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Abstract:
Plants depend on the enzyme ribulose-1,5-bisphosphate
carboxylase/oxygenase (Rubisco) for CO2 fixation. However, especially in
C3 plants, photosynthetic yield is reduced by formation of
2-phosphoglycolate, a toxic oxygenation product of Rubisco, which needs
to be recycled in a high-flux-demanding metabolic process called
photorespiration. Canonical photorespiration dissipates energy and
causes carbon and nitrogen losses. Reducing photorespiration through
carbon-concentrating mechanisms, such as C4 photosynthesis, or bypassing
photorespiration through metabolic engineering is expected to improve
plant growth and yield. The beta-hydroxyaspartate cycle (BHAC) is a
recently described microbial pathway that converts glyoxylate, a
metabolite of plant photorespiration, into oxaloacetate in a highly
efficient carbon-, nitrogen-, and energy-conserving manner. Here, we
engineered a functional BHAC in plant peroxisomes to create a
photorespiratory bypass that is independent of 3-phosphoglycerate
regeneration or decarboxylation of photorespiratory precursors. While
efficient oxaloacetate conversion in Arabidopsis thaliana still masks
the full potential of the BHAC, nitrogen conservation and accumulation
of signature C4 metabolites demonstrate the proof of principle, opening
the door to engineering a photorespiration-dependent synthetic
carbon-concentrating mechanism in C3 plants.
