Sugar Uptake and Starch Biosynthesis by Slices of Developing Maize Endosperm

Abstract

14C-Sugar uptake and incorporation into starch by slices of developing maize (Zea mays L.) endosperm were examined and compared with sugar uptake by maize endosperm-derived suspension cultures. Rates of sucrose, fructose, and D-and L-glucose uptake by slices were similar, whereas uptake rates for these sugars differed greatly in suspension cultures. Concentration dependence of sucrose, fructose, and D-glucose uptake was biphasic (consisting of linear plus saturable components) with suspension cultures but linear with slices. These and other differences suggest that endosperm slices are freely permeable to sugars. After diffusion into the slices, sugars were metabolized and incorporated into starch. Starch synthesis, but not sugar accumulation, was greatly reduced by 2.5 millimolar p-chloromercuribenzenesulfonic acid and 0.1 millimolar carbonyl cyanide m-chlorophenylhydrazone. Starch synthesis was dependent on kernel age and incubation temperature, but not on extemal pH (5 through 8). Competing sugars generally did not affect the distribution of 14C among the soluble sugars extracted from endosperm slices incubated in 14C-sugars. Competing hexoses reduced the incorporation of 14C into starch, but competing sucrose did not, suggesting that sucrose is not a necessary intermediate in starch biosynthesis. The bidirectional permeability of endosperm slices to sugars makes the characterization of sugar transport into endosperm slices impossible, however the model system is useful for experiments dealing with starch biosynthesis which occurs in the metabolically active tissue. from kernels after peeling away the pericarp. In this study, sugar uptake and metabolism in developing maize endosperm tissue slices was examined and compared with endospermderived suspension cultures, which have been studied previously (3). This comparison was made, in part, to address the intactness of the tissue slices and thus their suitability for sugar transport studies. We also examined the effect ofvarious incubation treatments on the incorporation of exogenous sugars into starch in endosperm tissue slices.

Post-Sieve Element Transport of Sucrose in Developing Seeds

“…In vitro studies of sugar influx by exposed endosperm of developing maize kernels suggested that sugar influx occurred by passive diffusion alone (Griffith et al 1987). However, this behaviour could result from mechanical damage to the uptake mechanism (Felker et al 1990; Thomas et al 1992). The developing wheat grain appears to be a more robust experimental model.…”

Fructan Accumulation and Sucrose Metabolism in Transgenic Maize Endosperm Expressing a Bacillus amyloliquefaciens SacB Gene

“…High levels of invertase activity localized at the base of a kernel have been demonstrated (Shannon, 1972), and the resulting hexose may be phosphorylated and enter the starch pathway independently of SS (Dohlert et al, 1988;Felker et al, 1990). However, it has also been shown that hydrolysis of SUC is not an absolute requirement for movement of SUC into the endosperm (Schmalstig and Hitz, 1987).…”

“…Felker et al (1990), using maize endosperm tissue slices, also suggested that hexose may enter the starch biosynthetic pathway, but tissue slices may not necessarily represent in vivo conditions. The questions of whether hexose is phosphorylated and then converted to starch in the amyloplast or if UDP-Glc is a critica1 metabolite in the starch biosynthetic pathway remain points of discussion.…”