Circadian Clock Regulation of Starch Metabolism Establishes GBSSI as a Major Contributor to Amylopectin Synthesis in Chlamydomonas reinhardtii  

Abstract

Chlamydomonas reinhardtii displays a diurnal rhythm of starch content that peaks in the middle of the night phase if the algae are provided with acetate and CO 2 as a carbon source. We show that this rhythm is controlled by the circadian clock and is tightly correlated to ADP-glucose pyrophosphorylase activity. Persistence of this rhythm depends on the presence of either soluble starch synthase III or granule-bound starch synthase I (GBSSI). We show that both enzymes play a similar function in synthesizing the long glucan fraction that interconnects the amylopectin clusters. We demonstrate that in log phase-oscillating cultures, GBSSI is required to obtain maximal polysaccharide content and fully compensates for the loss of soluble starch synthase III. A point mutation in the GBSSI gene that prevents extension of amylopectin chains, but retains the enzyme's normal ability to extend maltooligosaccharides, abolishes the function of GBSSI both in amylopectin and amylose synthesis and leads to a decrease in starch content in oscillating cultures. We propose that GBSSI has evolved as a major enzyme of amylopectin synthesis and that amylose synthesis comes as a secondary consequence of prolonged synthesis by GBSSI in arrhythmic systems. Maintenance in higher plant leaves of circadian clock control of GBSSI transcription is discussed.

Two distinct waxy alleles impact the granule-bound starch synthase in sorghum

“…Alternatively, the wx b GBSS protein still might be capable of synthesizing amylopectin within starch granules, but not amylose. It has been previously shown that GBSS has the ability to synthesize or extend both types of starch polymers (Delrue et al 1992;Denyer et al 1996;Ral et al 2006). In maize, GBSS activity in isolated starch granules did not strongly correlate with amylose content (Tsai 1974), which suggests that there are other factors that influence amylose content in starch granules in addition to GBSS.…”

Hydrogen Production in Chlamydomonas: Photosystem II-Dependent and -Independent Pathways Differ in Their Requirement for Starch Metabolism

“…Although this decrease in the starch content compared to the wild type is not explained at the moment, this could be due to subtle differences in transcriptional regulation of the complementing transgene by comparison to the wild-type gene. Indeed, transcription of STA6 is known to be under circadian clock control and exerts a tight control in the carbon flux to starch (Zabawinski et al, 2001;Ral et al, 2006). Limitations in the indirect pathway more likely rely on enzymatic steps involved from starch breakdown to nonphotochemical reduction of PQs.…”

Induction of Photosynthetic Carbon Fixation in Anoxia Relies on Hydrogenase Activity and Proton-Gradient Regulation-Like1-Mediated Cyclic Electron Flow in Chlamydomonas reinhardtii

“…Chlorophyll a plus b concentration was determined with a l 20 spectrophotometer (Perkin Elmer). Starch was extracted according to Ral et al (2006). Starch amounts were determined spectrophotometrically using the Starch Kit (Roche, R-Biopharm).…”