75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

251 Ontogeny of the Arabidopsis Circadian Clock
Patrice Salome, C. Robertson McClung
Dartmouth College, Hanover, (NH), USA
A functional circadian clock allows an organism to synchronize with the daily succession of day and night, and to
anticipate changes in the environment. In Arabidopsis, much research has focused on the entrainment of the clock to lightdark
(LD) or warm-cold (HC) cycles. Very little is known, however, on the requirements leading to the generation of a
rhythm. In zebrafish, prior entrainment is critical for onset of rhythmic behavior. In Drosophila or mammals, entrainment
is dispensable for rhythmicity, but not for synchronization with the environment. With the use of the LUCIFERASE
reporter gene driven by a number of clock-regulated genes, we set out to investigate the minimal requirements for
onset of rhythmic expression in Arabidopsis seedlings. Seedlings grown in constant light were rhythmic, demonstrating
that, unlike zebrafish, entrainment is dispensable for rhythmicity. In addition, exposure to light is not a requirement for
rhythmicity, as etiolated seedlings displayed rhythms, although with weaker amplitudes, in all genes tested including
the LIGHT HARVESTING COMPLEX PROTEIN B (LHCB). Although rhythms were detected in seedlings grown in
constant light and constant darkness, the relative synchronization among individual seedlings was not as precise as that
seen after LD or HC cycles, especially for the clock-regulated genes CATALASE 3 (CAT3) and LHCB. The observed
phase of all genes in the absence of entrainment also appeared to be out of phase with entrained seedlings. Onset of
circadian rhythmicity could be observed as early as 1 day after release from stratification, as seen with the expression
pattern of the genes PSEUDO RESPONSE REGULATOR 7 (PRR7), CIRCADIAN CLOCK ASSOCIATED 1 (CCA1)
and TIMING OF CAB 1 (TOC1). Interestingly, rhythmic expression of CCA1, LATE and ELONGATED HYPOCOTYL
(LHY) and TOC1 was not detected in germinating, non-stratified seeds until 3-4 days after imbibition, when PRR7
could be readily detected 1-2 days after imbibition. We speculate that germinating seedlings use imbibition to initiate
circadian rhythmicity before light exposure. While still buried in the soil, seedlings become synchronized with the outside
environment by HC cycles and show a weak amplitude critical for rapid adaptation to new conditions. The first exposure
to light will greatly increase the amplitude of circadian oscillations, providing seedlings with high levels of the needed
transcripts at this early stage of development.
252 The interaction of the holoparasite, Cuscuta reflexa, with Arabidopsis thaliana
Dierk Scheel, Mandy Birschwilks
Leibniz Institute of Plant Biochemistry, Halle, Germany
The interaction of Cuscuta reflexa with Arabidopsis thaliana represents a compatible host–parasite combination. After
twinning around the influorescence stem, Cuscuta produces a haustorium that penetrates the host tissue. In early stages
of development the searching hyphae at the tip of the haustorial cone are connected to the host tissue by interspecific
plasmodesmata. Ten days after infection translocation of the fluorescent dyes, Texas red and 5,6-Carboxyfluorescein,
demonstrates the existence of continuous connections between xylem and phloem of host and parasite. Cuscuta reflexa
is the dominant sink in this host-parasite-system. A set of different ecotypes of Arabidopsis thaliana is being screened
for altered infection phenotypes with Cuscuta relexa. One out of 15 ecotypes analyzed so far apparently displays a novel
type of resistance against the parasite.