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New Gravitropic Transduction Mutants |
When Arabidopsis
inflorescence stems are gravistimulated at 4°C for several hours,
they remain unbent until they are returned to room temperature (RT).
Cold can thus be used to uncouple gravity perception from gravitropic
response. Wyatt et al. (pp. 1426-1435) utilized this cold
effect to select for mutants with an altered gravitropic signal
transduction and/or storage mechanism. They have identified several
gps (gravity persistent signal) mutants at three independent
loci (GPS1, GPS2, and GPS3). All three
mutants had an altered response after gravistimulation at 4°C.
gps1-1 did not bend in response to the 4°C gravity
stimulus upon return to RT. gps2-1 responded to the 4°C
stimulus but bent in the opposite direction. gps3-1
over-responded after return to RT, continuing to bend to an angle
greater than wild-type (WT) plants. The gps mutants may
represent three independent aspects of signal transduction in the
gravitropic response: perception or retention of the gravity signal
(gps1-1), determination of the polarity of the response (gps2-1), and the rate of the response (gps3-1).
All three mutants exhibited normal gravi- and phototropic responses
when stimulated at RT. At 4°C, starch-containing statoliths
sedimented normally in both WT and the gps mutants, but
auxin transport was abolished at 4°C. By identifying the genes
affected in the gps mutants, it may be possible to identify
components of early signal transduction that link the biophysical
signal of statolith movement to the biochemical effects that establish
differential auxin transport.
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Maize (Zea mays) Leaves Shun Their Neighbors |
The distance between maize plants is typically greater between
rows (70 cm) than within rows (16-23 cm). This rectangular arrangement
creates a heterogeneous light environment for phytochrome absorption:
Plants receive higher red light (R) to far-red light (FR) ratios from
inter-row than intra-row spaces. Maddonni et al. (pp.
1181-1189) placed mirrors reflecting FR close to isolated plants
to simulate the presence of neighbors in the field and noted a distinct
displacement of the laminae of these "pseudo-shaded" specimens in
the horizontal plane. Cultivar-specific differences in this response
were observed: One maize hybrid showed an increased proportion of
leaves toward inter-row spaces, whereas another retained random leaf
orientation. Growth chamber experiments indicated that leaf
reorientation in the sensitive cultivar was a local rather than a
systemic reaction to FR. The observation that at least some
responses to R/FR can be beneficial to crop performance
contradicts the current paradigm that phytochrome-mediated responses to low R/FR are a relic from wild conditions that are detrimental to crop yield. The maize cultivar showing the stronger leaf
orientation response exhibited weaker responses in tillering and plant height, two other phytochrome-regulated parameters. Therefore, the simple strategy of overexpressing phytochrome is likely
to eliminate detrimental as well as beneficial effects of plant
responses to the R-to-FR ratio. The response-specific genes operating
downstream of phytochromes, however, may provide a way to selectively
eliminate those responses that have a negative impact on yield, while
retaining those that make a positive contribution.
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Spittlebug (Philaenus spumarius) "Spittle" Reflects Xylem
Sap Composition |
It is not easy to extract xylem sap without contamination from
neighboring tissues and cells. It is especially difficult to obtain
xylem sap from strongly transpiring plants because of cavitation. In
this issue, Malone et al. (pp. 1436-1442) describe a method
for the continuous, nondestructive analysis of xylem-borne mineral nutrients in intact transpiring plants. The method uses the
meadow spittlebug (Fig. 1), a
xylem-feeding homopteran, in combination with ion chromatography. This
insect feeds upon a wide range of plant species and organs. Its excreta
can be collected at all times of the day and night, and its mineral ion
content can be analyzed rapidly, and without purification, by ion
chromatography. The excreta have a mineral content virtually identical
to that of xylem sap. Cages suitable for containing the insects and
collecting excreta from any desired location on plants in both the
laboratory and the greenhouse are described. Example results are
presented from fully mature pepper (Capsicum annuum) plants
that illustrate the dynamics, over several days, in the xylem
concentrations of Na+, K+,
NH4+,
Mg2+, Ca2+,
Cl
,
NO3,
PO43, and
SO42. This sensitive and
nondestructive method should facilitate studies of macronutrient uptake
and transport in a range of plants and environments.
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Figure 1.
The spittle-resembling excreta of the meadow
spittlebug accurately reflects the ion content of xylem sap.
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mRNA Processing and Abscisic Acid (ABA) Signaling |
The abh1 mutant of Arabidopsis exhibits
hypersensitivity in a variety of ABA-mediated responses.
ABH1 encodes the larger of two subunits of an
Arabidopsis mRNA cap-binding complex (the smaller subunit, AtCBP20, is
required for the in vitro binding of ABH1 to mRNA). In this issue,
Hugouvieux et al. (pp. 1276-1287) show that ABH1
and AtCBP20 are expressed coincidentally and extensively in
Arabidopsis, and that the effects of abh1 on growth
phenotype are relatively minor (most notably, the development of leaf
serrations). In guard cells, ABH1 is mainly localized in the nucleus,
consistent with its role in mRNA processing. Stomatal apertures are
smaller in abh1 than in WT at low, but not high, humidity,
but ABH1 expression is not affected by exogenous ABA or by
drought stress. These findings suggest that the reduced stomatal
apertures in abh1 at low humidity are due to the heightened
sensitivity of certain components of the ABA signal transduction
network. Indeed, two ion currents that mediate ABA-induced stomatal
closure are altered in abh1 guard cells compared with WT.
Double-mutant analyses of the ABA-hypersensitive signaling mutants,
era1-2 and abh1, showed complex genetic
interactions, suggesting that ABH1 and ERA1 (farnesyl transferase
-subunit) do not modulate the same negative regulator in ABA
signaling. The recent isolation of two other ABA-hypersensitive
mutants, hyl1 and sad1, both of which also encode
RNA-associated proteins, suggests an integral role for RNA processing
in ABA signal transduction. Both abh1 and sad1
show similar sensitivity to exogenous ABA, whereas the hyl1
mutant did not affect stomatal apertures in response to ABA. These data
strengthen the hypothesis that the mRNA-processing proteins ABH1 and
SAD1 (a Sm-like snRNP protein) function as negative regulators in guard
cell ABA signaling.
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A Homeobox Gene Regulates Auxin Transport |
Much evidence associates the plant hormone auxin with vascular
tissue development in dicots, but the existence of such a link in
monocots is more tenuous. Previous studies have shown that the
auxin-inducible homeobox gene Oshox1 of rice
(Oryza sativa) is a positive regulator of procambial
cell fate commitment, and its overexpression reduces the sensitivity of
polar auxin transport (PAT) to inhibition by
1-N-naphthylphthalamic acid (NPA). In this issue,
Scarpella et al. (pp. 1349-1360) extend these findings to
two additional classes of PAT inhibitors (PATIs), and show that WT rice
leaves formed under conditions of PAT inhibition display vein
hypertrophy, reduced distance between longitudinal veins, and increased
distance between transverse veins. Although class-specific differences
were noticed in the effects of the three classes of PATIs, the results,
collectively, indicate a role for PAT in vascular patterning in rice.
As expected, Oshox1 overexpression largely prevented these
PATI-induced vascular-patterning defects. The normal sensitivity to
PATIs of other processes that occur in root vascular tissue, such as
lateral root development and acropetal PAT, were also prevented by
Oshox1 overexpression. The effects of Oshox1
overexpression apparently only extend to vascular
tissues: Root elongation and gravitropic responsiveness, both of which
depend on basipetal PAT in the root cortex, displayed WT NPA
sensitivity in rice seedlings overexpressing Oshox1.
Evidence is presented that the overexpression of Oshox1
reduces the affinities of the NPA-binding protein toward NPA.
Oshox1 may promote fate commitment in procambial cells by
increasing their auxin conductivity properties and by simultaneously
stabilizing this newly acquired state against modulations of PAT by
endogenous NPA-like regulators.
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How a Fern Hyperaccumulates As |
Inorganic As species released from both natural
and anthropogenic sources are widespread environmental toxins.
Arsenate is the predominant As species in aerobic soils, whereas
arsenite dominates under anaerobic conditions. Arsenate acts as a
phosphate analog and can disrupt phosphate metabolism, whereas arsenite reacts with sulfhydryl groups of enzymes and tissue proteins, leading
to inhibition of cellular function and death. Phytoremediation offers
one approach to reducing As concentrations in contaminated soils. The
brake fern Pteris vittata is unusual in its ability to
hyperaccumulate As in its shoots. In this issue, Wang et al. (pp.
1552-1561) investigated the interactions of arsenate and
phosphate on the uptake and distribution of As and P in P. vittata. Long-term hydroponic experiments revealed that As
accumulated in the shoots of P. vittata at concentrations up
to 27 g kg1 dry weight (DW),
although phytotoxic symptoms started to appear once the concentration
exceeded 10 g kg1 DW. The tolerance of
P. vittata to As is far greater than that of
non-hyperaccumulating plant species, which typically have a threshold
concentration for phytotoxicity of between 0.005 and 0.1 g
kg1 DW. Internal detoxification of As is
obviously an important feature of this species and, in this respect,
P. vittata differs sharply from As-resistant grasses, which
tolerate As largely by excluding it. The authors show that arsenate
(but not arsenite, contrary to a previous report) is taken up by
P. vittata via phosphate transport systems as in other
species and that this absorbed arsenate is reduced to arsenite and
sequestered in the fronds primarily as As(III). It still remains to be
determined whether arsenite is complexed prior to xylem loading and
xylem transport in P. vittata, and whether phytochelatin
complexation plays a role in As sequestration in its vacuoles.
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