Figure 1.

Schematic presentation of sugar metabolism in source and sink tissues during the day and night, and localization of HXK and FRK enzymes in eudicots and monocots. Triose-phosphate (Triose-P), the product of photosynthetic CO₂ fixation in the Calvin cycle, is exported to the cytoplasm. Consecutive cytoplasmic enzymatic steps lead to the formation of G6P independent of HXK and FRK. Further metabolism of G6P yields sucrose, which remains in the cytosol, is temporarily stored in the vacuole or is exported to the apoplast. G6P metabolism may also yield T6P and trehalose. Within the chloroplast, Triose-P is used for the formation of starch during the day. During the dark period, starch is degraded to maltose, glucose, and glucose-1-phosphate (G1P). Maltose cleavage also releases glucose, and vacuolar and cytosolic sucrose might be cleaved by cytosolic (cINV) and vacuolar (vINV) invertases to produce glucose and fructose. While fructose can be phosphorylated by FRK, glucose must be phosphorylated by HXK. Orange stained HXK and FRK indicate the localization of these enzymes in monocots and eudicots. Cytosolic HXK (stained green) is also found in monocots. Sucrose transported to the apoplast, during the day or night enters the phloem via sucrose transporters. Within the vascular tissues, sucrose can be cleaved by sucrose synthase (SUS) to support vascular development or transported to other sink tissues. Sucrose unloading in sink tissues may proceed symplasmically via plasmodesmata or through the apoplast via sucrose transporters. Alternatively, sucrose might be cleaved by apoplastic (cell wall) invertase (cwINV) to produce glucose and fructose that would enter sink cells via specific hexose transporters. The enzymatic steps of sugar metabolism in sink tissues are similar to those found in source tissues. In addition to the metabolic function of HXK, this enzyme also senses the presence of glucose and represses the expression of photosynthetic genes in the nucleus of source photosynthetic tissues. The origin of the glucose in photosynthetic tissues that is sensed by HXK is not known. Potential sources are presented by dashed lines: cleavage of sucrose or trehalose within the cytosol, or apoplastic cleavage of sucrose followed by uptake of the released hexoses. The presumed role of fructose and FRK in vascular tissues is indicated by the gray lines. Some of the sucrose transported in the phloem is cleaved by SUS to support cellulose and cell wall synthesis and vascular development. The released fructose is phosphorylated by FRK, but if SUS cleaves too much sucrose, the concentration of fructose will increase and inhibit both SUS and FRK activity, thereby affecting the amount of sucrose allocated for vascular development. ADP, adenosine diphosphate; ADP-G, ADP-glucose; cINV, cytosolic invertase; cwINV, cell wall invertase; F1,6BP, fructose 1,6-biphosphate; F6P, fructose 6-phosphate; FRK, fructokinase; G6P, glucose 6-phosphate; G1P, glucose 1-phosphate; HXK, hexokinase; Suc-P, sucrose-phosphate; SUS, sucrose synthase; T6P, trehalose 6-phosphate; Triose-P, triose-phosphate; UDP, uridine diphosphate; UDP-G, UDP-glucose; vINV, vacuolar invertase. Blue circles represent transporters.