Sucrose synthase controls both intracellular ADP glucose levels and transitory starch biosynthesis in source leaves.
The prevailing model on transitory starch biosynthesis in source leaves assumes that the plastidial ADPglucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule, ADPG. However, recent investigations have shown that ADPG linked to starch biosynthesis accumulates outside the chloroplast, presumably in the cytosol. This finding is consistent with the occurrence of an 'alternative' gluconeogenic pathway wherein sucrose synthase (SuSy) is involved in the production of ADPG in the cytosol, whereas both plastidial phosphoglucomutase (pPGM) and AGP play a prime role in the scavenging of starch breakdown products. To test this hypothesis, we have compared the ADPG content in both Arabidopsis and potato wild-type (WT) leaves with those of the starch-deficient mutants with reduced pPGM and AGP. These analyses provided evidence against the 'classical' model of starch biosynthesis, since ADPG levels in all the starch-deficient lines were normal compared with WT plants. Whether or not SuSy is involved in the synthesis of ADPG accumulating in leaves was tested by characterizing both SuSy-overexpressing and SuSy-antisensed transgenic leaves. Importantly, SuSy-overexpressing leaves exhibited a large increase of both ADPG and starch levels compared with WT leaves, whereas SuSy-antisensed leaves accumulated low amounts of both ADPG and starch. These findings show that (i) ADPG produced by SuSy is linked to starch biosynthesis; (ii) SuSy exerts a strong control on the starch biosynthetic process; and (iii) SuSy, but not AGP, controls the production of ADPG accumulating in source leaves.
Muñoz
Francisco José
FJ
Agrobioteknologiako Instituta, Nafarroako Unibertsitate Publikoa and Consejo Superior de Investigaciones Científicas, Mutiloako etorbidea zenbaki gabe, 31192 Mutiloabeti, Nafarroa, Spain.
Baroja-Fernández
Edurne
E
Morán-Zorzano
María Teresa
MT
Viale
Alejandro Miguel
AM
Etxeberria
Ed
E
Alonso-Casajús
Nora
N
Pozueta-Romero
Javier
J
D000245:Adenosine Diphosphate Glucose / Q000378:metabolism*; D017360:Arabidopsis / Q000378:metabolism; D029681:Arabidopsis Proteins; D051599:Glucose-1-Phosphate Adenylyltransferase; D005964:Glucosyltransferases / Q000378:metabolism*; D009713:Nucleotidyltransferases / Q000378:metabolism; D018515:Plant Leaves / Q000378:metabolism*; D011198:Solanum tuberosum / Q000378:metabolism; D013213:Starch / Q000096:biosynthesis*
D029681:Arabidopsis Proteins; D000245:Adenosine Diphosphate Glucose; D013213:Starch; D005964:Glucosyltransferases; C022699:sucrose synthase; D009713:Nucleotidyltransferases; C496715:ADP-glucose pyrophosphorylase, Arabidopsis; D051599:Glucose-1-Phosphate Adenylyltransferase