Circadian regulation of chloroplasts

•Circadian timing signals pass from the nuclear-encoded circadian clock to chloroplasts.•Circadian signalling by sigma factors occurs across the green lineage.•The circadian clock regulates light signalling to chloroplasts.•There are circadian-regulated retrograde signals from chloroplasts to the nucleus.

Circadian rhythms produce a biological measure of time that increases plant performance. The mechanisms that underlie this increase in productivity require investigation to provide information that will underpin future crop improvement. There is a growing body of evidence that a sophisticated signalling network interconnects the circadian oscillator and chloroplasts. We consider this in the context of circadian signalling to chloroplasts and the relationship between retrograde signalling and circadian regulation. We place circadian signalling to chloroplasts by sigma factors within an evolutionary context. We describe selected recent developments in the integration of light and circadian signals that control chloroplast gene expression.