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

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 (R_dark) differ among
species using a conventional C₃ photosynthetic pathway. By contrast, less attention
has been given to quantify rates of R_dark 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 NADPME 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 R_dark (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 R_dark were also quantified, and the variation of leaf R_dark did not align
with the observed differences in the mitochondrial traits. This suggested that rates of
leaf R_dark 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 between leaf R_dark 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 R_dark 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 R_dark, 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 R_dark and respiratory metabolism,
and enhanced our understanding on what drives variation of leaf R_dark. 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 R_dark representation in current vegetation-climate
models