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Abstract

Growth is driven by newly fixed carbon in the light, but depends at night on reserves, like starch, that are laid down in the light. Unless plants coordinate their growth with diurnal changes in the carbon supply, they will experience acute carbon starvation during the night. Protein synthesis represents a major component of cellular growth. Polysome loading was investigated during the diurnal cycle, an extended night and low CO2 in Arabidopsis Col-0 and in the starchless pgm mutant. In Col-0, polysome loading was 60-70% in the light, 40-45% for much of the night and <20% in an extended night, whilst in pgm it fell to <25% early in the night. Quantification of rRNA species using qRT-PCR revealed that polysome loading remained high for much of the night in the cytosol, was strongly light-dependent in the plastid, and was always high in mitochondria. The rosette sucrose content correlated with overall and with cytosolic polysome loading. Ribosome abundance did not show significant diurnal changes. However, compared to Col-0, pgm had decreased and increased abundance of plastidic and mitochondrial ribosomes, respectively. Incorporation of label from 13CO2 into protein confirmed that protein synthesis continues at a diminished rate in the dark. Modelling revealed that a decrease in polysome loading at night is required to balance protein synthesis with the availability of carbon from starch breakdown. Costs are also reduced by using amino acids that accumulated in previous light period. These results uncover a tight coordination of protein synthesis with the momentary supply of carbon.