JXB Advance Access originally published online on October 26, 2007
Journal of Experimental Botany 2007 58(13):3765-3773; doi:10.1093/jxb/erm227
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RESEARCH PAPER |
Physiological basis of different allelopathic reactions of cucumber and figleaf gourd plants to cinnamic acid
1Department of Horticulture, Huajiachi Campus, Zhejiang University, Kaixuan Road 268, Hangzhou, PR China 310029
2Key Laboratory of Horticultural Plant Growth, Development and Biotechnology, Agricultural Ministry of China, Kaixuan Road 268, Hangzhou, PR China 310029
* To whom correspondence should be addressed. E-mail: jqyu{at}zju.edu.cn
Received 13 June 2007; Revised 15 August 2007 Accepted 22 August 2007
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Abstract |
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To provide an insight into the mechanism of interspecific interactions mediated by allelochemicals, cucumber and figleaf gourd seedlings were compared on their response to cinnamic acid, an autotoxin from root exudates of cucumber. Reactive oxygen species metabolism and plasma membrane H+-ATPase activity were examined in roots upon exposure to cinnamic acid. This exposure resulted in significant increases in activities of NADPH oxidase, superoxide dismutase, guaiacol peroxidase, and catalase, as well as in O
production and H2O2 content, in cucumber roots but not in figleaf gourd roots. Notably, the cucumber roots produced significant amount of reactive oxygen species (ROS) immediately after cinnamic acid treatment, consequently increasing membrane peroxidation, decreasing membrane H+-ATPase activity, and losing root viability. By contrast, no such changes were observed in figleaf gourd roots. All these results indicated that there was an interspecies difference in the recognition of allelochemicals, which induced oxidative stress accompanied by root cell death in cucumber, an autotoxic plant, but not in figleaf gourd, a cucumber relative. Key words: Allelochemical, autotoxicity, cell viability, H+-ATPase, reactive oxygen species, oxidative stress, specific recognition
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Introduction |
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Allelopathy, the chemical inhibition of one plant species by another, represents a form of chemical warfare between plants competing for limited light, water, and nutrient resources (Bais et al., 2003). Allelopathy is believed to be involved in many natural and manipulated ecosystems and plays a role in the evolution of plant communities, exotic plant invasion, and replant failure (Yu and Matsui, 1994; Ridenour and Callaway, 2001; Inderjit and Duke, 2003).
Despite the ecological and agronomic importance of allelopathy, relatively little is known concerning the mechanism or the adaptive strategies adopted by plants in defence against allelochemicals (Bais et al., 2006). Early studies have elucidated that allelochemicals are delivered into an environment, especially the rhizosphere, by processes such as leaching from the aerial plant parts, volatile emissions, root exudation, and the breakdown of plant residue litter (Bertin et al., 2003). Many phytotoxic allelochemicals have been isolated, identified, and found to influence a number of physiological reactions, for example, transpiration, water utilization, photosystem II (PSII) efficiency, nutrient uptake, dark respiration, ATP synthesis, cell cycle, phytohormone metabolism, reactive oxygen species generation, and gene expression, etc (Inderjit and Duke, 2003; Blum, 2005).
It is well-known that plants generate more reactive oxygen species (ROS) when exposed to stressful conditions such as sub-optimal temperature, high light, salt, and pathogen infection (Yamamoto et al., 2003; Halliwell, 2006; Rhoads et al., 2006). These ROS are either toxic by-products of aerobic metabolism or key regulators of growth, development, and the defence pathway (Mehdy et al., 1996; Laloi et al., 2004; Mittler et al., 2004). Toxic ROS can affect membrane permeability, cause damage to DNA and protein, induce lipid peroxidation, and ultimately lead to programmed cell death. Recent findings about the biochemical and physiological effect of natural phytotoxins, have shed light on the rhizosphere interactions (Weir et al., 2004). Several studies including our early studies have shown that allelochemical stress can cause oxidative damage, as evidenced by enhanced activity of ROS-scavenging enzymes and increased degree of membrane lipid peroxidation (Baziramakenga et al., 1995; Politycka, 1996; Yu et al., 2003; Lara-Nunez et al., 2006; Ye et al., 2004, 2006). Furthermore, Bais et al. (2003) found that allelochemicals such as catechin could trigger a wave of ROS, lead to a Ca2+ signalling cascade, induce genome-wide changes of gene expression, and ultimately result in the death of the root in susceptible species.
Autointoxication is a special kind of allelopathy which occurs within the same plant species. There exist a lot of genetic variations in the autotoxic potentials of Cucurbitaceous plants (Yu et al., 2000). Some species such as cucumber, watermelon, and melon show autotoxic potential, while others do not. Indeed, a difference in autotoxic potential was also observed among cucumber genotypes (Asao et al., 1998). The poor plant growth in monocropping of cucumber or watermelon is supposed to be related to autointoxication partly arising from root exudates (Yu et al., 2000). Autotoxins including benzoic acid and cinnamic acid have been identified from root exudates of cucumber plants and they had detrimental effects on ion uptake and enhanced the incidence of Fusarium wilt by triggering oxidative stress in cucumber (Yu and Matsui, 1994, 1997; Ye et al., 2004, 2006). Interestingly, root exudates of cucumber or watermelon showed high phytotoxicity to cucumber and watermelon themselves, respectively, but not to other species such as figleaf gourd (Yu et al., 2000). Apparently, there is an interspecific difference in the response to phytotoxic constituents in root exudates as observed in other studies (Weir et al., 2003). The mechanisms involved, however, have not been extensively investigated. It is hypothesized that ROS status is an important mechanism involved in the interspecific difference in response to allelochemicals. Cinnamic acid is the principal autotoxin in root exudates of cucumber identified (Yu and Matsui, 1994) and the model allelochemicals used in many studies (Baziramakenga et al., 1995; Politycka, 1996; Yu and Matsui, 1997; Ye et al., 2004, 2006; Blum, 2005). Accordingly, the effects of cinnamic acid (CA) on growth, ROS generation rate, and ROS-scavenging enzyme activity were investigated in cucumber and figleaf gourd seedlings. Meanwhile, ROS generation and cell viability in roots were also monitored by cellular approaches that have not been fully employed in allelopathy study (Bais et al., 2003).
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Materials and methods |
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Growth bioassay
Two species, cucumber (Cucumis sativus L. cv. Jinyan No. 4) and figleaf gourd (Cucurbita ficifolia Bouché), were used. Seeds of both species were directly sown in vermiculite and grown for 7 d in a non-heated greenhouse at Zhejiang University, China. Average day/night temperatures were 25/20 °C. Groups of seedlings were then transplanted into a container (40x25x15 cm) filled with 8.0 l of Enshi nutrient solution (Yu and Matsui, 1997), and maintained at a relative humidity of 95–100%, a photosynthetic photon flux density (PPFD) of 500 µmol m–2 s–1, and a temperature of 25–30 °C for 6 d. When the cucumber seedling was at the two-leaf stage, cinnamic acid (CA) dissolved in ethanol was added into the nutrient solution at concentrations of 0, 0.05, 0.10, 0.15, 0.20, or 0.25 mM. The final concentration of ethanol in each solution including the control was 0.1% (v/v), at which concentration ethanol has a negligible effect on cucumber plants (Yu and Matsui, 1997). Each treatment had eight plants and was in triplicate. Three days later, plants were harvested, oven-dried at 80 °C for 3 d and then weighed. Meanwhile, samples were taken for the determination of reactive oxygen species and associated enzyme activity, lipid peroxidation, as well as NADPH oxidase activity.
Determination of NADPH oxidase activity of plasma membranes (PMs)
Root PM was isolated using the two-phase aqueous polymer partition system as described by Larsson et al. (1987). Briefly, root samples were homogenized in 4 vols of the extraction buffer (50 mM TRIS–HCl, pH 7.5, 0.25 M sucrose, 1 mM AsA, 1 mM EDTA, 0.6% polyvinylpyrrolidone (PVP), and 1 mM PMSF). The homogenate was filtered through four layers of cheesecloth, and the resulting filtrate was centrifuged at 10 000 g for 15 min. Microsomal membranes were pelleted from the supernatant by centrifugation at 50 000 g for 30 min. The pellet was suspended in 0.33 M sucrose, 3 mM KCl, and 5 mM potassium phosphate, pH 7.8. The plasma membrane fraction was isolated by adding the microsomal suspension to an aqueous two-phase polymer system to give a final composition of 6.2% (w/w) Dextran T500, 6.2% (w/w) PEG 3350, 0.33 M sucrose, 3 mM KCl, and 5 mM potassium phosphate, pH 7.8. Three successive rounds of partitioning yielded the final upper phase. The upper phase produced was diluted 5-fold in TRIS–HCl dilution buffer (10 mM, pH 7.4) containing 0.25 M sucrose, 1 mM EDTA, 1 mM dithiothreitol, 1 mM ASC, and 1 mM PMSF. The fractions were centrifuged at 120 000 g for 30 min. The pellets were then resuspended in TRIS–HCl dilution buffer and used immediately for further analysis. All procedures were carried out at 4 °C. Protein content of PM was determined according to the method of Bradford (1976) with bovine serum albumin (BSA) as standard. To determine NADPH oxidation rate, 50 µl of the isolated PM vesicles was added to a reaction mixture consisting of 50 mM HEPES–KOH (pH 7.8), 100 µM EDTA, and 1 µM KCN in a final volume of 1 ml (Pinton et al., 1994). KCN was added to block peroxidase activity. Reactions were initiated by the addition of 100 µM NADPH. The NADPH oxidation rate was based on a decrease of A340 after incubation at 30 °C for 5 min.
Antioxidant enzyme activity and reactive oxygen species determination
Antioxidant enzyme extraction and the activity determination were carried out as described by Zhou et al. (2004). Generally, each 0.5 g of root material was homogenized in 3 ml of 25 mM HEPES buffer (pH 7.8) containing 0.2 mM EDTA and 2% (w/v) PVP. The homogenate was centrifuged for 20 min at 12 000 g and the supernatant obtained was used for enzyme analysis. All operations were carried out at 0–4 °C. An aliquot of the extract was used to determine protein content following Bradford (1976), using BSA as a standard.
Total superoxide dismutase (SOD, EC 1.15.1.1) activity was measured by the photochemical method as described by Giannopolitis and Ries (1977). One unit of SOD activity was defined as the amount of enzyme required to cause a 50% inhibition of the rate of 
-nitroblue tetrazolium chloride reduction at 560 nm. The activity of guaiacol peroxidase (GPX, EC 1.11.1.7) was assayed according to the method of Cakmak and Marschner (1992). Ascorbate peroxidase (APX, EC 1.11.1.1
[EC]
1) activity was measured according to Nakano and Asada (1981) by monitoring the rate of ascorbate oxidation at 290 nm (E=2.8 mM cm–1). Catalase (CAT, EC 1.16.1.6) activity was assayed in a reaction mixture containing 25 mM phosphate buffer (pH 7.0), 10 mM H2O2, and the enzyme. The decomposition of H2O2 was followed at 240 nm (E=39.4 mM cm–1) (Cakmak and Marschner, 1992). Since protein content is comparable between the samples of both species, the same aliquot of the extract was used for the enzyme assay.
O was measured as described by Elstner and Heupel (1976) by monitoring the nitrite formation from hydroxylamine in the presence of O. The content of H2O2 was measured by monitoring the A410 of titanium-peroxide complex following the method described by MacNevin and Uron (1953).
Determination of lipid peroxidation and plasma membrane H+-ATPase activity measurement
For the measurement of lipid peroxidation in roots, the thiobarbituric acid (TBA) test was used, which determines malondialdehyde (MDA) as an end-product of lipid peroxidation (Zhou et al., 2004). The plasma membrane (PM)-enriched and tonoplast (TP)-enriched vesicles were prepared as described by Kasamo (1986) with some modifications. In brief, roots were cut into pieces and immediately homogenized in isolation medium (1/2, w/v) containing 300 mM sucrose, 5 mM EDTA, 0.5 mM EGTA, 2 mM 1,4-dithiothreitol (DTT), 1.5% (w/v) PVP, 60 mM HEPES–TRIS (pH 7.5). The homogenate was filtered through four layers of cheesecloth and centrifuged at 8 000 g for 20 min. The supernatant was loaded on top of a gradient consisting of 45/33/15% sucrose and centrifuged at 80 000 g for 150 min. The TP-enriched membrane fraction and the PM-enriched membrane fraction were collected from the 15/33% and 33/45% sucrose interfaces, diluted in three times by volume of a buffer containing 20 mM HEPES–TRIS (pH 7.5), 0.5 mM EDTA, 0.5 mM EGTA, and then centrifuged at 10 000 g for 60 min. The pellets were collected and suspended in 0.5 ml of solution containing 20 mM HEPES–TRIS (pH 7.5), 300 mM sucrose, 0.5 mM EGTA, 3 mM MgCl2.6H2O. The preparation procedures were carried out at 4 °C. The activity of H+-ATPase was then determined by measuring the release of Pi according to Ohinishi et al. (1975).
In vivo determination of reactive oxygen species and root viability
For the measurement of root viability and ROS formation, seeds of both cucumber and figleaf gourd were incubated at 25 °C for 5 d. Roots approximately 6 cm long of seedlings were selected and infiltrated with a mixture of fluorescein diacetate (FDA, 12.5 µg ml–1) and propidium iodide (PI 5 µg ml–1) or 17 µM 5-(and -6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate acetyl ester (CM-H2DCFDA, Molecular Probes) in the presence of 0.25 mM CA for 10 min, as described by Ezaki et al. (2000) and Fryer et al. (2002). Fluorescence intensity was observed using a Zeiss Axioskop 2 epifluorescence LSM 510 confocal microscope (Carl Zeiss Mikroskopie, Jena, Germany) fitted with a Spot CCD camera (Diagnostic Instruments, Sterling Heights, MI) with 488 nm excitation, 488 nm dichroic and 510–560 nm emission, respectively. FDA fluorescence decreases as the dye leaks from the dead cells. CM-H2DCFDA fluorescence increases as the dye is oxidized by ROS to dichlorofluorescein (DCF).
Statistical methods
Measurements were performed randomly using three replicates except that the biomass assay was carried out using four replicates. SAS 8.0 (SAS Institute, Cary, North Carolina) for windows was used for statistical analysis. The data were analysed with a one-way analysis of variance and differences between treatments were separated by the Tukey's HSD test at the P 
0.05 level.
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Results |
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Plant growth
The inhibitory effect of CA on cucumber growth is concentration-dependent. Exposure to 0.05 mM CA resulted in a significant inhibition on root growth but not on shoot growth in cucumber. At a concentration of 0.25 mM, CA decreased biomass of the shoot and root by 43.4% and 53.4%, respectively. CA, however, had little effect on the growth of figleaf gourd, even at the highest concentration (Fig. 1).
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ROS generation and metabolism
Further study was carried out to investigate whether these effects were mediated by altered root redox status. ROS generation, and ROS-scavenging capacity in roots of both species were determined upon exposure to CA at 0.25 mM. In the absence of CA, there was no significant difference in the activity of root NADPH oxidase between cucumber and figleaf gourd. Exposure to CA, however, induced an increase (by 84.2%) of NADPH oxidase activity in cucumber roots. By contrast, such an increase was not found in roots of figleaf gourd at the same CA concentration (Fig. 2).
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To determine if the NADPH oxidase activity difference was accompanied by difference of ROS scavenging enzymes, the activity of root SOD, GPX, APX, and CAT was analysed. Compared with cucumber, figleaf gourd showed higher root SOD, GPX, and APX activities, but lower CAT level. As showed in Fig. 3, the activity of SOD, GPX, APX, and CAT in cucumber roots all increased, in a dose-dependent manner, by 15.6, 38.0, 38.0, and 40.4%, respectively at a CA concentration of 0.25 mM. However, such a dose-dependent response was not found in figleaf gourd roots, for which activities of these enzymes (SOD, GPX, APX, and CAT) remained almost unchanged over a range of CA levels (Fig. 3).
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An experiment was carried out to determine if the difference of NADPH oxidase and ROS-scavenging enzyme activity correlates to the root ROS status. In cucumber roots, both O
production and H2O2 accumulation increased, by 178.7% and 73.8%, respectively, after exposure to 0.25 mM CA for 3 d. No significant increase of O production or H2O2 accumulation, however, was found in the roots of figleaf gourd treated by CA at any concentrations (Fig. 4).
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Membrane peroxidation and oxidative stress
Having established that there was significant difference in ROS metabolism in roots between the two species, it was next investigated whether the change of ROS accumulation was associated with changes in root H+-ATPase activity and membrane lipid peroxidation of root cell. In the absence of CA, there was no difference of MDA concentration in roots between two species. Exposure to CA at different concentrations, however, resulted in a concentration-dependent increase of MDA in cucumber roots. By contrast, CA had little effect on the root MDA concentration in figleaf gourd (Fig. 5).
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The effects of CA on PM- H+-ATPase and TP-H+-ATPase activities were assayed and found to be dependent on plant species. For cucumber roots, both PM-H+-ATPase and TP-H+-ATPase activities decreased as the CA concentration increased, indicating that the exposure to CA induced membrane peroxidation of cucumber root. By contrast, the activity of the two enzymes was barely influenced by CA treatment in figleaf gourd (Fig. 6).
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Transient ROS generation and root cell viability
Transient ROS production in roots was monitored by imaging the ROS-sensitive fluorescent dye, dichlorofluorescein (DCF). Consistent with the quantitative measurement of ROS accumulation, cucumber roots accumulated less ROS than figleaf gourd roots in the absence of CA. Interestingly, ROS was mostly distributed in the elongation zone (CEZ) of figleaf gourd roots. CA induced a transient increase of intercellular ROS in cucumber roots as soon as the dye touched the roots (10 s) as characterized by the increased fluorescence intensity (the oxidized product by ROS). However, the ROS level was not altered by CA in roots of figleaf gourd since the fluorescence intensity remained almost unchanged (Fig. 7).
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To examine whether different changes in ROS level after exposure to CA was associated with changes in root cell viability, fluorescein diacetate (FDA) was used to detect the change in root cell viability. As shown in Fig. 8a, the fluorescence changed from green to red in CA-treated cucumber roots, indicating cell death within the roots. By comparison, such a dramatic change was not found in figleaf gourd (Fig. 8b), indicating the roots of figleaf gourd did not lose cell viability when they were exposed to CA. To characterize further whether the accumulation of ROS in cucumber roots represented a wave of cell death upon CA administration, the dynamic change in root cell viability was examined. As shown in Fig. 8c, fluorescence intensity in cucumber roots decreased with the time of CA exposure, indicating that cell death occurred as soon as CA permeated into the cucumber roots. Notably, this change of root viability occurred almost simultaneously with that of root ROS (Fig. 8c).
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Discussion |
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Although there were genotypic differences in cucumber, most examined genotypes including Jinyan No. 4 used in this study were sensitive to the allelochemicals exuded from roots (Asao et al., 1998; Yu and Matsui, 1994, 1997; Ye et al., 2004, 2006). In this study, there were great differences in the response to CA between cucumber roots and figleaf gourd roots in terms of shoot and root biomass accumulation (Fig. 1). CA significantly inhibited the growth of cucumber but did not inhibit the growth of figleaf gourd. Apparently, the two plant species responded differently to the presence of the autotoxic compounds of cucumber, thereby contributing to species selectivity. While the allelopathic effects on other plant species have been recognized as an important survival strategy, autointoxication has also been suggested as a useful mechanism to avoid future conspecific competitors both in annual and perennial plants (Canals et al., 2005). It is worth noting that CA was applied at suitable concentrations (0
0.25 mM) in this study, since the concentration for allelochemicals was c. 0.1 mM in soils after cucumber cultivation (Tsuchiya, 1990; JQ Yu et al., unpublished data) and may reach to much higher levels in soils enriched in plant residues (An et al., 2001b). Meanwhile, the presence of a variety of allelochemicals exuded from cucumber roots would exhibit synergism (Yu and Matsui, 1994; An et al., 2001a). Taken together, autointoxication might be involved in the replant failure in cucumber monocropping.
To date, the primary mechanisms of allelopathy have remained elusive. Several action modes have been suggested, including direct inhibition of PSII components and ion uptake, interruption of dark respiration, and ATP synthesis and ROS-mediated allelopathic mechanisms (Inderjit and Duke, 2003). Compared with interruption or inhibition of photosynthesis, allelochemical-induced peroxidation of root cell membranes by ROS is more likely, since the root is the first organ to be exposed to allelochemicals in the rhizosphere. In the present study, CA induced an accumulation of ROS (O and H2O2) in cucumber roots, but not in figleaf gourd roots (Figs 4, 7). This large difference in ROS generation could be an important factor that regulates the occurrence or absence of phytotoxicity in cucumber and figleaf gourd, respectively. Although evidence about allelochemical-induced oxidative stress together with increased activity of antioxidant enzymes is emerging (Simard, 1995; Politycka, 1996; Herrig et al., 2002; Baziramakenga et al., 1995; Yu et al., 2002; Ye et al., 2006), however, little information is available about the mechanisms by which allelochemicals induce ROS formation. Many studies have shown that increased plasma membrane NAD(P)H oxidase activity was associated with increased O and H2O2 production following biotic and abiotic stresses (Keller et al., 1999; Forman et al., 2002; Lara-Nunez et al., 2006). Here CA induced an increased activity of NADPH oxidase in cucumber roots, but little in figleaf gourd roots (Fig. 2). Accordingly, it is clear that CA could trigger ROS generation by root cell membranes in cucumber but not in figleaf gourd. In agreement with the increase of NADPH oxidase activity (Fig. 2), both O production and H2O2 accumulation increased with the CA concentration in cucumber roots but not in figleaf gourd (Fig. 4). Accordingly, it seems likely that NADPH oxidase is one of the generation sites for ROS in cucumber roots.
Plants possess both enzymatic (superoxide dismutase, various peroxidases, catalase, etc.) and non-enzymatic defence systems for the detoxification of various types of ROS (e.g. peroxides, superoxides) (Laloi et al., 2004). Increases of ROS-scavenging enzyme activities have been observed in cucumber roots after exposure to allelochemicals in previous studies (Baziramakenga et al., 1995; Politycka, 1996; Yu et al., 2002; Ye et al., 2006). In this study, figleaf gourd roots maintained higher activity of SOD, GPX, and APX than cucumber roots in the absence of CA. Importantly, increases of these enzyme activities were induced by CA only in cucumber roots, but not in figleaf gourd (Fig. 3). This discrepancy is apparently related to the difference in NADPH oxidase activity and the intrinsic capacity for ROS-scavenging between the two species. Cucumber roots exhibited higher NADPH oxidase activity, leading to higher O production and ROS-scavenging activity (SOD, GPX, APX, and CAT). The insensitive response of NADPH oxidase to change of CA concentration in figleaf gourd suggests that CA could induce little change in ROS generation and ROS-scavenging activity as shown in Fig. 3 and Fig. 4.
Although there was an increased level of antioxidants in roots after exposure to allelochemical stress, oxidative stress still occurred as described in an earlier study (Figs 7, 8; Yu et al., 2002). In Arabidopsis, (-)-catechin triggered a wave of ROS initiated in the roots, which led to a Ca2+ signalling cascade triggering genome-wide changes in gene expression and, ultimately, death of the roots (Bais et al., 2003; Weir et al., 2006). Consistently, changes of ROS and cell viability occurred coincidently in cucumber roots in the experiment (Figs 7, 8). It is well known that ROS accumulation causes damage to DNA and proteins, induces lipid peroxidation, and ultimately leads to programmed cell death. The finding that CA treatment resulted in significant decrease of PM-H+-ATPase and TP-H+-ATPase activities in cucumber roots, but not in figleaf roots (Fig. 6), also supports the hypothesis that allelochemical-induced ROS accumulation is extremely harmful for the membrane protein (Friebe et al., 1997; Hejl and Koster, 2004; Ye et al., 2006). In agreement with Bais et al. (2003), it was found that the species-specific ROS accumulation patterns reflected the differences in other physiological parameters such as H+-ATPase, root cell viability, and root growth. Moreover, it was found that cells in the elongation zone (CEZ) lost their viability more significantly than cells in apex (Fig. 8). ROS formation was more significant in CEZ than in the root apex (Fig. 7). Accordingly, ROS-induced cell death occurred in cucumber roots, but not in figleaf gourd roots. Interestingly, Bais et al. (2003) found that exotic plants show allelopathic potential by triggering ROS accumulation and programmed cell death in receiver plant species but not in donor plant species. This may be one of the fundamental differences between allelopathy and autotoxicity.
In summary, this study has demonstrated that one autotoxic plant experienced an increased ROS generation and accumulation in roots, leading to increased membrane peroxidation, decreased H+-ATPase activity, and finally loss of root viability while another species was largely insensitive to this chemical with little change in ROS metabolism. Further study on allelochemical uptake, compartmentalization, and detoxification is necessary to elucidate the mechanism involved in this specific recognition ability.
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Acknowledgements |
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We are grateful to JM Vivanco and H Ye for their critical reading and revision of the manuscript. This work was supported by the National Natural Science Foundation of China (30235029; 300070522) and the National Outstanding Youth Foundation (30230250).
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Abbreviations |
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APX, ascorbate peroxidase; CA, cinnamic acid; CAT, catalase; CEZ, cells in the elongation zone; GPX, guaiacol peroxides; MDA, malondialdehyde; PSII, photosystem II; PM, plasma membranes; ROS, reactive oxygen species; SOD, superoxide dismutase.
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