Glucose- and ADPGlc-dependent starch synthesis in isolated cauliflower-bud amyloplasts. Analysis of in the interaction of various potential precursors

ADP-glucose drives starch synthesis in isolated maize endosperm amyloplasts: characterization of starch synthesis and transport properties across the amyloplast envelope

“…Figure 3 illustrates that rising concentrations of ADPGlc do not inhibit Glc6P-dependent starch synthesis even though it is well documented that Glc6P-dependent starch synthesis is totally dependent on the uptake of ATP into the amyloplast [4]. This result confirms recent results from our group showing that Glc6P-dependent starch synthesis in isolated amyloplasts from cauliflower buds is barely inhibited by increasing concentrations of ADPGlc in the incubation medium [12]. The low degree of inhibition induced by ADPGlc agrees with the inability of ADPGlc to interfere with the plastid ATP\ADP transporter (Table 3) [37], and is probably due to a minor contamination of commercially available ADPGlc with ADP [37], acting as a substrate for the plastid ATP\ADP transporter (Table 3) [37].…”

“…In contrast, the low rate of ADPGlc-driven starch synthesis in isolated cauliflower amyloplasts is saturated at concentrations below 1 mM [12]. The cytosolic concentration of ADPGlc in maize endosperm is not known, but it is clear from the results given in Table 2 and Figure 1 that ADPGlc-dependent starch synthesis by isolated maize endosperm amyloplasts can be much faster than the rate of Glc6P-dependent starch synthesis [4].…”

“…The capability of Glc1P-dependent starch synthesis has also been demonstrated for isolated cauliflower bud amyloplasts [20]. Detailed analysis, however, indicated that, under physiological concentrations, Glc6P is the most likely precursor for starch biosynthesis [12]. In the case of amyloplasts purified from cauliflower buds, Glc1P is not imported via a hexose phosphate translocator but by another so far unidentified membrane protein [20].…”

“…This result was later confirmed for amyloplasts purified from a soya-bean cell suspension culture [9]. In contrast, in plastids isolated from pea embryos, cauliflower buds, maize endosperm or green-pepper fruits the most efficient carbon precursor for starch synthesis is Glc6P, supplied at physiological concentrations [4,[10][11][12][13].…”

“…(1) No enzyme reaction was known that allowed cytosolic synthesis of ADPGlc (for a review see [15]). (2) In some types of amyloplasts, exogenously supplied ADPGlc induced only low rates of starch biosynthesis [12]. Recently, a new ADPGlc pyrophosphorylase (ADPGlcPPase) isoenzyme has been discovered which is located in the cytosol of barley and maize endosperm tissue, representing between 85 and 95 % of the total activity [16,17].…”