LeFRK2, the gene encoding the major fructokinase in tomato fruits, is not required for starch biosynthesis in developing fruits

The LeFRK2 gene product has been proposed to play a role in transient starch accumulation in developing tomato ( Lycopersicon esculentum) fruits. To analyze the role of LeFRK2 in starch biosynthesis, we produced transgenic tomato plants with sense and antisense expression of StFRK, the potato homologue to LeFRK2. Fruits of homozygous plants expressing sense or antisense StFRK exhibited suppression of LeFRK2, concomitantly with specific elimination of the FKI isozyme, indicating that FKI is the gene product of LeFRK2. The activity of fructokinase was reduced in antisense and sense homozygous fruits and increased in fruits of sense hemizygous plants. The modified activities of fructokinase led to small but significant changes both in the steady state levels of sugars and in the level of phosphorylated sugars in fruits. However, fruits lacking FKI had increased rather than decreased starch content. We therefore concluded that LeFRK2 is not required for transient starch accumulation in tomato fruits. Rather, LeFRK2 might have a role in the regulation of sucrose import into tomato fruits.