specific Amylopectin – -1,4 & -1,6-glucan –10, ,000 glucose units –highly branched, glucoses/branch potato: oval, 100 µm in diameter rice: angular, 10 µm in diameter acceptors for addition of further glucose units start (reducing end) polymer of glucose units

12 Fructans Some plants store other compounds Most common are the fructans –water-soluble, non reducing polymers of fructose – fructose units, joined to one glucose Leaves, flowers and underground storage organs –Asteraceae (dahlias, jerusalem artichokes) –Liliaceae (onions, asparagus) –Iridaceae (irises) Leaves of temperate Gramineae –C3 grasses - barley, oats, rye grass –major feedstuff for cattle & sheep in temperate zones –But store starch in the seed

13 How are Sucrose and Starch Synthesised? Elucidated by careful biochemical studies Determination of enzyme activities –what reactions are catalysed, thermodynamic considerations Correlation with flux through pathways –is the activity which can be measured sufficient to account for the process taking place?

14 Enzymes of Sucrose Metabolism Sucrose UDP-Glucose Fructose 6P Sucrose P Pi Sucrose P Synthase Sucrose P Phosphatase UDP Fructose + UDP-Glucose Sucrose Synthase UDP Fructose + Glucose Invertase

15 Relationship between enzyme activities and sucrose synthesis Sucrose is made via Sucrose-P 14 CO 2 incorporation experiments show label goes from UDP-Glucose sucrose-P sucrose Activity (µmol/hr per g FW) Tissue Sucrose P Sucrose synthasesynthase Spinacia oleracea leaf Lolium temulentum leaf Pisum sativum root stele Pisum sativum root cortex

16 Location of sucrose synthesis Subcellular fractionation of pea leaves Homogenize tissue in isotonic buffer 36000g supernatant Spin at 36000g 36000g pellet Spin at 2000g 2000g pellet 2000g supernatant Activity per fraction (µmol/h) Cell fraction SPS Rubisco 2000g pellet g pellet Supernatant ie SPS is in the cytosol

17 Permeability of Chloroplast Envelope How does fixed carbon get to the cytosol? –Uptake of labelled compounds into isolated chloroplasts time (sec) hexose P 3 PGA nmol/mg chl Triose phosphate and 3-Pglycerate can cross envelope at rates comparable to photosynthesis CH 2 OPCH 2 OP CH 2 OP | | | CHOHC=O CHOH | | | COOHCH 2 OH CHO 3-P glycerateDHAPGlyceraldehyde 3-P

18 Phosphate Translocator Competition experiments –uptake of 3-carbon compounds by same carrier –strict counter exchange for Pi Export of carbon –Major compound exported is DHAP – 20X more than Ga3P 3-PGA taken up Pi released time (sec) nmol/mg chl

19 Synthesis of UDP-Glucose Triose P is converted to hexose P by gluconeogenesis This must be converted to substrate for sucrose P synthase (UDP-G) CH 2 OH OH O-P-O-P-O-uridine OH O O-O- O-O- O O Glucose 1-P UDP-G pyrophosphorylase UTP PPi

20 Pathway of sucrose synthesis from CO 2 DHAP CO 2 Ga3P 3PGA RuBP 1,3 bisPGA CO 2 FBP Ga3P sucrose P F6P G6P Pi DHAP sucrose UTP PPi G1P UDGP

21 Enzymes of Starch Synthesis 1,4 glucan n+1 Glucose 1-P 1,4 glucan n Starch phosphorylase Pi ADPglucose Starch synthase 1,4 glucan n Starch Glucose 1-P PPi ATP ADPG PPiase

22 Starch Synthesis in vivo Hard to measure starch synthase and phosphorylase in vivo –Phosphorylases act in degradative direction –Arabidopsis starchless mutant Plants ADPG PPiase Starch Wild type 54 ± 57.3 ± 0.4 Mutant 0 0 F 1 (WT x Mutant) 50 ± 46.0 ± 0.5 Glucose units added to non-reducing end, from ADP-G, forming -1,4 links ADP ADP-G starch synthase

23 The First Plant Biochemist Gregor Mendel round versus wrinkled peas

24 wrinkled (rr) peas have less amylopectin than wt - collapse on drying Mendels Wrinkled Peas One isozyme of BE has 0.8 kbp transposon in r locus (rugosus) [Alison Smith, John Innes Centre] Weight (mg) BE (µmol.min -1 ) Starch (mg) RR rr RR rr Branching Enzyme Activity in embryos

25 Branching Enzyme Branching enzyme forms the -1,6 links start branching enzyme starch synthase -1,4 link -1,6 link

26 chloroplast source cell Synthesis of Transitory Starch sucrose starch ADP-G PPiase, Starch synthase and BE all found in chloroplast export to phloem PGA 1,3 bisPGA Triose P CO 2 RuBP

27 Lecture 9 - Summary Sugars translocated –Mainly sucrose, but also other tri- and tetrasaccharides and sugar alcohols Storage carbohydrate –starch or fructans Elucidating a metabolic pathway –determining enzyme activities, correlating with flux measurements Subcellular location –cell fractionation Regulation –there must be some!

28 Genetic or biochemical modifications of starch are or may be used for... Modified starch Phosphate content water absorbency improve starch granule integrity (cross linker) modified starch fried snacks (crispness / browning) thickener / gelling agent biodegradable packing material film coating + amylose Improve freeze- thaw of frozen food paper strength adhesive livestock feed addition + amylopectin

Download ppt "Management of Reserves Alison Smith CO 2 fixation produces triose P conversion to sucrose for translocation to sink tissues nutrient."

Similar presentations

Ads by Google