Anthropogenic activities are rapidly changing the composition and thermal conditions of the global atmosphere, leading to fundamental trade-offs in photosynthetic carbon metabolism. Unique challenges of photosynthetic carbon fixation under future climate conditions – mainly elevated temperatures – include increased rates of photorespiration, which limit C3 photosynthetic performance. Photorespiration initiates when oxygen binds to rubisco, the enzyme responsible for carbon fixation, instead of carbon dioxide. The resulting oxygenation reaction produces 2-phosphoglycolate, an intermediate that inhibits carbon assimilation and allocation. To reduce the inhibition of carbon metabolism, photorespiration detoxifies and recycles 2-phosphoglycolate through a set of reactions that salvage most of the initial carbon fixed to rubisco. While the rate of photorespiratory influx is set by rubisco, downstream photorespiratory enzymes must process the subsequent photorespiratory intermediates that are produced. If photorespiratory influx outpaces downstream metabolic capacity, a reduction of photosynthetic net carbon fixation is anticipated due to insufficient conversion rate and accumulation of various biologically active intermediates. Managing these photorespiratory intermediates is important to maintain plant vigor, especially in dynamic environments where photorespiratory influx is unpredictable. Currently, it is unclear whether photorespiratory biochemistry, downstream of rubisco, adjusts to handle a greater carbon influx through photorespiration. The work in this dissertation looks past rubisco, which has been a prominent focus in engineering efforts, to explore downstream photorespiratory enzymes that directly manage carbon flux through photorespiration. I first investigate the hallmarks of a temperature-tolerant photorespiratory pathway in Rhazya stricta, a C3 shrub native to the hot-arid regions in the Middle East. I found that R. stricta supports higher rates of photorespiration under elevated temperatures and that these higher rates of photorespiration correlate with increased activity of key photorespiratory enzymes; phosphoglycolate phosphatase and catalase, compared to Nicotiana tabacum. I then studied the acclimation potential of photorespiratory capacity in Betula papyrifera, with particular interest in whether enzyme activities acclimate to changes in photorespiratory influx. I found no plasticity in photorespiratory capacity when B. papyrifera was exposed to 6 different CO2 concentrations and temperatures scenarios, and that a fixed capacity is maintained under each growth condition. The fixed capacity is likely due to the existence of safety factors in the pathway that manages unpredictable photorespiratory influx in dynamic environments. Finally, I explored whether replacing native catalase in Arabidopsis thaliana with a foreign catalase isoform conferred any benefit to photorespiratory carbon recycling efficiency. To explore this question, we generated three transgenic independent expression lines of Heliobacter pylori catalase in A. thaliana cat2-KO to determine their in vivo and in vitro function. I found that two out of the three transgenic lines have similar amounts of CO2 loss from photorespiration compared to wildtype line and were able to rescue the cat2-KO growth phenotype, while having less catalase activity than wildtype. The findings from this dissertation contribute to the long-range goal of engineer photorespiration to improve photosynthetic carbon fixation under future climate conditions in C3 crop systems.
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