Plant Physiol. (1998) 116: 511-518
Hormonal Control of Parthenocarpic Ovary Growth by the Apical
Shoot in Pea1
María J. Rodrigo2 and
José L. García-Martínez*
Instituto de Biología Molecular y Celular de Plantas,
Universidad Politécnica de Valencia-Consejo Superior de
Investigaciones Científicas, Camino de Vera s/n,
46022-Valencia, Spain
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ABSTRACT |
The
role of the apical shoot as a source of inhibitors preventing fruit
growth in the absence of a stimulus (e.g. pollination or application of
gibberellic acid) has been investigated in pea (Pisum
sativum L.). Plant decapitation stimulated parthenocarpic growth, even in derooted plants, and this effect was counteracted by
the application of indole acetic acid (IAA) or abscisic acid (ABA) in
agar blocks to the severed stump. The treatment of unpollinated ovaries
with gibberellic acid blocked the effect of IAA or ABA applied to the
stump. [3H]IAA and [3H]ABA applied to the
stump were transported basipetally, and [3H]ABA but not
[3H]IAA was also detected in unpollinated ovaries. The
concentration of ABA in unpollinated ovaries increased significantly in
the absence of a promotive stimulus. The application of IAA to the stump enhanced by 2- to 5-fold the concentration of ABA in the inhibited ovary, whereas the inhibition of IAA transport from the
apical shoot by triiodobenzoic acid decreased the ovary content of ABA
(to approximately one-half). Triiodobenzoic acid alone, however, was
unable to stimulate ovary growth. Thus, in addition to removing IAA
transport from the apical shoot, the accumulation of a promotive factor
is also necessary to induce parthenocarpic growth in decapitated
plants.
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INTRODUCTION |
The ovaries of nonparthenocarpic varieties do not grow normally
after anthesis unless they are pollinated. The application of
plant-growth regulators can substitute for pollination and induce
parthenocarpic fruit development (Goodwin, 1978
). In the garden pea
(Pisum sativum L.), parthenocarpic growth can be stimulated by the application of GAs, auxins, and cytokinins
(García-Martínez and Hedden, 1997). However, only
applied GAs produce parthenocarpic fruits morphologically similar to
fruits with seeds (Vercher et al., 1984; Carbonell and
García-Martínez, 1985). Furthermore, the inhibition of
fruit growth by inhibitors of GA biosynthesis and its reversal by
applied GAs (García-Martínez et al., 1987; Santes and
García-Martínez, 1995), and the correlation between the
content of GAs in different tissues of fruit and the growth rate of the
pod (García-Martínez et al., 1991; Rodrigo et al., 1997) suggest that GAs, probably GA1, are the
hormones that control the development of the pericarp of seeded fruits.
The growth of vegetative organs competes with fruit growth, and the
removal of vegetative parts enhances fruit development (Quinlan and
Preston, 1971; Matsui et al., 1978; García-Martínez and
Beltrán, 1992). In pea parthenocarpy can be induced by severing the shoot just above the unpollinated ovary (Carbonell and
García-Martínez, 1980). The decapitation causes the
diversion of GAs from mature leaves to the unpollinated ovary, which
may be the cause of parthenocarpic growth (Peretó et al., 1988;
García-Martínez et al., 1991). However, parthenocarpic
growth after decapitation could also be due to the removal of
inhibitors from the apical shoot, as occurs in the release of lateral
buds (Tamas, 1995).
In this work we have investigated the role of the apex as a source of
inhibitors for the growth of unpollinated pea ovaries after anthesis.
We present evidence that IAA, transported basipetally from the apical
shoot, prevents fruit growth in the absence of pollination. The effect
of IAA on the inhibition of parthenocarpic growth is indirect and is
probably mediated by ABA.
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MATERIALS AND METHODS |
Plants of pea (Pisum sativum, line V1 cv Alaska type;
seeds purchased originally from Asgrow, Complejo Agrícola de
Semillas, Madrid, Spain) were grown in the greenhouse as described
previously (García-Martínez et al., 1991). Self-pollinated
flowers were tagged on the day of anthesis (d 0). Flowers were
emasculated 2 d before the day equivalent to anthesis (d 
2).
Decapitation of the plant was performed on the day equivalent to
anthesis (d 0) by cutting the internode just above the first flower
(Carbonell and García-Martínez, 1980). Roots were removed
at d 0 on decapitated plants by severing the stem just below the first
true leaf (node 3, counting from the cotyledons as node zero), and the
cut end was introduced into a vial with water, which was replenished
daily.
Phloem exudate from apical shoots was obtained using the EDTA exudation
technique (King and Zeevaart, 1974; Hanson and Cohen, 1985). Pea apex
explants, including the youngest unexpanded leaf and 2 cm of the
internode immediately below the leaf, were made when the first flower
was at anthesis. The explants were cut under a solution of 10 mm Na2-EDTA and 10 mm
Tris-HCl, pH 8, and placed into 1.5-mL Eppendorf vials containing 250 µL of the same solution (one explant per vial). The explants were
placed in closed boxes lined with wet filter paper and left for 24 h at 22°C in the dark. The liquid from the vials was collected and
stored at 20°C until hormone analysis. The plant material for
identification and quantification of hormones was frozen in
liquid N2 and stored at 65°C until extracted.
Application of Plant-Growth Regulators
IAA (Sigma), (±)-ABA (Sigma), and mixtures of IAA plus
[3H]IAA (851 GBq/mmol, Amersham) and (±)-ABA
plus [3H]-(±)-ABA (2.00 TBq/mmol, Amersham)
were applied to the plants in 1.2% agar blocks (small cylinders 6 mm
in diameter and about 0.2 mL of total volume made with a 5-mL glass
graduated pipette), in 0.1 m Mes buffer, pH 5.8. The agar
blocks were stuck to the upper part of the severed stumps of
decapitated plants at d 0. The blocks were removed after 24 h and
fresh ones were applied to the stumps after renewing the cut. The agar
blocks were covered with aluminum foil caps to reduce desiccation and
to protect hormones from light degradation. Control plants were treated
with agar blocks without hormones.
TIBA (Sigma) was applied at 0.6% (w/w) in lanolin paste containing
15% (w/v) of Tween 20. The lanolin paste was spread as a ring about 5 mm wide at the internode immediately above the first flower.
GA3 (Sigma) at different doses was applied to
ovaries from emasculated flowers in 20 µL of aqueous solution
at d 0.
Extraction and Purification of IAA and ABA
IAA and ABA were extracted and purified following the procedure
described by Li et al. (1992) with some modifications. Frozen plant
material was ground to a fine powder in a mortar with liquid N2, and homogenized with a polytron (model PT-10,
Kinematica, Kriens-Luzern, Switzerland) in 80% methanol with
butylhidroxytoluene (20 mg/L) (1:10, w/v), stirred overnight at 4°C,
and reextracted with methanol (1:5, w/v) for 30 min. Phloem exudates
were made up to 20 mL with water and adjusted to pH 3.0 before
partitioning against ethyl acetate, as described below.
To check recoveries during extraction and purification
[3H]IAA (about 1000 Bq) and
[3H]ABA (about 1000 Bq) were added to the
homogenized samples (except extracts from material treated with agar
blocks containing [3H]IAA or
[3H]ABA). Methanol was removed under a vacuum
at 40°C, and the aqueous residue was partitioned at pH 3.0 three
times against an equal volume of ethyl acetate. The aqueous phase was
discarded and the pooled ethyl acetate phases were evaporated at 40°C
to dryness under vacuum. The residue was dissolved in 5 mL of 70%
methanol, prepared with water at pH 8.5, and passed through a
C18 cartridge (Bond Elut, Varian, Harbor City,
CA) washed previously with 5 mL of methanol and 10 mL of the 70%
methanol solution. The column was washed with 10 mL of 70% methanol
and the combined effluent and wash solutions were dried under a vacuum
at 40°C.
HPLC
The residues from partially purified extracts were dissolved in
0.4 mL of 30% methanol, injected onto a 5-µm column (ODS
Ultrasphere, Beckman; 25 cm long, 0.46 cm i.d.), and eluted from the
column by a two-slope linear aqueous methanol gradient containing 50 µL/L acetic acid at a flow rate of 1 mL/min. The first-stage linear gradient consisted of 37 to 46% methanol in 5 min, and the second of
46 to 71% methanol in 30 min, then the column was washed with methanol
for 10 min. The fractions from HPLC corresponding to IAA (about 13-15
min) and ABA (about 17-19 min) were grouped according to retention
times of radioactive internal standard markers and taken to dryness
under vacuum. Radioactivity in HPLC fractions from transport
experiments (Figs. 5 and 6) was measured by scintillation counting
using scintillation liquid (OptiPhase Hisafe 3, Wallac Scintillation
Products, Loughborough, Leicestershire, UK).
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| Figure 5.
Transport of [3H]IAA applied to the
stumps of plants decapitated above the first flower. Agar blocks
containing 10 4 m IAA plus 2100 Bq
[3H]IAA were applied at d 0 and 1 to the stumps of 11 decapitated plants bearing 1 emasculated flower. The material was
collected at d 2 for extraction and analysis of radioactive compounds
by HPLC. The data are HPLC traces of total radioactivity recovered in
the 3 cm of internode immediately above the flower, where the treatment
was given (top panel), the 5 cm of internode immediately below the
flower (middle panel), and the peduncles plus sepals (bottom panel).
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| Figure 6.
Transport of [3H]ABA applied to the
stumps of plants decapitated above the first flower. Agar blocks
containing 103 m ABA plus 1300 Bq
[3H]ABA were applied at d 0 and 1 to the stumps of eight
decapitated plants bearing one emasculated flower. The material was
collected at d 4 for extraction and analysis of radioactive compounds
by HPLC. The data are HPLC traces of total radioactivity recovered in
the 3 cm of internode immediately above the flower where the treatment
was given (first panel), the 5 cm of internode immediately below the
flower (second panel), the peduncles plus sepals (third panel), and the
ovary (fourth panel). DPA, Dihydrophaseic acid.
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Identification of IAA and ABA by GC-MS
Before GC-MS, the dried aliquots from pooled HPLC fractions were
methylated with ethereal diazomethane for 30 min, dried under a stream
of N2, transferred with methanol to 200-µL
glass vials (Teknokroma, Barcelona, Spain), dried again under a
vacuum, and dissolved in 5 µL of methanol.
The analyses were carried out with a gas chromatograph (model 5890, Hewlett-Packard) using a 5% phenylmethyl silicone capillary column
(Ultra-2, Hewlett-Packard; 25 m long, 0.2 mm i.d., and 0.33 µm
in film thickness) linked to a mass-selective detector (model 5971A,
Hewlett-Packard). For IAA analysis the initial oven temperature was
maintained at 60°C for 2 min, then increased at 20°C/min to
200°C, and at 4°C/min to 290°C. In the case of ABA, after 1 min
at 60°C the temperature was raised at 4°C/min to 200°C, and then
at 20°C/min to 250°C. The MS ion source was operated at 70 eV and
170°C, the injector temperature at 220°C, and the interface
temperature at 250°C. The samples were run using full-scan acquisition mode.
Quantitative Analysis by GC-MS with Selected Ion Monitoring
For quantitative analysis appropriate amounts of
[2H3]ABA and
[13C6]IAA (gifts from Dr.
P. Hedden) as internal standards were added to the samples at the time
of extraction. The samples were purified as described above and
analyzed by CG-MS in selected ion monitoring mode. The peak-area ratios
of the following pairs of ions were determined to calculate the
concentrations of IAA and ABA by reference to calibration curves:
130/136 and 189/195 for IAA, and 162/165 and 190/193 for ABA.
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RESULTS |
Growth of Pollinated and Parthenocarpic Ovaries
Pollinated and parthenocarpic fruits induced by
GA3 had similar rates of growth and final weight,
at least until d 6 (Fig. 1). Plant
decapitation also stimulated the development of unpollinated ovaries,
although their growth was slower and their final weight was about
one-half of that of pollinated or GA3-treated
ovaries (Fig. 1). Unpollinated, nonstimulated ovaries did not grow
significantly after d 2 and eventually degenerated.
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| Figure 1.
Growth of pollinated ovaries (P), unpollinated
ovaries (U), unpollinated and GA3 treated ovaries (G), and
unpollinated ovaries on decapitated plants (D). Emasculation of the
flowers was carried out at d 2, and decapitation of the plants and
GA3 treatment (2 µg/ovary) were carried out at d 0. The
bars indicate ± se.
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To investigate the role of the roots and mature leaves on the
parthenocarpic fruit growth after decapitation, the stem was severed at
the level of the first internode and all of the leaves were removed.
The excision of roots at the time of decapitation did not affect fruit
set, although the final size of the parthenocarpic fruits was reduced
(about 35% shorter) (Table I). On the
contrary, when all of the mature leaves of decapitated plants were
removed, fruit set was prevented, both in the presence and
absence of roots (Table I).
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Table I.
Effect of roots and mature leaves on parthenocarpic
fruit development in decapitated plants
Flowers were emasculated on d 2, and decapitation and removal of
roots and leaves were carried out on d 0. Values were recorded on d 12, and the lengths give the means ± se. Fruit set on
intact plants was always 0%. Fruit set means the number of ovaries set over the total number of ovaries per treatment in each experiment.
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Effect of IAA and ABA on Parthenocarpic Fruit Growth Induced by
Plant Decapitation
The application of IAA or ABA in agar blocks to the severed stump
of decapitated plants counteracted the parthenocarpic growth induced by
decapitation, and this inhibition was proportional to the amount of
applied hormone (Fig. 2). The maximum
inhibition of fruit growth by IAA was attained with
101 mm (about 70% weight
reduction), without further effect for higher doses. ABA inhibited very
efficiently both fruit set and fruit growth at 1 mm (Fig.
2). The percentage of fruit set inhibition by IAA varied between
experiments but was always lower than that caused by ABA (see also Fig.
4). The growth of pollinated ovaries was also stimulated by plant
decapitation, probably due to a reduced competition for photosynthates,
but in this case neither IAA nor ABA applied in agar blocks (even at 1 mm) to the severed stump had any effect on fruit set and
growth (data not presented).
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| Figure 2.
Inhibition by IAA and ABA of parthenocarpic fruit
growth induced by plant decapitation. Agar blocks with different
concentrations of IAA and ABA (0, 103, 102,
101, and 1 mm) were applied at d 0 and 1 to
the severed stumps of plants decapitated at d 0. Each treatment was
applied to eight plants. The weight of fruits set was recorded on d 6. Values in parentheses indicate the number of fruits set when fewer than eight. The points are average values of developed fruits ± se.
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| Figure 4.
Effect of GA3 on parthenocarpic fruit
development on decapitated plants treated with IAA or ABA. The severed
stumps were treated at d 0 and 1 with agar blocks containing IAA
(101 mm), ABA (1 mm), or no
hormone (control). GA3 (0, 0.02, 0.2, and 2 µg) was
applied to ovaries on d 0. Each treatment was applied to five plants.
Values in parentheses indicate the number of fruits set when less than
five. The points are the means of developed fruits ± se.
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The growth of the axillary bud at the leaf immediately below the first
flower was measured to determine the release of apical dominance after
decapitation above that flower. Severing the apical shoot at d 0 promoted the growth of axillary buds (Fig.
3). The application of IAA but not ABA to
the stump counteracted the release of apical dominance (Fig. 3).
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| Figure 3.
Effect of IAA and ABA applied in agar blocks to
the severed stumps of decapitated plants on the growth of axillary
buds. Decapitation was carried out at d 0, IAA and ABA were applied at
d 0 and 1, and the length of the first lateral shoot immediately below
the first flower was measured at d 8. The values are the means of at
least seven replicates ± se. Int., Intact; C,
control; I, IAA; and A, ABA.
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The inhibitory effect of IAA and ABA applied to the severed stump on
parthenocarpic fruit growth was negated by treating the ovaries with
GA3 (Fig. 4). The
IAA (101 mm) inhibition was fully
counteracted with a dose of 0.2 µg GA3/ovary, whereas at least 2 µg GA3/ovary was required to
overcome the effect of ABA (1 mm).
Identification and Quantitation of IAA and ABA in the Shoot
IAA and ABA were identified in different pea tissues by comparing
full-scan mass spectra of methylated extracts with those of authentic
compounds. Tables II and III present the
relative abundance of the main characteristic ions for both hormones,
respectively, and show that IAA and ABA were present in extracts from
apical shoots (at the time of anthesis) and in 24-h phloem exudates
from apical shoots. Additionally, ABA was identified in unpollinated ovaries at d 4 (Table III).
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Table II.
Identification of IAA in methylated methanolic
extracts and phloem exudates from apical shoots
The shoots were collected at d 0 and the phloem exudate was
collected for 24 h, as described in ``Materials and Methods''.
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Table III.
Identification of ABA in methylated methanolic
extracts and phloem exudates from apical shoots, and in methanolic
extracts from unpollinated ovaries
The shoots were collected at d 0 and the phloem exudate was
collected for 24 h, as described in ``Materials and Methods''.
The ovaries were collected at d 4.
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IAA and ABA were quantitated in methanolic extracts and phloem exudates
of apical shoots in two independent experiments (Table IV). The total amounts of IAA (4.7-5.3 ng per organ)
and ABA (approximately 2.1-2.4 ng per organ) present in the apical
shoot (methanolic extracts) at time 0 were quite similar to the amounts
recovered in the phloem exudates after 24 h of incubation (average
values of 3.8 and 1.4 ng per organ, respectively).
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Table IV.
Quantification of IAA and ABA in methanolic extract
and phloem exudate of apical shoots
The shoots were taken at the time of anthesis of the first flower. In
each experiment the phloem exudate was collected from apical shoots
(nine in Experiment 1; seven in Experiment 2) incubated for 24 h
at 22°C in the dark.
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Transport of IAA and ABA Applied to the Severed Stump of
Decapitated Plants
To determine the transport of IAA and ABA from the apical part of
the plant, mixtures of radioactive (2100 Bq of
[3H]IAA or 1300 Bq of
[3H]ABA) and unlabeled
(104 m IAA or
103 m ABA, concentrations shown
previously to inhibit parthenocarpic fruit growth) hormones were
applied in agar blocks at d 0 to the stumps of plants decapitated at
the internode immediately above the first flower. Two (IAA) or 4 (ABA)
d later, the two internodes immediately below the site of application,
the peduncles plus sepals, and the ovary were collected to study the
distribution of radioactivity.
In plants treated with [3H]IAA, a radioactive
peak with the same HPLC elution volume as IAA and containing unlabeled
IAA from the plant extract (confirmed by GC-MS) was detected in the two internodes beneath the site of treatment (Fig.
5). Other radioactive peaks of unknown
nature (probably IAA conjugates and degradation products) were also
observed in the chromatograms. A minor peak with the same elution
volume as [3H]IAA was found in extracts from
peduncles plus sepals (Fig. 5), but no radioactivity could be detected
in ovary extracts (data not presented).
When the plants were treated with [3H]ABA, the
extracts from the internodes and the ovary but not those from the
peduncle plus sepals contained a radioactive peak with the same
retention time as ABA, also coinciding with unlabeled ABA from the
plant extract (confirmed by GC-MS) (Fig.
6). In addition, the chromatograms corresponding to the internodes and peduncle plus sepals but not those
from ovaries contained a radioactive peak that in some cases was the
major component of the chromatogram (Fig. 6). Fractions associated with
this peak were combined, analyzed by GC-MS, and shown to contain the
characteristic ions of dihydrophaseic acid (data not shown), a
metabolite of ABA (Neill and Horgan, 1987).
Quantitation of ABA in Developing Ovaries
The application of ABA to the stump of decapitated pea plants
inhibited the parthenocarpic growth of unpollinated ovaries located
below the stump (Fig. 2), and [3H]ABA applied
to the stump was transported into the unpollinated ovary (Fig. 6). It
was also found previously that the application of ABA directly to the
ovary prevents its parthenocarpic growth (García-Martínez and Carbonell, 1980). Therefore, the
concentration of ABA in unpollinated, parthenocarpic
(GA3-treated and those on decapitated plants),
and pollinated ovaries during their early growth was determined in two
separate and independent experiments. Similar results were obtained in
both experiments, and data from one are presented in Figure
7. The level of ABA was relatively high
at d 2 (about 80 ng/g) and decreased progressively until d 2 in all
kinds of ovaries. Then the concentration of ABA remained almost
constant and relatively low in growing fruits, at least until d 4. The
concentration of ABA in unpollinated, nongrowing ovaries increased
considerably at d 4 (to 90 ng/g), and was associated with the beginning
of ovary senescence.
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| Figure 7.
Concentration of ABA in pollinated ovaries (P),
unpollinated ovaries (U), unpollinated and GA3-treated
ovaries (G), and unpollinated ovaries on decapitated plants (D) during
early growth. Decapitation and GA3 treatment (2 µg/ovary)
were carried out at d 0. FW, Fresh weight.
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The IAA from the Apical Shoot Affects the ABA Content of
Unpollinated Ovaries
As shown before (Fig. 2), the application of IAA in agar blocks to
the stump of decapitated plants inhibited parthenocarpic ovary growth.
This inhibition was associated with increased ABA content in the ovary:
more than twice at d 1, and about five times at d 2 (Table
V).
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Table V.
Concentration of ABA in unpollinated ovaries growing
on decapitated plants treated with agar blocks without and with IAA
The flowers were emasculated on d 2, the plants were
decapitated on d 0, and agar blocks with 1 mm IAA were
applied to the severed stump on d 2 and 1. The ovaries, at
least 11 per treatment and day, were collected on d 1 and 2.
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When TIBA (an inhibitor of auxin transport) was applied in lanolin
paste at d 2 to the apical shoot of intact plants on the internode
immediately above the unpollinated ovary, an approximately 50%
decrease of ABA content in the ovary was found at d 0 compared with
control plants (Table VI). However, the decrease of ABA
content in the ovaries did not stimulate parthenocarpic fruit
development, and all of the ovaries degenerated eventually. Repeated
applications of TIBA to the apical shoot retarded the degeneration of
the ovaries but did not promote their growth (data not presented).
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Table VI.
Concentration of ABA in unpollinated ovaries on
plants treated with TIBA
Lanolin paste without () or with (+) TIBA (11.4 mm) was
applied to the internode immediately above the unpollinated ovary on
d 2, and the ovaries (at least 20 per treatment) were collected on d 0.
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DISCUSSION |
Decapitation of the pea plant stimulates parthenocarpic growth of
unpollinated ovaries (Fig. 1), which would degenerate otherwise, and
the application of IAA or ABA in agar blocks to the severed stump
inhibits the decapitation stimulus (Fig. 2). The inhibition by IAA or
ABA was counteracted by applying GA3 directly to
the unpollinated ovary (Fig. 4). We also identified IAA and ABA in the
apical shoot and phloem exudates (Tables II and III), and found that
both endogenous (Table IV) and 3H-labeled (Figs.
5 and 6) IAA and ABA were transported basipetally from the apex. These
data indicate that IAA and/or ABA from the apical shoot may be the
factor(s) that prevents the growth of unpollinated ovaries after
anthesis unless, as a result of pollination and fertilization
(García-Martínez and Hedden, 1997), GAs accumulate in
the ovary.
The inability of IAA or ABA to inhibit the growth of pollinated ovaries
when applied to the severed stump of decapitated plants may indicate
that following pollination, these organs contain saturating levels of
active GAs to counteract the inhibitory effect of IAA or ABA
transported from the apical shoot. This is in agreement with the
observation that the level of GA1 (the presumed
active GA) in the pod of pollinated ovaries (approximately 1.2 ng/g
fresh weight) is sufficient to stimulate the maximum growth capability of the ovary, and that the presence of higher concentrations of GA1 in the ovary (as a result of exogenous
applications) does not elicit further growth (Rodrigo et al., 1997).
This is probably the reason that the inhibitory effect of IAA and ABA
from the apical shoot is not perceived under standard experimental
conditions in the presence of developing seeds. The efficiency of the
application of IAA and ABA in agar blocks to the stump is in contrast
to previous results (Carbonell and García-Martínez,
1980), in which no inhibitory effect of IAA or ABA was found when
applied in lanolin paste to decapitated pea plants. Clearly, the
application of these hormones to a cut surface in agar (rather than in
lanolin) protected from light degradation and desiccation with aluminum
foil facilitates their basipetal transport in the plant.
It is not possible to decide from the data described above which are
the physiological roles of IAA and ABA in the inhibition of ovary
growth. The following evidence, however, indicates that IAA transported
from the apical shoot controls in unpollinated ovaries the content of
ABA, which would act as a secondary messenger preventing parthenocarpic
fruit set and growth. It has been shown previously that the application
of ABA but not IAA to unpollinated ovaries counteracts the stimulative
effect of GA3 (García-Martínez and Carbonell, 1980). Also, the content of ABA in unpollinated ovaries
increases if they are not stimulated by pollination or GA3 treatment, and this increase is prevented by
plant decapitation (Fig. 7). Furthermore, the application of IAA to the
stump enhances the content of ABA in the ovary (Table V), and the
inhibition of IAA transport from the apical shoot by application of
TIBA to the internode immediately above the flower decreases the ABA content in the ovary (Table VI). The observed inhibition of
parthenocarpic growth by ABA when applied to the stump of decapitated
plants (Fig. 2) was probably due to the basipetal transport of the
applied hormone into the unpollinated ovary (Fig. 6).
The blockage of IAA transport from the apical shoot by TIBA in entire
plants, although preventing the accumulation of ABA in the ovary, was
not sufficient to stimulate parthenocarpy in intact plants. Therefore,
both the absence of basipetal IAA transport and the presence of a
promotive factor are necessary to induce fruit growth in the absence of
pollination. GAs, the transport of which from mature leaves is diverted
to the unpollinated ovary, where they accumulate after removing the
apical shoot, have been proposed as promotive factors in the case of
plant decapitation (Peretó et al., 1988;
García-Martínez et al., 1991). Cytokinins can induce
parthenocarpy when applied to unpollinated pea ovaries (Eeuwens and
Schwabe, 1975; García-Martínez and Carbonell, 1980; Sponsel, 1982), and the transport of these hormones from the roots is
under the control of IAA from the shoot (Bangerth, 1994; Li et al.,
1995). However, the observation that decapitation induces parthenocarpy
in the absence of roots (the main purported source of cytokinins; Chen
et al., 1985), as far as the mature leaves were present in the plant
(Table I), does not support the idea of cytokinins being involved in
stimulating fruit growth on decapitated plants. The possibility cannot
be discarded that mature leaves and/or the stem, organs also capable of
cytokinin biosynthesis (Chen et al., 1985), are a source of these
hormones.
The induction of parthenocarpy in pea by removing the apical shoot is
similar to the apical dominance phenomenon, in which the inhibition of
axillary buds is released by decapitation and negated by IAA is applied
to the stump (Cline, 1991). The decapitation of the pea plant above the
first flower also stimulated the growth of axillary buds (Fig. 3).
Also, IAA transported basipetally from the apical shoot inhibited
parthenocarpic growth without apparently entering and accumulating in
the ovary (Fig. 5), as occurs in the inhibition of axillary buds
(Tamas, 1995), indicating that the effect of IAA on parthenocarpy is
indirect and mediated by another inhibiting factor. It is not possible
to discard the possibility, however, that the absence of appreciable
radioactivity in the ovary could be due to the rapid metabolization of
IAA to a product lost during the purification procedure. A role for ABA
in mediating the effect of IAA has also been proposed in the apical
dominance phenomenon (Gocal et al., 1991). We found that the
application of ABA to the stump inhibited parthenocarpic growth but had
no effect on axillary bud elongation (Fig. 3). This indicates that either ABA from the shoot may not be transported into the axillary buds, or that different mechanisms operate in the inhibition of axillary buds and unpollinated ovaries by IAA transported from the
apical shoot.
In conclusion, the results presented here indicate that the transport
of IAA from the apical shoot prevents the parthenocarpic growth of
unpollinated ovaries, and that this inhibitory effect may be mediated
by ABA. Additionally, a promotive factor is also necessary for the
growth of parthenocarpic fruits, which in the case of plant
decapitation could be GAs and/or cytokinins transported from mature
leaves.
| |
FOOTNOTES |
1
This work was supported by the Dirección
General de Investigación Científica y Técnica
(grant no. PB93-0133 to J.L.G.-M.) and by the Ministerio de
Educación y Ciencia of Spain (scholarship to M.J.R.).
2
Present address: Max Planck Institut für
Züchtungsforschung, Carl-von-Linné-Weg 10, Köln
50829, Germany.
*
Corresponding author; e-mail jlgarcim{at}upv.ibmcp.es; fax
34-6-387-7859.
Received June 12, 1997;
accepted September 17, 1997.
| |
ABBREVIATIONS |
Abbreviation:
TIBA, 2,3,5-triiodobenzoic acid.
| |
ACKNOWLEDGMENTS |
We thank Rafael Martínez-Pardo and Antonio Villar for
technical assistance in the greenhouse, Peter Hedden for the gift of 13C-labeled IAA and
2H-labeled ABA, and Isabel
López-Díaz for critical reading of the manuscript.
| |
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Abstract
Introduction
