shocks is limited to the earliest stage of disk
evolution. As such, different sources of shock
waves would be required to account for the observed ~3 My age range of chondrule formation
inferred from our study.
Our revised chronology of the formation of
solids and their thermal processing refutes the
long-held view of a time gap between the formation of CAIs and chondrules, thereby allowing for the possibility that the energy required
for melting CAIs and chondrules may have been
associated with the same physical process. Statistical studies based on astronomical observations
of young stellar objects within star-forming regions indicate that the median lifetime of disks
around low-mass stars is ~3 My (37). These
time scales are comparable to the timing of
melting of disk solids inferred from our Pb-Pb
dates (Fig. 4), suggesting that the formation of
CAIs and chondrules may reflect a process
intrinsically linked to the secular evolution of
protoplanetary disks (38) and is not unique to
our solar system. Transfer of mass from the
disk to the central protostar is the most energetic
process during the lifetime of the protoplanetary
disk. Although the energy generated during this
process is only gradually released, part of which
is converted into kinetic energy expressed as
magnetically driven bipolar outflows from the
protostar (39), a substantial amount of it is available for the thermal processing of solids during
transient mass-accretion events. Indeed, models
of the innermost structure of protoplanetary disks
predict temperatures in excess of 1400 K within
1 astronomical unit for mass accretion rates as
low as ~10−6 M☉ year−1 (40). Because the conservation of energy requires dissipation per unit
of area of the disk that scales as the inverse cube
of the distance from the central star, accretion-based processes may produce similar thermal regimes over a large range of accretion rates, albeit
at different orbital radii. Whether accretion-based
processes can provide thermal histories for CAIs
and chondrules that are consistent with their
heating and cooling rates, as well as the chronology provided here, requires robust numerical
simulations of the evolving thermal structure of
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Acknowledgments: All the data reported in this paper are
presented in the supplementary materials. The Centre for Star
and Planet Formation is financed by the Danish National
Research Foundation. We thank C. Paton for help in the mass
spectrometer laboratory and J. K. Jørgensen for discussion.
Materials and Methods
Figs. S1 to S22
Tables S1 to S4
3 July 2012; accepted 14 September 2012
Chloroplast Biogenesis Is
by Direct Action of the
Qihua Ling,* Weihua Huang,*† Amy Baldwin,‡ Paul Jarvis§
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Development of chloroplasts and other plastids depends on the import of thousands of nucleus-encoded
proteins from the cytosol. Import is initiated by TOC (translocon at the outer envelope of chloroplasts)
complexes in the plastid outer membrane that incorporate multiple, client-specific receptors. Modulation
of import is thought to control the plastid’s proteome, developmental fate, and functions. Using forward
genetics, we identified Arabidopsis SP1, which encodes a RING-type ubiquitin E3 ligase of the chloroplast
outer membrane. The SP1 protein associated with TOC complexes and mediated ubiquitination of TOC
components, promoting their degradation. Mutant sp1 plants performed developmental transitions that
involve plastid proteome changes inefficiently, indicating a requirement for reorganization of the TOC
machinery. Thus, the ubiquitin-proteasome system acts on plastids to control their development.
Chloroplasts belong to a family of plant organelles called plastids, which includes everal nonphotosynthetic variants (such as
Department of Biology, University of Leicester, Leicester LE1
*These authors contributed equally to this work.
†Present address: Shanghai Institute of Plant Physiology and
Ecology, Shanghai Institutes for Biological Sciences, Chinese
Academy of Sciences, Shanghai 200032, China.
‡Present address: School of Medicine, Cardiff University,
Cardiff CF14 4YS, UK.
§To whom correspondence should be addressed. E-mail:
etioplasts in dark-grown seedlings and carotenoid-rich chromoplasts in fruits) (1). A specific feature
of the plastid family is the ability to interconvert
in response to developmental and environmental cues—for example, during de-etiolation or
fruit ripening (1). Such plastid interconversions
are linked to reorganization of the organellar
proteome (2, 3).
Over 90% of the thousands of proteins in
plastids are nucleus-encoded and imported from
the cytosol posttranslationally (1). The translocon
at the outer envelope of chloroplasts (TOC) recognizes chloroplast pre-proteins and initiates