The feedback mechanism outlined in Sect. 4.2 operates in the short term, adjusting the activity of the existing photosynthetic apparatus to the capacity of export and sink activity, but mechanisms at the level of gene transcription play a more important role in the long term. They modify photosynthetic capacity and can override regulation by light, tissue type, and developmental stage (Smeekens & Rook 1998). Leaves of Triticum aestivum (wheat) fed with 1% glucose have a lower photosynthetic capacity as well as lower levels of mRNA coding for several Calvin-cycle enzymes, including the small subunit of Rubisco (Jones et al. 1996). Regulation of photo-synthetic gene expression by carbohydrates plays an important role in the control of the activity of the ''source'' (leaves) by the demand in the ''sink'' (e.g., fruits) (Paul & Foyer 2001). Sensing of carbohydrate levels is mediated by a specific hexokinase, which is an enzyme that phosphorylates hexose while hydro-lyzing ATP (Smeekens 2000). This regulation at the level of gene transcription plays a role in the acclimation of the photosynthetic apparatus to elevated concentrations of atmospheric CO2 (Sect. 12), and, more generally in adjusting photosynthetic capacity to environmental and developmental needs.
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