Analysis of tomato seedling cell death in response to copper and paraquat induction

Analise da morte cellular das plântulas do tomate na resposta à indução com cobre e paraquat

RESEARCH ARTICLE ARTIGO

Antoni Rueda^I; Yony Roman^I; Mario Lobo^II; Carlos Pelaez^I

^IUniversidad de Antioquia, Medellín, Colombia

^IICorpoica, Rionegro, Antioquia, Colombia

ABSTRACT

Plant defense responses to stress have attracted interest because of their similarity to reported mammalian stress responses. To investigate plant responses to abiotic stress, caspase 10-like enzymatic activities of Lycopersicon hirsutum and Lycopersicon esculentum were assayed in response to copper chloride and paraquat induction. Caspase 10-like activity was greatest at 40 mM CuCl₂ at 9 h after elicitation in both L. hirsutum and L. esculentum, for which respective response slopes (Δ absorbance per Δ minute) were 0.0054 and 0.0022 . The response for L. hirsutum was less variable and significantly greater than for L. esculentum. Elicitation of caspase 10-like activity by paraquat was greatest at around 2 h after treatment in both species with slopes of 0.013 and 0.0012, respectively for L. hirsutum and L. esculentum. The direct determination of caspase 10-like activity in tomato seedlings treated with copper and paraquat, using a specific substrate for mammals, suggested that the observed responses were related to apoptosis processes under abiotic induction.

Key words:Lycopersicon, abiotic elicitation, apoptosis, caspase activity

RESUMO

A pesquisa relacionada com a resposta inata das plantas á estresses tem atraído muito interesse por a similitude com as respostas reportadas nos mamíferos. Assim, foi proposta uma aproximação ás respostas de defesa aos estresses abióticos através da avaliação no modelo biológico Lycopersicon spp. da atividade enzimática caspase 10 em resposta da indução com CuCl₂ y paraquat. Os ensaios da atividade caspase 10 mostraram um máximo de atividade na concentração 40 mM, 9 horas depois da elicitação, com uma pendente de 0,0054 e 0,0022 para L. hirsutum y L. esculentum respectivamente. A atividade comparativa dos genótipos sugere que, embora os dois genótipos presenteassem atividade, a resposta é menos variável de maior magnitude para L. hirsutum. A elicitação com paraquat presenteou a maior atividade caspase 10 em torno das 2 horas depois do estímulo, com uma pendente de 0,013 e 0,0012 para L. hirsutum y L. esculentum respectivamente. A determinação direita da atividade caspase 10 nas plântulas de tomate tratadas com cobre e paraquat, usando um substrato específico para mamíferos, mostrou que estes mecanismos estão relacionados provavelmente com processos de apoptose depois da indução abiótica.

Palavras-chave:Lycopersicon, apoptose, atividade de caspase, eliciação abiótica.

INTRODUCTION

Stress involves the presence of an external factor that exerts a negative influence on the optimal development of the plant (Azcon-Bieto & Talon, 2003); the stress can be biotic or abiotic (Hahn, 1996; Hammerschmidt, 1999; Grayer & Kokubun, 2001; Zhaoa et al., 2005). Abiotic factors, such as heavy metals, xenobiotic compounds, and herbicides, contribute to adverse effects on plants. Although copper is an essential microelement for plants, it has toxic effects in high concentrations (Burda et al., 2002). The metal has been proposed as an inhibitor of the electron supply chain in the primary plastoquinone A acceptor (QA) and pheophytins and in the secondary plastoquinone B acceptor (QB). It has also been suggested that copper inhibits plant photosystem processes through tyrosine residue (Tyrz) oxidation (Yruela et al, 1996; Burda et al., 2002; Ke, 2007). Paraquat (1,1´-dimethyl-4,4´-bipyridinium dichloride) was reported to act by diverting electrons to the supply chain in photosystem I, especially during the transit of electrons from the cysteine sulfur complex toward ferredoxin, thus interrupting the reduction of nicotinamide adenine dinucleotide (NAD⁺) to NADH In addition, copper and paraquat have been associated with an oxidative burst, which theoretically triggers structural damage to cells (Mithofer, 2004). Despite these frequently reported mechanisms, detailed studies of other possible mechanisms leading to cell death have not been reported. In this research new aspects regarding the action mechanisms of these elicitors are proposed.

In recent years, a parallel has been drawn between the apoptotic processes in mammals and the hypersensitive reactions in plants (Danon et al., 2000; Godbole et al., 2003; Hofius, 2007). Functionally and mechanistically, two forms of cell death have been recognized. In animals, apoptotic and necrotic death have been defined using morphological, biochemical and, more recently, molecular criteria. Necrosis is a mechanism of cell death that involves swelling of cells and organelles, followed by plasma membrane rupture and release of cytoplasmic material (Golstein et al., 2003). Apoptosis was originally defined in 1972 based on specific morphological changes that occur during a genetically controlled cell death initiated by environmental stimuli (Kerr et al., 1972). In a simplified view, apoptosis is associated with chromatin condensation, cell shrinkage, plasma membrane blebbing, internucleosomal DNA cleavage, the formation of apoptotic bodies and finally the engulfment and degradation in phagocytic lysosomes of adjacent cells (Clarke & Clarke, 1995). Each newly discovered step was successively incorporated in a cell suicide pathway referred to as programmed cell death (PCD), which implies the activation of genome-encoded biochemical pathways and is part of the life cycle of multicellular animals and plants. By controlling cell numbers and turnover, forming and deleting structures, and eliminating damaged or infected cells, PCD is critical to normal development and immune responses (Baehrecke, 2002). Different meanings have been ascribed to the terms apoptosis and PCD (Vaux, 1999). For example, some researchers restrict apoptosis to the morphological phenotype observed in some forms of animal PCD and for others PCD means developmental cell death. In the present research, apoptosis is considered from the biochemical point of view, differentiating it from PCD, for which a genome-encoded biochemical pathway has been shown to lead to cell destruction.

A critical question in the study of cell death in plants is the extent to which hallmarks of apoptosis and of animal PCD can be found. Core components of the apoptotic machinery are caspases, a family of cysteine proteases. Caspases are activated by extrinsic death receptor pathways, or by intrinsic pathways that involve mitochondrial and endoplasmic reticulum (ER) release of proapoptotic factors, and the regulatory functions of BCL-2 protein family members (Hengartner, 2000). The caspases constitute a family of cysteine proteases that use a cysteine residue as the catalytic nucleophile and that share an exquisite specificity for cleaving target proteins at sites next to aspartic acid residues (Boatright & Salvesen, 2003). Caspases are essentially proteases with a cysteine residue acting as the catalytic amino acid (Earnshaw et al., 1999; Yan & Shi, 2005). Although plant caspase genes have not yet been identified, their existence has been indicated by many studies in which caspase inhibitors block the hypersensitive response (HR) and other defense mechanisms (Danon, 2000). Recently, treatment of tomato cell suspensions with AlCl₃ was reported to induce programmed cell death (PCD), a conclusion reached based on elicitation of the cells and treatment of the cells with different caspase inhibitors (Yakimova et al., 2007). In another work, specific peptide inhibitors of AC-YVAD-CMK and AC-DEVD-CHO caspases led to suppressed bacteria-induced PCD in tobacco leaves (Del Pozo & Lam, 1998). In addition, an apoptosis system in free cells of oats induced with victorin toxin exhibited apoptosis, nuclear collapse and DNA fragmentation in isolated nuclei (Kusaka et al., 2004). The aim of the present work was to study the responses of Lycopersicon spp. by evaluating the caspase 10-like enzymatic activities in response to CuCl₂ and paraquat induction.

MATERIALS AND METHODS

Genetic resource

Two tomato species were used: the wild-type species Lycopersicon hirsutum (PI 251305), which was reported to be resistant to several plant pathogens, and the cultivated species Lycopersicon esculentum (L-507, Licato type). These plants were obtained from the Sistema de Bancos de Germoplasma Vegetal de la Nación Colombiana (Colombian National Plant Germplasm Bank System), which is managed by the Corporación Colombiana de Investigación Agropecuaria, Corpoica (Colombian Corporation for Agricultural Research).

Regeneration and Material Propagation

The tomato seeds were sown in a sterile environment with nutrients. After germination, the seedlings were transplanted into trays of sterile soil with controlled water supply in a greenhouse with natural light, and later to flower pots in which they were allowed to develop to the reproductive phase.

Elicitation and evaluation of ex situ materials

Young leaflets of Lycopersicon hirsutum and Lycopersicon esculentum seedlings were sprayed with solutions of 40 mM CuCl₂ or 1% paraquat. For assays of caspase 10-like activity, leaflets of 4 seedlings (experimental unit) were harvested at 0, 2.5, 5.5, 8.5, 15 and 24 h after elicitation. The kinetics of the caspase 10-like activity was investigated for two replicates at each time of sampling. The harvested leaflets were weighed and immediately macerated at -70°C (-94°F) for storage and later use, or at 20°C (-4°F) for caspase 10-like activity assays. Two repetitions of each caspase 10 experiment were conducted.

Measurement of caspase 10-like activity

An extraction buffer to obtain the enzymes from the frozen leaves was prepared with 250 mM NaCl, 50 mM NaH₂PO₄.H₂O, 10% glycerol, 1 mM EDTA, 1% BSA, and 1 mM DTT at pH 8.0 (the extraction buffer was optimized from the proteolytic activity measured at different pH values). The caspase 10-like activity was measured using a kinetic method with N-Acetyl-Ala-Glu-Val-Asp-7-amido-4-trifluoromethyl coumarin, which is a specific substrate for caspases. To assay the enzymatic kinetics, the reaction was stopped at 0, 2, 4, 6, 8, 10, 15, 20 and 25 min in a reaction consisting of a 1:1 ratio of 25 µL of the enzymatic reactions and 25 µL of the stop solution composed of 1% CH₃COONa and 4.5 mM CH₃COOH at pH 4.5. The reaction was read at 380 nm. The enzymatic activity was determined as the initial velocity (slope) and is expressed as follows: Δ absorbance per Δ min of the catalytic product from the reaction of 0.075 µmoles of substrate with 125 µL of extract, which was obtained from the macerated sample (1:2 mg leaf tissue:µL of buffer). Additionally, dose-response assays were developed using concentrations of 10, 40 and 80 mM CuCl₂ and 3, 1 and 0.25 % paraquat.

Analysis of the results

To analyze the caspase 10-like activity data, linear regressions were conducted for each time point (Statgraphics Centurion XV). The slope for each sampling point (enzymatic activity) was used to construct a graph as a function of time after the elicitation. To determine the reproducibility of the caspase assays, variance analyses were conducted with the SAS statistical package version 8.0 for independent-in-time assays. To this end, analyses were done under the completely randomized model arrangement.

RESULTS

Caspase 10-like activity assays during copper elicitation

For use of the extraction buffer in assays, its pH was optimized in relation to enzyme activity (Figure 1). The effects of seedling induction with copper and paraquat on caspase 10-like activity of L. hirsutum and L. esculentum were assessed. Seedling elicitation of both species with a 40 mM CuCl₂ solution showed caspase 10-like activities at 3, 6 and 9 h after elicitation (Figure 3). Observations for the copper-treated seedlings indicated that a process of physiological death took place during the experiment (Figure 2). This process can be associated with caspase 10-like activity which was about three times greater for L. hirsutum than for L. esculentum (Figures 3-4).

Dose-response effect of caspase 10-like activity during copper elicitation

The dose effect was assessed 9 h after elicitation, when the highest level of activity was observed for copper elicitation. For these assays, effects of 10, 40 and 80 mM CuCl₂ were measured in two independent experiments. A dose-response effect was evident for both tomato species. In L. hirsutum the level of activity was higher for the 40 mM concentration than for 80 mM, and no activity was observed for the 10 mM concentration (Figure 5). The dose-response behavior of L. esculentum differed from that of L. hirsutum. While the highest level of activity was observed for the 40 mM concentration, considerable activity was observed for the10 mM dose, and no activity was observed for the 80 mM concentration (Figure 6). The dose-response assays revealed that the 40 mM dose used during the kinetic experiments was adequate to induce caspase 10-like activity.

Caspase 10-like activity during paraquat elicitation

The elicitation with paraquat was performed in two independent time-course assays, which revealed activity for both tomato species (Figures 7-8). A comparison between the activity of the taxa showed differences in favor of L. hirsutum, which exhibited a maximum caspase 10-like activity around 2.5 h compared to L. esculentum. Nevertheless, the response was much higher in magnitude for L. hirsutum, which suggested that the mechanisms associated with apoptosis were activated to a greater extent for this species. These results isuggest events associated with apoptosis, rather than an exclusive cell damage sequence as a result of an oxidative burst.

Dose-response effect of caspase 10-like activity during paraquat elicitation

Dose-response effects were measured at 3 h after paraquat elicitation when the level of caspase 10-like activity was greatest. Two independent experiments were conducted in which the concentrations of 0.25, 1 and 3% paraquat were used. The responses were similar for the two tomato species. The level of caspase 10-like activity was higher for seedlings treated with 1% paraquat than for those treated with 3% paraquat treatment, and no activity was found in the 0.25% treatment (Figures 9-10).

Reproducibility of caspase 10-like activity assays under induction with copper and paraquat

The analysis did not show any statistically significant differences between the independent assays but did reveal differences in activity between the tomato species at 8.5 h after copper elicitation (Table 1). To confirm the reproducibility of the data, a Pearson correlation analysis was performed. The independent caspase activity assays for L. hirsutum had a high positive correlation of 0.86 with a Pr> value of 0.057; L. esculentum had a positive correlation of 0.65, which was not significant (Table 2). The reproducibility of the paraquat assays was similar to that for copper. No differences were observed between the assays (Table 1) or between the tomato species. The correlation coefficients revealed a higher significance, which corroborated the absence of differences between the assays, and also revealed a high degree of similarity in the nature of the phenomena (Table 2).

DISCUSSION

In the present work, we developed a protocol for the direct assessment of caspase 10-like activities in a Lycopersicon biological model, which lays the groundwork for future studies of plant reactions associated with apoptosis. L. esculentum and L. hirsutum, the tomato species studied, represent genotypes that are susceptible and resistant to stress respectively, especially biotic stresses such as pathogen attacks by pathogens or insects. On the other hand, abiotic stress, which does not have a co-evolutive relation, can induce the expression of mechanisms closely linked to the immune response. Research during the past few years has elucidated cell death processes in plants, and included studies on caspases and caspase inhibitors (Del Pozo & Lam, 1998; Kusaka et al., 2004). Recent findings corroborate the existence of highly conserved mechanisms in animals and plants. Measurements on cell suspensions of reactive oxygen species (ROS) are often used to assess cell death and effects due to caspase inhibitors. Although the mechanism of copper toxicity has not yet been described in detail, it is generally accepted that damage is generated following an oxidative stress route (Ke, 2007).

For the two tomato species in our tests, we propose a mechanism associated with apoptotic processes under elicitation with Cu²⁺. In this model, we propose that copper induces specific caspase 10-like activities, which are associated with processes of apoptosis initiation. This type of induction probably would activate routes of cell death, which are considered to be exclusive of co-evolutive systems, similar to the plant-pathogen relationship. The direct determination of caspase 10-like activities on tomato seedlings using a specific substrate for mammals revealed the existence of highly conserved mechanisms in animals and plants. These mechanisms are related to apoptosis processes in tomatoes and are subsequent to abiotic induction. The caspase 10-like activity assays showed a maximum activity at 40 mM CuCl₂ and 8.5 h after the elicitation. The comparative observations for the two tomato species indicated that caspase 10-like activity is more stable in L. hirsutum and significantly higher than L. esculentum.

Observations in the elicitation experiments with paraquat suggest that plant interaction with a xenobiotic compound such as paraquat can activate processes in the genome, which are initially exclusive for plant-pathogen and plant-insect interactions. While paraquat inhibits the transit of electrons toward ferredoxin in photosystem I and destroys the cell with a consequent oxidative burst, a fully detailed mechanism of action has not been elucidated. A xenobiotic compound such as paraquat may be able to activate encoded processes in the genome. The finding of specific caspase 10-like activity in the two tomato species suggest a hitherto unreported mechanism for the action of paraquat.

Subsequent to a signaling process related to oxidative stress, the mechanisms associated with apoptosis and eventually with PCD processes would be activated. L. hirsutum differentially activated more processes that are linked to apoptosis and eventually to PCD. The elicitation with paraquat showed differences in the time reduction with regard to copper induction, suggesting that the acceleration of an apoptosis process was possibly associated with mechanisms related to higher production of ROS. The dose-response assays for both copper and paraquat suggest enzymatic processes that require adequate conditions for expressing the highest level of caspase 10-like activity.

ACKNOWLEDGMENTS

The authors acknowledge CORPOICA for providing the genetic resources. They also thank Grupo Interdisciplinario de Estudios Moleculares (GIEM) of Universidad de Antioquia for supporting this research and acknowledge Colciencias for support under its national Ph.D. program.

Received 11 August 2010

Accepted 21 June 2011

Author for correspondence: Carlos Pelaez, e-mail: [e-mail]

Abbreviations: HR - hypersensitivity response; PCD - programmed cell death; ROS - reactive oxygen species.

TPP 159

Section Editor: John C. Sutton

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Analysis of tomato seedling cell death in response to copper and paraquat induction

Analise da morte cellular das plântulas do tomate na resposta à indução com cobre e paraquat

Publication Dates

• Publication in this collection
  24 Aug 2011
• Date of issue
  June 2011

History

• Received
  11 Aug 2010
• Accepted
  21 June 2011