Respiration. Lecture overview What do plants do with the energy they get? Mitochondria Glycolysis...

Respiration

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

Net primary production (NPP)

•NPP = GPP – Plant Respiration

What controls respiration?

R = R + R +R plant growth maint ion

● 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

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

Respiration: occurs in

mitochondria

From Rost et al., “Plant biology,” 2nd edn.

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 H2O60 ADP + 60 Pi --> 60 ATP + 60 H2O

What are the major steps?1) glycolysis2) citric acid cycle3) electron transport/oxidative phosphorylation

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

Steps of respiration

• Glycolysis

• Pentose phosphate pathway

• Citric acid cycle

• Oxidative phosphorylation

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 andinto the citric acid cycle

- if no O2,then “fermentation” (see later) occurs, pyruvate is reduced to lactic acid and/or ethanol

If aerobic - pyruvate from glycolysis is converted to acetyl

CoA, which enters the citric acid cycle

Citric Acid Cycle

• takes place in mitochondrial matrix, except succinate dehydrogenase

• most NADH is produced

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

Mitochondrial electron transport

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.

Relationship of respiratory intermediates to other plant biosynthetic pathways

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

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)

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!

Mobilizing the energy in stored oil involves the glyoxylate cycle and gluconeogenesis

• 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

Figure 7.13

a/k/a _____________

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.

Aerenchyma formation in flooded roots

Flooding induced aerenchyma

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

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).

Example of specialized heavy equipment, often employing laser-guided gradient sensing, to install subsurface plastic drainage

pipes in arable farmland.

Mitochondria are major sites of ROS production following stress

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

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

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

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

Fungi also contain rotenone insensitive external and internal NAD(P)H dehydrogenases (“class 2”) – lower

efficiency alternatives to complex 1 – also thermogenic

Organization of alternative internal and external NADH dehydrogenases

Uncoupling proteins (UCPs) are responsible for “uncoupled” respiration in plants

Energy dissipation by the uncoupling proteins

Transmembrane arrangements of UCPs

Features of UCPs

► Thermogenic

► Several forms exist

► Occur in plants and fungi

► 40% homology with mammalian UCP

► Proton carrier or FFA carriers

► FFA stimulate activity

► Inhibition (purine nucleotides) increases ROS formation, adding FFA decreases ROS formation

Dead-horse arum (Helicodiceros muscivorus)

More reasons to heat up!

Real thermoregulation, very rare in plants!

Desiccation for 2.5 h followed by rehydration increases heat production in the dark for more than 4 h in Peltigera

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!

Mobilizing the energy in stored oil involves the glyoxylate cycle and gluconeogenesis

• 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

Figure 7.13

a/k/a _____________