Spatial Patterns of Sucrose-Inducible and Polygalacturonic Acid-Inducible Expression of Genes That Encode Sporamin and r{beta}-Amylase in Sweet Potato

Two major proteins of tuberous roots of sweet potato, sporaminand rß-amylase, were detected in storage parenchymacells, which contain a large amount of starch. In both the leavesand petioles of sweet potato, the sucrose-induced accumulationof mRNAs for sporamin and rß-amylase, and of starchoccurred in a wide variety of cells, first in cells within andaround the vascular tissue and then in various cells distalto them, with the exception of epidermal cells. In the mesophyllcells of leaves treated with sucrose, the accumulation of largenumbers of well-developed starch granules occurred in the preexistingchloroplasts. These results, together with the previous observationthat the sucrose-induced accumulation of sporamin, of rß-amylaseand of starch occurs with similar dependency on the concentrationof sucrose, suggest that an excess supply of sugars to varioustypes of cell triggers a cellular transition that induces thesimultaneous accumulation of these reserve materials that arenormally present in tuberous roots. Accumulation of mRNAs forsporamin and rß-amylase, but not the accumulationof starch, in leaves and petioles can be also induced when leaf-petiolecuttings are supplied with low concentrations of polygalacturonicacid (PGA) at their cut edges. The spatial patterns of accumulationof mRNAs for sporamin and rß-amylase in leaves andpetioles after treatment with PGA were found to be similar tothose observed upon treatment with sucrose. These results suggestthat most of the cells in leaves and petioles have the capacityto respond to both a carbohydrate metabolic signal and a PGA-derivedsignal that is transmitted by diffusion from the vascular system. ⁴Present address: Department of Molecular Biology, NationalInsustitute of Agrobiological Resources, Tsukuba City, Ibaraki,305 Japan.