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Abstract

The circadian clock controls the expression of thousands of genes, many of which are involved in photosynthesis and metabolism. The proper daily timing of physiological and cellular processes by the circadian clock confers a fitness advantage, as a disruption of circadian period results in plants with lower photosynthetic capacity and lower biomass. In animals, the circadian clock and the metabolic state of the cell are intimately interconnected: one of the mammalian clock proteins acts as a sensor of intracellular heme, and modulates the expression of other clock genes accordingly. To determine whether changing heme levels could also impact the plant circadian clock, we tested how clock parameters (circadian period, phase and amplitude) were affected when seedlings are grown under limited iron supply. We found that the phase and amplitude of several luciferase reporters (for clock genes and output genes) remained unchanged between 0-100 μM Fe-HBED. However, the clock responded to a lowering of available iron by lengthening the period of all reporters tested by up to 3 hours over iron replete conditions. This response was specific to iron, as conditions of low copper, manganese or zinc did not affect period length. Loss of function in the clock genes CCA1, LHY, TOC1, ZTL or GI all responded to lower available iron by lengthening their fre-running period, indicating that the encoded clock proteins do not require heme as a cofactor. However, a mutant in the chloroplast-localized heme oxygenase HO1 failed to adjust its circadian period to available iron levels, suggesting that, just like in animals, heme might play a critical signaling role in connecting the circadian clock and the cell metabolic state. We will discuss our efforts to characterize how the circadian clock responds to changing iron levels, and how this might impact plant fitness.