From light to dark: linking the roles of mitochondria in C4 photosynthesis with dark respiration

Abstract

Respiration is the powerhouse that provides energy and carbon skeletons for biosynthesis, cellular maintenance and active transport in plants. Increasing attention has been given to how rates of leaf respiration measured in the dark (Rdark) differ among species using a conventional C3 photosynthetic pathway. By contrast, less attention has been given to quantify rates of Rdark in species using a more efficient C4 photosynthetic pathway. Mitochondria in C4 plants play distinct roles depending on the biochemical types of photosynthesis (NADP-ME, NAD-ME and PCK types). While C4 NAD-ME and PCK type photosynthetic pathways require mitochondria to decarboxylate C4 acid (such as malate) and to generate ATP for photosynthesis, C4 NADP-ME type photosynthesis is similar to C3 photosynthesis in that it does not directly involve mitochondria. This thesis explored how the roles of mitochondria in C4 photosynthesis may have altered mitochondrial traits associated with their ultrastructure, abundance, metabolic capacities, gas exchange rates and metabolite profiles in C4 monocots, compared to their C3 counterparts.

Experiments were conducted to examine to what extent rates of Rdark (measured in the day after 30-minute dark adaptation) were associated with mitochondrial traits in three C4 monocots representing the three photosynthetic types. Results have shown that the involvement of mitochondria in C4 NAD-ME and PCK type photosynthetic pathways has altered the abundance, size, ultrastructure and associated enzymatic capacity in bundle sheath cells compared to that of in C4 NADP-ME type leaves. Rates of leaf Rdark were also quantified, and the variation of leaf Rdark did not align with the observed differences in the mitochondrial traits. This suggested that rates of leaf Rdark were primarily governed by substrate availability and cellular maintenance demands, in a manner that is somewhat independent of mitochondrial roles in C4 photosynthesis.

Subsequent experiments were conducted to investigate how the relationship be- tween leaf Rdark and substrate availability/maintenance demands may vary over a day-night cycle in 12 C3 and C4 monocots. The key findings were: (1) respiratory metabolism measured after 30-minute dark-adaptation in the day was not equivalent to the night-time metabolism; and, (2) day-night variation in Rdark was correlated with changes in metabolite profiles (particularly organic acids but not including soluble carbohydrates). The results also highlighted that C4 NAD-ME and PCK type species used malate at night to fuel leaf Rdark, instead of the conventional soluble carbohydrates. Such usage of malate agreed with the roles of mitochondria play in NAD-ME and PCK type photosynthesis.

Overall, the results in this thesis demonstrated the influence of mitochondrial involvement in some types of C4 photosynthesis on leaf Rdark and respiratory metabolism, and enhanced our understanding on what drives variation of leaf Rdark. Findings in this thesis have ramifications on leaf respiration modelling on a terrestrial scale, provide information on potential sites for mitochondrial engineering needed to further supercharge C4 rice, and serve as a foundation for any future large-scale study on C4 vegetation aiming to improve Rdark representation in current vegetation-climate models.