Plant and Cell Physiology Advance Access originally published online on January 24, 2006
Plant and Cell Physiology 2006 47(4):447-456; doi:10.1093/pcp/pcj011
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An Important Pool of Sucrose Linked to Starch Biosynthesis is Taken up by Endocytosis in Heterotrophic Cells
1 Agrobioteknologia Instituta, Nafarroako Unibertsitate Publikoa, Gobierno de Navarra and Consejo Superior de Investigaciones Científicas, Mutiloako etorbidea zenbaki gabe, 31192 Mutiloabeti, Nafarroa, Spain
2 University of Florida, IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850, USA
* Corresponding author: E-mail, javier.pozueta{at}unavarra.es; Fax: +34-948232191.
We have recently shown the occurrence of endocytic sucrose uptake in heterotrophic cells. Whether this mechanism is involved in the sucrose–starch conversion process was investigated by comparing the rates of starch accumulation in sycamore cells cultured in the presence or absence of the endocytic inhibitors wortmannin and 2-(4-morpholynyl-)-8-phenyl-4H-1 benzopyran-4-1 (LY294002). These analyses revealed a two-phase process involving an initial 120 min wortmannin- and LY294002-insensitive starch accumulation period, followed by a prolonged phase that was arrested by the endocytic inhibitors. Both wortmannin and LY294002 led to a strong reduction of the intracellular levels of both sucrose and the starch precursor molecule, ADPglucose. No changes in maximum catalytic activities of enzymes closely linked to starch and sucrose metabolism occurred in cells cultured with endocytic inhibitors. In addition, starch accumulation was unaffected by endocytic inhibitors when cells were cultured with glucose. These results provide a first indication that an important pool of sucrose incorporated into the cell is taken up by endocytosis prior to its subsequent conversion into starch in heterotrophic cells. This conclusion was substantiated further by experiments showing that sucrose–starch conversion was strongly prevented by both wortmannin and LY294002 in both potato tuber discs and developing barley endosperms.
(Received December 5, 2005; Accepted January 17, 2006)
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