Nucleotide sugars and starch synthesis in spadix of Arum maculatum and suspension cultures of Glycine max

The aim of this work was to determine the relative contributions of ADPglucose and UDPglucose to starch synthesis in two non-photosynthetic tissues, the developing club of the spadix of Arum maculatum and suspension cultures of Glycine max. Rates of starch accumulation during growth are compared with estimates of the maximum catalytic activities in vitro of ADPglucose starch synthase, ADPglucose pyrophosphorylase, UDPglucose pyrophosphorylase and UDPglucose starch synthase. The latter could only be measured at high concentrations (10–30 mM) of UDPglucose. Clubs of Arum and cells of Glycine contained 292 and 6.8 nmol UDPglucose per gram fresh weight, respectively. The corresponding figures for ADPglucose were 29 and 0.4. From the above data it is argued that in both Arum club and Glycine cells the activity of UDPglucose starch synthase is too low to make any quantitatively significant contribution to starch synthesis. The activities of ADPglucose starch synthase and pyrophosphorylase were high enough to mediate the observed rates of starch accumulation. It is suggested that starch synthesis in these tissues is via ADPglucose.