Glucose 1‐phosphate is efficiently taken up by potato (Solanum tuberosum) tuber parenchyma cells and converted to reserve starch granules

Abstract

Summary Reserve starch is an important plant product but the actual biosynthetic process is not yet fully understood. Potato (Solanum tuberosum) tuber discs from various transgenic plants were used to analyse the conversion of external sugars or sugar derivatives to starch. By using in vitro assays, a direct glucosyl transfer from glucose 1‐phosphate to native starch granules as mediated by recombinant plastidial phosphorylase was analysed. Compared with labelled glucose, glucose 6‐phosphate or sucrose, tuber discs converted externally supplied [14C]glucose 1‐phosphate into starch at a much higher rate. Likewise, tuber discs from transgenic lines with a strongly reduced expression of cytosolic phosphoglucomutase, phosphorylase or transglucosidase converted glucose 1‐phosphate to starch with the same or even an increased rate compared with the wild‐type. Similar results were obtained with transgenic potato lines possessing a strongly reduced activity of both the cytosolic and the plastidial phosphoglucomutase. Starch labelling was, however, significantly diminished in transgenic lines, with a reduced concentration of the plastidial phosphorylase isozymes. Two distinct paths of reserve starch biosynthesis are proposed that explain, at a biochemical level, the phenotype of several transgenic plant lines.

Glucose-1-Phosphate Transport into Protoplasts and Chloroplasts from Leaves of Arabidopsis

“…Following the import of Glc-1-P into the amyloplasts, the subsequent incorporation into starch is mainly mediated by the plastidial phosphorylase isozyme (Pho1; Fettke et al, 2010). By contrast, in mesophyll cells, Glc-1-P imported into the chloroplast leads to the formation of ADPglucose, which then acts as a glucosyl donor for starch synthases (Table II).…”

“…2A). Thus, the superior efficiency of the Glc-1-P-dependent labeling of starch is not unique to heterotrophic cells (compare with Fettke et al, 2010). Light stimulates the flux from both external Glc-1-P and Glc to starch, but even in darkened protoplasts, the Glc-1-P-dependent labeling of starch is higher than that of Glc in illuminated protoplasts.…”

“…In order to determine whether or not the imported Glc-1-P is converted to starch (as is the case in heterotrophic potato tuber cells; Fettke et al, 2010), mesophyll protoplasts from Arabidopsis leaves were incubated for 20 min with equimolar concentrations (20 mM each) of [U-…”

Biosynthesis of Reserve Starch

“…Several reports have supported this idea. First, Fettke et al (2010aFettke et al ( , 2012 showed that G1P was efficiently and directly incorporated into reserve starch granules in potato tuber cells, suggesting the G1P pathway functions in parallel with the authentic AGPase pathway in starch biosynthesis. Second, Hwang et al (2010) proved that rice Pho1 could elongate MOS to be used by SS in the glucan synthetic direction even under physiological conditions of high Pi/G1P concentrations.…”

Initiation Process of Starch Biosynthesis

“…They also assumed that some unknown factor(s) which can function and/or is(are) expressed only at higher temperatures above 20 ı C support(s) the role of Pho1. Fettke et al (2010Fettke et al ( , 2012 have examined a specific incorporation behavior of glucose 1-phosphate (G1P) by potato tuber discs from various transgenic lines and found that the exogenously added G1P was converted to native starch granules in tubers as mediated by Pho1 (Fettke et al 2010). The rate of incorporation of G1P was much higher than glucose, glucose 6-phosphate (G6P), or sucrose.…”