REPORTS
helical banister through the center of the
staircase. The cross-sectional thickness of
this banister is 0.5 mm. Intricate shapes with
thin walls and hollow sections can be fabricated this way, and there is no geometrical
constraint. We have also produced parts that
approach 1 kg in weight. Examples are
shown in figs. S1 and S2. There is no theoretical maximum part size; the limit will be
determined by the magnitude of the build
platform and furnace constraints.
Data for selected parameters for a variety of
other near net shape manufacturing routes for
aluminum are collected in Table 1 and are
compared with those of rapid manufacturing.
Large parts can be fabricated by casting and
forging, whereas mechanical working by forging or extrusion develops high tensile strength.
The comparative advantage of rapid manufacturing is the ability to quickly fabricate parts of
almost any shape. Because no tooling is required, the lead time is short and there are no
geometrical constraints. This table also shows
preliminary tensile properties for aluminum by
rapid manufacturing. The tensile strength using
the Al-Cu infiltrant is 230 MPa, although the
ductility is negligible (fig. S3). The Al-Si-Mg
infiltrant provides limited ductility but lower
strength. In the Al-Cu alloy, the fracture propagates through the AlCu2 eutectic particles,
whereas the material fails by fracture along the
nitride matrix interface in the Al-Si-Mg alloy
(fig. S4). This suggests that it may be possible
to induce some ductility by control of the nitride thickness, design of the matrix microstructure, or selection of the infiltrant. This is currently under investigation.
References and Notes
1. P. Calvert, R. Crockett, Chem. Mater. 9, 650 (1997).
2. U. Lakshminarayan, K. McAlea, D. Girouard, R. Booth,
in Advances in Powder Metallurgy and Particulate
Materials, M. Phillips, J. Porter, Eds. [Metal Powder
Industries Federation (MPIF), Princeton, NJ, 1995], pp.
13.77–13.85.
3. K. Dalgarno, T. Stewart, Rapid Prototyp. J. 7, 173
(2001).
4. T. B. Sercombe, G. B. Schaffer, P. Calvert, J. Mater. Sci.
34, 4245 (1999).
5. C. W. Souvignier, T. B. Sercombe, S. H. Huo, P. Calvert,
G. B. Schaffer, J. Mater. Res. 16, 2613 (2001).
6. T. B. Sercombe, Mater. Sci. Eng. A 341, 163 (2003).
7. R. G. Iacocca, N. Meyers, in Advances in Powder
Metallurgy and Particulate Materials, H. Ferguson,
D. T. Whychell Jr., Eds. (MPIF, New York, 2000), pp.
12.09 –12.17.
8. T. B. Sercombe, P/M Sci. Technol. Briefs 3, 22 (2001).
9. D. Uzunsoy, I. T. H. Chang, P. Bowen, Powder Metall.
45, 251 (2002).
10. R. Soundararajan, G. Kuhn, R. Atisivan, S. Bose, A. Bandyopadhyay, J. Am. Ceram. Soc. 84, 509 (2001).
11. W. Bradley et al., “Rapid Prototyping of EDM Electrodes of
Zirconium-Diboride/Copper,” Rapid Prototyping/Laser Applications in the Automotive Industries, 16 to 19 June
1997, Florence, Italy (Automotive Automation Limited,
Croydon, UK, 1997), pp. 181–184.
12. M. Wohlert, D. Bourell, G. Lee, J. Beaman, in Processing and Fabrication of Advanced Materials V, T. S.
Srivatsan, J. J. Moore, Eds. (TMS, Cincinnati, OH,
1996), pp. 293–302.
13. Z. A. Munir, J. Mater. Sci. 17, 2733 (1979).
14. Z. A. Munir, Powder Metall. 24, 177 (1981).
15. P. K. Higgins, Z. A. Munir, Powder Metall. Int. 14, 26 (1982).
16. H. J. Mathieu, M. Datta, D. Landolt, J. Vac. Sci. Technol. 3, 331 (1985).
17. I. Olefjord, A. Karlsson, from conference Aluminum
Technology ’86, 11 to 13 March 1986, London, T.
Sheppard, Ed. (Institute of Metals, London, 1986), pp.
383–391.
18. P. Marcus, C. Hinnen, I. Olefjord, Surf. Interface Anal.
20, 923 (1993).
19. A. Nylund, I. Olefjord, Surf. Interface Anal. 21, 290
(1994).
20. I. E. Anderson, J. C. Foley, J. F. Flumerfelt, from
conference Powder Metallurgy Aluminum and Light
Alloys for Automotive Applications, 10 to 11 November 1998, W. F. Jandeska, R. A. Chernenkoff, Eds.
(MPIF, Dearborn, MI, 1998), pp. 75– 82.
21. A. Ozbilen, A. Unal, T. Sheppard, from conference
Physical Chemistry of Powder Metals–Production and
Processing, 16 to 18 October 1989, St. Mary’s, PA, W.
M. Small, Ed. (Minerals, Metals and Materials Society,
Warrendale, PA, 1989), pp. 489 –505.
22. P. Nielsen, Y. L. Liu, N. Hansen, 1993 Powder Metallurgy World Congress, Kyoto, Japan ( Japan Society of
Powder and Powder Metallurgy, Japan, 1993), pp.
58 – 61.
23. Y. Kim, W. M. Griffith, F. H. Froes, J. Metals 37, 27
(1985).
24. H. Scholz, P. Greil, J. Mater. Sci. 26, 669 (1991).
25. K. Kondoh, A. Kimura, R. Watanabe, Powder Metall.
44, 161 (2001).
26. K. Kondoh, A. Kimura, R. Watanabe, Powder Metall.
44, 253 (2001).
27. M. Okumiya, Y. Tsunekawa, T. Murayama, Surf. Coat.
Technol. 142, 235 (2001).
28. G. B. Schaffer, B. J. Hall, Metall. Mater. Trans. A 33,
3279 (2002).
29. P. Sthapitanonda, J. L. Margrave, J. Phys. Chem. 60,
1628 (1956).
30. C. G. Goetzel, J. Groza, in ASM Handbook, vol. 7, Powder
Metal Technologies and Applications (ASM International,
Materials Park, OH, 1998), pp. 541–564.
31. S. Kalpakjian, S. R. Schmid, Manufacturing Engineering
and Technology (Prentice Hall, Upper Saddle River,
NJ, ed. 4, 2001).
32. ASM Specialty Handbook, Aluminum and Aluminum
Alloys (ASM International, Materials Park, OH, 1993).
33. ASM Handbook, Volume 20 Materials Selection and
Design (ASM International, Materials Park, OH, 1997).
34. G. B. Schaffer, D. Apelian, Aluminium Powder Metallurgy: Process, Properties and Design Solutions ( The
Aluminum Association Inc., Washington, DC, 2000).
35. Supported by 3D Systems, Inc., and the Aluminium
Powder Company.
Supporting Online Material
www.sciencemag.org/cgi/content/full/301/5637/1225/DC1
Figs. S1 to S4
19 May 2003; accepted 15 July 2003
Melt Segregation and Strain
Partitioning: Implications for
Seismic Anisotropy and Mantle Flow
B. K. Holtzman,1* D. L. Kohlstedt,1* M. E. Zimmerman,1
F. Heidelbach,2 T. Hiraga,1 J. Hustoft,1
One of the principal means of understanding upper mantle dynamics involves
inferring mantle flow directions from seismic anisotropy under the assumption
that the seismic fast direction (olivine a axis) parallels the regional flow direction. We demonstrate that (i) the presence of melt weakens the alignment
of a axes and (ii) when melt segregates and forms networks of weak shear zones,
strain partitions between weak and strong zones, resulting in an alignment of
a axes 90° from the shear direction in three-dimensional deformation. This
orientation of a axes provides a new means of interpreting mantle flow from
seismic anisotropy in partially molten deforming regions of Earth.
Most of the chemical exchange between the
interior and the surface of Earth occurs at
plate boundaries and above plumes. In these
regions, the interactions resulting from coupled fluid migration and mantle flow must be
understood in order to interpret mantle structure (from seismology) and processes of
melt-rock reaction (from melt chemistry).
Anisotropy in the velocity of elastic shear (S )
waves and the polarization of compression
(P) waves propagating through the upper
mantle can be caused by preferred orientation
of olivine and/or the alignment of melt pockets or melt-rich planes (1). Past and/or present
1
Department of Geology and Geophysics, University
of Minnesota, Minneapolis, MN 55455, USA. 2Bayerisches Geoinstitut, Bayreuth, D-95440, Germany.
*To whom correspondence should be addressed. Email: [email protected] (B.K.H.); [email protected]
(D.L.K.)
flow directions in the mantle are generally
assumed to be parallel to the seismically fast
direction, corresponding to the a axis of olivine (2–4). This assumption has recently
been questioned on the basis of shear deformation experiments on olivine under high
stress and high water-content conditions (5).
However, many partially molten mantle regions, such as spreading centers, are relatively dry, low-stress environments (6). We report the discovery of an alternative mechanism by which the parallelism between the
seismic fast axis and the flow direction
breaks down.
We deformed partially molten mantle
rocks under anhydrous conditions. Samples
were placed between pistons cut at 45° and
deformed in a geometry approximating simple shear at high pressure and temperature
(P ⫽ 300 MPa, T ⫽ 1523 K) in a gasmedium deformation apparatus (7, 8) (fig.
www.sciencemag.org SCIENCE VOL 301 29 AUGUST 2003
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REPORTS
S1; table S1). The samples consisted of olivine and mid-ocean ridge basalt (MORB) and
olivine and MORB with either an additional
melt (FeS) or solid (chromite) phase. The
additional phases lower the permeability relative to that of olivine and MORB samples by
partially plugging the melt channels, without
substantially changing the rheological properties (7, 8). In sheared samples of olivine
and MORB, melt pockets align at ⬃20° to the
shear plane and distribute uniformly across
the sample (9). During deformation of samples with reduced permeability, melt segregates from an initially homogeneous distribution into melt-rich layers or “bands” oriented
at ⬃20° to the shear plane, separated by
melt-depleted regions or “lenses.” (Fig. 1, A
and B). When melt segregates, strain concentrates in the weak melt-rich bands. The interaction of strain partitioning and melt segregation leads to self-organization of the
melt-rich bands. These bands form anastomosing networks (Fig. 1), with larger bands
at higher angles connected by smaller
bands at lower angles. Because the distribution of band angles remains independent
of strain, the melt in the bands must be
moving relative to the solid. The spacing
between bands is governed by the compaction length of the sample [supporting online
material (SOM) Text, section 2], consistent
with theory for porous flow in a permeable
viscous deforming medium (7, 10).
Crystallographic preferred orientations
(CPOs) of olivine grains were measured for
1 undeformed and 10 sheared samples with
electron backscatter diffraction (EBSD)
analysis (8, 11). In samples of olivine and
MORB deformed in shear, the CPOs develop an “axial” pattern in which the b axes
concentrate normal to the sample shear
plane (the walls of the pistons) and the a
and c axes form girdles in the shear plane
(Fig. 2A). In samples with an added third
phase (olivine and MORB with either chromite or FeS melt), all of which formed
networks of melt-rich bands, the CPO is
characterized by (i) significant concentrations of a axes normal to the shear direction; (ii) tight clusters of b axes oriented
15° to 20° from the pole to the sample shear
plane, back-rotated relative to the orientation caused by simple shear; and (iii) significant concentrations of c axes rotated
15° to 20° counterclockwise from the sample shear plane (90° from the b axes) (Fig.
2B). In six samples deformed to shear
strains of ␥ ⫽ 1.1, 2.1, and 3.3 to 3.5, the
intensity of CPOs increases with increasing
strain. In addition, all samples progressively decrease in thickness by up to 20% with
increasing strain, accommodated by minor
but important lengthening of the sample
normal to the shear direction.
Observations from detailed studies of
CPOs and microstructures by scanning and
transmission electron microscopy (SEM and
TEM) constrain the microstructural processes
active in the samples (Fig. 1B). A map of
CPOs in the vicinity of a similar melt-rich
band exhibits no significant spatial variations, suggesting that deformation in the
bands does not modify the CPO (SOM Text,
section 3). TEM observations reveal that dislocations with Burgers vectors parallel to the
a axis outnumbered those with Burgers vectors parallel to the c axis (SOM Text, section
4). The density of dislocations is not elevated in olivine grains adjacent to stronger
chromite grains, suggesting that the pres-
Fig. 1. Microstructures. (A) Reflected-light image of a sample of olivine and chromite and 4%
MORB sheared between tungsten pistons. The
melt-rich bands are visible as the darker regions
oriented 10° to 20° to the sample walls, forming an anastomosing network. (B) SEM backscattered electron image of one band. The
chromite grains (white) are smaller than the
olivine grains (gray). Chromite tends to sit in
melt (black) tubules, reducing the permeability.
Fig. 2. Pole figures for a, b, c
axes for sheared samples. The
shear plane is horizontal and
shear sense is at top right. (A)
Sheared sample of olivine and
4% MORB. (B) Sheared sample
of olivine and chromite and 6%
MORB, shown in Fig. 1. The CPO
measured in a sample of olivine
and MORB with FeS melt is identical to this one. (C) Schematic
diagrams of three sets of three
pole figures for a melt-free olivine aggregate (top) [after (3)],
olivine and oriented melt (middle), and olivine and segregated
melt (bottom), all deformed at
high T (1473 to 1573 K) and P (300 MPa) to shear
strains larger than ␥ ⫽ 1. The reference position for
the back-rotation of the CPO is indicated by the
outlined areas. A similar obliquity between the
shear plane and the axis aligned in the shear direction can develop due to recrystallization, discussed
further in SOM Text, section 5, but is not the origin
of the back-rotation observed here. The relative
seismic velocities are Va ⬎ Vc ⬎ Vb. At right is a
spatial representation of the third CPO.
1228
ence of chromite does not influence dislocation dynamics.
A commonly observed CPO for olivine
deformed in simple shear at high temperature
is characterized by a axes parallel to the shear
direction, b axes normal to the shear direction, and c axes normal to the shear direction
in the shear plane (Fig. 2C) (3) (SOM Text,
section 5). This pattern reflects the fact that in
dry olivine, a slip on b planes is significantly
weaker than a slip on c planes, which is
weaker than c slip on b planes (12). In contrast, in partially molten samples deformed at
similar temperature and stress conditions as
described in (3), the more diffuse CPO pattern is distinguished by a and c axis girdles in
the shear plane with strong concentrations of
b axes normal to the shear plane. Further,
when melt segregates into bands, the b axes
rotate 20° antithetic to the sense of shear, and
concentrations of a and c axes appear 90°
from their “normal” positions, referred to
below as the “a-c switch” (Fig. 2C).
First, we must explain how the oriented
melt pockets weaken a and c axis concentrations relative to CPOs from melt-free samples (Fig. 2B). Melt pockets provide a fastdiffusion path for olivine components,
enhancing the importance of diffusionaccommodated creep and possibly grain
boundary sliding in the direction of shear,
whereas movement of dislocations accommodates the remainder of the interactions between grains, providing a mechanism for
modifying the von Mises criterion (13).
Strong anisotropy in melt-pocket orientation
(MPO) may randomize the orientations of a
and c axes, while leaving orientations of slip
planes (b axes) intact (“the MPO-CPO effect”). As long as melt pockets are oriented,
29 AUGUST 2003 VOL 301 SCIENCE www.sciencemag.org
REPORTS
which should occur at shear stresses higher
than 0.1 MPa (14), olivine CPOs in the mantle will be affected by the presence of melt.
Second, we argue that the transition in
CPOs in going from samples of olivine and
MORB (Fig. 2A) to samples with melt segregated into networks of bands (Fig. 2B) is
due to changes in the flow pattern rather than
a change in the deformation mechanism. The
latter CPO is similar to type-B CPO defined
in (5), which the authors of that paper attribute to a change in the behavior of dislocations at high water fugacity and high stress
conditions. However, neither of these conditions applies to our experiments. Several
lines of evidence disfavor a change in dislocation dynamics (e.g., a change from
dominant a slip to c slip on b planes) as an
explanation for the CPO observed in our
experiments, discussed in (15). We propose
a kinematic explanation for the a-c switch,
on the basis of three points, as follows:
1) The total strain in the sample partitions
between the melt-rich bands and meltdepleted lenses. Although the bands comprise
only ⬃20% of the total sample volume, the
strain rate, and thus the strain, is higher in the
weaker bands than in the stronger lenses. In
the bands, shear strain is oriented at ⬃20° to
the sample shear plane and, therefore, in the
lenses the shear plane must be back-rotated
relative to the sample shear plane (Fig. 3C).
This back-rotation is observed in the orientation of b axes in the CPO.
2) The observed CPO predominantly reflects deformation in the melt-depleted lenses, implying that the deformation in the bands
is not contributing to or strongly modifying
the CPO (16). Observations that the CPO is
barely modified in the vicinity of a band
(SOM Text, section 3) support this point.
3) The deformation that produces the CPO
in the melt-depleted lenses is not simple
shear, but it involves substantial components
of strain normal to the shear direction. Alignment of a axes normal to the shear direction
suggests that a slip on the b plane occurs
normal to the shear direction (17). Because
much of the shear strain is accommodated in
the bands, the components of strain normal to
the shear direction in the lenses have much
greater expression in the CPO than they
would if the bands were absent. This expression may be enhanced further by the MPOCPO effect. Thus, the “mechanism” for a-axis orientation is the kinematic effect of strain
partitioning, not a change in the dominant
slip system.
The influence of melt segregation and
strain partitioning on CPO development will
be even more effective in partially molten
regions of Earth where deformation is more
three-dimensional than in our experiments.
The olivine a axes will rotate if the geometry
of the overall flow in the region permits or
requires an elongation of the melt-depleted
lenses normal to the shear direction. An
anisotropic network of melt-rich layers will
affect the seismic properties of regions large
in comparison to a seismic wavelength (␭) if
the separation between layers (␦S) is much
less than a seismic wavelength (i.e., ␦S ⬇
100-3 m ⬍⬍ ␭ ⬇ 104 m). The configuration
(comprising average thickness, spacing, angle, and topology) of the network depends on
the physical properties of the solid and fluid,
the kinetics of the processes governing their
interactions and transport, and the geometric
(kinematic) boundary conditions of regional
flow. Some of these properties are encompassed in the first-order compaction length
scaling argument discussed in (7) (SOM
Text, section 2). Others remain to be studied
experimentally and theoretically.
Fig. 3. Representation of observed melt distribution and internal strain partitioning in experimentally deformed samples.
(A) Synthesis of the configuration of melt bands. Bands form
anastomising networks with
larger bands at higher angles relative to the shear plane (flat red
arrow) connected by smaller
bands at lower angles. Smaller
arrows indicate that the samples
flatten and widen with shear. In
three dimensions, the melt-rich
layers connect and surround
melt-depleted lenses. (B) Strain
partitioning between bands
(anastomosing layers) and
lenses. The flat arrows indicate
the total shear and the component concentrated in the
bands. The narrow arrows indicate alignment of olivine a axes normal to the shear direction in the lenses. The black lines
mark the orientation of the shear plane in the lenses, “back-rotated” relative to the sample
shear plane due to strain partitioning.
A literal extrapolation of the process discussed here may help to explain the complex
seismic anisotropy observed in many partially molten regions of the upper mantle (18).
Several potential examples include (i) vertical fast-direction measurements beneath
the Reykjanes Ridge south of Iceland (19),
(ii) trench parallel fast axes measured in the
mantle wedge above subduction zones (20),
or (iii) tangential patterns of anisotropy
around plume heads, e.g., Iceland (21).
Each of these observations has produced a
range of hypotheses, a discussion of which
is beyond the scope of this paper (22).
However, the processes discussed here suggest detailed field-based (23) and seismological predictions and tests, which may
influence our interpretations of the dynamics of partially molten regions of Earth.
References and Notes
1. D. Mainprice, Tectonophysics 279, 161 (1997).
2. A. Nicolas, N. I. Christensen, Formation of Anisotropy
in Upper Mantle Peridotites—A Review, Geodynamic
Series (American Geophysical Union, Washington,
DC, 1987), vol. 16.
3. S. Zhang, S. Karato, Nature 375, 774 (1995).
4. M. Bystricky, K. Kunze, L. Burlini, J.-P. Burg, Science
290, 1564 (2000).
5. H. Jung, S. Karato, Science 293, 1460 (2001).
6. G. Hirth, D. L. Kohlstedt, Earth Planet. Sci. Lett. 144,
93 (1996).
7. B. K. Holtzman, N. J. Groebner, M. E. Zimmerman,
S. B. Ginsberg, D. L. Kohlstedt, Geochem. Geophys.
Geosyst. 4, 8607 (2003).
8. Material and Methods are available as supporting
material on Science Online.
9. M. E. Zimmerman, S. Zhang, D. L. Kohlstedt, S. Karato,
Geophys. Res. Lett. 26, 1505 (1999).
10. D. McKenzie, J. Petrol. 25, 713 (1984).
11. B. L. Adams, S. I. Wright, K. Kunze, Metall. Trans. 24A,
819 (1993).
12. W. B. Durham, C. Goetze, J. Geophys. Res. 82, 5737 (1977).
13. M. S. Paterson, in Physics of Strength and Plasticity, A. S.
Argon, Ed. (MIT Press, Cambridge, MA, 1969), pp. 377–392.
14. M. J. Daines, D. L. Kohlstedt, J. Geophys. Res. B102,
10257 (1997).
15. The fabric in Fig. 2B would normally be interpreted as
slip on the (010)[001] system. Such a change in
dominant slip system could result from locally high
stresses due to the strong chromite grains activating
the (010)[001] slip system in olivine, which is much
stronger than (010)[100] (12). However, this possibility is negated by the fact that the same CPO exists
in the olivine and MORB with FeS sample, which
forms bands with a weak melt phase (i.e., FeS) in
place of strong chromite inclusions. Furthermore,
TEM images of the dislocation structures revealed no
evidence for a preponderance of dislocations with
[001] Burgers vectors or with high local stresses (i.e.,
increased dislocation density). Because no evidence
exists for a change in the relative strength of the slip
systems, another explanation for the a-c switch must
be invoked.
16. The absence of variation in the CPO around and
within a band supports the conclusion that deformation in the bands does not contribute to or strongly
modify the CPO (SOM Text, section 3). At least two
reasons argue against formation of the observed CPO
in the melt-rich bands: (i) To maintain a stable orientation during simple shear, the melt within the
bands must move relative to the solid (7). Thus, the
strain associated with a band at a given location will
probably not be high enough to modify the CPO. (ii)
The deformation mechanisms may be different in the
two regions because of increased melt fraction in the
melt-rich bands. Granular flow or grain boundary
sliding accommodated by diffusion with rigid rota-
www.sciencemag.org SCIENCE VOL 301 29 AUGUST 2003
1229
REPORTS
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
tion of the grains in the melt-rich bands would
reduce the strength of the CPO produced in the
lenses, but would not produce a different CPO.
The whole sample undergoes general shear, with a
larger amount of simple shear (⬃300%) and a smaller component of axial shortening (⬃20%) (i.e.,
“transpression”). Models incorporating the influence
of transpressional geometry of deformation on olivine CPOs produce girdles in a and c axes because the
flattening component efficiently orients a and c axes
radially (24). Because the simple shear component of
deformation is partitioned between the bands and
lenses, in the lenses the shear plane has a high angle
to the principal compressive stress (lenses ␤ ⫽ 45° ⫹
20°, bands ␣ ⫽ 45° – 20°) and, thus, the flattening
component of strain is greater. The effect of the
highly asymmetric strain partitioning is superimposed on the effect of transpression on the CPO and
possibly on the MPO-CPO effect; the bands accommodate much of the strain in the shear direction,
whereas the lenses accommodate most of the strain
normal to the shear direction. Furthermore, the
bands may accommodate some differential movement between the lenses normal to the shear direction, but cannot accommodate internal deformation
of the lenses. The internal deformation of the lenses
creates the observed alignments of a axes.
“Complex” often implies that seismic fast directions
are not parallel to plate motions. However, for this
proposed relation between flow and anisotropy to be
useful, we must also explain why and when “normal”
relations between flow and anisotropy exist. In the
MELT experiment study area (25), seismic fast directions are parallel to plate motions. In purely passive
upwelling, as in the East Pacific Rise, the flow may be
sufficiently two-dimensional that the extrusion required to re-align a axes may not exist.
J. B. Gaherty, Science 293, 1645 (2001).
X. Yang, K. M. Fischer, G. A. Abers, J. Geophys. Res.
B100, 18165 (1995).
I. Bjarnason, P. G. Silver, G. Rumpker, S. C. Solomon,
J. Geophys. Res. 107, 2382 (2002).
In all three examples, researchers have discussed the
potential role of high water fugacity and high stress
in modifying the relation between seismic anisotropy
and flow direction, as discussed in (5). The waterstress mechanism and the one proposed here are
likely to be mutually exclusive. When melting begins,
olivine quickly dehydrates due to the strong partitioning of water into the melt (26), which is a likely
scenario in a sub-ridge melting region and a plume
head. Thus, a water-dependent mechanism may not
apply to partially molten regions of the mantle.
In natural rock samples, new criteria must be used to
identify the CPO reported here. Without knowledge
of the shear direction and evidence of strain partitioning in a network of melt-rich bands, it would not
be possible to distinguish this CPO from any other
produced by a slip on b planes. To identify the effects
of strain partitioning, one would have to establish the
orientation of the a axes in relation to either the
network geometry or some regional or macroscopic
indicator of shear sense.
A. Tommasi, B. Tikoff, A. Vauchez, Earth Planet. Sci.
Lett. 168, 173 (1999).
C. J. Wolfe, S. C. Solomon, Science 280, 1230 (1998).
K. Koga, E. Hauri, M. Hirschmann, D. Bell, Geochem.
Geophys. Geosyst. 4, 1019 (2003).
Support for this research was provided by NSF EAR0126277, NSF OCE-0002463, NSF OCE-0327143, a
Japanese Society for the Promotion of Science Fellowship (to T.H.), a Fulbright Fellowship to France (to
B.K.H.), and DAAD grant 315-ab and NSF grant INT0123224 for collaborations between members of the
University of Minnesota and the Bayerisches Geoinstitut.
Supporting Online Material
www.sciencemag.org/cgi/content/full/301/5637/1227/DC1
Materials and Methods
SOM Text
Figs. S1 to S3
Table S1
References
22 May 2003; accepted 26 June 2003
1230
Cleavage of Arabidopsis PBS1 by
a Bacterial Type III Effector
Feng Shao,1*† Catherine Golstein,2* Jules Ade,2*
Mark Stoutemyer,2 Jack E. Dixon,1† Roger W. Innes2‡
Plant disease-resistance (R) proteins are thought to function as receptors for
ligands produced directly or indirectly by pathogen avirulence (Avr) proteins.
The biochemical functions of most Avr proteins are unknown, and the mechanisms by which they activate R proteins have not been determined. In Arabidopsis, resistance to Pseudomonas syringae strains expressing AvrPphB requires RPS5, a member of the class of R proteins that have a predicted nucleotide-binding site and leucine-rich repeats, and PBS1, a protein kinase. AvrPphB
was found to proteolytically cleave PBS1, and this cleavage was required for
RPS5-mediated resistance, which indicates that AvrPphB is detected indirectly
via its enzymatic activity.
The molecular mechanisms by which pathogens trigger disease resistance in plants are
poorly understood. The “gene-for-gene” hypothesis was proposed more than 40 years
ago by Flor, whereby plant disease resistance
occurs only in the simultaneous presence of a
specific resistance (R) gene in the plant and
its corresponding avirulence (Avr) gene in the
pathogen (1), which suggests that a direct
Avr-R interaction activates disease resistance. Isolation of more than 30 R genes from
different plant species has revealed that most
encode proteins containing a predicted
nucleotide-binding site and leucine-rich repeats (NB-LRR) (2). Although remarkable
progress has been made in cloning corresponding pairs of NB-LRR genes and Avr
genes (2), a direct physical interaction between NB-LRR proteins and pathogen Avr
proteins has only been shown twice (3, 4).
Because several Avr genes are known to enhance pathogen virulence in susceptible
plants (5), an alternative model has been proposed whereby Avr proteins interact with and
modify a specific target (or targets) in the
plant to promote disease in the absence of
their corresponding R proteins (2, 6). R proteins serve to “guard” these virulence targets
and thereby activate resistance signaling
pathways when these targets are modified by
specific Avr factors. Although evidence supporting this guard hypothesis is emerging
(7–9), little is known about the targets of
pathogen Avr proteins or how these targets
are modified.
1
Department of Biological Chemistry, Medical School
and Life Sciences Institute, University of Michigan,
Ann Arbor, MI 48109, USA. 2Department of Biology,
Indiana University, Bloomington, IN 47405, USA.
*These authors contributed equally to this work.
†Present address: Departments of Pharmacology, Cellular and Molecular Medicine, School of Medicine, and
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA.
‡To whom correspondence should be addressed. Email: [email protected]
We have shown that the AvrPphB protein
of Pseudomonas syringae belongs to a novel
family of cysteine proteases and that its protease
activity is required for induction of a hypersensitive response (HR, a diseaseresistance response characterized by localized cell death at
the point of pathogen attack) in Arabidopsis
plants that carry the NB-LRR gene RPS5 (10,
11). AvrPphB is likely secreted into host cells
by the P. syringae type III secretion system,
because mutations in this system block AvrPphB-induced resistance in bean plants (12).
Consistent with this, transgenic expression of
AvrPphB in the cytoplasm of Arabidopsis induces an RPS5-mediated HR (13). In the context of the guard model, a plant substrate of the
AvrPphB protease may activate RPS5 upon
cleavage by AvrPphB. A candidate for this AvrPphB substrate is the Arabidopsis PBS1 protein
kinase, which is specifically required for AvrPphB/RPS5-mediated resistance (13, 14).
To test this hypothesis, we coexpressed
AvrPphB and a hemagglutinin (HA)-tagged
form of PBS1 in Arabidopsis leaves using an
Agrobacterium-mediated transient transformation system (15). Coexpression of PBS1HA and AvrPphB(C98S), a protease inactive
mutant of AvrPphB in which Cys98 is replaced by Ser (10, 16), in pbs1-1 mutant
plants generated full-length PBS1-HA protein (60 kD, Fig. 1A). Expression of wildtype AvrPphB with PBS1-HA markedly
reduced the 60-kD band intensity, which suggests that PBS1 is degraded when elicited
with AvrPphB.
RPS5-mediated resistance also requires
the RAR1 gene (17), which has been hypothesized to function in a proteasome-dependent
protein degradation pathway (18). We tested
whether RAR1 and RPS5 are required for
the degradation of PBS1 by coexpressing
AvrPphB and PBS1-HA in rar1-20 and
rps5-2 Col-0 mutants, and in the Landsberg
erecta (Ler) accession, which lacks RPS5. A
28-kD HAcontaining band was detected in
these lines (Fig. 1A), which indicates that
29 AUGUST 2003 VOL 301 SCIENCE www.sciencemag.org