Source Capacity

Focusing on limiting factors for biomass production, we encounter the question of possible limitations of primary energy fixation in the source organs of the plant. Certainly energy uptake by the absorption of light is not a limiting factor, as photosynthetic fixation already reaches a maximum at moderate photon flux densities. If abiotic factors like water availability or CO2 supply are not restrictive, regulatory features of carbohydrate metabolism could become limiting.

During photosynthesis, photosynthates are exported from the chloroplast as triose phosphates that are condensed to hexoses and ultimately converted to sucrose. The sucrose is usually exported to sink organs of the plant such as growing leaves, the root system and storage organs. In terms of regulation, two metabolites are of central importance. The first is inorganic phosphate, which is released from fructose bisphopsphate and in the last step of sucrose synthesis, and is needed as a transport equivalent for every triose phosphate that is exported from the plastid. The second is fructose 2,6-bisphosphate (F-2,6-PP), which is produced from fructose 6-phosphate (F6P) and is a regulator of the activity of the enzyme frucosebisphosphatase (FBPase), which catalyzes the main regulatory reaction of sucrose synthesis.

Under conditions when sucrose accumulates in the cytosol, fructose 6-phosphate (F6P) concentrations increase and as a consequence, F-2,6-PP is produced and causes inhibition of F6P production. This restrains triose phosphate withdrawal from the plastid and initiates transitory starch production in most plants. Alternatively, the sucrose accumulation can be circumvented by import into the vacuole (Fig. 15.1).

The fact that metabolites control the reactions complicates genetic modification of the pathway. Attempts to increase flux by hydrolyzing pyrophosphate to force the reaction of fructose-6-phosphate 1-phosphotrans-ferase (PFP) towards F6P production failed, because pyrophosphate is needed in later steps of sucrose transport to sink tissues.1,2

Fig.15.1. Regulation of carbohydrate metabolism in mesophyll cells (source tissue). Metabolites: F-6-P: fructose-6-phosphate; F-1,6-PP: fructose-1,6-bisphosphate; F-2,6-PP: fructose-2,6-bisphosphate; G-1-P: glucose-1-phosphate; G-6-P: glucose-6-phosphate; P: phosphate; Pi: inorganic phosphate; PPi: pyrophosphate; S: sucrose; S-6-P: sucrose-6-phosphate; Triose-P: triose phosphates; UDP: uridine diphosphate; UTP: uridine triphosphate. UDP-G: UDP-glucose. Enzymes: FBPase: fructose-bisphosphatase; PFK: phosphofructo-kinase; PFP: fructose-6-phosphate 1-phosphotransferase; SPS: sucrosephosphate-synthase.

Fig.15.1. Regulation of carbohydrate metabolism in mesophyll cells (source tissue). Metabolites: F-6-P: fructose-6-phosphate; F-1,6-PP: fructose-1,6-bisphosphate; F-2,6-PP: fructose-2,6-bisphosphate; G-1-P: glucose-1-phosphate; G-6-P: glucose-6-phosphate; P: phosphate; Pi: inorganic phosphate; PPi: pyrophosphate; S: sucrose; S-6-P: sucrose-6-phosphate; Triose-P: triose phosphates; UDP: uridine diphosphate; UTP: uridine triphosphate. UDP-G: UDP-glucose. Enzymes: FBPase: fructose-bisphosphatase; PFK: phosphofructo-kinase; PFP: fructose-6-phosphate 1-phosphotransferase; SPS: sucrosephosphate-synthase.

Another means of controlling sucrose production is the activity of the last enzyme in the pathway, sucrosephosphate-synthase (SPS), which usually has a low activation status and is regulated aHosterically. Posttranslational phosphorylation of SPS reduces affinity for the substrate UDP-G. Again, genetic modification is hampered, because overexpression of the enzyme does not necessarily increase its activity.3

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