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Prepare a 10 ’ talk for Friday March 3 on plant defense responses or describe interactions between plants& pathogens, pests or symbionts. Plant defense responses Hypersensitive response Systemic acquired resistance Innate immunity Phytoalexin synthesis Defensins and other proteins
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Prepare a 10’ talk for Friday March 3 on plant defense responses or describe interactions between plants& pathogens, pests or symbionts Plant defense responses • Hypersensitive response • Systemic acquired resistance • Innate immunity • Phytoalexin synthesis • Defensins and other proteins • Oxidative burst Some possible pests • Nematodes • Rootworms • Aphids • Thrips • Gypsy moths • hemlock woolly adelgid Some possible pathogens • Agrobacterium tumefaciens • Agrobacterium rhizogenes • Pseudomonas syringeae • Pseudomonas aeruginosa • Viroids • DNA viruses • RNA viruses • Fungi • Oomycetes Some possible symbionts • N-fixing bacteria • N-fixing cyanobacteria • Endomycorrhizae • Ectomycorrhizae
Photosynthesis 2 sets of rxns in separate parts of chloroplast
Light-independent (dark) reactions Overall Reaction: 3 CO2 + 3 RuBP + 9 ATP + 6 NADPH = 3 RuBP + 9 ADP + 9 Pi + 6 NADP+ + 1 Glyceraldehyde 3-P
Light-independent (dark) reactions 1) fixing CO2 2) reversing glycolysis 3) regenerating RuBP
fixing CO2 1) RuBP binds CO2
fixing CO2 • 1) CO2 is bound to RuBP • 2) rapidly splits into two 3-Phosphoglycerate • therefore called C3 photosynthesis • detected by immediately killing cells fed 14CO2
fixing CO2 • 1) CO2 is bound to RuBP • 2) rapidly splits into two 3-Phosphoglycerate • 3) catalyzed by Rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase) • the most important & abundant protein on earth • Lousy Km • Rotten Vmax! • Makes lots of mistakes!
Reversing glycolysis converts 3-Phosphoglycerate to G3P consumes 1 ATP & 1 NADPH
Reversing glycolysis • G3P has 2 possible fates • 1) 1 in 6 becomes (CH2O)n
Reversing glycolysis • G3P has 2 possible fates • 1) 1 in 6 becomes (CH2O)n • 2) 5 in 6 regenerate RuBP
Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell)
Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose
Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi
Reversing glycolysis 1 in 6 G3P becomes (CH2O)n either becomes starch in chloroplast (to store in cell) or is converted to DHAP & exported to cytoplasm to make sucrose Pi/triosePO4 antiporter only trades DHAP for Pi mechanism to regulate PS
Regenerating RuBP • G3P has 2 possible fates • 5 in 6 regenerate RuBP • necessary to keep cycle going
Regenerating RuBP Basic problem: converting a 3C to a 5C compound feed in five3C sugars, recover three5C sugars
Regenerating RuBP Basic problem: converting a 3C to a 5C compound must assemble intermediates that can be broken into 5 C sugars after adding 3C subunit
Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C
Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C
Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C 7C + 3C = 5C + 5C
Regenerating RuBP making intermediates that can be broken into 5 C sugars after adding 3C subunits 3C + 3C + 3C = 5C + 4C 4C + 3C = 7C 7C + 3C = 5C + 5C Uses 1 ATP/RuBP
Light-independent (dark) reactions build up pools of intermediates , occasionally remove one very complicated book-keeping
Light-independent (dark) reactions build up pools of intermediates , occasionally remove one very complicated book-keeping Use 12 NADPH and 18 ATP to make one 6C sugar
Regulating the Calvin Cycle Rubisco is main rate-limiting step
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubiscoactivase: Rubisco must be carbamylated & bind Mg2+ to be active!
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase : • uses ATP to activate rubisco
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • Rubisco must be carbamylated & bind Mg2+ to be active! • RuBP binds & inactivates uncarbamylatedrubisco • Rubiscoactivase removes this RuBP
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • Rubisco must be carbamylated & bind Mg2+ to be active! • RuBP binds & inactivates uncarbamylatedrubisco • Rubiscoactivase removes this RuBP • In the dark many species phosphorylate carboxyarabinotol to form carboxyarabinitol 1-phosphate which binds the rubisco active site
Regulating the Calvin Cycle • Rubiscoactivase removes this RuBP • In the dark many species phosphorylate carboxyarabinotol to form carboxyarabinitol 1-phosphate which binds the rubisco active site • Rubiscoactivase also removes CA1P in the light • CA1P phosphatase then removes the PO4
Regulating the Calvin Cycle Availability of CO2 Demand is set by mesophyll, stomata control supply Ci is usually much lower than Ca A vsCi plots tattle on the Calvin cycle
Regulating the Calvin Cycle A vsCi plots tattle on the Calvin cycle • In linear phase rubisco is limiting • When curves RuBP or Pi regeneration is limiting
Regulating the Calvin Cycle Currently Rubisco usually limits C3 plants Will increase plant growth until hit new limiting factor
Regulating the Calvin Cycle Currently Rubisco usually limits C3 plants Will increase plant growth until hit new limiting factor Free-Air CO2 Enrichment Experiments show initial gains, but taper off w/in a few years Now are limited by nutrients or water
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH: rubisco is most active at pH > 8 • (in dark pH is ~7.2)
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH: rubisco is most active at pH > 8 • b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH: rubisco is most active at pH > 8 • b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark • Mg2+ moves from thylakoid lumen to stroma to maintain charge neutrality
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • a) pH: rubisco is most active at pH > 8 • b) [Mg2+]: in light [Mg2+] in stroma is ~ 10x greater than in dark • c) CO2 is an allosteric activator of rubisco that only binds at high pH and high [Mg2+] • also: stomates open in the light
Regulating the Calvin Cycle • Rubisco is main rate-limiting step • indirectly regulated by light 2 ways • 1) Rubisco activase • 2) Light-induced changes in stroma • Several other Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also activated by high pH & [Mg2+]
Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark
SH SH light oxidized enzyme (inactive) S - S reduced thioredoxin 2Fdox 2e- PSI + PSII S - S SH SH oxidized thioredoxin reduced enzyme (active) 2Fdred • Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark • in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme
Regulating the Calvin Cycle • Several Calvin cycle enzymes (e.g.Fructose-1,6-bisphosphatase) are also regulated by thioredoxin • contain disulfide bonds which get oxidized in the dark • in light, ferredoxin reduces thioredoxin, thioredoxin reduces these disulfide bonds to activate the enzyme • How light reactions talk to the Calvin cycle SH SH light oxidized enzyme (inactive) S - S reduced thioredoxin 2Fdox 2e- PSI + PSII S - S SH SH oxidized thioredoxin reduced enzyme (active) 2Fdred
PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + phosphoglycolate
PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate Releases CO2 without making ATP or NADH
PHOTORESPIRATION • Releases CO2 without making ATP or NADH • Called photorespiration : undoes photosynthesis
PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate C3 plants can lose 25%-50% of their fixed carbon
PHOTORESPIRATION Rubisco can use O2 as substrate instead of CO2 RuBP + O2 <=> 3-phosphoglycerate + Phosphoglycolate C3 plants can lose 25%-50% of their fixed carbon Both rxns occur at same active site
PHOTORESPIRATION C3 plants can lose 25%-50% of their fixed carbon phosphoglycolate is converted to glycolate : poison!
Detoxifying Glycolate 1) glycolate is shuttled to peroxisomes
Detoxifying Glycolate • 1) glycolate is shuttled to peroxisomes • 2) peroxisomes convert it to glycine • produce H2O2