Plant Physiol. (1998) 117: 695-701
Saline Stress Alters the Temporal Patterns of Xylem
Differentiation and Alternative Oxidase Expression in Developing
Soybean Roots1
Mirna Hilal,
Ana M. Zenoff,
Graciela Ponessa,
Hortensia Moreno, and
Eddy M. Massa*
Departamento Bioquímica de la Nutrición, Instituto
Superior de Investigaciones Biológicas (Consejo Nacional de
Investigaciones Científicas y Tecnológicas-Universidad
Nacional de Tucumán), and Instituto de Química
Biológica Dr. Bernabé Bloj, Chacabuco 461, San Miguel de
Tucumán, 4000 Argentina (M.H., A.M.Z., H.M., E.M.M.); and Departamento de Morfología Vegetal, Fundación Miguel
Lillo, M. Lillo 251, San Miguel de Tucumán, 4000 Argentina
(G.P.)
 |
ABSTRACT |
We conducted a coordinated
biochemical and morphometric analysis of the effect of saline
conditions on the differentiation zone of developing soybean
(Glycine max L.) roots. Between d 3 and d 14 for
seedlings grown in control or NaCl-supplemented medium, we studied (a)
the temporal evolution of the respiratory alternative oxidase (AOX)
capacity in correlation with the expression and localization of AOX
protein analyzed by tissue-print immunoblotting; (b) the temporal
evolution and tissue localization of a peroxidase activity involved in
lignification; and (c) the structural changes, visualized by light
microscopy and quantified by image digitization. The results revealed
that saline stress retards primary xylem differentiation. There is a
corresponding delay in the temporal pattern of AOX expression, which is
consistent with the xylem-specific localization of AOX protein and the
idea that this enzyme is linked to xylem development. An NaCl-induced
acceleration of the development of secondary xylem was also observed.
However, the temporal pattern of a peroxidase activity localized in the
primary and secondary xylem was unaltered by NaCl treatment. Thus, the
NaCl-stressed root was specifically affected in the temporal patterns
of AOX expression and xylem development.
 |
INTRODUCTION |
Salinity is an environmental stress that limits growth and
development in plants. The response of plants to excess NaCl is complex
and involves changes in their morphology, physiology, and metabolism.
Most studies have been descriptive and have not elucidated mechanisms
by which salinity inhibits plant growth (Cheeseman, 1988
; Munns, 1993
).
There are multiple genes that seem to act in concert to increase NaCl
tolerance, and certain proteins involved in salinity stress protection
have been recognized (Bohnert and Jensen, 1996
; Hare et al., 1996
).
Within any organ there exists a range of both cell types and cell ages
and, therefore, the metabolic functions and the responses to
environmental stimuli may be expected to vary with these different patterns of localization and developmental stages. Plant roots provide
an attractive experimental system for investigating salinity effects on
growth and other parameters for the following reasons: (a) they have a
definable growing region in the tip and a separate nongrowing region
consisting of mature, elongated cells, some distance behind the tip
(Ishikawa and Evans, 1995
); and (b) root cells can be directly exposed
to different NaCl concentrations by changing the root
medium.
Previously, it was reported that excess NaCl in the growth medium
induces structural changes in bean roots, as well as leakage of ions
correlated with alterations of the cell membranes (Cachorro et al.,
1995
). It was also reported that NaCl treatment leads to changes in the
lipid composition of bean roots (Cachorro et al., 1993
; Zenoff et al.,
1994
; Surjus and Durand, 1996
) and affects the proton-extrusion
activity, which appears to be partially dependent on a
H+-ATPase associated with the plasmalemma (Zenoff
et al., 1994
).
Knowledge about respiratory metabolism during saline stress is scarce
(Fernandes De Melo et al., 1994
). In this context, the role of the
nonphosphorylating alternative pathway, which is a common feature of
higher plant respiration (Moore and Siedow, 1991
; Siedow and Umbach,
1995
), has not been elucidated. This pathway can be induced by a number
of treatments generally described as stress conditions, and thus it was
suggested that the AOX pathway may be part of a stress response in
plants (Purvis and Shewfelt, 1993
; Day et al., 1995
). The participation
of the AOX pathway in response to NaCl stress has been analyzed in
barley leaves (Jolivet et al., 1990
), but the reported data are
difficult to interpret in part because they were based on
considerations, the validity of which has been questioned (Millar et
al., 1995
; Day et al., 1996
).
An approach toward understanding the mechanisms of saline effects in
young roots is to follow the time course of a series of biochemical,
physiological, and structural events in the early stages of
development. We studied the effect of NaCl treatment on the
differentiation zone of developing soybean roots by analyzing the
temporal evolution of AOX capacity and peroxidase activity, in
correlation with the tissue localization of these enzymes and NaCl-induced structural changes. These coordinated analyses during a
defined growth period revealed that saline stress specifically delays
or advances the temporal evolution of determined parameters and has no
effect on the temporal pattern of others, leading to a plant that is
not only smaller than the control but also with different biochemical
and morphological characteristics.
 |
MATERIALS AND METHODS |
Plant Growth and Saline Stress
Soybean (Glycine max L. var UFV-8) seeds were
germinated for 3 d at 28°C in sterile sand that was moistened
with tap water. Then the seedlings were transferred to hydroponic
culture in 25% Hoagland medium supplemented with 120 mM
NaCl (saline stress) or without the NaCl supplement (control). Plants
were grown at 28°C under greenhouse conditions and harvested when
indicated for each experiment during the period between d 0 (sowing)
and d 14 of development. The nutrient medium was renewed every 3 d. This standard protocol was followed for all of the experiments, except for that experiment whose results are shown in Figure 3.

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| Figure 3.
Temporal evolution of AOX capacity in the
differentiation zone of roots from seedlings grown in sand containing
control ( ) or NaCl-supplemented ( ) Hoagland medium from d 0 to
12. Each value is the mean ± SD of two separate
measurements. Fw, Fresh weight.
|
|
In the experiment shown in Figure 3, the seedlings were germinated and
grown (at 28°C) in sand containing control or NaCl-supplemented Hoagland medium (140 mL/kg sand) over the whole period from 0 to
12 d of development. The sand was periodically moistened with distilled water.
Selection of the Root Region Studied
The differentiation zone of the primary root was studied. To
verify that the selected zone from both the control and stressed roots
was identical at the different developmental stages, a segment about 4 mm long was marked gently with a pen in the differentiation zone of the
primary root in 3-d-old seedlings grown in parallel with those used for
the biochemical and morphological analyses. One-half of the marked
seedlings was transferred to the control medium and the other half was
transferred to the NaCl-supplemented medium, and the localization of
the selected segment was observed during the following growth period.
This segment remained without substantial length change and was
localized almost in the middle of the primary root in both the control
and the stressed seedlings over the period studied.
Assays of AOX Capacity and Peroxidase Activity
AOX capacity was measured as described previously (Hilal et al.,
1997
) in slices of the selected segment from the primary root-differentiation zone of control and stressed seedlings during the
growth period between d 2 and 14.
Peroxidase activity was determined in extracts of the selected root
segments, as described by Peyrano et al. (1997)
, using the substrate
syringaldazine. The specific activity was expressed as the increase in
A530 per minute and milligram of protein.
Protein concentration was measured by the procedure of Lowry et al.
(1951)
.
Tissue Prints
Tissue printing of cross-sections from the differentiation zone of
primary roots (selected as indicated above) and specific immunostaining
with anti-AOX monoclonal antibody were performed as described
previously (Hilal et al., 1997
) at d 8 of plant growth under control or
saline conditions.
Tissue prints of the same root zone were also made on d 3 and d 10 of
control and stressed seedlings to detect activity of syringaldazine
oxidase, a peroxidase associated with lignification (Goldberg et al.,
1983
). The assay conditions were as described by Peyrano et al. (1997)
Mophometric Analysis of the Root-Differentiation Zone
The selected segments from the root-differentiation zone of
control and stressed seedlings were fixed in filtered control or
NaCl-supplemented Hoagland medium, respectively, with 3%
glutaraldehyde for 6 h at 4°C and then postfixed overnight with
1% osmium tetroxide in 0.1 M phosphate buffer (pH 7.2).
The samples were dehydrated with a graded series of ethanol, ending
with 100% acetone, and then embedded in Spurr's medium (Spurr, 1969
)
and polymerized overnight in a 60°C oven. Cross-sections (0.5 µm)
were prepared with an ultramicrotome and stained with toluidine blue
(Richardson et al., 1960
) before visualization with a light microscope.
The number of xylem vessels and the areas occupied by the xylem and
phloem in the stele and the intercellular-to-cellular-area ratios in
the cortex were determined from images of the root cross-sections digitized with a charged-coupled device 200E video camera (Videoscope International, Washington, DC) coupled to a Macintosh Quadra 700 computer. Image analysis and quantitation were performed with NIH Image
1.45 software (Rasband W, National Institutes of Health, Bethesda, MD).
 |
RESULTS |
Figure 1 shows the appearance of
control and NaCl-stressed seedlings at d 8 of growth. Roots of plants
treated with NaCl were shorter and had fewer secondary roots than the
controls. Saline stress decreased the growth rate of soybean seedlings,
a well-known phenomenon.

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| Figure 1.
Control (left) and NaCl-stressed (right) soybean
seedlings at d 8 of growth; magnification ×0.4.
|
|
Temporal Evolution of AOX Capacity in Control and NaCl-Stressed
Roots
AOX capacity in the differentiation zone of control roots greatly
decreased between d 3 and 8, as already reported (Hilal et al., 1997
),
whereas in the stressed roots AOX capacity remained high at d 8 (Fig.
2) and declined several days later than
in the controls. At d 6 of development, when some of the stressed
seedlings were transferred to the control medium, their root AOX
capacity decreased earlier than that of the seedlings maintained in
saline medium (Fig. 2). When some of the control seedlings at d 6 of development were transferred to the saline medium, they retained their
root AOX capacity for a longer period than those remaining in the
control medium. Results in Figure 2 show that saline stress delays the
decline of AOX capacity in developing roots, but it does not induce an
increase of this capacity.

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| Figure 2.
Temporal evolution of AOX capacity in the
differentiation zone of roots from control ( ) and NaCl-stressed
( ) seedlings. At d 6 of growth, a group of the seedlings was
transferred from the control to the saline medium ( ) or from the
saline to the control medium ( ). Each value is the mean ± SD of three separate measurements. Fw, Fresh weight.
|
|
In the experiment shown in Figure 2, saline stress was initiated at d 3 of plant growth when AOX capacity in the root-differentiation zone was
highest (Hilal et al., 1997
). A different protocol was followed in the
experiment presented in Figure 3. In this
case, the seedlings were grown on sand containing control or
NaCl-supplemented Hoagland medium over the whole period from d 0 to 12 of development. As shown in Figure 3, AOX capacity in the
differentiation zone of the control roots was maximal at d 3 to 5, whereas in the stressed roots the peak of AOX capacity was shifted to
2 d later. Thus, the temporal pattern of AOX capacity in
developing roots is delayed by saline stress.
Localization of AOX by Tissue-Print Immunoblots
Control roots at d 8 showed no specific immunostaining in the
differentiation zone using tissue-print immunoblots (Fig.
4B) because, as already reported (Hilal
et al., 1997
), AOX protein is no longer expressed at this developmental
stage. However, in roots of NaCl-stressed 8-d-old seedlings, the xylem
strongly reacted with the anti-AOX monoclonal antibody (Fig. 4D),
indicating that AOX protein was still present in this tissue. This
correlates with the delayed decline of AOX capacity in stressed roots
(Fig. 2) and shows that the xylem-specific localization of AOX (Hilal et al., 1997
) is conserved under saline stress. Figure 4, A and C,
illustrates total protein, as evidenced by amido black staining of
tissue prints from control and NaCl-stressed roots, respectively.

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| Figure 4.
Localization of AOX protein. Tissue prints of
cross-sections from the differentiation zone of control (A and B) and
NaCl-stressed (C and D) roots at d 8 of growth. A and C, Amido black
stains of total protein. B and D, Immunostains specific for AOX. x,
Xylem; p, phloem; and c, cortex. Bars = 250 µm.
|
|
Morphometric Analysis of Developing Roots
To determine whether the NaCl-induced delay in AOX expression was
associated with retarded root differentiation, root anatomy was
examined by light microscopy of cross-sections from the differentiation zone, which had been previously fixed and embedded. As shown in Figure
5, the most notable effect of the saline
stress was to retard primary xylem differentiation. The appearance of
protoxylem and metaxylem in the stressed roots at d 8 of growth was
similar to that in the 3-d-old seedlings rather than to that in the
control roots at d 8. This effect was quantified with an image analyzer and the data are summarized in Table I.
The total area of the xylem in the cross-sections of the root
differentiation zone was significantly smaller in the control 8-d-old
seedlings than at d 3 of growth, whereas in the NaCl-stressed 8-d-old
seedlings, the xylem area remained similar to that at d 3 of
development. Also, the number of vessels remained constant in the
NaCl-stressed roots, whereas the number decreased significantly in
control roots between d 3 and 8 of growth. The changes in the xylem of
control roots shown in Table I reflect the normal differentiation of protoxylem to metaxylem over the period between d 3 and 8 of plant growth. These changes did not occur in the NaCl-stressed roots, indicating delayed primary xylem differentiation.

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| Figure 5.
Anatomy of control and NaCl-stressed roots. Light
photomicrograph of cross-sections from the differentiation zone of
fixed and embedded roots. A, At d 3 (after germination in sand). B, Control at d 8. C, NaCl stressed at d 8. x, Xylem; p, phloem; and
c, cortex. Bar = 260 µm; all panels are shown at the same magnification.
|
|
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|
Table I.
Morphometric analysis of cross-sections from the
differentiation zone of the primary root
Cross-sections similar to those shown in Figure 5 were analyzed as
indicated in ``Materials and Methods''. Data are the means ± SD of at least four seedlings from each group. Values in
the same line with different lowercase letters are significantly
different (P < 0.05) by the Student's t test.
|
|
No appreciable effect of the saline stress was observed in the phloem
(Fig. 5; Table I). In the cortex the intercellular-to-cellular-area ratio was significantly decreased in the NaCl-stressed roots (Table I),
reflecting a reduction in the apoplast in response to the increased
NaCl concentration in the growth medium.
Temporal Evolution and Tissue Localization of Peroxidase Activity
in Developing Roots
To determine whether the above results reflect a direct effect of
saline stress on the seedling growth rate leading to a delayed evolution of every parameter linked to root development, we analyzed the effect of NaCl on a peroxidase involved in lignification (Goldberg et al., 1983
). This enzyme activity, measured with the substrate syringaldazine, was not affected in tomato roots under saline conditions (Peyrano et al., 1997
).
As shown in Figure 6, peroxidase activity
in the differentiation zone of control roots presented two maxima: one
at d 3 to 4, coincident with the peak of AOX capacity reported by Hilal et al. (1997)
, and the other at d 9 to 10. The temporal pattern of
peroxidase activity was unaffected by the saline conditions.

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| Figure 6.
Temporal evolution of peroxidase activity in the
differentiation zone of control ( ) and NaCl-stressed ( ) roots.
Each value is the mean ± SD of three separate
measurements. prot, Protein.
|
|
The tissue localization of this peroxidase is shown in Figure
7. The enzyme was concentrated in the
xylem at d 3 and 10 (Fig. 7, B, E and H). An unexpected result revealed
by tissue prints in Figure 7 was the accelerated development of
secondary xylem in the NaCl-stressed roots (Fig. 7, G and H) compared
with the control roots (Fig. 7, D and E). The development of the
secondary xylem in the control roots was slower and its appearance at d 18 became similar to that of the NaCl-stressed roots at d 10, as
evidenced by tissue-print analysis (not shown).

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| Figure 7.
Localization of peroxidase activity. Tissue prints
of cross-sections from the root differentiation zone: at d 3, after
germination in sand (A, B, and C); at d 10, controls (D, E, and F); and
at d 10, NaCl stressed (G, H, and I). A, D, and G, Toluidine blue stain
of total protein. B, E, and H, Stain for peroxidase activity. C, F, and
I, Blanks for peroxidase activity, omitting the substrate H2O2. PX, Primary xylem; SX, secondary xylem;
P, phloem; and C, cortex. Bar = 350 µm; all panels are shown at
the same magnification.
|
|
 |
DISCUSSION |
The reduction in the apoplast of stressed roots relative to the
controls (Table I) is in agreement with previous data on the effects of
NaCl in bean roots (Cachorro et al., 1995
) and probably reflects an
adaptive response to avoid NaCl loading (Wegner and Raschke, 1994
).
Data in this paper revealed that saline stress alters the temporal
pattern of xylem differentiation, leading to the delayed development of
the primary xylem (derived from the pro-cambium) and precocious
development of the secondary xylem (derived from the cambium). Thus,
saline stress had opposite effects on the temporal evolution
of primary and secondary xylem, two tissues with different ontogenic
processes.
AOX protein, which has a xylem-specific localization (Hilal et al.,
1997
), exhibited a delayed pattern of expression that was apparently
linked to primary xylem development. In this regard, it should be noted
that depending on the developmental stage at which exposure to salinity
is initiated, three different situations were observed: (a) when NaCl
treatment was initiated before the increase in AOX capacity (Fig. 3),
there was a shift in the peak and, thus, values either lower or higher
than the controls could be obtained at different days of growth; (b)
when NaCl treatment was initiated when AOX capacity was high (Fig. 2),
there was a delay in the decline of AOX capacity and, thus, values
higher than the controls were obtained between d 4 and 12; and (c) when NaCl treatment was initiated after AOX decline (Fig. 2), there was no
NaCl-induced enhancement of AOX capacity. Therefore, it is clear that
salinity delays developmental processes linked to AOX expression. Once
such events have occurred, NaCl is not able to modify AOX capacity.
On the contrary, the temporal evolution of a peroxidase activity
localized in the xylem was not affected by saline stress even though
this enzyme presented a peak of activity at d 3 to 4 of root
development (Fig. 6), coincident with the peak of AOX capacity.
Therefore, saline stress does not alter the evolution of every
parameter that has a temporal pattern linked to root development or
seedling age.
In conclusion, this work is the first demonstration, to our knowledge,
of NaCl-induced retardation in primary xylem differentiation associated
with a delayed pattern of AOX expression, as well as subsequent
acceleration in the secondary xylem differentiation. The net result is
that the NaCl-stressed plant is not only smaller than the control one
but has specific modifications in various biochemical and morphological
parameters.
 |
FOOTNOTES |
1
This work was partially supported by the Consejo
de Investigaciones de la Universidad Nacional de Tucumán and by
the Consejo Nacional de Investigaciones Científicas y
Tecnológicas of Argentina.
*
Corresponding author; e-mail massa{at}insibio.unt.edu.ar; fax
54-81-24-8025.
Received December 31, 1997;
accepted March 23, 1998.
 |
ABBREVIATIONS |
Abbreviation:
AOX, alternative oxidase.
 |
ACKNOWLEDGMENTS |
We thank Dr. Thomas E. Elthon (University of Nebraska, Lincoln)
for providing the anti-AOX monoclonal antibody and Carolina Schlick
(Laboratorio de Microscopía Electrónica del Noroeste, Tucumán, Argentina) for collaborating in sample preparation for light microscopy. Seeds were generously provided by Graciela Salas from
the Estación Experimental O. Colombres (Tucumán,
Argentina).
 |
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