Plant Molecular Biology 52: 981–997, 2003.
© 2003 Kluwer Academic Publishers. Printed in the Netherlands.
Light and circadian regulation in the expression of LHY and Lhcb genes in
Athanasios-Dimitrios Kaldis, Panagiotis Kousidis, Konstantinos Kesanopoulos and Anastasia
Institute of Biology, NCSR ‘Demokritos’, 15310 Aghia Paraskevi, Attiki, Greece (
author for correspondence;
Received 3 February 2003; accepted in revised form 13 May 2003
Key words: circadian regulation, Lhcb, light induction, Phaseolus, PvLHY
In order to understand some aspects of the circadian clock function in Phaseolus vulgaris, we analyzed the temporal
transcript proﬁle of Lhcb genes, typical clock reporters in plants, and that of PvLHY, an orthologue of Arabidopsis
thaliana LHY which is a putative transcription factor of Lhcb genes. Under different light regimes, Lhcb and PvLHY
exhibit a clear circadian pattern of expression. Moreover, the rhythm of Lhcb genes appears to be tightly coupled
to that of PvLHY with the latter having a slightly earlier phase. This supports the idea that the oscillating capacity
of PvLHY may be one of the causes of the rhythmic expression of Lhcb genes in bean. In addition to their circadian
regulation, Lhcb and PvLHY are induced by light with similar and relatively slow induction kinetics. Moreover,
this light induction is gated by the circadian oscillator: minimal responses occur at times around peaks of the pre-
existing rhythm, while maximal ones occur at troughs of the pre-existing rhythm. This pattern of gating is opposite
to that observed in Arabidopsis. The failure to block the light induction pathways at pre-existing troughs appears
to have a detrimental effect to the subsequent circadian rhythmicity. Brieﬂy, the overall regulation of PvLHY and
Lhcb genes by light and the circadian clock reveals different strategies between Phaseolus and Arabidopsis in the
adaptation to photoperiodic conditions.
Abbreviations: Lhcb, genes encoding the apoproteins of PSII light-harvesting complexes (LHCB); PvLHY,geneof
Phaseolus vulgaris with high similarity to the LHY (LATE ELONGATED HYPOCOTYL) clock gene of Arabidopsis
Circadian rhythms in biological functions are ob-
served in a variety of very different organisms such
as cyanobacteria, fungi, plants, insects and mammals.
They are deﬁned as rhythmic oscillations that occur
in day/night cycles and persist with a period of ca.
24 h even after shifting organisms to constant envir-
onmental conditions such as constant light or constant
darkness. The rhythmic expression of Lhcb genes,
coding for the apoproteins of PSII light-harvesting
The nucleotide sequence reported will appear in the EMBL, Gen-
Bank and DDBJ Nucleotide Sequence Databases under the acces-
sion number AJ420902.2 (PvLHY).
complexes, is one of the most profound circadian
rhythms in plants (Tavladoraki et al., 1989; Beator and
Kloppstech, 1994). Lhcb expression is greatly elevated
around midday, while in the middle of the night it falls
to almost undetectable levels (Kellmann et al., 1993).
The circadian regulation of a biological function may
serve its appropriate timing (Harmer et al., 2000). In
this respect, since the function of LHCB antennae is to
harvest the surrounding light for photosynthesis, the
circadian peak of Lhcb expression into the daytime
appears to have adaptive signiﬁcance. Moreover, Lhcb
rhythm seems to be driven by an endogenous mech-
anism that enables plants to anticipate the beginning
of the day or the beginning of night. For example,