Date Published: February 2, 2017
Publisher: Public Library of Science
Author(s): Marouane Baslam, Edurne Baroja-Fernández, Adriana Ricarte-Bermejo, Ángela María Sánchez-López, Iker Aranjuelo, Abdellatif Bahaji, Francisco José Muñoz, Goizeder Almagro, Pablo Pujol, Regina Galarza, Pilar Teixidor, Javier Pozueta-Romero, Lam-Son Phan Tran.
Although there is a great wealth of data supporting the occurrence of simultaneous synthesis and breakdown of storage carbohydrate in many organisms, previous 13CO2 pulse-chase based studies indicated that starch degradation does not operate in illuminated Arabidopsis leaves. Here we show that leaves of gwd, sex4, bam4, bam1/bam3 and amy3/isa3/lda starch breakdown mutants accumulate higher levels of starch than wild type (WT) leaves when cultured under continuous light (CL) conditions. We also show that leaves of CL grown dpe1 plants impaired in the plastidic disproportionating enzyme accumulate higher levels of maltotriose than WT leaves, the overall data providing evidence for the occurrence of extensive starch degradation in illuminated leaves. Moreover, we show that leaves of CL grown mex1/pglct plants impaired in the chloroplastic maltose and glucose transporters display a severe dwarf phenotype and accumulate high levels of maltose, strongly indicating that the MEX1 and pGlcT transporters are involved in the export of starch breakdown products to the cytosol to support growth during illumination. To investigate whether starch breakdown products can be recycled back to starch during illumination through a mechanism involving ADP-glucose pyrophosphorylase (AGP) we conducted kinetic analyses of the stable isotope carbon composition (δ13C) in starch of leaves of 13CO2 pulsed-chased WT and AGP lacking aps1 plants. Notably, the rate of increase of δ13C in starch of aps1 leaves during the pulse was exceedingly higher than that of WT leaves. Furthermore, δ13C decline in starch of aps1 leaves during the chase was much faster than that of WT leaves, which provides strong evidence for the occurrence of AGP-mediated cycling of starch breakdown products in illuminated Arabidopsis leaves.
A substrate or “futile” cycle is a metabolic cycle of simultaneous synthesis and breakdown of a compound for which the net balance consists solely on the dissipation of energy [1,2]. In carbon cycles, energy dissipation occurs mainly, but not exclusively, through the net hydrolysis of ATP. In some cases substrate cycles consume up to 70% of the ATP produced by the cell [3–6]. They operate in microorganisms , plants [7,8], yeasts  and animals , playing roles such as heat generation, buffering of metabolite concentrations, improvement of sensitivity in metabolic regulation, and control of the direction of flow in bidirectional pathways [1,2]. In particular, there is a great wealth of genetic, radiotracer, stoichiometric analysis and stable isotope labeling data supporting the occurrence of metabolic cycles resulting from the simultaneous synthesis and breakdown of storage carbohydrates such as trehalose in fungi [11,12], sucrose and starch in heterotrophic organs of plants [7,13,14] and glycogen in animals [15–17], yeasts  and bacteria [18–24].