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. 2017 Aug 16;7(1):8307.
doi: 10.1038/s41598-017-07132-9.

Resolving the Central Metabolism of Arabidopsis Guard Cells

Free PMC article

Resolving the Central Metabolism of Arabidopsis Guard Cells

Semidán Robaina-Estévez et al. Sci Rep. .
Free PMC article


Photosynthesis and water use efficiency, key factors affecting plant growth, are directly controlled by microscopic and adjustable pores in the leaf-the stomata. The size of the pores is modulated by the guard cells, which rely on molecular mechanisms to sense and respond to environmental changes. It has been shown that the physiology of mesophyll and guard cells differs substantially. However, the implications of these differences to metabolism at a genome-scale level remain unclear. Here, we used constraint-based modeling to predict the differences in metabolic fluxes between the mesophyll and guard cells of Arabidopsis thaliana by exploring the space of fluxes that are most concordant to cell-type-specific transcript profiles. An independent 13C-labeling experiment using isolated mesophyll and guard cells was conducted and provided support for our predictions about the role of the Calvin-Benson cycle in sucrose synthesis in guard cells. The combination of in silico with in vivo analyses indicated that guard cells have higher anaplerotic CO2 fixation via phosphoenolpyruvate carboxylase, which was demonstrated to be an important source of malate. Beyond highlighting the metabolic differences between mesophyll and guard cells, our findings can be used in future integrated modeling of multi-cellular plant systems and their engineering towards improved growth.

Conflict of interest statement

The authors declare that they have no competing interests.


Figure 1
Figure 1
(A) A comparison of the predicted metabolic state of G and M cells and (B) detailed depiction of the sucrose futile cycle (SFC) predicted to take place in G cells. (A) Reactions colored in red (resp. black) carry significantly larger mean flux values in G (resp. M) cells. Reactions depicted in gray cannot be discriminated in terms of mean flux values between the two cell types. The numbers on the reactions correspond to the indices in Supplementary Table S1. The abbreviations used in this figure correspond to: PEP, Phosphoenolpyruvate, Pyr, Pyruvate, Mal, Malate, OAA, Oxaloacetate, Glu, Glutamate, Gln, Glutamine, α – KG, α – Ketoglutarate, FD, Ferredoxin, FD, reduced Ferredoxin, DHAP, Dihydroxyacetone phosphate, G3P, Glyceraldehyde 3-phosphate, G1P, Glucose 1-phosphate, G6P, Glucose 6-phosphate. (B) This cycle is composed by five reactions in which sucrose is preferentially degraded into glucose and fructose by the activity of invertase (Inv, index number 45 in AraCOREred) and resynthesized following activities of hexokinase (HXK, index number 31), phosphoglucomutase (PGM, index number 40), UDP-glucosepyrophosphorylase (UGPase, index number 41) and sucrose synthase (SuSy). In M cells, Glucose 6-phosphate was synthetized exclusively by the action of Glucose 6-phosphate isomerase (GPI, index number 39). Values in parenthesis correspond to the predicted mean flux values for each reaction, values in red correspond to G cells while values in black to M cells. A detailed comparison of the flux values for the reactions in the CBC is provided in Table S3.
Figure 2
Figure 2
Evidence for the higher anaplerotic CO2 fixation in G cells in comparison to M cells. M cells (black bars) and G cells (red bars) were fed with 13-NaHCO3 and harvested after 30 min and 60 min in the light. The abundance of mass isotopomers of aspartate m3 (left side) and malate m3 (right side) in mesophyll cells (M) or guard cells (G) after 30 and 60 min in the light is displayed. The anaplerotic reaction catalysed by phosphoenolpyruvate carboxylase (PEPc) and the subsequent steps catalysed by aspartate aminotransferase (AspAT) and malate dehydrogenase (MDH) are highlighted in the center of the figure. Small spheres represent carbon atoms labelled directly by the activity of PEPc (green spheres) or by the reflux of this 13C by the activity of the tricarboxylic acid cycle (black spheres). Asterisks indicate values that are significantly different between mesophyll and guard cells by Student’s t-test (P < 0.05) in the same time point. Data presented are mean ± standard deviation (n = 3).
Figure 3
Figure 3
13C-enrichment in primary metabolites. M cells (black bars) and G cells (red bars) were fed with 13-NaHCO3 and harvested after 30 min and 60 min in the light. Asterisks indicate values that are significantly different between M and G cells, (Student’s t-test, P < 0.05) for the same time point. Data presented are mean ± standard deviation (n = 3). The complete list of the 13C-enrichment is presented in Supplementary Table S11. Abbreviations: metabolites: GABA, gamma-aminobutyric acid; Suc, sucrose. Enzymes: CA, carbonic anhydrase; PEPc, phosphoenolpyruvate carboxylase; Rbcs, ribulose-1,5-biphosphhate carboxylase/oxygenase. Amino acids are abbreviated using the standard three-letters code.

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