Isotope ratio mass spectrometric and genetic evidence for the occurrence of starch degradation and cycling in illuminated Arabidopsis leaves

Trabajo presentado en la XIII Reunión de Biología Molecular de Plantas, celebrada en Oviedo (España), del 22 al 24 de junio de 2016Substrate or “futile” cycles are metabolic cycles of synthesis and degradation of a compound resulting in ATP consumption and dissipation of energy. Although there is a great wealth of data supporting the occurrence of storage carbohydrate cycling in many organisms, previous 14CO2 pulse-chase studies indicated that starch degradation and cycling do not operate in illuminated Arabidopsis leaves. In this work we show that leaves of different starch breakdown mutants cultured under continuous light conditions accumulate higher levels of starch than WT leaves, which shows that starch degradation operates 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 time-course analyses of the stable isotope carbon composition (δ13C) of starch in leaves of 13CO2 pulsed-chased wild type (WT) and AGP lacking aps1 plants. Maximum δ13C values of starch in aps1 leaves reached at the end of the pulse were exceedingly higher than those of WT leaves. Furthermore, δ13C reduction in starch of aps1 leaves during the chase was much more rapid than that of WT leaves. Notably, aps1/mex1 leaves impaired in the export of maltose derived from starch breakdown displayed a high-maltose phenotype. Results presented in this work provide strong evidence for the occurrence of simultaneous synthesis and breakdown of starch and the operation of starch cycling through a mechanism involving AGP in illuminated Arabidopsis leaves.Peer reviewe