SPATIAL PATTERNS OF SUCROSE-INDUCIBLE AND POLYGALACTURONIC ACID-INDUCIBLE EXPRESSION OF GENES THAT ENCODE SPORAMIN AND BETA-AMYLASE IN SWEET-POTATO

Two major proteins of tuberous roots of sweet potato, sporamin and bet a-amylase, were detected in storage parenchyma cells, which contain a large amount of starch. In both the leaves and petioles of sweet potat o, the sucrose-induced accumulation of mRNAs for sporamin and beta-amy lase, and of starch occurred in a wide variety of cells, first in cell s within and around the vascular tissue and then in various cells dist al to them, with the exception of epidermal cells. In the mesophyll ce lls of leaves treated with sucrose, the accumulation of large numbers of well-developed starch granules occurred in the preexisting chloropl asts. These results, together with the previous observation that the s ucrose-induced accumulation of sporamin, of beta-amylase and of starch occurs with similar dependency on the concentration of sucrose, sugge st that an excess supply of sugars to various types of cell triggers a cellular transition that induces the simultaneous accumulation of the se reserve materials that are normally present in tuberous roots. Accu mulation of mRNAs for sporamin and beta-amylase, but not the accumulat ion of starch, in leaves and petioles can be also induced when leaf-pe tiole cuttings are supplied with low concentrations of polygalacturoni c acid (PGA) at their cut edges. The spatial patterns of accumulation of mRNAs for sporamin and beta-amylase in leaves and petioles after tr eatment with PGA were found to be similar to those observed upon treat ment with sucrose. These results suggest that most of the cells in lea ves and petioles have the capacity to respond to both a carbohydrate m etabolic signal and a PGA-derived signal that is transmitted by diffus ion from the vascular system.