Tobacco guard cells fix CO2 by both Rubisco and PEPcase while sucrose acts as a substrate during light‐induced stomatal opening

• Published in: Plant, cell and environment Vol. 38; no. 11; pp. 2353 - 2371
• Main Authors: Daloso, Danilo M., Antunes, Werner C., Pinheiro, Daniela P., Waquim, Jardel P., AraÚJo, Wagner L., Loureiro, Marcelo E., Fernie, Alisdair R., Williams, Thomas C. R.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.11.2015
• Subjects: Acid production; bicarbonates; carbon; carbon dioxide; Carbon Dioxide - metabolism; Carbon dioxide fixation; fructose; glucose; guard cell metabolism; guard cells; Kinetics; metabolism; Metabolites; Nicotiana - cytology; Nicotiana - metabolism; Organic acids; phosphoenolpyruvate carboxylase; Phosphoenolpyruvate Carboxylase - metabolism; Plant Cells - metabolism; Plant Cells - physiology; Plant Stomata - physiology; Plant Stomata - radiation effects; proteomics; ribulose-bisphosphate carboxylase; Ribulose-Bisphosphate Carboxylase - metabolism; Stable isotope labelling; stable isotopes; Starch; stomatal movement; sucrose; Sucrose - metabolism; tobacco; transcriptomics; tricarboxylic acid cycle; Stable isotope labelling; guard cell metabolism; organic acids
• ISSN: 0140-7791; 1365-3040; 1365-3040
• DOI: 10.1111/pce.12555

Transcriptomic and proteomic studies have improved our knowledge of guard cell function; however, metabolic changes in guard cells remain relatively poorly understood. Here we analysed metabolic changes in guard cell‐enriched epidermal fragments from tobacco during light‐induced stomatal opening. Increases in sucrose, glucose and fructose were observed during light‐induced stomatal opening in the presence of sucrose in the medium while no changes in starch were observed, suggesting that the elevated fructose and glucose levels were a consequence of sucrose rather than starch breakdown. Conversely, reduction in sucrose was observed during light‐ plus potassium‐induced stomatal opening. Concomitant with the decrease in sucrose, we observed an increase in the level as well as in the 13C enrichment in metabolites of, or associated with, the tricarboxylic acid cycle following incubation of the guard cell‐enriched preparations in 13C‐labelled bicarbonate. Collectively, the results obtained support the hypothesis that sucrose is catabolized within guard cells in order to provide carbon skeletons for organic acid production. Furthermore, they provide a qualitative demonstration that CO2 fixation occurs both via ribulose‐1,5‐biphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPcase). The combined data are discussed with respect to current models of guard cell metabolism and function. Although signaling pathways in guard cells have been the subject of extensive investigation, several important questions regarding the metabolism of these cells remain unanswered. Here, we provide evidence that sucrose act as substrate during light induced stomatal opening and suggest that the carbon released is probably used for organic acid production. Furthermore, an isotope labeling experiment reveals that CO2 fixation occurs both via RubisCO and PEPcase in guard cells.