Enhancement of sink activity by controling the gene expression and the activity of invertase and sorbitol dehydrogenase

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The role of acid invertase for cell enlargement and its regulation of gene expression were investigated using the systems of fruit enlargement of Japanese pear and of petal enlargement of rose. (1) Two full length vacuolar type acid invertase cDNA (PsV-AIV1, PsV-AIV2) were cloned from Japanese pear fruit. PsV-AIV1 related the early growth concerning active cell division and PsV-AIV2 related mainly cell enlargement with sugar accumulation. (2) The enlargement of rose petal was initiated by forming high turgor pressure which was formed by converting sucrose to hexose via the rising of invetase activity with increase of gene expression. The rising of activity was induced by auxin treatment.The regulation of NAD-SDH gene expression and the accumulation of sorbitol were investigated using straw berry fruit. (1) The full length cDNA of NAD-SDH of strawberry fruit (FaSDH) was cloned and the activity also detected. However, relating sorbitol-6-phosphate dehydrogenase the gene existed but the protein and activity were not detected. The kinetics of NAD-SDH activity revealed that sorbitol was synthesized by the reduced reaction of fructose. (2) NAD-SDH activity was stimulated by fructose, sorbitol or auxin treatment. However, there were not clear changes of mRNAs by them. (3) The mRNA of NAD-SDH were composed of the pre-mature mRNA containing intron and the mature mRNA. The stimulation of NAD-SDH activity by fructose, sorbitol or auxin depended on the stimulation of splicing rate after transcription.As described above, it was indicated that acid invertase and sorbitol dehydrogenase play an important role for the cellular enlargement and sugar accumulation and their activities were regluated by the substrates and phytohormone.