Sucrose-Induced Expression of Genes Coding for the Tuberous Root Storage Protein, Sporamin, of Sweet Potato in Leaves and Petioles

Abstract

Sporamin, a major tuberous root protein of sweet potato, was found to accumulate in large quantities in excised leaves and petioles when such explants were supplied with high concentrations of sucrose. Although a small amount of sporamin could be detected in leaves and petioles treated with 1% or lower concentrations of sucrose, the maximum level of induction required sucrose at a concentration of 3% or higher. The appearance of sporamin polypeptides in leaves and petioles treated with 3% sucrose followed a lag period of about one day, while a significant amount of sporamin mRNAs was already detectable in petioles after one day of treatment with sucrose. Addition of silver nitrate to the medium did not affect the accumulation of sporamin, suggesting that this induction is not due to the effect of ethylene induced by wounding of the tissue. The accumulation of sporamin could also be induced by glucose and by fructose, but not by man-nitol, suggesting that changes in carbohydrate and/or energy metabolism in the cell may be involved in the induction. Callus tissues obtained by treatment of leaf segments with 1-naphthaleneacetic acid did not accumulate sporamin even though these cells were cultured on agar medium that contained 3% sucrose. However, when callus tissues were allowed to grow after transfer to a medium that contained 6-benzylaminopurine and sucrose, accumulation of large amounts of sporamin was induced. These results suggest that, while expression of genes coding for sporamin can be induced in organs other than the tuberous root by a process that does not accompany the differentiation of tissue, the induction of expression of sporamin genes by sucrose requires that cells be competent in some specific, but as yet unidentified, way.