Running a little late: chloroplast Fe status and the circadian clock.

In plants, the circadian clock is known to regulate a wide variety of processes and it is clear that the clock-conferred ability of plants to anticipate daily changes in environmental conditions confers significant fitness advantages. Two new studies highlight complex reciprocal interactions between the clock and chloroplast iron status and provide significant new insight into the role of iron in control of circadian period length. Iron (Fe) uptake and homeostasis are essential for plant growth and survival. These processes are carefully regulated to ensure adequate supplies of Fe to maintain metabolism while avoiding the negative effects of Fe over-accumulation. Indeed, it is clear that Fe uptake, efflux, long-distance transport and storage are controlled via transcriptional and posttranscriptional regulatory mechanisms and in recent years, a number of new components of the Fe-deficiency signalling cascade have been identified (Walker and Connolly, 2008; Palmer and Guerinot, 2009). In plants, the chloroplast represents a major sink for Fe; approximately 90% of the Fe found in leaves is contained within plastids, where it is associated with the photosystems, haem and chlorophyll. In addition, significant amounts of Fe are stored in chloroplasts in the form of ferritin-Fe. Chloroplast Fe homeostasis poses significant challenges owing, in part, to the fact that while Fe is essential for chloroplast function, excess Fe participates in the generation of hydroxyl radicals, which may exacerbate oxidative stress associated with normal functioning of photosynthetic electron transport reactions. It is not yet known how plants sense Fe but it is assumed that photosynthetic organisms possess a mechanism to gauge chloroplast Fe status.