Journal of Experimental Botany, Vol. 51, No. 348, pp. 1267-1275,
July 2000
© 2000 Oxford University Press
Original papers |
Early physiological and cytological events induced by wounding in potato tuber
1 Dipartimento Biologia Vegetale, Università ‘La Sapienza’, Largo Cristina di Svezia 24, 00165 Rome, Italy
2 Istituto Dermatologico San Gallicano, IRCCS, via San Gallicano 25, 00153 Rome, Italy
3 Dipartimento di Genetica e Biologia Molecolare, Università ‘La Sapienza’, via Lancisi 29, 00191 Rome, Italy
4 Dipartimento Biologia Vegetale, Università ‘La Sapienza’, P.le Aldo Moro 5, 00185 Rome, Italy
Received 10 January 2000; Accepted 13 March 2000
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Abstract |
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The response of potato tuber (Solanum tuberosum L. cv. Kennebec) to mechanical wounding was investigated at different times. Changes in the levels of indole-3-acetic acid (IAA), polyunsaturated fatty acids (PUFAs) and lipid hydroperoxides (LOOHs) were monitored up to 120 min after wounding and related to the cytological events occurring up to 24 h. Twenty minutes after injury, an increase in IAA and LOOH levels and a decrease in the levels of PUFAs was observed. Wounding induced mitoses in differentiated (parenchyma) cells starting at 120 min, and promoted an increase of mitotic activity in the meristematic cells (procambium and bud dome), after 360 min. The inhibition of the increase in LOOHs and IAA by lipoxygenase (LOX) inhibitors, as well as the ability of in vitro peroxidated linoleic acid to enhance IAA production, suggest a close relationship among lipoperoxidation, IAA and mitotic activity in the response of potato tuber cells to injury, resulting in a specific growth response, i.e. bud growth and periderm formation.
Key words: Indole-3-acetic acid, lipids, lipid hydroperoxides, mitosis, wounding.
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Introduction |
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Some of the factors involved in the events following plant–pathogen interactions are similar to those seen following abiotic stresses such as mechanical wounding. Among these, the role of lipoxygenase (LOX) in both biotic (Croft et al., 1990
; Slusarenko et al., 1993) and abiotic stresses (Siedow, 1991; Geerts et al., 1994) has been reported. In fact, the early products derived by the activation of LOX, such as lipid hydroperoxides (LOOHs), reactive oxygen species (Croft et al., 1990) and subsequent metabolites, such as jasmonic and traumatic acids (Esquerre-Tugaye et al., 1993; Bell et al., 1995; Vijayan et al., 1998), are involved in the defence mechanisms to both pathogenic attack and abiotic stress. These reactive species, although toxic at high concentrations, could be considered secondary messengers in plant growth and development at lower concentrations (Jones, 1994). This general picture has led to the speculation that biotic and abiotic stresses can follow similar pathways, but with a different regulation.
In previous studies (Castoria et al., 1992; Fanelli et al., 1992), hypersensitive browning and phytoalexin accumulation induced in potato tuber slices by arachidonic acid (AA), an elicitor of the hypersensitive response (HR), have been reported. Furthermore, the elicitor AA significantly enhances important cytological events such as necrosis, nuclear hypertrophy, cell wall lignification, and tracheary element differentiation in potato tubers (Altamura et al., 1994). In particular, the key role played by 5-S-hydroperoxyeicosatetraenoic acid (HPETE), one of the products resulting from the activity of LOX on AA, has been demonstrated among the cascade of HR events (Castoria et al., 1992). However, AA is not produced in potato, but rather by the pathogen Phytophthora infestans and therefore has to be considered an exogenous elicitor and HPETE an endogenous secondary messenger capable of triggering the events of HR in Solanum tuberosum tubers.
Wounding is an abiotic stress in plants and the cutting of potato tubers is a common agricultural practice for propagating potato plants, favouring the budding event (Hartmann et al., 1990). Wounding is responsible for a very rapid hydrolysing of polyunsaturated fatty acids (PUFAs) in membrane phospholipids by lipolytic acyl hydrolase (LAH) and phospholipase (PhL) (Theologis and Laties, 1981; Thompson et al., 1987; Lee et al., 1997). The release of endogenous fatty acids can lead to membrane deterioration by their peroxidation through the LOX activity.
Several aspects of plant life, such as cell elongation, proliferation, xylem formation, and organogenesis are dependent on the activities of auxins and, in particular, of indole-3-acetic acid (IAA) (Davies, 1995). To date, little is known about the relationship between lipid peroxidation and IAA regulation after wounding, and in this paper their possible relationship has been investigated. The time-course of IAA accumulation, the role of LOX after tissue injury, the relationship between LOX by-products and IAA production, and the subsequent cytological events in the regions located below-bud of wounded tubers were studied.
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Plant material
Certified potato tubers (Solanum tuberosum L. cv. Kennebec, kindly supplied by PAT-FRUT, Castel D'Aiano, Bologna, Italy) 2–3-months-old, were stored at 4 °C and were brought to room temperature 24 h prior to use. In all the experiments tubers were wounded by slicing (3 cm diameter and ~0.5 cm thick) below-bud (eye region) and weighed (5 g).
Lipid analysis
Potato tuber slices of 5 g fresh weight (FW) each, were washed three times in sterilized distilled water and extracted three times with 15 ml CHCl3/CH3OH (2:1, v/v) in the presence of 100 µg of butylated hydroxytoluene (BHT) as an antioxidant. After filtration and evaporation under vacuum, the extracted lipids were fractionated by thin layer chromatography (TLC) and the polar lipid (PL) and free fatty acid (FFA) fractions were recovered as previously reported (Passi et al., 1986) with slight modification. Tricosanoic acid (C23:0) was added as internal standard before extraction. The extracted lipids were trans-esterified by boron-trifluoride (BF3 10% in CH3OH). The resulting fatty acid methyl-esters were analysed by gas chromatography (GC) on a capillary column FFA-P 50 mx0.32 mmx0.52 µm (Passi et al., 1994).
In other experiments, the time-course (up to 120 min) of changes in linoleic (C18:2) and linolenic (C18:3) acids from the PL fraction was carried out as reported above.
IAA detection
Analyses of IAA were performed at different times (0, 10, 20, 30, 60, and 120 min) on 5 g FW of tuber sliced below-bud. Samples were extracted three times for 1 h with 20 ml ethylacetate in the presence of 100 µg BHT as an antioxidant and 5 µg of 2,4 dichlorophenoxyacetic acid (2,4 D) as an internal standard. After filtration and evaporation under vacuum, the extracted solutes were silylated. In other experiments the IAA concentration was monitored under the same experimental conditions after the addition to the potato slice of in vitro peroxidized C18:2, C18:3 and the LOX inhibitor salicylhydroxamic acid (SHAM, 1 mM). The trimethyl-silyl-ether (TMS)-derivatives were analysed by GC-MS on a 5890 Hewlett-Packard (Palo Alto, CA, USA) gas chromatograph coupled with a 5970 Hewlett-Packard Mass Spectrometer using a Supelco (Bellefonte, PA, USA) SPBTM-1 fused silica capillary column (30 mx0.20 mmx0.30 µm) and splitless injection of 2 µl of sample. Helium was used as the carrier gas. For the analyses the oven temperature was programmed from 110 °C to 240 °C at a rate of 5 °C min-1, and from 240 °C to 280 °C, held for 20 min, at a rate of 30 °C min-1 with an injector temperature of 250 °C. Quantitative analyses was performed in SIM (Single Ion Monitoring) mode, selecting the ions having mz-1 319, 304, 202 for the trimethylsilyl derivative of IAA and ions of mz-1 292, 257, 219 for the derivative of 2,4 D. Calibration curves were constructed by plotting the ratios of the integrated peak areas of IAA and 2,4 D against their amount and performing a linear regression using equal weighting. The method was linear (R>0.99) in the analysed concentration range of 1–50 ng IAA µl-1.
Lipid hydroperoxide detection
LOOH formation was carried out at different times (0, 10, 20, 30, 60, and 120 min) on lipids extracted from 5 g FW of tuber slices and assayed spectrophotometrically using N,N-diethyl-1,4-phenylene-diammoniumsulphate (DEPD) according to Nazzaro-Porro et al. (Nazzaro-Porro et al., 1986). The effect of LOX inhibitors, i.e. SHAM and nordihydroguaiaretic acid (NDGA), on LOOH formation was also assayed. The inhibitors were applied at 1 mM concentration to the upper surface of the potato discs.
In vitro enzymatic peroxidation of linoleic and linolenic acids and their assay on potato tuber slices
A non-purified peroxidized C18:2 or C18:3 was obtained by reacting 300 µg of C18:2 or C18:3 (Sigma Chemical Co.) with 500 Units of LOX (EC. 1.13.11.12) (Sigma Chemical Co.). The solution was acidified with HCl to pH 4.0 and extracted three times with peroxide-free diethylether. The extracted solute was purified by TLC according to Castoria et al. (Castoria et al., 1992). The reactivity of linoleic and linolenic acid peroxides with DEPD was measured and a quantity of peroxide having an absorbance similar to that found in potato slices was added to the upper surface of the potato discs. This amount correspond to 8–10 µg of C18:2 peroxidized and to 2–3 µg of C18:3 peroxidized, using ter-butylhydroperoxide as standard.
Isoenzyme analysis of LOX
Five grams (FW) of tuber sliced below-buds were analysed after 0, 20, 120, and 360 min. The samples were homogenized with 200 ml of an extraction buffer (TRIS-HCl 20 mM pH 7.0, ethylenediaminetetraacetic acid 2 mM, ß-mercaptoethanol 14 mM, MgSO4 10 mM, DL-dithiothreitol 1 mM, phenazine methosulphate 1 mM) using a method modified from Chalot et al. (Chalot et al., 1989) and centrifuged at 5000 g for 15 min at 4 °C. Total protein was evaluated following the method of Lowry et al. (Lowry et al., 1951).
Electrophoresis was carried out using a horizontal starch gel consisting of 12% (w/v) hydrolysed potato starch powder (Connaught Medical Research Laboratory, Toronto, Canada). LOX enzymes were detected at pH 8.7 using a discontinuous TRIS-citrate buffer system (Poulik, 1957), and at pH 7.0 by phosphate-citrate buffer (Harris, 1966). The detection of LOX enzymes was carried on with borate buffer (pH 8.7) and TRIS-HCl buffer (pH 7.0) using linoleic acid as substrate and DEPD as reagent.
Lipoxygenase spectrophotometric assay
LOX activity was measured following the conjugated diene formation as increase of absorbance at 234 nm by a Beckman DU530 spectrophotometer. Potato tubers (8 g) were homogenized in 0.1 M acetate buffer (pH 4.5), NaCl 500 mM, dithiothreitol 2 mM, ethylenediaminetetraacetic disodium salt 1 mM, glycerin 10%, ascorbic acid 5 mM, phenylmethylsulphonyl fluoride 1 mM. The homogenate was filtered and centrifuged at 20 000 g for 15 min at 4 °C. The supernatant was collected and proteins were determined by Bradford reagent for protein determination (Sigma). LOX activity was measured in a final volume of 2 ml containing TRIS-HCl 0.1 M (pH 7.0), 100 µl supernatant, 100 µg C18:2 as substrate.
Histological analysis
For each time point, five explants, each of about 260 mm3 by volume, belonging to the eye, were excised from five randomly chosen discs (3 cm in diameter, 0.5 cm thick) at 0, 20, 120, 360 min and 24 h after wounding. In other experiments SHAM 1 mM was added on the tuber disc, soon after injury. The explants were fixed, dehydrated, embedded in paraffin (melting point 52–54 °C; Pabisch, Milano, Italy), sectioned at 10 µm intervals, and stained as described previously (Altamura et al., 1991). Counts of mitoses and of cells containing pre-prophasic nuclei were carried out in 1250–1300 longitudinal sections randomly chosen from among the five explants sectioned at each time point. The counts were expressed as mean number (±SE)/mean volume of each section (40.5x107 µm3). The distribution of starch grains in tuber tissues and changes in grain dimensions were analysed on the same sections under polarized light. All the observations were carried out with a DAS Mikroskop Leica DMRB (Leica, Wetzlar, Germany). At each time point, the number of cell layers of the suberized periderm was determined on the same sections used for the counts of mitoses and on free-hand sections, stained with Sudan IV (Sigma Chemical Co.) according to Jensen (Jensen, 1962).
Statistical analysis
In all experiments, mean values were compared using Student's t-test.
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Fatty acid composition of PL and FFA fractions of potato tuber slices
The analyses of the fatty acid composition of PLs and FFAs demonstrated that linoleic (C18:2) and linolenic (C18:3) acids were the only PUFAs present in potato tubers (Table 1
).
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IAA time-course after wounding in potato tuber slices
There was a significant increase in the concentration of IAA 20 min (P<0.01) after wounding (from 249.1±34 ng g-1FW to 484.8±35 ng g-1FW) (Fig. 1). Subsequently, IAA decreased up to 120 min (150.4±30.5 ng g-1FW).
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Time-course of changes in PUFAs of PL and LOOHs in wounded potato tuber slices
Since PLs are the main lipid fraction in potato tubers (Lepage, 1968) only this class of lipids was studied. The percentage of C18:2+C18:3 in the PL fraction decreased significantly (P<0.001) following tissue injury (Fig. 2) up to 20 min. This value was restored 120 min after wounding. At the same time LOOH formation significantly (P<0.01) increased soon after tissue injury (absorbance from 0.25±0.04 at time 0 up to 1.02±0.03 at 20 min) (Fig. 2). Up to 120 min the LOOHs level was inversely related to the amount of PUFAs (R=-0.91 and P<0.01).
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Detection of lipid hydroperoxides in wounded potato tuber slices in the presence and absence of SHAM and NDGA
In order to verify whether the production of reactive species rising from enzymatic or non-enzymatic action might be responsible for the fall in the percentage of PL- PUFAs, the effect of LOX inhibitors SHAM and NDGA (Peña-Cortes et al., 1993; Macrì et al., 1994) was examined (Fig. 3). In untreated samples, LOOHs peaked at 20 min after wounding whereas, in the presence of SHAM, a reduction (~50% in absorbance) was observed. However, a slight concentration of lipid hydroperoxides was still detected, probably due to the presence of reactive species not originated from the LOX activity (Fig. 3). NDGA, also acting as an antioxidant (Andrikopoulos et al., 1991), exhibited a much higher (P<0.01) inhibitory effect on lipid hydroperoxide production up to 20 min.
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Isoenzyme analysis of LOX in potato tuber slices at different times after tissue injury
LOX isoforms, with different electrophoretic mobilities, were seen in the isoenzyme analysis at pH 7.0 (Fig. 4). The one having the highest electrophoretic mobility starting from a constitutive level at time 0 (2.01 U mg-1 protein) showed a strong activation at 20 min (9.41 U mg-1 protein) after wounding remaining activated up to 120 min either evaluated at pH 7.0 or to a lesser extent at pH 8.7. The bulk of LOX activity was measured by spectrophotometric assay.
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Effect of exogenously added linoleic and linolenic peroxides on IAA concentration
To verify the role of LOOHs in IAA production, the effect of in vitro peroxidated C18:2 and C18:3 on the tuber slices was tested. Linoleic acid hydroperoxides (8–10 µg) exogenously added significantly increased (P<0.01) the IAA concentration at 20 min (from 470 ±32 ng in the control to 780.5±30 ng in the treated sample) (Fig. 5), followed by a decrease at 30 min up to 120 min. On the other hand the linolenic acid hydroperoxides, added at 2–3 µg because it represents about 30% of PUFA in PL fraction, did not influence the IAA concentration with respect to the control (Fig. 5).
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Effect of SHAM in the presence of the exogenously added linoleic peroxides on IAA concentration
The enhancing effect of linoleic peroxides on IAA concentration (Fig. 5
) was lost when these compounds were added in the presence of LOX inhibitor SHAM (Fig. 6). In fact, in the treated samples the IAA concentration at 20 min (455±21 ng) was comparable with the control (465±33 ng) and maintained the same trend at the other time points. On the other hand, in the presence of SHAM, the IAA concentration decreased (P<0.01) at 20 min and it was followed by a non-significant increase up to 120 min.
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Histological analysis
At time zero the eye contained a developed central bud, not yet showing cell elongation in the pith meristem and, on average, two small lateral buds. All the buds were connected with a very superficial procambium. In the procambial strands located in the middle and deep portions of the explant, and far from the buds, mitoses could be observed after 120 min and the number of these increased up to 24 h (Table 2). In the procambia, near and below the bud domes, mitotic activity did not seem to be affected by wounding (Table 2). An average of 3.5 layers of suberized cells (Fig. 7A) were present in the periderm at time 0. The number of these layers increased with time from 4 (at 20 min) to 6 layers (at 24 h) (Fig. 7B). A number of periderm layers with no suberized cell walls was present below the suberized layers. In these layers mitotic activity, was absent at time 0, sporadic at 120 min, significantly increased at 360 min, and then decreased slightly, but not significantly (Table 2). Initially (120 and 360 min) all the observed mitoses occurred in the first three non-suberized layers (Fig. 7C). Subsequently (24 h) they appeared in the fourth layer, and seemed to be associated to a phellogen-like activity. In the dome of the central and lateral buds mitoses reached a highly significant value (P<0.01) at 360 min after injury, increasing up to 24 h, but with a lower significance level (P<0.05) (Table 2). In no tissue and at no time point did synchronization in mitotic activity occur.
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After 120 min the cells located in the deep zone of the tuber explant were filled by starch, while the superficial zone exhibited only a few small starch grains surrounding some of the nuclei. The greater the distance from the tuber surface, the greater was the dimensions of the grains. Furthermore, the nuclei surrounded by the small grains seemed in a pre-prophasic stage, being hypertrophic, highly chromophil and with a single, prominent nucleolus (Fig. 7D
). The cells containing such nuclei were considered to indicate reactivation of the mitotic cycle. Although they also occurred occasionally at time 0, they increased significantly (P<0.01) at 20 min, with a mean number of 350±40 (x10-3) per section and three times more at 120 min. They were observed 6–7 layers below the suberized periderm and in the tuber portion far from the buds. In the same tissues binucleate cells (Fig. 7E) and cells with multilobed nuclei occurred.
At the final sample point (24 h), two axillary buds were present on the flanks of the central and of the lateral buds of the eye (Fig. 7F), and intense mitotic activity was observed on these buds. At this time, the central and lateral buds started to elongate (Fig. 7F) and the pith cells below the dome expanded, and this event resulted in the macroscopic appearance of the buds on the tuber.
In a following experiment the effect of SHAM (1 mM), added soon after injury, on mitotic activity was compared with the effect of wounding per se. No significant difference was observed in the mitotic activity between treated and untreated samples at time 0 [2±1 (x10-3) and 5±3 (x10-3) mitoses, respectively].
The treatment with SHAM (1 mM), at 360 min, inhibited mitotic activity compared with the control [6±2 (x10-3) and 38±6 (x10-3), respectively]. At 24 h the mitotic activity increased further in the control [66±10 (x10-3)] while in the SHAM-treated explants it was nearly unchanged [8±3 (x10-3)]. Further mitotic activity in the SHAM samples was observed in the bud dome only, while this was not restricted to this area in the wounded control in the absence of SHAM as in the previous experiment (Table 2).
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Discussion |
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The results show a relationship between PUFAs peroxidation, with LOOHs formation, and IAA production in potato tuber at different times after wounding. This relationship suggests that membrane modifications, due to abiotic stress, lead to the production of a growth factor (IAA) with subsequent cytological effects. The LOOHs act as secondary messengers capable of triggering at least some biological responses. These time-courses of IAA, linoleic and linolenic acid are in agreement with the results reported by other authors, although under different experimental conditions (Gaspar et al., 1994
; Theologis and Laties, 1981). This suggests that such phenomena could be a part of a general physiological response to an abiotic stress. The relationship between lipid hydroperoxides and IAA production was supported both by the good direct correlation (R=0.90) between these parameters and by the IAA response to the addition of synthetic lipid hydroperoxides. In particular, the results obtained with the lipid hydroperoxides added exogenously suggest a specificity of action of linoleic versus linolenic acid in IAA increase in potato tubers. This specificity of response seems to be similar to that obtained in an animal system (Nagy et al., 1998).
A generator of LOOHs is the LOX that works in general in combination with LAH and PhL (Thompson et al., 1987; Croft et al., 1990). The isoenzyme analyses show a high activity of a LOX isozyme either at pH 7.0 or pH 8.7 up to 120 min after wounding. Moreover, the results obtained in the presence of LOX inhibitors, SHAM and NDGA, further confirm the LOX involvement in LOOHs production. However, a role of reactive oxygen species in the peroxidation of membrane lipids cannot definitively be excluded. It is known that one of the biochemical responses to wounding is the activation of the superoxide-generating NADPH oxidase system (Thompson et al., 1987; Doke et al., 1991; Bolwell and Wojtaszek, 1998) and the early formation of reactive oxygen species could be partly responsible for the peroxidation of fatty acids released from membrane phospholipids (Kappus, 1985; Fanelli et al., 1992). A slight amount of LOOHs was observed after wounding even in the presence of SHAM, whereas the NDGA, which can also act as an antioxidant, severely inhibited LOOH generation. Nevertheless, the severe inhibitory effect of SHAM on LOX activity accounts for the key role played by this enzyme in the lipoperoxidation process up to 120 min after wounding.
The enhancing effect of in vitro peroxidated C18:2 on IAA concentration supports their close relationship in this kind of stress response and seems to minimize a potential involvement of jasmonic acid soon (20 min) after injury. This was furtherly confirmed by the failure of in vitro peroxidated C18:3 to enhance IAA concentration at the same time points. Furthermore, among the three families of LOX present in the tuber and root of potato, LOX-1, that works with C18:2 as substrate, is most prevalent (Royo et al., 1996). The LOOHs deriving from C18:2 can not act as precursors of jasmonic acid, hydroperoxylinolenic acid being its sole known precursor (Sembdner and Parthier, 1993). Moreover, the presence of C16:3 in the potato leaves and its involvement with dinor-oxo-phytodienoic acid as a hexadecanoic signal related to the jasmonate family has been reported (Weber et al., 1997). Dinor-oxo-phytodienoic acid transforms C18:2 into 
-ketol 13-hydroxy-12-oxo-9 (z) octadecenoic acid. This compound seems to be implicated in the activation of the LOX 2 (4 h after wounding) with the subsequent formation of jasmonic acid. However, LOX 2 is poorly expressed in potato tubers (Royo et al., 1996), and therefore this pathway can not be considered in this experimental model.
It is only possible to speculate as to how the LOOHs promote the increase of IAA. Linoleic hydroperoxides can lead to an increase of IAA both by activating DNA transcription factors with subsequent de novo synthesis of IAA or by the activation of hydrolase enzymes that promote the cleavage of IAA conjugates. Preliminary studies have shown that an esterase activity constitutively present was enhanced, after wounding, up to 120 min (unpublished results), supporting a theory that a putative cleavage of conjugated IAA can be stimulated up to 20 min after slicing. However, at least in animal models, it has been shown that reactive species activate transcription factors such as NF-kB which, in turn, induce the expression of several genes including those of growth factors (Legrand et al., 1998; Ginn-Pease and Whisler, 1998).
The main cytohistological result is that wounding causes mitotic activity in differentiated cells competent to produce periderm, and enhances mitotic activity in specific meristematic cell populations. In fact, the peaks of mitotic activity in the periderm competent cells and in the bud-dome meristematic cells occur at 360 min, after the mean peak (20 min) and the decrease (120 min) of IAA. This trend suggests that endogenous IAA might be the signal for preparing and activating these cells to mitosis. In fact, it is widely known that the initiation and execution of cell division is dependent on auxin which affects the expression of specific ‘early’ genes, as well as that of genes encoding for mitotic cyclins (Abel and Theologis, 1996; Doerner et al., 1996).
The increase in the mitotic activity specifically observed in the cells of the axillary bud-domes at 24 h and the subsequent elongation of the pith meristem cells of the buds might be positively affected by endogenous IAA. The positive role of this hormone both in increasing mitosis and in cell elongation (Kende and Zeevaart, 1997), in particular in a stress environment (Cosgrove, 1997), is known.
Starch digestion was observed to occur early in the most superficial layers of the tuber. It is known that this event exhibits an important role in sustaining reactivation to mitosis (Maeda and Thorpe, 1979; Altamura et al., 1991). Furthermore, a positive effect of IAA in modulating starch digestion for sustaining bud growth has also been hypothesized, for example, in tomato cotyledons cultured in vitro (Branca et al., 1994).
Combining both the biochemical and cytohistological results, it is hypothesized that the stimulation of budding and periderm formation in wounded potato tuber could be part of an overall stress response system starting from a metabolic input derived by reactive species. Thus, potato tuber wounding leads to the production of reactive species, that can be considered as a stress signal triggering an increase in IAA that, in turn, results in budding and periderm formation. The modulation of a number of factors such as LOX activity, lipid hydroperoxides and other reactive species levels, also present in pathogenic stress, may be responsible for auxin messages to cell differentiation.
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Acknowledgments |
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This work was supported by funds from University ‘La Sapienza’ of Rome, Italy (Progetti di Ateneo).
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5 To whom correspondence should be addressed. Fax: +39 06 6833878 (call before sending). E-mail: afabbri{at}axrma.uniroma1.it
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