Plant lecture 12 - Regulation/ evolution of C4/CAM

Question 1

Evolution of oxygenic photosynthesis + Rubisco

Answer 1

• Early atmosphere of earth = high CO2 conc. but negligible O2
• Atmospheric O2 ↑ steeply during great oxidation event
• All organisms that do oxygenic photosynthesis use Rubisco for the primary CO2 assimilation reaction, despite its inefficiencies
• Originally thought 2 types of ancestral Rubisco: 1A + 1B
• Interestingly, Higher plants have 2x specificity compared to 1A, 1B + bacteria. Indicates constraints under Rubisco operates, only 2x ↑ over ↑ ↑ time
• Cyanobacterium ↑ CO2 conc. of around 250um, have different CO2 conc. mechanism based on pumping HCO3- out into chloroplast. Highest turnover number + photosynthesis (then CC4 then C4 then C3)

Question 2

Evolution of C3 to C4

Answer 2

• BS cells enlarge and mitochondria (with GDC) become localized to inner wall of BS cells
• In M cells, GDC lost from mitochondria + concentrated in BS cells, where chloroplasts become appressed to mitochondria
• In M cells, Rubisco lost from chloroplasts and isoform of PEPC becomes highly expressed; BS cells acquire chloroplast C4-decarboxylase (NADP-ME) and lose PSII
• Any photorespiratory metabolism now restricted to BS cells and resulting CO2 efficiently refixed by Rubisco

Question 3

Single celled C4

Answer 3

• Only few examples e.g. large-celled halophyte Bienertia cycloptera
• Immunoblotting showed Rubisco = localised to basal part of cell + malic E = restricted to basal part
• PPDK = restricted to upper cell + PEPC is restricted round cell periphery
• Thought once C4 acid formed, decarboxylated by malic E + pyruvate formed in base of cell. Intermediates diffuse up through cytosol to top part of cell where PPDK converts pyruvate → PEP

Question 4

C4 evolution

Answer 4

• Reconstruction of CO2 conc. during Cenozoic era
• Showed significant ↓ in CO2 during Eocene + Oligocene epochs
• Thought where C4/CAM emerged
• Different E discriminate against 13C to different degrees (PEPC discriminates less than Rubisco). Through teeth of fossil equids, found towards end of Miocene become more like C4
• Thought due to aridification of earth’s climate
• Through molecular evolutionary sequence analysis, thought CAM evolved at same time of aridification