Early evolution of the land plant circadian clock

While angiosperm clockscan be described as an intricate network of interlocked transcriptional feedback loops, clocksof green algae have been modelled as a loop of only two genes. To investigate the transition from a simple clockin algae to a complex one in angiosperms, we performed an inventory of circadian clockgenes in bryophytesand charophytes. Additionally, we performed functional characterization of putative core clockgenes in the liverwort Marchantia polymorphaand the hornwort Anthoceros agrestis. Phylogenetic construction was combined with studies of spatiotemporal expression patterns and analysis of M. polymorpha clockgene mutants. Homologues to core clockgenes identified in Arabidopsis were found not only in bryophytesbut also in charophytes, albeit in fewer copies. Circadian rhythms were detected for most identified genes in M. polymorpha and A. agrestis, and mutant analysis supports a role for putative clockgenes in M. polymorpha. Our data are in line with a recent hypothesis that adaptation to terrestrial life occurred earlier than previously expected in the evolutionary history of charophyte algae. Both gene duplicationand acquisition of new genes was important in the evolution of the plant circadian clock, but gene loss has also contributed to shaping the clockof bryophytes.