Increase in sorbitol biosynthesis in stressed Japanese pear leaves

To clarify the mechanism of stress response in leaf tissue of Japanese pear, sugar content, conversion from glucose to sorbitol, and S6PDH mRNA expression were investigated in leaf discs incubated under salt or low temperature stress. Sorbitol contents increased in the leaf discs under both stresses. The recovery of ¹⁴C in sorbitol also increased in the leaf discs incubated with ¹⁴C-glucose under both stresses. Glucose, fructose, and sucrose content also increased in the leaf discs although the recoveries of ¹⁴C in those sugars in the leaf discs did not change. The result suggests that Japanese pear leaves respond to salt and low temperature stress and dominantly synthesize sorbitol. Next, the PCR fragment of putative S6PDH cDNA was cloned by RT-PCR from Japanese pear leaves because sorbitol-6-phosphate dehydrogenase (S6PDH) could be a key enzyme in sorbitol biosynthesis as described in apple. The nucleotide sequence was identical to 98% S6PDH cDNA from apple seedlings. Northern blot analysis suggested that S6PDH mRNA expression was source-specific and was induced by salt and low temperature stress. Collectively, sorbitol accumulation is induced by salt and low temperature stress, and the accumulation is probably due to an increase in sorbitol biosynthesis that is related to the expression of S6PDH gene.