TY  - JOUR
A1  - Fettke, Jörg
A1  - Leifels, Lydia
A1  - Brust, Henrike
A1  - Herbst, Karoline
A1  - Steup, Martin
T1  - Two carbon fluxes to reserve starch in potato (Solanum tuberosum L.) tuber cells are closely interconnected but differently modulated by temperature
JF  - Journal of experimental botany
N2  - Parenchyma cells from tubers of Solanum tuberosum L. convert several externally supplied sugars to starch but the rates vary largely. Conversion of glucose 1-phosphate to starch is exceptionally efficient. In this communication, tuber slices were incubated with either of four solutions containing equimolar [U-C-14]glucose 1-phosphate, [U-C-14]sucrose, [U-C-14]glucose 1-phosphate plus unlabelled equimolar sucrose or [U-C-14]sucrose plus unlabelled equimolar glucose 1-phosphate. C-14-incorporation into starch was monitored. In slices from freshly harvested tubers each unlabelled compound strongly enhanced C-14 incorporation into starch indicating closely interacting paths of starch biosynthesis. However, enhancement disappeared when the tubers were stored. The two paths (and, consequently, the mutual enhancement effect) differ in temperature dependence. At lower temperatures, the glucose 1-phosphate-dependent path is functional, reaching maximal activity at approximately 20 degrees C but the flux of the sucrose-dependent route strongly increases above 20 degrees C. Results are confirmed by in vitro experiments using [U-C-14]glucose 1-phosphate or adenosine-[U-C-14]glucose and by quantitative zymograms of starch synthase or phosphorylase activity. In mutants almost completely lacking the plastidial phosphorylase isozyme(s), the glucose 1-phosphate-dependent path is largely impeded. Irrespective of the size of the granules, glucose 1-phosphate-dependent incorporation per granule surface area is essentially equal. Furthermore, within the granules no preference of distinct glucosyl acceptor sites was detectable. Thus, the path is integrated into the entire granule biosynthesis. In vitro C-14-incorporation into starch granules mediated by the recombinant plastidial phosphorylase isozyme clearly differed from the in situ results. Taken together, the data clearly demonstrate that two closely but flexibly interacting general paths of starch biosynthesis are functional in potato tuber cells.
KW  - glucose 1-phosphate
KW  - phosphorylase
KW  - potato tubers
KW  - starch
KW  - starch synthase
Y1  - 2012
U6  - https://doi.org/10.1093/jxb/ers014
SN  - 0022-0957
VL  - 63
IS  - 8
SP  - 3011
EP  - 3029
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Schopper, S.
A1  - Muhlenbock, P.
A1  - Sorensson, C.
A1  - Hellborg, L.
A1  - Lenman, M.
A1  - Widell, S.
A1  - Fettke, Jörg
A1  - Andreasson, Erik
T1  - Arabidopsis cytosolic alpha-glycan phosphorylase, PHS2, is important during carbohydrate imbalanced conditions
JF  - Plant biology
N2  - Arabidopsis thaliana has two isoforms of alpha-glycan phosphorylase (EC 2.4.1.1), one residing in the plastid and the other in the cytosol. The cytosolic phosphorylase, PHS2, acts on soluble heteroglycans that constitute a part of the carbohydrate pool in a plant. This study aimed to define a physiological role for PHS2. Under standard growth conditions phs2 knock-out mutants do not show any clear growth phenotype, and we hypothesised that during low-light conditions where carbohydrate imbalance is perturbed, this enzyme is important. Soil-grown phs2 mutant plants developed leaf lesions when placed in very low light. Analysis of soluble heteroglycan (SHG) levels showed that the amount of glucose residues in SHG was higher in the phs2 mutant compared to wild-type plants. Furthermore, a standard senescence assay from soil-grown phs2 mutant plants showed that leaves senesced significantly faster in darkness than the wild-type leaves. We also found decreased hypocotyl extension in in vitro-grown phs2 mutant seedlings when grown for long time in darkness at 6 degrees C. We conclude that PHS2 activity is important in the adult stage during low-light conditions and senescence, as well as during prolonged seedling development when carbohydrate levels are unbalanced.
KW  - Lesion formation
KW  - low light stress conditions
KW  - phosphorylase
KW  - PHS2
KW  - senescence
KW  - soluble heteroglycans
Y1  - 2015
U6  - https://doi.org/10.1111/plb.12190
SN  - 1435-8603
SN  - 1438-8677
VL  - 17
IS  - 1
SP  - 74
EP  - 80
PB  - Wiley-Blackwell
CY  - Hoboken
ER  -