INDUCTION OF SUCROSE SYNTHASE IN POTATO TISSUE-CULTURE - EFFECT OF CARBON SOURCE AND METABOLIC REGULATORS ON SINK STRENGTH

Sucrose synthase activity and growth (i.e., dry weight, starch, and protein levels) of cultured potato cells (Solanum tuberosum L., cv. Norchip) were greater when sucrose was the sole carbon source compared with the hexoses glucose or fructose. Variation in sucrose synthase activity during the culture period represented a coarse regulation of the number of enzyme molecules rather that a fine control regulating the catalytic activity of pre-existing enzyme molecules. Significant levels of sucrose synthase occurred only when the intracellular concentration of sucrose reached 2 to 3 mM. Induction of sucrose synthase occurred during the lag phase of the potato suspension culture. During rapid periods of cellular division, total sucrose synthase activity remained relatively constant per cell. Cultures treated with various regulatory compounds (GA3, ABA, cAMP) and metabolic inhibitors (chloramphenicol) demonstrated marked differences in growth, soluble protein, endogenous sucrose concentration, and in the expression of sucrose synthase activity. ABA (10(-5)M) was the most efficient in lowering the level of this enzyme in cultured cells. The effect that biologically-active substances have on regulating the level of sucrose synthase may be indirectly mediated through their influence on the cell's ability to take up and accumulate sucrose.