Figure 2.

Carbon flux to the cortical root cells during arbuscular–mycorrhizal interactions. Photosynthates flow through the mycorrhizal plant from the leaves to the arbusculated cortical cells in the roots. The catabolism of Suc in the arbusculated and other cortical cells close to them promotes Suc mass flow and enables the translocation of hexoses, Suc, and lipids to the periarbuscular space towards the fungal arbuscule, imposing a carbon sink (Updated from Wipf et al., 2019; Roth and Paszkowski, 2017; and Manck-Götzenberger and Requena, 2016 [8,34,39]). Blue arrows trace the current carbon flow routes, the discontinued orange arrows showthe “futile” cycles of sucrose catabolism and synthesis, the colored barrels designate the carbohydrate transporters. Enzymes are indicated in numbered circles as: (1) Sucrose synthase; (2) Neutral invertase; (3) Apoplastic invertase; (4) Vacuolar invertase; (5) Glycerol-3-phosphate acyl transferase. Numbers in rounded rectangles denote specific metabolic pathways: (1) Suc as a source for aerobic respiration; (2) Glycolytic pathway to render phosphoenolpyruvate; (3) lipid synthesis mediated by the plastid Type I FAS molecular complex. Diamonds indicate potential carbon fluxes as: (1) symplastic and (2) apoplastic routes of hexoses entry to sink cells. Sucrose * indicates the sucrose biosynthesized in the arbusculated cell.