Rhythms of chloroplast magnesium import contribute to daily metabolic feedback

Organisms have evolved stable yet dynamic circadian systems to coordinate biological rhythms which contribute to fitness. Circadian oscillators generate rhythms via transcriptional and post-transcriptional control of key cellular processes such as metabolism and nutrient transport that fluctuate across the course of the day. In addition, intracellular ions and metabolites can act as signalling molecules and protein cofactors which provide feedback to fine-tune biological timekeeping (Haydon et al., 2013). This feedback control is critical to match cellular processes with the external environment and optimise growth and productivity. A rice magnesium transporter OsMGT3 facilitates rhythmic Mg import into chloroplasts and generates diel oscillations of chloroplast Mg which contributes to efficient CO assimilation by supporting rubisco activity (Li et al., 2020). It has now been shown that two core circadian oscillator proteins, OsPRR59 and OsPRR95, directly repress the expression of OsMGT3 and control rhythms of chloroplast Mg concentration. This Mg contributes to rhythms of carbon fixation, which in turn, promotes the expression of these oscillator genes through a metabolic signal cascade involving sugar and superoxide (O) (Chen et al., 2022). This work highlights the bidirectional communication between chloroplast Mg and the core circadian oscillator in the nucleus, acting via a sugar-activated O signal which regulates cellular timekeeping, similar to that recently identified in Arabidopsis (Román et al., 2021; Chen et al., 2022).