Summary
Evidence suggests that guard cells have higher rate of
phosphoenol pyruvate carboxylase (PEPc)-mediated dark
CO2 assimilation than mesophyll cells. However, it is
unknown which metabolic pathways are activated following dark
CO2 assimilation in guard cells. Furthermore, it remains
unclear how the metabolic fluxes throughout the tricarboxylic acid (TCA)
cycle and associated pathways are regulated in illuminated guard cells.
Here we used 13C-HCO3 labelling of
tobacco guard cells harvested under continuous dark or during the
dark-to-light transition to elucidate principles of metabolic dynamics
downstream of CO2 assimilation. Most metabolic changes
were similar between dark-exposed and illuminated guard cells. However,
illumination increased the 13C-enrichment in sugars
and metabolites associated to the TCA cycle. Sucrose was labelled in the
dark, but light exposure increased the 13C-labelling
into this metabolite. Fumarate was strongly labelled under both dark and
light conditions, while illumination increased the13C-enrichment in pyruvate, succinate and glutamate.
Only one 13C was incorporated into malate and citrate
in either dark or light conditions. Our results collectively suggest
that the PEPc-mediated CO2 assimilation provides carbons
for gluconeogenesis, the TCA cycle and glutamate synthesis and that
previously stored malate and citrate are used to underpin the specific
metabolic requirements of illuminated guard cells.
Key words: 13C-labelling analysis, glutamate,
metabolic network, metabolic regulation, phosphoenol pyruvate
carboxylase.