Spatial patterns of sucrose-inducible and polygalacturonic acid-inducible expression of genes that encode sporamin and rβ-amylase in sweet potato

Two major proteins of tuberous roots of sweet potato, sporamin and rβ-amylase, were detected in storage parenchyma cells, which contain a large amount of starch. In both the leaves and petioles of sweet potato, the sucrose-induced accumulation of mRNAs for sporamin and rβ-amylase, and of starch occurred in a wide variety of cells, first in cells within and around the vascular tissue and then in various cells distal to them, with the exception of epidermal cells. In the mesophyll cells of leaves treated with sucrose, the accumulation of large numbers of well-developed starch granules occurred in the preexisting chloroplasts. These results, together with the previous observation that the sucrose-induced accumulation of sporamin, of rβ-amylase and of starch occurs with similar dependency on the concentration of sucrose, suggest that an excess supply of sugars to various types of cell triggers a cellular transition that induces the simultaneous accumulation of these reserve materials that are normally present in tuberous roots. Accumulation of mRNAs for sporamin and rβ-amylase, but not the accumulation of starch, in leaves and petioles can be also induced when leaf-petiole cuttings are supplied with low concentrations of polygalacturonic acid (PGA) at their cut edges. The spatial patterns of accumulation of mRNAs for sporamin and rβ-amylase in leaves and petioles after treatment with PGA were found to be similar to those observed upon treatment with sucrose. These results suggest that most of the cells in leaves and petioles have the capacity to respond to both a carbohydrate metabolic signal and a PGA-derived signal that is transmitted by diffusion from the vascular system.