2. Metabolic conversion of translocation sugars in
fruit
1) Sucrose-metabolizing enzymes and their regulation
(1) Loading and unloading of sucrose
Sucrose translocated in fruit is generally generated in
leaves by sucrose phosphate synthase (SPS). SPS was
first isolated in spinach leaves and its properties (Huber
and Huber, 1996) and subsequent regulation of activity
by light were clarified (Comparot et al., 2003). Sucrose
synthesized in leaves is loaded into phloem by the H+
-
sucrose co-transporter whose presence was predicted by
Giaquinta (1977). Riesmier et al. (1994) cloned the gene
of this transporter and clarified its function. Loaded
sucrose is translocated to fruit flesh through the phloem
and is unloaded into the parenchyma tissue. Two types
of unloading in fruit are 1) the symplasmic pathway in