FiveApproaches to

Manipulate Carbon Fixation in

C3 Crops

Introduction: Carbon fixation efficiency in C3 plants is limited by environmental factors such as temperature, CO2 concentration, water availability, and C3 leaf cell anatomical characteristics. A focus of research in plant physiology is to manipulate how crop C3 plants may respond to increasing global temperatures and CO2 levels. Rubisco (ribulose 1,5-biphosphate carboxylase/oxygenase), an enzyme concentrated in leaves, is hugely important to life on earth because it catalyzes the first step of carbon fixation (carboxylation) in photosynthesis. Ironically, much of C3 plant photosynthetic inefficiency can be attributed in part to rubisco, as it will also catalyze oxygen fixation (oxygenation). The competing processes of carboxylation in photosynthesis and oxygenation in photorespiration can result in a loss of 30% to 50% of the carbon fixed in C3 plants (Fig1). The purpose of this poster is to introduce some of the approaches investigated to increase crop yields in a world expected to have a warmer and high CO2 future.

“Got Carbon?” Fixing Carbon Fixation: Transgenic ApproachesBrenda Bott, Pat Delwiche, Joe Kinscher, JoAnn Miller – Modeling the Molecular World 2, 2011

Rubisco Protein Engineering•The large subunit (LSU) of rubisco is the catalytic site•Attempts to genetically engineer rubisco by altering amino acids of the LSU have failed•A strategy for engineering an enzyme with an increased specificity factor has yet to be realized

Screening Natural Variants of Rubisco

•Specificity for CO2 as great as 3 times that of rubisco in crop plants have been found in red algae rubisco•Introduction of red algae rubisco into plants did not result in increase carboxylation•The absence of certain red algae chaperonin proteins is most likely cause•Mediterranean C3 plants that grow in hot, arid, saline environments have been found to have rubisco with CO2 specificity factors higher than crop plants•This presents an opportunity for engineering this rubisco into transgenic crop plants

Manipulation of the Photorespiratory Cycle

•This is unlikely to improve carbon fixation•Photorespiration mutants cannot grow in air•Blocking oxygenation has been found to be lethal•Photorespiration has been found to aid in nitrogen fixation

Introduce CO2 Concentrating Mechanisms into C3 Plants

•Attempts to breed C4 traits into C3 plants with the goal to improve carbon fixation have been unsuccessful•Genes encoding 4 enzymes in the C4 pathway have been engineered into C3 plants•Some plants experienced stunted growth and leaf bleaching (potato)•Other plants experienced no effect or little increase in growth (tobacco and rice)

Anatomical Considerations

• a single-cell system such as that found in C3 plants is unlikely to make engineering C3 plants for CO2 sequestering unsuccessful•C3 cells are leaky to CO2

• spatial separation of CO2 uptake and carbon assimilation as found in C4 plants is important

Expressing the cyanobacteria ictB gene in crop plants

•The ictB gene is assumed to be involved in inorganic carbon buildup in cyanobacteria•Transgenic Nicotiana (tobacco) plants with the ictB gene where found to have higher photosynthetic rates than control plants•Similar results where found in Arabidopsis (thale cress) plants (Fig. 3). Also, the final dry weight of the transgenic plants was greater than the control plants•These results suggest that introducing the ictB gene into crop plants may hold the potential to increasing crop yield in hot and arid climates.

C3 Plantsbarley, sunflower, rice, tomatoes, wheat, oats

C4 Plantscorn , sugar cane, millet, sorghum,

Fig. 2: Examples of C3 and C4 Plants and Pathways

References: 1. Raines. Plant, Cell and Environment (2006) 29, 331-339 2. Andersson. Journal of Molecular Biology (1996) 259, 160-174 3. Spencer et al. Plant Physiology (Oct. 1999) 121, 579-588

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Fig. 1: Competing Processes Leads to Inefficiency

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Fig. 3

carboxylation oxygenation

Conclusion: A major focus in research , to increase the ability of C3 crop plants to photosynthesize in high temperatures and dry conditions, has been to decrease the oxygenation reaction of rubisco. As shown by this poster, it has been difficult to obtain efficacious results. One approach, using naturally occurring rubiscos with high CO2 specificity factors such as those found in Mediterranean C3 plants and in cyanobacteria, has been shown to improve photosynthetic efficiency by approximately 20%. In the future, improving carbon fixation by crop plants should not be limited to rubisco studies.