The ability to infect new hosts can drive the
evolution and specialization of secreted
Critically, the mean fluid flow provides the
fixed-direction bias field that breaks the time-reversal invariance of the system. The two
resonant modes of the system are unequally
excited by an input, despite the symmetric
port configuration. Careful design, by tuning of the mean fluid flow, enables the interference of these two modes to create a sound
null at one of the ports, resulting in complete
transmission to the other port.
In this case, the three-port device that was
fabricated functions as a circulator, in which
an input signal on one port at the design fre-
quency transmits all of its acoustic energy to
its neighbor port in the direction of the mean
flow, and none to its neighbor port opposite
the flow. The resulting nonreciprocity is eas-
ily seen: An input signal on port 1 is trans-
mitted fully to port 2, whereas the same input
signal on port 2 is fully transmitted to port
3 (not port 1, as would happen in a recipro-
cal device). A circulator like this can easily
be converted into a two-port isolator with a
matched termination on one port. The mea-
sured sample exhibits an impressive 30 dB of
sound isolation in a device built from simple,
A one-way device for sound and vibra-
tions has broad implications. Unidirectional
acoustic wave propagation has obvious uses
in noise control, acoustic sensors, and manip-
ulation of acoustic scattering. At smaller spa-
tial scales, mechanical vibrations (called pho-
nons in their quantum-mechanical form) are
responsible for heat transport in solid mate-
rials as well. The work of Fleury et al. adds
to the bank of ideas that can be applied to
manipulating heat flow in a nonreciprocal
fashion (although still subject to the laws of
thermodynamics) in what is called a thermal
diode (7, 8). Challenges in device scaling and
bandwidth control remain in applying the
mean-flow–based concept demonstrated here
to more specific scenarios, but that all-impor-
tant first step toward general-purpose linear
acoustic nonreciprocity has now been taken.
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2. H. B. G. Casimir, Rev. Mod. Phys. 17, 343 (1945).
3. A. A. Maznev et al., Wave Motion 50, 776 (2013).
4. D. Jalas et al., Nat. Photonics 7, 579 (2013).
5. K. G. Budden, Radio Waves in the Ionosphere (
Cambridge Univ. Press, Cambridge, UK, 1961).
6. D. M. Pozar, Microwave Engineering ( Wiley, Hoboken, NJ,
ed. 4, 2012).
7. N. A. Roberts, D. G. Walker, Int. J. Therm. Sci. 50, 648
8. N. Li et al., Rev. Mod. Phys. 84, 1045 (2012).
Plants can be attacked by a vast range of pathogen classes, causing substan- tial agricultural losses. The
Phytophthora (meaning “plant killer”) genus is a
particularly destructive pathogen that causes
root and stem base decay in a wide range of
plants. Phytophthora infestans, which precipitated the Irish potato famine, originated
in Central Mexico and is closely related to
other Phytophthora species with distinct
host ranges (1, 2). Pathogen effectors that
are secreted during infection play a key
role in disease biology, but effector-induced
adaptation to new hosts is an understudied topic. On page 552 of this issue, Dong
et al. investigate how Phytophthora effector
proteins evolve the ability to specialize on
new hosts (see the figure) (3).
Phytophthora is a genus of oomycetes that
exhibit filamentous growth on plants. Oomy-
cetes share some phenotypes with fungi, but
are phylogenetically related to photosynthetic
brown algae and are thought to have initially
emerged from marine environments. The
Phytophthora genus comprises 10 main lin-
eages designated as clades 1 to 10. Clade 1c,
a subdivision of clade 1, includes P. infestans
(infecting potato and tomato) and P. mirabi-
lis (infecting 4 o’clock weeds), indicating that
these species share a recent common ancestor
(2). Genome comparisons between P. infes-
tans and P. mirabilis highlight alterations and
patterns of selection in repetitive DNA con-
taining rapidly evolving families of virulence
genes (such as effectors) (4).
The 82 effectors undergoing positive
selection between P. infestans and P. mirabilis are promising candidates shaping host
specialization (4). Dong et al. focused their
efforts on the EPIC1 effector, which is abundantly secreted during infection of tomato
and inhibits extracellular papain-like proteases (including RCR3) that are involved in
plant immune perception (5).
Dong et al. now report that the P. mirabilis epiC1 ortholog (PmepiC1) shows signatures of positive selection, suggesting that
this effector has evolved to function in
Mirabilis jalapa, the 4 o’clock plant, following
the split between P. mirabilis and P. infestans.
Using activity-based profiling with a probe
that targets papain proteases, the authors
demonstrate that recombinant EPIC1 from P.
infestans effectively inhibits tomato and wild
potato RCR3 proteases, whereas PmEPIC1
does not. The authors identify two PmEPIC1
M. jalapa targets with homology to RCR3
(MRP1 and MRP2). Subsequent experi-
Department of Plant Pathology, University of California,
Davis, Davis, CA 95616, USA. E-mail: firstname.lastname@example.org
Phytophthora clade 1c
effector = AncEPIC1
Route to specialization. A host jump within Phytophthora 1c led to the emergence of P. mirabilis, which can
infect M. jalapa. Dong et al. show that effector specialization following the host jump is associated with the
R112 mutation in PmEPIC1, enabling effective inhibition of the M. jalapa MRP2 protease. Polymorphic resi-
dues controlling specificity for the host and pathogen are highlighted. Adapted from fig. S18 in (3). PHO