Sugar-mediated semidian oscillation of gene expression in the cassava storage root regulates starch synthesis Page: 2 of 24
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Starch branching enzyme (SBE) activity in the cassava storage root exhibited a
diurnal fluctuation, dictated by a transcriptional oscillation of the corresponding
SBE genes. The peak of SBE activity coincided with the onset of sucrose
accumulation in the storage, and we conclude that the oscillatory mechanism
keeps the starch synthetic apparatus in the storage root sink in tune with the flux
of sucrose from the photosynthetic source. When storage roots were uncoupled
from the source, SBE expression could be effectively induced by exogenous
sucrose. Turanose, a sucrose isomer that cannot be metabolized by plants,
mimicked the effect of sucrose, demonstrating that downstream metabolism of
sucrose was not necessary for signal transmission. Also glucose and glucose-1-P
induced SBE expression. Interestingly, induction by sucrose, turanose and
glucose but not glucose-1-P sustained an overt semidian (12-h) oscillation in SBE
expression and was sensitive to the hexokinase (HXK) inhibitor glucosamine.
These results suggest a pivotal regulatory role for HXK during starch synthesis.
Abscisic acid (ABA) was another potent inducer of SBE expression. Induction by
ABA was similar to that of glucose-1-P in that it bypassed the semidian
oscillator. Both the sugar and ABA signaling cascades were disrupted by okadaic
acid, a protein phosphatase inhibitor. Based on these findings, we propose a
model for sugar signaling in regulation of starch synthesis in the cassava storage
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Jansson, Christer; Baguma, Yona; Sun, Chuanxin; Boren, Mats; Olsson, Helena; Rosenqvist, Sara et al. Sugar-mediated semidian oscillation of gene expression in the cassava storage root regulates starch synthesis, article, January 15, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc894515/m1/2/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.