Respiration

1. Respiration

2. Lecture overview • What do plants do with the energy they get? • Mitochondria • Glycolysis (or oxidative pentose phosphate shunt) • Citric acid cycle • Oxidative phosphorylation • Fermentation • Gluconeogenesis • Respiration and stress

3. Net primary production (NPP) • NPP = GPP – Plant Respiration

5. R = R + R +R plant growth maint ion What controls respiration? ● Growth – building new biomass ●Maintenance– maintaining tissues ●Ion – transport across membranes is energetically expensive. Depends on form of nutrients taken up, (e.g., cost of reducing NO3- to NH4+), resource availability… - all processes that require energy, ATP

6. What is maintenance respiration? • Respiration associated with repair • Proteins • Membranes • Other • 85% is associated with maintaining proteins, which turn over at a rate of about 6% per day • Thus, there is a strong correlation between protein content and respiration rate in non-growing tissues • Maintenance respiration is probably about 50% of total plant respiration

7. Respiration: occurs in mitochondria From Rost et al., “Plant biology,” 2nd edn.

9. Plant respiration - chapter 11 of Taize Simple definition: The oxidation of sugars to produce usable energy (ATP), reductant (NADH), and carbon “skeletons” for biosynthesis. C12H22O11 + 12O2 --> 12 CO2 + 11 H2O 60 ADP + 60 Pi --> 60 ATP + 60 H2O What are the major steps? 1) glycolysis 2) citric acid cycle 3) electron transport/oxidative phosphorylation

11. Often, starch is the most important substrate for respiration, and it moves from the amyloplasts or chloroplasts by various mechanisms: - as glucose by a glucose transporter -as triose-P by the triose-P/Pi antiporter

12. Steps of respiration • Glycolysis • Pentose phosphate pathway • Citric acid cycle • Oxidative phosphorylation

13. Glycolysis - conversion of carbohydrates into pyruvate producing some ATP and NADH in cytoplasm and plastids - can happen in presence or absence of O2 - if O2, then pyruvate converted to acetyl CoA and into the citric acid cycle - if no O2,then “fermentation” (see later) occurs, pyruvate is reduced to lactic acid and/or ethanol

15. If aerobic - pyruvate from glycolysis is converted to acetyl CoA, which enters the citric acid cycle

16. Citric Acid Cycle • takes place in mitochondrial matrix, except succinate dehydrogenase • most NADH is produced

17. Electron transfer chain • Inner mitochondrial membrane • NADH made into ATP using an “ATP synthase” – basically a proton pump in reverse • Called oxidative phosphorylation • Four major complexes involved

19. Mitochondrial electron transport

21. Note: not all carbon entering respiratory pathway ends up as carbon dioxide – many important carbon skeletons are made e.g. for making proteins, lipids, DNA, cellulose etc.

22. Relationship of respiratory intermediates to other plant biosynthetic pathways

23. Oxidative Pentose Phosphate Pathway • Alternative to glycolysis • Also called the hexose monophosphate shunt • Converts glucose to Triose-P – this enters the last stages of glycolysis, carbon then enters citric acid cycle as normal • Only 5 – 20% respiration occurs this way • But – makes useful intermediates needed for making DNA, RNA and phenolics • Appears important during plant recovery from stress

25. Fermentation • Without oxygen, citric acid cycle and oxidative phosphorylation cannot work • “Fermentation” metabolizes pyruvate to give some ATP, CO2 and ethanol or lactic acid • Only 4% as efficient as the oxidative phosphorylation, and ethanol and lactic acid may be toxic • Note the “Pasteur effect” (absent in flooding tolerant plants)

26. Oilseeds are able to convert stored oil to carbohydrate • Many seeds store a significant portion of photoassimilate as oil, not carbohydrate • This oil is mobilized as an energy source upon germination • e.g., canola (45% oil by dry weight versus maize 5%) • Oil – not water soluble, not transportable • Most plants convert oil droplets (triglycerides)  sucrose to mobilize its energy • Animals cannot interconvert lipids and carbohydrates! • Again, this gives plants metabolic flexibility in allocating carbon between lipids and carbohydrates • Seeds can be smaller because lipids store more energy per gram!

27. Mobilizing the energy in stored oil involves the glyoxylate cycle and gluconeogenesis Figure 7.13 • Triglyceride conversion to sucrose involves 3 organelles + cytosol • Fatty acids are removed from triglyceride by lipase • FA imported into glyoxysome – specialized plant organelle • Cleaved at every 2nd C to generate acetyl CoA via ß-oxidation • Glyoxylate cycle take home messages: • Borrowing oxaloacetate from the mitrochondrion allows citrate synthesis from fatty acids • It’s a cycle! Regeneration of OAA in mt keeps acetyl CoA incorporation high • The products of the cycle enter gluconeogenesis to generate sucrose in the __________ • Glycerol from triglyceride also enters gluconeogenesis for sucrose biosynthesis • NADH enters oxidative phosphorylation a/k/a _____________

29. Left - typical late spring waterlogging of poorly drained field of peas (Pisum sativum) in Cambridgeshire, UK. Right – close-up of the injury sustained by leaves of a pea plant after several days soil waterlogging.

35. Aerenchyma formation in flooded roots

36. Flooding induced aerenchyma

37. In a desperate attempt to get ATP, glycolysis is stimulated (the “Pasteur effect”) resulting in pyruvate accumulation, and the accumulation of the by-products of anaerobic metabolism

41. Trial illustrating the superior tolerance to 10 days complete submergence of a line of rice (FR13A) derived from an old Indian farmer variety (left line of green plants) compared with two other lines of lowland rice (right).

42. Example of specialized heavy equipment, often employing laser-guided gradient sensing, to install subsurface plastic drainage pipes in arable farmland.

43. Mitochondria are major sites of ROS production following stress

44. Following stress, components of the mitochondrial electron transfer chain become damaged (e.g. ATP synthase). H2O2 generation in mitochondria is proportion to the pmf Skulachev, 1998

45. Respiration in Plants ► Energy coupled ► “Uncoupled” – ΔΨ is dissipated ► “Non-coupled” – electrons do not form ΔΨ – original electron transfer chain is modified, or other respiratory enzymes are involved Uncoupled or non-coupled respiration can reduce ROS formation following stress

46. The alternative oxidase (AOX) is mainly responsible for non-coupled respiration in plants

48. Feature of the AOX ► Thermogenic ► CN insensitive, SHAM inhibits ► AOX1 (stress induced); 2a,b; 3 ► ROS stimulate expression ► Over-expression reduces ROS formation ► Anti-sense AOX increases ROS formation ► Amount of protein not correlated to engagement in respiration

49. Fungi also contain rotenone insensitive external and internal NAD(P)H dehydrogenases (“class 2”)– lower efficiency alternatives to complex 1 – also thermogenic

50. Organization of alternative internal and external NADH dehydrogenases