1. AGR2451 Lecture 11 - M. Raizada “Using and Storing Plant Carbon” 1. Photosynthesis results in high energy cofactors (ATP/NADPH) to allow chemical reactions to occur (via enzymes encoded by genes). 2. The Calvin Cycle uses these cofactors to remove C from CO2, add it to a 5C acceptor, to build 2x3C and regenerate 5C. 3. Carbon-fixation from CO2 occurs via RuBisCo, the most abundant and perhaps slowest enzyme known. 4. O2 competes with CO2 for binding to RuBisCo. QUESTIONS?? ------------------------------------------------------------------------- 5. 3C is produced in the chloroplast. The initial 3C can be broken down to a very reactive 2C (acetyl Co-A) or can form the 6C glucose subunit in the cytoplasm. What are the 4 major fates of the 3C molecule? (see handout): Slide 11.1

2. What are the 4 major fates of the 3C molecule? (see handout): Slide 11.2

3. 6. Glucose (6C) glucose is a temporary storage form for C-C bonds. 7. Sucrose (12C) The transport form of C-C in a plant is typically 12C (ie.sucrose), transported in the phloem. This is why the sap of the trunk of a Maple tree is sweet or why the stalks of sugarcane are sweet. 8. Storage Compounds For long-term storage, long chains of C-C bonds are formed as fats/oils, or starches. Long carbon chains are very stable and unreactive especially fats/oils stored in seeds (high in calories), stored in specialized oil body organelles. Why is carbon stored in long chains (starches/oils) instead of simpler subunits (such as glucose)? Slide 11.3

4. 9. Plant Cell Walls Glucose subunits joined together by a large enzyme complex on the outside of the plasma membrane. P.648 and p.659 Biochemistry and Molecular Biology of Plants Slide 11.4 Cellulose microfibres Construction of Cellulose cell wall

5. 10. Plant Structure vs. Metabolic Function Draw Plant *** In breeding or during evolution, the needs of primary metabolism (carbon fixation, etc.) drove plant development, not the other way around. Structures exist for carbon/nitrogen/H20. Carbon metabolism, alone, explains much of the plant structure. Slide 11.5

6. 11. Energy may be stored in the cell walls of leaf cells,in the roots, not just seeds/fruits. Grazing livestock mostly feed, not on seeds, but on the cell walls of leaves. Thus, it is important to view the entire plant as a nutrient-storage organism. 12. Carbon Partitioning Carbon reserves may be remobilized when the plant has a need (eg. grain-fill) or to any growing organ. What are some of the primary carbon “sinks”? 1. 2. 3. 4. 5. Therefore, the control of carbon transport or “partitioning” of carbon to these sinks is critical to agriculture and breeding. eg. vegetative growth vs. seeds. Harvest Index?? ____________________________________ -Up to 30-40% of fixed carbon is transported to the roots, some of It excreted into the soil for use by microbes/fungi. 13. Gross productivity?? ______________________________ -40-80% of photosynthate is lost in respiration or photorespiration -There has been ~no change in Gross Productivity after 10,000 years of human selection! Breeding has simply allowed the carbon to be partitioned to edible organs, increasing the Harvest Index. Slide 11.6

7. 14. How efficient is the transfer of plant carbon to humans? 25-70% of plant carbon lost as respiration, 30-60% is undigestable, leaving up to 2-15% for insects and herbivores. Starting with ________ calories of sunlight in an alfalfa field, _____ calories are produced as hay for cow feed, resulting in ____calories of cow matter, producing __ calorie of human matter. In natural ecosystems, 0.2 to 3.5% of incident light energy is converted into dry matter =net productivity. This can be as high as 4% in agricultural C4 crops such as maize and sugarcane Through animal chain, up to 90% of plant calories are lost due to respiration or undigested material: chickens (75% lost) but less For beef (4-6 kg plant protein = 1 kg beef). Plants, Genes and Agriculture,pp.183 M. Chrispeels and D.Sadava Jones and Bartlett Publishers, Boston, 1994 Slide 11.7

8. 15. What ratio of storage molecules do plants produce? Plant Dry Weight: 44% = Carbon (44% is oxygen, 6% is hydrogen, 1-4% is N, 0.5-6% is Potassium, 0.2-3.5% is Calcium, and P, Mg and S are each less than 1%) -59% of leaf dry weight = carbohydrates, 6% are lipids, 19% are proteins, 9% are minerals and 7% are others. Corn seeds have higher lipids (oils). What constitutes 50% of the leaf protein? _______ Breeding programs alter carbohydrate/sugar/fat and protein ratios, especially in seeds. 16. What ratio of carbon storage molecules do humans consume? Plants, Genes and Agriculture,pp.111 M. Chrispeels and D.Sadava Jones and Bartlett Publishers, Boston, 1994 118

9. 16. Today, plant storage carbon is being manipulated for human health, for renewable industrial products and energy. possible improvement is to develop monounsaturated soils that may reduce risk of heart attacks and strokes: Canola is a mutation in a gene of Brassica napus (rapeseed) that decreased a monosaturated fatty acid (22:1) called Erucic acid which constituted 50% of the oil but caused heart disease much work on developing better oils for animal feed fatty acids can be used to produce nylon, plastics, lubricants, soaps, paints, detergents, adhesives, and perhaps biofuels -ethanol/methanol from corn (current car gasoline is largely from ancient plants) oil found in distinct oil bodies in plant cells breeding and genetic engineering can produce novel materials Examples demo -- biodegradeable foam packing chips (starch) plastic granules produced in plants (in Arabidopsis)using a transgene from bacteria = biodegradeable plastic Slide 11.9 Figures 1 and Figure 3 from: Biochemistry and Molecular Biology of Plants (W.Gruissem, B. Buchanan and R.Jones p.525 ASPP, Rockville MD, 2000