, 1828 (2008);
et al.Bethany L. Ehlmann,
Orbital Identification of Carbonate-Bearing Rocks
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Large clutch volume–adult body mass ratios do
not occur in dinosaurs more distantly related to
birds, such as allosauroids (26). Troodontids and
oviraptorids further differ from other more basal
dinosaurs in featuring relatively larger eggs, mo-
Consequently, two factors may have contributed
to the evolution of paternal care: (i) increased
energy demands of larger, sequentially ovulated
eggs, necessitating females to focus strictly on
greater thermal incubation needs of embryos, re-
maternal and biparental care systems occur with-
in extant crocodilians, the nature of parental care
within more basal theropods and dinosaurs in
general remains ambiguous.
Paternal care in both troodontids and ovi-
raptorids (Fig. 2E) implies that this reproductive
system originated before the origin of flight and
was primitive for Aves. Biparental care of
Neognathes would then represent a derived
condition. Although paternal care has previously
been suggested as the ancestral condition for
extant birds (3, 24, 27, 29), it has largely been
envisioned as evolving within primitive birds,
potentially in conjunction with superprecocial
chicks (24, 27). In extant birds, the three parental
care strategies correspond to statistically distinct
clutch volume–adult body mass relationships
(table S2), with paternal care associated with the
largest clutches, maternal care with intermediate-
size clutches, and biparental care with the
smallest clutches for most adult sizes. This sug-
gests a trade-off in parental investment between
overall clutch mass and total parental care.
References and Notes
1. T. H. Clutton-Brock, The Evolution of Parental Care
(Princeton Univ. Press, Princeton, NJ, 1991).
2. B. S. Tullberg, M. Ah-King, H. Temrin, Philos. Trans.
R. Soc. London Ser. B 357, 251 (2002).
3. J. D. Ligon, The Evolution of Avian Breeding Systems
(Oxford Univ. Press, Oxford, 1999).
4. S. J. J. F. Davies, Ratites and Tinamous (Oxford Univ.
Press, Oxford, 2002).
5. D. J. Varricchio, F. Jackson, J. J. Borkowski, J. R. Horner,
Nature 385, 247 (1997).
6. M. A. Norell, J. M. Clark, L. M. Chiappe, D. Dashzeveg,
Nature 378, 774 (1995).
7. See supporting material on Science Online.
8. H. Motulsky, A. Christopoulos, Fitting Models to
Biological Data Using Linear and Nonlinear Regression
(Oxford Univ. Press, Oxford, 2004).
9. W. R. Branch, R. W. Patterson, J. Herpetol. 9, 243
10. L. H. S. Van Mierop, S. M. Barnard, J. Herpetol. 10, 333
11. O. Lourdais, T. C. M. Hoffman, D. F. DeNardo, J. Comp.
Physiol. B 177, 569 (2007).
12. L. A. Somma, Smithsonian Herpetological Information
Service 81 (1990).
13. R. Shine, in Biology of the Reptilia: Volume 16, Ecology
B, C. Gans, R. B. Huey, Eds. (Liss, New York, 1988),
14. D. J. Varricchio, F. D. Jackson, J. Vertebr. Paleontol. 24,
15. C. S. Wink, R. M. Elsey, J. Morphol. 189, 183 (1986).
16. K. Simkiss, Calcium in Reproductive Physiology
(Chapman and Hall, London, 1967).
17. A. Chinsamy, Palaeontol. Afr. 27, 77 (1990).
18. M. H. Schweitzer, J. L. Wittmeyer, J. R. Horner, Science
308, 1456 (2005).
19. A. H. Lee, S. Werning, Proc. Natl. Acad. Sci. U.S.A. 105,
20. P. C. Sereno, Science 284, 2137 (1999).
21. G. M. Erickson, K. C. Rogers, D. J. Varricchio, M. A. Norell,
X. Xu, Biol. Lett. 3, 558 (2007).
22. R. Pahl, D. W. Winkler, J. Graveland, B. W. Batterman,
Proc. R. Soc. London Ser. B 264, 239 (1997).
23. D. J. Varricchio, F. D. Jackson, in Feathered Dragons,
P. J. Currie, E. B. Koppelhus, M. A. Shugar, J. L. Wright,
Eds. (Indiana Univ. Press, Bloomington, 2004),
24. A. Elzanowksi, in Acta XVIII Congressus Internationalis
Ornithologici, V. D. Ilyichev, V. M. Gavrilov, Eds. (Academy
of Sciences of the USSR, Moscow, 1985), pp. 178–183.
25. J. R. Horner, in Dinosaurs Past and Present, Volume II,
S. J. Czerkas, E. C. Olson, Eds. (Univ. of Washington
Press, Seattle, 1987), pp. 51–63.
26. I. Mateus et al., C. R. Acad. Sci. Paris IIA 325, 71 (1997).
27. T. Wesolowski, Am. Nat. 143, 39 (1994).
28. S. L. Vehrencamp, Behav. Ecol. 11, 334 (1999).
29. J. Van Rhijn, Neth. J. Zool. 34, 103 (1984).
30. We thank J. Horner, A. Chinsamy-Turan, L. Hall,
H. Akashi, J. Rotella, and P. T. Varricchio Sr. Supported by
NSF grants EAR-0418649 and DBI-0446224 (G.M.E.).
Supporting Online Material
Materials and Methods
Tables S1 to S5
14 July 2008; accepted 14 November 2008
Orbital Identification of
Carbonate-Bearing Rocks on Mars
Bethany L. Ehlmann,1John F. Mustard,1Scott L. Murchie,2Francois Poulet,3Janice L. Bishop,4
Adrian J. Brown,4Wendy M. Calvin,5Roger N. Clark,6David J. Des Marais,7Ralph E. Milliken,8
Leah H. Roach,1Ted L. Roush,7Gregg A. Swayze,6James J. Wray9
Geochemical models for Mars predict carbonate formation during aqueous alteration.
Carbonate-bearing rocks had not previously been detected on Mars’ surface, but Mars
Reconnaissance Orbiter mapping reveals a regional rock layer with near-infrared spectral
characteristics that are consistent with the presence of magnesium carbonate in the Nili Fossae
region. The carbonate is closely associated with both phyllosilicate-bearing and olivine-rich rock
units and probably formed during the Noachian or early Hesperian era from the alteration of
olivine by either hydrothermal fluids or near-surface water. The presence of carbonate as well
as accompanying clays suggests that waters were neutral to alkaline at the time of its formation
and that acidic weathering, proposed to be characteristic of Hesperian Mars, did not destroy
these carbonates and thus did not dominate all aqueous environments.
instruments found no large-scale or massive
carbonate-bearing rocks (4, 5). Carbonate in
veins within Martian meteorites (6) and possibly
at <5% abundance in Mars dust (1, 4) indicates
that it is present as a minor phase. The lack of
carbonate-bearing rock outcrops is puzzling in
light of evidence for surface water and aqueous
alteration, which produced sulfate and phyllo-
lthough telescopic measurements hinted
at the presence of carbonate on Mars
(1–3), subsequent orbiting and landed
silicate minerals (5, 7). Carbonate is an expected
weathering product of water and basalt in an
atmosphere with CO2 (8, 9), and large-scale
deposits, which might serve as a reservoir for
atmospheric CO2, were predicted for Mars (10).
Lack of carbonate among identified alteration
minerals has compelled suggestions that either
(i) a warmer, wetter early Mars was sustained by
greenhouse gases other than CO2(11, 12); (ii)
liquid water on Mars’ surface in contact with its
to form substantial carbonate (13) (thus implying
that minerals such as phyllosilicates must have
formed in the subsurface); or (iii) formation of
carbonate deposits was inhibited or all such
deposits were destroyed by acidic aqueous ac-
report the detection of carbonate in a regional-
scale rock unit by the Mars Reconnaisance
Orbiter’s (MRO’s) Compact Reconnaissance
Imaging Spectrometer for Mars (CRISM) and
discuss the implications for the climate and hab-
itability of early Mars.
In targeted mode, CRISM acquires hyper-
spectral images from 0.4 to 4.0 mm in 544 chan-
nels at a spatial resolution of 18 meters per pixel
(17). In addition to diverse hydrated silicates
(18), CRISM identified a distinct, mappable
1Department of Geological Sciences, Brown University,
Providence,RI02912,USA.2Johns Hopkins University/Applied
Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD
Paris Sud 11, 91405 Orsay, France.4SETI Institute and NASA
Ames Research Center, 515 North Whisman Road, Mountain
Engineering, University of Nevada, MS 172, 1664 North
MS 964, Box 25046, Denver Federal Center, Denver, CO
80225, USA.7NASA Ames Research Center, Mountain View,
of Technology, MS 183-301, 4800 Oak Grove Drive, Pasadena,
CA 91109, USA.9Department of Astronomy, Cornell Uni-
versity, 610 Space Sciences Building, Ithaca, NY 14853,
3Institut d'Astrophysique Spatiale, Université
19 DECEMBER 2008VOL 322
on January 5, 2009
spectral class of hydrated material in the Nili
Fossae region and two nearby areas (Fig. 1) for
which a match to known mineral reflectance
spectra was not initially evident (19). This spec-
structural H2O and also characteristic absorptions
at 2.3 and 2.5 mm and a broad 1-mm absorption
(Fig. 2). Similar spectra have been obtained with
the Observatoire pour la Minéralogie, l’Eau, les
Glaces, et l’Activité (OMEGA) imaging spec-
trometer on Mars Express (20).
Mg carbonate is the best candidate to ex-
plain the distinctive features of this spectral
class. Paired absorptions at 2.3 and 2.5 mm are
overtones and combination tones of C-O stretch-
ing and bending fundamental vibrations in the
mid-infrared (21). The wavelength of their min-
ima identifies the major metal cation in the
carbonate (Fig. 3) (21, 22). Anhydrous carbon-
ates with mostly Mg exhibit minima at shorter
wavelengths (2.30 and 2.50 mm) than those with
mostly Ca (2.34 and 2.54 mm) and Fe (2.33 and
2.53 mm) (22) and match the spectral class iden-
tified by CRISM (Fig. 3). We know of no other
mineral spectrum that has all of the proper-
ties of this class in terms of band position and
width in the 2.0-to-2.6-mm spectral region
(Fig. 2, B and C). The overall spectral shape,
position, and relative strengths of the 2.3- and
2.5-mm absorption bands are consistent among
CRISM spectra (Figs. 2B and 3), which sug-
gests that the distinctive spectral class is gen-
erated by the presence of a single phase rather
than a mixture of many alteration minerals (23).
In a mixture, the relative strengths of individual
bands would be expected to vary with varia-
tion in the relative abundances of the mineral
Fig. 1. (A) Global map of carbonate detections by CRISM (green circles) on a
Thermal Emission Imaging System (THEMIS) satellite day infrared (IR) image with
Mars Orbiter Laser Altimeter elevation data (blue is low and red is high). (B)
THEMIS day IR image of the Nili Fossae region with olivine mapped by using
OMEGA orbits <4500 using the olivine parameter (43). Targeted CRISM images
examined in this study are shown and outlined in yellow where Fe-Mg
phyllosilicates were found and in white where they were not.Green circles indicate
carbonate detections. Where image footprints have substantial overlap, only one
circle is shown.
ratioed CRISM I/F
laboratory reflectance (offset)
0.4 0.8 1.21.6 2.02.42.83.2 3.6 4.0
CRISM ratioed I/F
Fig. 2. CRISM andcandidate mineral visibleandnear-IR spectra.
CRISM data have been photometrically and atmospherically
corrected as in (18) and then processed with the noise-reduction
algorithm of (47). (A) CRISM reflectance spectra from hundreds of
pixel regions of interest from image HRL000040FF. Black line,
cap rock. (B) Spectral ratios, which highlight differences in
composition between two units, are shown for the regions of
interest in five different CRISM images (top to bottom,
FRT00003FB9, FRT0000A09C, FRT000093BE, HRL000040FF,
and FRT0000B072) that contain the spectral class. The bold
spectrum is the ratio from (A). (C) Laboratory spectra (RELAB
brucite, the zeolite analcime, nontronite, serpentine, and chlorite. (D) CRISM spectral ratios over the full
wavelength range, using the same denominator, for terrains from HRL000040FF inferred to be carbonate-
bearing (black)and olivine-bearing(gray).(E)Laboratoryspectrafor fayalitic olivine,magnesite,andamixture
VOL 322 19 DECEMBER 2008
on January 5, 2009
components. In some CRISM spectra of what
we infer to be carbonate-bearing materials (Fig.
2B), intimate or spatial mixing is indicated by a
wavelength shift and narrowing at 2.3 mm and
broadening at 2.5 mm that is accompanied by
the appearance of a weak band at 2.4 mm. These
collectively indicate the presence also of iron-
magnesium smectite [for example, nontronite
(Fig. 2C, orange)], which has been previously
identified in the region (18–20, 24).
and CRISM (19) observations, a carbonate min-
eral identification was rejected because the data
lacked the strong 3.4- and 3.9-mm overtone ab-
sorptions seen in some laboratory data of calcite,
and their mixtures (25). However, we found the
highly correlated 2.3- and 2.5-mm bands in many
CRISM observations and thus reexamined the
3-to-4-mm region in both CRISM and laboratory
data. Absorptions at 3.45 and 3.9 mm in the
CRISM spectra are present in terrains with the
those absorptions [Fig. 2D and supporting online
material (SOM) text] but are quite subtle and
become apparent only after averaging of spectra
from hundreds of pixels. Laboratory data show
that absorptions from 3 to 4 mm in carbonate are
not always strong (Fig.2E,purple and blue). The
presence of water, coatings, or additional min-
putative carbonate-bearing spectral class, the
presence of a water-bearing phase (or phases) is
indicated by 1.9-mm(Fig. 2B) anda deep 3.0-mm
absorption (Fig. 2D). Hydrous carbonates (car-
bonates whose structures incorporate water) fre-
quently have no 3.4- or 3.9-mm bands (2,26) and
are a kinetically favored low-temperature alter-
ation product from solutions with Mg and CO3
water and OH near 3 mm in hydrated phases (for
example, hydrous carbonates or clays) when
mixed with anhydrous carbonate can subdue the
3-to-4-mm carbonate absorptions [Fig. 2E, blue;
magnesite + hydromagnesite in (21, 29)]. Addi-
tionally, remote detections in the 3-to-4-mm
region are complicated by a thermal emission
contribution that reduces band strength (30, 31)
and also by instrument effects. CRISM’s signal-
to-noise ratio is more than four times lower at
wavelengths >2.7 mm, and interpretation of that
out-of-order light (17) and a probable detector
artifact at 3.18 mm. The combination of subtle
absorption features at 3.4 and 3.9 mm and the
distinctive 2.3- and 2.5-mm bands is consistent
with the presence of carbonate.
The CRISM spectra also display a strong
broad band near 1.1 mm, which is generated by
electronic transitions of Fe2+(32). Magnesite
(MgCO3) and siderite (FeCO3) form a complete
solid solution, and a strong broad electronic
bandcentered near1.1 mm isapparentwitheven
<1weight percent (wt %) ironwithout changing
the position of the 2.3- and 2.5-mm bands (21)
(Fig. 2E, dark green). Alternatively, the strong
1.1-mm band in the putative CRISM carbonate
spectra might result from small amounts of oli-
vine, which is commonly associated with the
and the Fe-rich smectite nontronite (5 wt %)
produces a spectrum similar to that observed by
CRISM (Fig. 2E, blue).
Mars’surface have been examined for this phase
Montes contain small exposures of carbonate-
bearing rocks, but the largest and most clearly
defined exposures are in the Nili Fossae region,
1). The Mg carbonate is present in relatively
bright rock units exposed over <10 km2, which
allows detection by OMEGA and CRISM but
probably precludes definitive detection by the
Thermal Emission Spectrometer (TES) with its
larger spatial footprint. The carbonate-bearing
(34), and their brightness in nighttime thermal
Imaging Science Experiment (HiRISE) images
indicates that they occur in lithified deposits. The
around the fossae, rocks exposed on the sides
of valleys in the Jezero crater watershed and
elsewhere, and sedimentary rocks within Jezero
The carbonate-bearing rocks are relatively
bright-toned and are commonly fractured (Fig. 4).
Like the regional smectite and olivine deposits
in Nili Fossae (18, 36, 37), the carbonate-bearing
rocks consistently lie stratigraphically beneath an
unaltered mafic cap unit (Fig. 4). All CRISM
images examined that exhibit carbonate also
examples, the carbonate-bearing unit is clearly
in some cases the relationship is indeterminate
and lateral variations between smectite and car-
phases. In places where both carbonates and
aluminum phyllosilicates can be mapped clearly,
the carbonate-bearing unit is always stratigraph-
ically lower. The carbonate-bearing unit appears
nearby olivine-bearing units (18), namely beneath
the mafic cap unit but above Fe-Mg smectite-
2.24 2.262.28 2.32.322.342.36
continuum removed band center, 2.3 µm
continuum removed band center, 2.5 µm
Fig. 3. Scatter plotofcontinuum-removedabsorption bandpositionsforanhydrous carbonates,hydrous
carbonates, and other minerals with 2.3- and 2.5-mm absorptions. CRISM data that we identify as
from Gaffey (green circles) (22), Hunt and Salisbury (red squares) (21), RELAB spectra measured by
Band centers of CRISM spectra are known to T0.01 mm.
19 DECEMBER 2008VOL 322
on January 5, 2009
WhereasFe-Mgsmectitesare foundina broad
region extending westward to the Antoniadi basin
Nili Fossae (Fig. 1). This area is the most olivine-
rich region so far observed on Mars (38, 39) and
indicate that extensive surface fluvial activity
extended into the early Hesperian (36).
We propose two possible formation settings
to explain the origin, stratigraphy, and distri-
groundwater percolating through fractures in the
ultramafic rock and altering olivine. This may
have occurred at only slightly elevated temper-
atures, as determined by the geothermal gradient.
Alternatively, hot olivine-rich rocks excavated
volcanic flows (39, 40) may have been deposited
on top of water-bearing phyllosilicate rocks of
the Noachian crust and may have initiated local
hydrothermal alteration in a zone along the con-
tact. The magnesite thus might occur in veined
structures throughout olivine-rich rock, a relation-
ized; however, CRISM has not yet conclusively
An alternative explanation is that exposed
olivine-rich rocks were weathered at surface am-
bient temperatures, perhaps during the surface
fluvial activity in Nili Fossae that continued after
the Isidis impact into the early Hesperian (36).
The transformation of olivine-rich rocks to mag-
nesite under cold dry conditions on Mars might
in Antarctica (41), which produces magnesite and
iron oxide mineral assemblagesas rock rinds and/
or coatings. A more water-rich surface-formation
scenario would be that carbonate precipitated
in shallow ephemeral lakes (42) from waters en-
riched in Mg2+relative to other cations by perco-
lation through ultramafic olivine-bearing rocks.
Either scenario implies that surface conditions in
chemical weathering during the late Noachian or
early Hesperian eras.
The Nili Fossae carbonates do not appear to
have sequestered large quantities of CO2. With
sedimentary units within Jezero crater (35), we
found no evidence of classicbedded sedimentary
carbonate rocks resembling those on Earth.
having formed in response to specific local con-
ditions. Although olivine is globally distributed
on Mars (43, 44), ultramafic rocks and their
substantial interaction with water may have been
necessary to generate carbonate in sufficient
quantities to be detected from orbit at resolutions
rocks found at Nili Fossae, and perhaps also
indicated by TES (4).
The existence of carbonate in rocks on Mars
implies that neutral-to-alkaline waters existed at
the time of their formation. Such conditions are
consistent with those indicated by Fe-Mg smec-
titeformationduringtheNoachian (5,11,24) but
contrast with the acid, low-water-activity con-
ditions thought to prevail over at least some of
Mars during later time periods (5, 45). The
survival of the Nili Fossae carbonates indicates
that they escaped destruction by exposure to
acidic conditions, which would have dissolved
the carbonate. Because aqueous activity in the
Nili Fossae region extended into the Hesperian
era (36), these carbonate-bearing rock units in-
dicate that not all aqueous crustal environments
proposed to be characteristic of the planet during
the Hesperian era, approximately 3.5 billion years
ous environments in a variety of geologic settings
in which waters ranged from the acidic to the
alkaline. Such diversity bodes well for the pros-
pect of past habitable environments on Mars.
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of the 2.3- and 2.5-mm features and their relative
strengths. However, alteration assemblages are typically
compositionally variable. Especially over the 105km2
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with OMEGA codetections of both olivine and hydration
or phyllosilicate were investigated by the coauthors where
CRISM images were available. Large Nili Fossae–scale
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A. Baldridge, J.-P. Bibring, D. Jouglet, A. Fraeman,
S. Wiseman, A. McEwen, G. Marzo, P. McGuire, and
M. Wyatt for thoughtful discussions during manuscript
preparation and E. Cloutis and others who have made
quality spectral libraries available and contributed to the
building of the NASA/Keck Reflectance Experiment
Laboratory (RELAB) spectral database. We are grateful
for the ongoing efforts of the MRO science and
engineering teams, in particular the CRISM team, which
enable these discoveries.
Supporting Online Material
Figs. S1 and S2
18 August 2008; accepted 3 November 2008
The Circadian Clock in Arabidopsis
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Allan B. James,1José A. Monreal,1Gillian A. Nimmo,1Ciarán L. Kelly,1Pawel Herzyk,2,3
Gareth I. Jenkins,1Hugh G. Nimmo1*
The circadian oscillator in eukaryotes consists of several interlocking feedback loops through which
the expression of clock genes is controlled. It is generally assumed that all plant cells contain
essentially identical and cell-autonomous multiloop clocks. Here, we show that the circadian
clock in the roots of mature Arabidopsis plants differs markedly from that in the shoots and that
the root clock is synchronized by a photosynthesis-related signal from the shoot. Two of the
feedback loops of the plant circadian clock are disengaged in roots, because two key clock
components, the transcription factors CCA1 and LHY, are able to inhibit gene expression in shoots
but not in roots. Thus, the plant clock is organ-specific but not organ-autonomous.
fitness (1–3). The eukaryotic clock involves gene
expression feedback loops, with both negative
and positive elements, and cytosolic signaling
any organisms have circadian clocks
that temporally regulate their physi-
ology and behavior and contribute to
molecules (4–7). In the model plant Arabidopsis,
the clock mechanism is thought to include at
least three interlocking feedback loops (5, 8, 9).
The central loop comprises two partially redun-
dant MYB domain transcription factors, CIR-
CADIAN CLOCK ASSOCIATED1 (CCA1)
and LATE ELONGATED HYPOCOTYL (LHY),
which inhibit expression of a pseudo-response
regulator TIMING OF CAB EXPRESSION1
(TOC1) (also known as PSEUDO-RESPONSE
REGULATOR1, PRR1), whereas TOC1 activates
expression of CCA1 and LHY by an unknown
mechanism (5, 10–12). In the morning-phased
loop, CCA1 and LHYactivate the expression of
PSEUDO-RESPONSE REGULATOR7 (PRR7)
and PSEUDO-RESPONSE REGULATOR9 (PRR9)
(13, 14); the evening-phased loop involves TOC1
and GIGANTEA (GI) (see legend to fig. S12
for further information). These conclusions are
basedonexperimentsusing whole seedlingsgrown
in the presence of sucrose, without consideration
of organ specificity. Yet, one major function of
the plant clock involves the temporal partition-
ing of metabolic pathways via the control of out-
put gene expression (15), and metabolism is
inherently organ-specific. We therefore analyzed
the circadian clock separately in shoots and roots
of mature, hydroponically grown Arabidopsis
Following transfer of plants from 12 hours
light/12 hours dark (LD) to constant light (LL),
LHYand CCA1 transcripts continued to oscillate
in both shoots and roots for three full cycles,
with some damping (Fig. 1A and fig. S1), as de-
termined by quantitative real-time reverse tran-
scription polymerase chain reaction (qPCR).
Notably, the period was some 2 hours longer
in roots than in shoots; analysis of LHY protein
(fig. S2) gave a similar result. PRR9 and PRR7
transcripts oscillated in both organs, with the time
of peak expression later in roots than in shoots
(fig. S3). TOC1 transcripts in shoots oscillated
in LL, in antiphase to those of CCA1 and LHY,
as expected. In marked contrast, TOC1 transcripts
in roots dipped slightly during the first subjec-
tive day in LL, then remained at a high level
without oscillations (Fig. 1B and table S1). In
shoots, oscillations in TOC1 protein were de-
tectable for at least two cycles, whereas in roots
TOC1 was present, with little variation, for 72
1Division of Molecular and Cellular Biology, Faculty of Bio-
medical and Life Sciences, University of Glasgow, Glasgow
G12 8QQ, UK.
Biomedical and Life Sciences, University of Glasgow, Glasgow
G12 8QQ, UK.3The Sir Henry Wellcome Functional Genomics
Facility, Faculty of Biomedical and Life Sciences, University of
Glasgow, Glasgow G12 8QQ, UK.
*To whom correspondence should be addressed. E-mail:
2Division of Integrated Biology, Faculty of
19 DECEMBER 2008 VOL 322
on January 5, 2009