When arrived in the storage cells, sucrose metabolism is achieved by several different enzymes performing the same or similar reactions and which operate in parallel. The symplastic enzymes directly involved are in direction of sucrose synthesis: sucrose-phosphate synthase (SPS) (in concert with sucrose-phosphate phosphatase) and sucrose synthase (SS), and in direction of sucrose breakdown: soluble acid invertase (SAI), neutral invertase (NI) and sucrose synthase (SS) (Fig.3). The reaction products, glucose, fructose and UDP-glucose can be recycled to precursors of sucrose synthesis again by hexokinase, phosphoglucoisomerase (PGI) and UDP-glucose pyrophosphorylase (UDPG-PPase). All these enzymes are supposed to be cytosolic with exception of the soluble acid invertase, which is vacuolar. In addition a cell wall bound acid invertase will hydrolyse apoplastic sucrose. The presence of these enzymatic activities in parallel in storage tissue had been already found by the Australian group (3) and a enzymatic cycle of sucrose synthesis and degradation had been postulated. In cell suspension culture the actual rate of sucrose cycling could be determined and was found to be up to fourfold higher than the net rate of sucrose storage (12). In tissue slices of internodes of different developmental stages the cycling of sucrose could be quantified, too (13-14). The rate of cycling depended strongly on the age of the internodes (Fig.4): It was highest in young internodes (immature tissue) and decreased successively with ripening of the internodes. The participation of sucrose synthase and sucrose-phosphate synthase in sucrose synthesis could be calculated after simultaneous feeding of labeled glucose and non-labeled fructose and determination of label partitioning between glucose and fructose moiety in sucrose (sucrose from sucrose synthase will be labeled in glucose only, sucrose from sucrose-phosphate synthase will be equally labeled in glucose and fructose). Sucrose synthesis in immature storage tissue is equally achieved by both enzymes, degradation mainly by acid invertase. During ripening, where a clear net production of sucrose is achieved in the tissue, i.e. where the rate of synthesis is twice the rate of hydrolysis, synthesis is dominated by sucrose-phosphate synthase. Hydrolysis by acid invertase in contrast is strongly decreased. In mature tissue all enzymatic rates become very small and similar. The net import of carbon from the phloem into the internodes (indicated in Fig.4 as arrow at the left) is equal to growth and energy demand of the tissue in immature and in mature state. Only during the ripening phase approximately half of the imported carbon is diverted to stored sucrose.
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