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Plant and Cell Physiology Advance Access published online on January 24, 2006
Plant and Cell Physiology, doi:10.1093/pcp/pcj011
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Plant and Cell Physiology 2006 © The Japanese Society of Plant Physiologists (JSPP); all rights reserved.
Received December 5, 2005
Accepted January 17, 2006

Regular Paper

An Important Pool of Sucrose Linked to Starch Biosynthesis Is Taken up by Endocytosis in Heterotrophic Cells

Edurne Baroja-Fernandez 1, Ed Etxeberria 2, Francisco José Muñoz 1, María Teresa Morán-Zorzano 1, Nora Alonso-Casajús 1, Pedro Gonzalez 2, and Javier Pozueta-Romero 1 *

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

* To whom correspondence should be addressed.
Javier Pozueta-Romero, E-mail: javier.pozueta{at}unavarra.es


  Abstract

We have recently shown the occurrence of endocytic sucrose uptake in heterotrophic cells (Etxeberria et al. Plant Cell Physiol. 46, 474-481). 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 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, the 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 further substantiated by experiments showing that sucrose-starch conversion was strongly prevented by both wortmannin and LY294002 in both potato tuber discs and developing barley endosperms.

Keywords: Acer pseudoplatanus; Hordeum vulgare; Solanum tuberosum; sink strength; sucrose-starch conversion; sucrose synthase.
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