Does the core circadian clock in the moss Physcomitrella patens (Bryophyta) comprise a single loop?
BACKGROUND
The endogenous circadian clock allows the organism to synchronize processes both to daily and seasonal changes. In plants, many metabolic processes such as photosynthesis, as well as photoperiodic responses, are under the control of a circadian clock. Comparative studies with the moss Physcomitrella patens provide the opportunity to study many aspects of land plant evolution. Here we present a comparative overview of clock-associated components and the circadian network in the moss P. patens.


RESULTS
The moss P. patens has a set of conserved circadian core components that share genetic relationship and gene expression patterns with clock genes of vascular plants. These genes include Myb-like transcription factors PpCCA1a and PpCCA1b, pseudo-response regulators PpPRR1-4, and regulatory elements PpELF3, PpLUX and possibly PpELF4. However, the moss lacks homologs of AtTOC1, AtGI and the AtZTL-family of genes, which can be found in all vascular plants studied here. These three genes constitute essential components of two of the three integrated feed-back loops in the current model of the Arabidopsis circadian clock mechanism. Consequently, our results suggest instead a single loop circadian clock in the moss. Possibly as a result of this, temperature compensation of core clock gene expression appears to be decreased in P. patens.


CONCLUSIONS
This study is the first comparative overview of the circadian clock mechanism in a basal land plant, the moss P. patens. Our results indicate that the moss clock mechanism may represent an ancestral state in contrast to the more complex and partly duplicated structure of subsequent land plants. These findings may provide insights into the understanding of the evolution of circadian network topology.
Holm
Karl
K
Program in Evolutionary Functional Genomics, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE-752 36 Uppsala, Sweden.
Källman
Thomas
T
Gyllenstrand
Niclas
N
Hedman
Harald
H
Lagercrantz
Ulf
U
D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D019068:Bryopsida / Q000235:genetics* / Q000378:metabolism; D056926:CLOCK Proteins / Q000235:genetics* / Q000378:metabolism; D002940:Circadian Rhythm / Q000235:genetics*; D055028:Comparative Genomic Hybridization; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D053263:Gene Regulatory Networks; D017343:Genes, Plant; D017440:Photoperiod; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018749:RNA, Plant / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D017422:Sequence Analysis, DNA
D010940:Plant Proteins; D018749:RNA, Plant; D056926:CLOCK Proteins