SCIENCE sciencemag.orgPHOTO: YULIA ROZHNO/SHUTTERSTOCK
SYMBIOSIS
A bacterial battleground
The white button mushrooms
that many of us enjoy eating
are subject to brown blotch
disease caused by the bacterium
Pseudomonas tolaasii. These
bacteria are spread by means of
biosurfactants, and damage is
caused by a family of pore-form-
ing cyclic lipopeptides called
tolaasins. However, protective
bacteria are also found on but-
ton mushrooms, and Hermenau
et al. show how helper bacteria
of the genus Mycetocola protect
their host. Metabolic profiling,
mass spectroscopy, structural
characterization, and infection
assays show that tolaasin is
linearized by the helper bacteria.
The authors also identify the
enzyme responsible and show
that a biosurfactant involved
in pathogen spreading is also
a cyclic lipopeptide that is
neutralized by linearization.
Understanding these protective
mechanisms could allow the
development of biocontrol strat-
egies to stop spoilage. —VV
Proc. Natl. Acad. Sci. U.S.A. 10.1073/
pnas.2006109117 (2020).ECOLOGY
Identifying the
weakest links
Faced with mass extinction,
how do we prioritize conser-
vation needs? Disruption of
pollination could compromise
reproduction for many plant
species, and reduction of seed
dispersal could have devas-
tating effects for many tree
species. All organisms interact,
some very specifically, but we
understand little about how the
functional disruption of one
partner influences the resilience
of a community. Simmons et al.
mapped species interactions
involved in 41 pollination and
seed dispersal communities to
find vulnerabilities. Although
specialist links were more vul-
nerable to habitat loss, they also
found that the more vulnerable
interactions made the greatest
contributions to community
tolerance. Thus, identifying the
most vulnerable links betweenone daughter cell staying with
the distal tip cell niche and
remaining naive and the other
becoming surrounded by sheath
cells and differentiating. —BAP
eLife 9 , e56383 (2020).BLACK HOLES
Merging across
the black hole mass gap
Stellar mass black holes form
in supernovae, and theory
predicts that they have a maxi-
mum of about 65 solar masses.
Supermassive black holes with
millions of solar masses are
widely observed in the centers
of galaxies but how they formed
remains a topic of debate. The
LIGO Scientific Collaboration
and Virgo Collaboration discuss
the gravitational wave event
GW190521, finding that it was
most likely due to a merger that
formed a black hole of about
142 solar masses. This could be
an example of the long-sought
intermediate-mass black holes.
One of the black holes that
merged was about 85 solarmasses, itself perhaps the
product of an earlier merger,
suggesting hierarchical growth.
—KTS
Astrophys. J. Lett. 900 , L13 (2020).ORGANIC CHEMISTRY
Cyclopropanes
through C–H activation
Despite the strain inherent
in triangular carbon rings,
these cyclopropane motifs are
surprisingly common in natural
products and pharmaceuticals.
Their synthesis often involves
the addition of highly reactive
carbenes to C=C double bonds.
Clemenceau et al. now report an
alternative route whereby a pal-
ladium catalyst and base pluck
hydrogens off of alkyl groups
straddling a benzylic center,
which then join together to form
the third edge of the triangle. The
reaction is highly sensitive to the
nature of the base, with pivalate
yielding the desired product and
carbonate leading to a compet-
ing cyclobutene motif. —JSY
J. Am. Chem. Soc. 142 , 15355 (2020).species could become important
targets for conservation. —CA
PLOS Biol. 18 , e3000843 (2020).STEM CELLS
Identity by association
Tissue homeostasis and
regeneration after injury involve
coordinated dynamics for the
self-renewal and differentiation
of cells. Generally, cells that
remain in their niche retain self-
renewing activity, whereas cells
that leave the niche take on a
specific cell fate. Using CRISPR-
Cas9 genome editing, live-cell
imaging, and RNA interference–
mediated gene knockdown,
Gordon et al. examined the
interactions among germ cells of
Caenorhabditis elegans, the dis-
tal tip cell niche, and the adjacent
Sh1 sheath cells that surround
the gonad. Thin, membranous
extensions from the distal tip cell
niche and sheath cells touched
the undifferentiated germ cells.
Cells that touched just the distal
tip cell niche remained naïve,
whereas cells at the interface
divided asymmetrically, withPLANT BIOCHEMISTRYCarotenoids drive
chromoplast development
C
arotenoid-rich cellular structures
called chromoplasts are a source
of the rich reds and yellows in
the fruits and flowers of plants.
Similar to green, photosynthetic
chloroplasts, chromoplasts derive from
plastids but are distinct in structure
and development. Llorente et al. found
that expressing an enzyme in leaf chlo-
roplasts that causes enrichment of the
carotenoid precursor phytoene disrupted
photosynthesis and led to structural
changes consistent with chloroplast-
to-chromoplast differentiation. This
intervention may mimic natural cues for
chromoplast development and could
be a route to increasing the carotenoid
content of green vegetables. —MAF
Proc. Natl. Acad. Sci. U.S.A. 117 , 21796 (2020).The vivid red and yellow colors of some flowers
and fruits, such as tomatoes, are produced by
carotenoid-rich cellular structures called chromoplasts.25 SEPTEMBER 2020 • VOL 369 ISSUE 6511 1581