Flowering in time: Mis-expression of GIGANTEA and effects on
photoperiodic flowering in Arabidopsis
Jo Putterill, Raechel Milich and Richard Moyle. School of Biological Sciences, University
of Auckland, Private Bag 92019, Auckland, New Zealand.
Long day (LD) plants such as Arabidopsis rely on the receiving light signals at the right time
for accelerated flowering in LD photoperiods. Physiological and genetic experiments indicate
that photoperiod detection in plants involves photoreceptors and an endogenous pacemaker,
the circadian clock. In addition to the timing of light, the duration, wavelength and intensity
of light are also important for rapid flowering in LD. The Arabidopsis GIGANTEA (GI) gene
is involved in photoperiodic flowering, as loss of function gi mutants flower late in LD
conditions, but are less affected in short days (SD). The GI gene is a plant-specific gene
encoding a novel, nuclear-localised protein. GI transcript levels are regulated by the circadian
clock, peaking 8-10 h into the light period, and by photoperiod. In LD, the GI transcript is
expressed for a longer duration during the light period than in SD, and peaks later in LD.
While GI expression is controlled by the circadian clock, gi mutations also affect circadian
rhythms in plants, possibly via the light input pathway, and reduce the level of expression of
several clock controlled genes such as LATE ELONGATED HYPOCOTYL, CIRCADIAN
CLOCK ASSOCIATED 1 (CCA1) and CONSTANS (CO).
To determine whether the strict circadian and photoperiodic control of the phase (timing) and
duration of GI expression is important for photoperiodic flowering, we mis-expressed GI in
transgenic plants using the Cauliflower Mosaic Virus 35S promoter and the promoters of
several circadian-regulated genes. Photoperiodic flowering was partially disrupted in some
transgenic lines with plants flowering much earlier than wild type in SD. The results of GI
mis-expression on photoperiodic flowering and circadian clock controlled genes in the
transgenic plants will be presented.
Results of mis-expression of a second Arabidopsis circadian-regulated gene, CONSTANS
LIKE 1 with ~67 % identity to the CO flowering-time gene will also be discussed time
permitting (work carried out in collaboration with Carl Strayer and Steve Kay, Scripps
Institute, San Diego). Antisense COL1 plants flowered at a similar time to wild type. Over
expression of COL1 did not appear to disrupt photoperiodic control of flowering, but did
accelerate the pace of the clock. Results will be presented that suggest that ectopic over
expression of COL1 acts on light input pathways to the circadian clock.