Circadian expression of the PpLhcb2 gene encoding a major light-harvesting chlorophyll a/b-binding protein in the...

Circadian clocks control the expression of Lhcb genes encoding the chlorophyll a/b-binding proteins broadly in seed plants. We show here that this regulation is also conserved in the primitive moss Physcomitrella patens. Northern blotting analyses revealed a robust daily oscillation of Lhcb mRNA levels in protonema cells in 12-h : 12-h light-dark cycles (12 : 12LD) that damped rapidly in continuous darkness (DD). In continuous light (LL), by contrast with typical profiles in higher plants, Lhcb mRNA levels only peaked during the first day and thereafter it showed constant levels. Reverse transcription (RT)-PCR analyses showed similar patterns of expression in LL for three distinct Lhcb genes (PpLhcb1, PpLhcb2 and Zlab1). Moreover, transgenic reporter strains expressing luciferase under the control of the PpLhcb2 promoter showed bioluminescence patterns consistent with the Northern and RT-PCR data. At a higher concentration (4.5%) of glucose in the medium, the reporter strain showed self-sustained rhythms in DD, which was entrained to a differently phased 12 : 12LD, revealing a circadian regulation on the transcription. Kinetics of bioluminescent peaks in 12 : 12LD from gametophore was different to those from protonema, indicating a developmental regulation on PpLhcb2. Together, the regulatory link between the clock and Lhcb genes in P. patens shows characteristics that appear to differ from those in higher plants.