Genome-wide interacting effects of sucrose and herbicide-mediated stress in Arabidopsis thaliana: novel insights into atrazine toxicity and sucrose-induced tolerance
© Ramel et al; licensee BioMed Central Ltd. 2007
Received: 09 July 2007
Accepted: 05 December 2007
Published: 05 December 2007
Soluble sugars, which play a central role in plant structure and metabolism, are also involved in the responses to a number of stresses, and act as metabolite signalling molecules that activate specific or hormone-crosstalk transduction pathways. The different roles of exogenous sucrose in the tolerance of Arabidopsis thaliana plantlets to the herbicide atrazine and oxidative stress were studied by a transcriptomic approach using CATMA arrays.
Parallel situations of xenobiotic stress and sucrose-induced tolerance in the presence of atrazine, of sucrose, and of sucrose plus atrazine were compared. These approaches revealed that atrazine affected gene expression and therefore seedling physiology at a much larger scale than previously described, with potential impairment of protein translation and of reactive-oxygen-species (ROS) defence mechanisms. Correlatively, sucrose-induced protection against atrazine injury was associated with important modifications of gene expression related to ROS defence mechanisms and repair mechanisms. These protection-related changes of gene expression did not result only from the effects of sucrose itself, but from combined effects of sucrose and atrazine, thus strongly suggesting important interactions of sucrose and xenobiotic signalling or of sucrose and ROS signalling.
These interactions resulted in characteristic differential expression of gene families such as ascorbate peroxidases, glutathione-S-transferases and cytochrome P450s, and in the early induction of an original set of transcription factors. These genes used as molecular markers will eventually be of great importance in the context of xenobiotic tolerance and phytoremediation.
Different classes of herbicide act on plants through direct induction of oxidative injury. Herbicides of the triazine, phenolic and urea families, which bind to the D1 protein, inhibit photosystem II (PSII), and block electron transfer to the plastoquinone pool , thus causing the production of triplet chlorophyll and singlet oxygen (1O2). In cyanobacterial cells, 1O2 has been shown to cause direct photodamage to PSII and D1 protein and to prevent PSII repair by suppressing elongation of D1 protein . Furthermore, 1O2 may generate other reactive oxygen species (ROS), such as hydroxyl radical (HO•) , and probably superoxide anion (O2•-) . 1O2 can also act as a signalling molecule inducing stress and necrotic responses in Arabidopsis . The lethal effects of PSII inhibitors can thus be ascribed to ROS injury rather than to nutritional stress and carbon starvation [1, 6].
Exogenous treatment with sucrose, and to a lesser extent with glucose, was found to confer to Arabidopsis plantlets a very high level of tolerance to the triazine herbicide atrazine [7–9]. Sugar-treated plants were able to maintain PSII activity and phototrophic growth in the presence of atrazine concentrations, up to 40 μM, that are otherwise lethal, in the absence of sugar treatment. Moreover, sucrose-protected atrazine-treated Arabidopsis plantlets maintained active growth and oxygen evolution [7, 8], thus suggesting that other mechanisms than phototrophic-photoheterotrophic transitions may be involved in sucrose-based protection against atrazine and ROS injury. Since notable differences of protection were conferred by glucose and sucrose for the same supply of carbon equivalents [7, 8], we reasoned that protection also involved other physiological effects than metabolic feeding to energy and anti-oxidative pathways.
The demonstration that sugars acted as regulators of gene expression in plants [10, 11] has led to the characterisation of a growing number of sugar-regulated genes. Thus, glucose or sucrose treatment in the absence of abiotic stress usually represses photosynthesis-related genes in plants  and in cyanobacterial cells . This is the case for psbA mRNA and D1 protein accumulation in higher plants [7, 12]. In the cyanobacterium Synechocystis, glucose feeding depresses the steady-state mRNA levels of PSII genes  and, under dark conditions, induces the destabilisation of psbA transcripts . Surprisingly, sucrose treatment of Arabidopsis plantlets in the presence of atrazine results in markedly enhanced accumulation of psbA mRNA and D1 protein, which could be interpreted as derepression of sugar-induced repression of photosynthesis-related genes . Moreover, application of ROS, especially H2O2, or changes of the glutathione redox state in the dark enhance psbA gene expression, which may thus help replenish D1 protein under conditions of oxidative stress . Given that atrazine treatment itself had negative effects on D1 protein levels, the observed derepression in the presence of sucrose and atrazine  was therefore likely to result from interactions between sugar and oxidative stimuli. On the other hand, typical markers of ROS response have been shown to respond to interacting sugar and oxidative cues. Thus, Sulmon et al.  showed that, during sucrose-induced protection against atrazine treatment, FSD1 (encoding a chloroplastic Fe-superoxide dismutase) gene expression, which was slightly increased by sugar treatment per se and did not respond to atrazine treatment per se, was greatly enhanced in the presence of both sucrose and atrazine.
However, the extent of these interacting effects is not known. As outlined by Thum et al.  in their study of light and carbon signalling, the general picture of how interactions between sucrose and xenobiotic affect gene regulation must be gleaned from large-scale transcriptomic studies. In order to characterize these interactions and to obtain further insight into the roles of exogenous sugars in the tolerance to herbicides and oxidative stress, a CATMA whole Arabidopsis genome array  approach was undertaken. The microarray analysis was used to characterise the responses of Arabidopsis plantlets treated for 24 h in the presence of atrazine alone, sucrose alone, and sucrose plus atrazine and was complemented with a time-course study of a subset of genes using quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR). This genome-wide approach revealed that the primary effects of atrazine affected seedling physiology at a much larger scale than usually described in the literature, with the potential impairment of protein translation and of ROS defence mechanism, and that, correlatively, sucrose conferred atrazine protection through important modifications of transcript levels, not only as an effect of sucrose itself on stress response genes, but also as a result of sucrose and atrazine interactions that revealed the induction of novel stress defence genes. Finally, since sucrose application, which enhances xenobiotic tolerance, accumulation in plants, and consecutively decontamination of surrounding soil, appears to be potentially useful for phytoremediation [9, 18, 19], characterization of gene markers related to xenobiotic protection will be important to analyse phytoremediation properties in the field.
Physiological effects of atrazine and sucrose treatments
In order to obtain insight into sucrose-induced atrazine tolerance, to characterize specific and beneficial effects of sucrose and to establish an analysis of gene functions under tolerance or stress conditions, the first step was to determine a treatment yielding plantlets at an early stage of injury, and presenting effects on gene transcription prior to advanced and visible effects of cell death. Thus, at 24 h of transfer, atrazine-treated plantlets could be compared to mannitol-treated, sucrose-treated, and sucrose plus atrazine-treated plantlets which had not yet undergone significant developmental changes resulting from the treatment and were therefore in a comparable physiological state (Additional file 1). In order to study the same developmental stage of plantlets, Arabidopsis plantlets were grown on Murashige and Skoog agar medium and transferred at the 1.02 development stage  to Murashige and Skoog agar medium supplemented with mannitol (80 mM) as osmotic control (M), mannitol (80 mM) plus atrazine (10 μM) as herbicide treatment (MA), sucrose (80 mM) as sugar effect control (S) and sucrose (80 mM) plus atrazine (10 μM) as protective treatment (SA). MA treatment induced complete bleaching of plantlets after 6 to 7 days of stress application, thus leading to seedling death within 8 days (Additional file 1). In contrast SA treatments allowed plantlets to maintain growth and development beyond 8 days of transfer. The herbicide treatment produced root growth inhibition upon 24 h of transfer, while the other conditions resulted in recovery of root growth within 2 days of treatment. Chlorophyll and carotenoid contents were identical for all treatments upon 24 h of transfer (Additional files 2 and 3). Pigment contents were maintained between 4 and 8 days of SA treatment whereas these values decreased between 4 and 8 days of MA treatment. This difference between MA and SA treatments resulted in complete death of MA plantlets and in maintenance of growth and development in SA plantlets. This was correlated with measurements of photosystem II efficiency (Fv/Fm), which stayed unchanged for all treatments after 24 h of treatment (Additional file 4). After 8 days of SA treatment, PSII still showed significant efficiency, while no PSII efficiency was detected for the herbicide treatment.
Given that MA treatment induced significant injuries on shoot physiology and root growth from 24 h to 8 days, thus resulting in various secondary effects related to injury and bleaching, we chose to study plantlets at 24 h of treatment where primary effects of atrazine exposure could be analysed.
Effects of atrazine and sucrose on global gene expression
Mannitol treatment (M) was chosen as the control for all comparisons in order to filter osmotic-responsive genes (Additional file 5). Differentially expressed genes, i.e. showing at least one P-value ≤ 0.05 after Bonferroni correction, in MA/M, SA/M or S/M comparisons were selected. Among the 24 576 gene-specific tags, corresponding to 22 089 genes plus 516 chloroplastic and mitochondrial probes, 5304 probes (24% of represented genes) were significantly differentially expressed in at least one of the 3 comparisons with only 44 belonging to the mitochondrial genome and 41 to the chloroplastic genome.
The normalized log2(ratio) generated by the statistical analysis was used to describe the extent of transcriptional change between the MA/M, SA/M and S/M comparisons. In order to focus the study on highly repressed or induced transcripts, genes whose log2(ratio) was greater than 1.585 or lower than -1.585 (corresponding to 3-fold change) in at least one comparison were considered as highly-responsive. These criteria gave a total of 810 highly-responsive genes available in Additional file 6. Among these selected genes, 191 and 164 loci were found to be, respectively, upregulated and downregulated by atrazine treatment. Sucrose alone was responsible for increase of expression of 147 genes and decrease of expression of 113 genes. Under tolerance conditions, atrazine and sucrose generated induction of 321 genes and repression of 94 genes. Contrary to other treatments, sucrose-treated atrazine-tolerant plantlets therefore showed strong increase in upregulated genes correlated with a decrease in downregulated genes, thus suggesting induction of specific protective mechanisms.
The expression profiles of 8 genes, belonging to various functional categories and showing various expression patterns in the microarray, were analysed by qRT-PCR under the same conditions as those of the microarray experiment (Additional file 7). In all cases, due to the normalization steps, the fold changes obtained with the qRT-PCR were higher than those on the microarray, but gave comparable expression profiles relatively to the different treatments.
Identification of protection-related functional categories
The majority of sucrose-induced transcripts (Figure 2A) were involved in protein synthesis/protein fate, nitrogen and sulphur metabolism, DNA and RNA processing, cell fate, development and biogenesis, and C-compound and carbohydrate metabolism. Such effects of exogenous carbohydrates have been described in previous studies [10, 16, 23].
Figure 2B shows direct or indirect modifications caused by atrazine alone. Induction of genes involved in stress response, detoxification and degradation of foreign compounds, disease virulence and defence was observed. Similar patterns of gene responses have been described in response to oxidative stress in Arabidopsis [5, 24, 25]. Genes involved in protein synthesis, protein fate were largely downregulated and represented about 30% of repressed genes, whereas expression of genes related to protein degradation/cell aging was slightly increased. It must be noted that atrazine induction of genes involved in oxidative stress response (2 induced genes) was significantly low in comparison with the corresponding sucrose (10 induced genes) and sucrose plus atrazine (23 induced genes) inductions. The irreversible cellular injury caused by atrazine may thus be related to modifications of oxidative stress responses and of protein dynamics besides direct molecular damage of oxidative stress leading to cell death which has been widely described [25, 26].
The concurrent presence of atrazine and sucrose was associated with large induction of gene expression particularly related to cellular communication and signal transduction mechanism, detoxification and degradation of foreign compounds, oxidative stress responses, and protein degradation/cell aging (Figure 2C). Genes linked to detoxification and degradation of foreign compounds, oxidative stress response and cellular communication and signal transduction mechanism were thus largely induced in comparison with the atrazine alone or sucrose alone treatments, thus suggesting strong synergic effects of sucrose and atrazine on protection pathways. Induction of DNA/RNA processing, nucleotide metabolism (15 induced genes) and transcription (20 induced genes) categories by sucrose plus atrazine was significantly higher than that by sucrose alone, and also contrasted with the significant repression of these categories in the atrazine alone treatment.
In order to distinguish induction or repression resulting from the effect of atrazine and sucrose combination, we used the comparison of mannitol-atrazine treatment against sucrose-atrazine treatment (MA/SA) for detecting expression variations resulting only from the effect of atrazine, and for expression variations resulting only from the effect of sucrose, we used the comparison of sucrose treatment and sucrose plus atrazine treatment (S/SA). Transcripts presenting a P-value > 0.05 (not differentially expressed) in these comparisons were subtracted from the list of 810 highly-responsive genes, and then all the genes whose expression was not modified by addition of atrazine in the presence of sucrose (not differentially expressed in the S/SA comparison) and which were constant between the stress condition and the tolerance condition (not differentially expressed in the MA/SA comparison) were also removed. Among the 410 resulting genes, thus largely controlled by the combination of atrazine and sucrose, only 16 genes were strongly downregulated (more than 3-fold) against 136 genes strongly upregulated (more than 3-fold). These data, which corroborated the Venn diagram distribution (Figure 1), reinforced the idea of induction of specific stress tolerance mechanisms against the harmful effects of the atrazine treatment.
Characterization of atrazine xenobiotic and oxidative effects: evidence for deleterious effects on gene regulation
Induction by atrazine of genes involved in xenobiotic and oxidative stress response
4-hydroxyphenylpyruvate dioxygenase (PDS1)
MATE efflux family protein
Hydroxyacylglutathione hydrolase, putative/glyoxalase II putative
ABC transporter (TAP1)
Glyoxalase I family protein
Atrazine also upregulated PDS1 transcripts (At1g06570, group IV), encoding a 4-hydroxyphenylpyruvate dioxygenase involved in biosynthesis of plastoquinones, tocopherols and carotenoids , which are essential elements of photosynthetic electron transport chain and of antioxidative systems. The antioxidant properties of tocopherols arise from their ability to scavenge lipid peroxy radicals before they react with lipid substrates. Carotenoids play a key role in protection of PSII against photoinhibition, since they are able to quench 1O2 responsible for photooxidative damage .
Repression by atrazine of genes involved in xenobiotic and oxidative stress response
L-ascorbate peroxidase 1, cytosolic (APX1)
Glutathione S-transferase, putative (AtGSTU20)
Cytochrome P450 family protein (CYP710A2)
PDIL, thioredoxin family protein
Glutathione S-transferase, putative (AtGSTF11)
MrsB5, methionine sulfoxide reductase domain-containing protein/SeIR domain-containing protein
Allene oxide synthase (AOS)
The cytosolic methionine sulfoxide reductase B5 MrsB5 (At4g04830, group I) was also downregulated by addition of atrazine. ROS-mediated oxidation of methionine into methionine sulfoxide (MetO) is a major component of oxidative damage to proteins. Methionine sulfoxide reductase (Msr) systems reduce MetO to protect plant cells from cytotoxic effects and thereby prevent excessive accumulation of oxidized proteins and premature death during defence mechanisms . Vieira Dos Santos et al.  showed that MsrB protein amount increased after photooxidative stress, thus suggesting a role in the protection of cells against oxidative damage. Vignols et al.  demonstrated that thioredoxins (TRX) directly interact with Msr in vivo and could then act as electron donor to Msr proteins, thus suggesting the existence of a linked antioxidative mechanism. However, our results showed that transcripts of At2g47470 (group III) encoding a protein disulfide isomerase-like (PDIL) protein, a member of a multigene family within the TRX superfamily, were repressed by atrazine as observed for MsrB5. TRX are involved in the regulation of cellular redox balance by reducing disulfide bridges, and in a large panel of mechanisms like defence, development and antioxidative responses . Meyer et al.  suggested a crosstalk between TRX and glutaredoxins, thus leading to a potential link between TRX, glutathione and glutathione reductase. One of the proposed TRX targets in Arabidopsis is the APX1 protein , whose transcripts were repressed in the presence of atrazine. In our analysis, all of these transcripts involved in xenobiotic and oxidative stress defence belonged principally to group III (Figure 3) and were downregulated by atrazine, thus probably preventing their protective role.
All of these results strongly suggested a lack of an efficient anti-oxidative response in the presence of atrazine. Moreover, atrazine treatment was associated with strong repression of genes involved in nucleic acid and protein dynamics (Additional file 9). Indeed, among the 810 highly-responsive genes selected for data analysis, 81 belonged to the protein dynamics category (Protein Synthesis/Modification or Degradation) and 60% of these genes were found to be downregulated by the herbicide. Among the 36 genes involved in nucleic acids dynamics, 35% of transcripts were repressed by atrazine, whereas the sucrose/atrazine treatment induced about 40% of them.
Selected atrazine-regulated genes that may be involved in atrazine injury
3-methylcrotonyl-CoA carboxylase 1 (MCCA)
Tubulin alpha-2/alpha-4 chain (TUA4)
Senescence-associated family protein
Expansin, putative (EXP15)
Serine carboxypeptidase S10 family protein
Cysteine proteinase, putative
ATP-dependent Clp protease proteolytic subunit, putative
FKBP15-2 immunophilin/FKBP-type peptidyl-prolyl cis-trans isomerase-related
Beta-expansin, putative (EXPB3)
NADH-cytochrome b5 reductase, putative
Glutamine synthetase (GS2)
Moreover, Table 3 shows that, in presence of atrazine, an ATP-dependent Clp protease (At4g17040, group I) was induced. Clp proteases in chloroplasts degrade misfolded or unassembled proteins in an ATP-dependent manner, in relation to the activity of molecular chaperones, in order to target specific polypeptide substrates and avoid inadvertent degradation of others . Accumulation of transcripts of Clp protease is upregulated during several stresses . Moreover, genes involved in amino acid catabolism, such as At1g03090 (group IV) encoding the 3-Methylcrotonyl-coenzyme A carboxylase (MCCase), At3g45300 (group IV) encoding a isovaleryl-CoA-dehydrogenase (IVD), and genes involved in nitrogen salvaging such as At5g35630 (group I) encoding glutamine synthetase GS2, were upregulated during atrazine stress. Their implication during protein degradation has been previously described [39–41]. This increase may reflect a situation of carbohydrate starvation [41, 42]. However, other typical markers of carbohydrate starvation and autolysis regulation , such as the At3g48920 MYB transcription factor (TF), catalase 3 (At1g20620) and APG8 autophagy genes (At3g06420, At4g16520), did not respond to atrazine treatment, thus confirming that atrazine effects could not be primarily ascribed to carbohydrate starvation.
Specific effects of combined sucrose plus atrazine treatment on tolerance-related gene regulation
Previous studies have already described the transcriptomics of sugar treatment in Arabidopsis thaliana [16, 23, 42, 43]. The sucrose-alone control treatment was therefore compared in detail with previous studies in order to detect any anomaly or specificity of the sucrose-treated plantlets used in the present study. Our conditions of sucrose treatment resulted in modification of typical markers of carbohydrate responses in accordance with previous studies (Additional file 10) [10, 23]. However, the observed gene expression modifications due to sucrose alone could not explain enhanced tolerance to atrazine, thus emphasising the importance of comparing sucrose-alone and sucrose plus atrazine treatments.
Genes potentially involved in sucrose-induced atrazine tolerance
Glutamyl-tRNA reductase 2/GluTR (HEMA2)
Glutathione S-transferase, putative (AtGSTU11)
AAA-type ATPase family protein
Cytochrome P450 family protein (CYP710A1)
Alternative oxidase 1a, mitochondrial (AOX1A)
Zinc finger (AN1-like) family protein (PMZ)
AAA-type ATPase family protein
SIB1, sigma factor binding protein
Protein kinase family protein
Protein phosphatase 2C, putative/PP2C, putative
Pyruvate decarboxylase, putative (PDC)
In2-1 protein, putative
Band 7 family protein
Among group V, cluster N (Figure 3) showed further evidence for specific effects of sucrose and atrazine interactions (Table 4). This cluster contained genes encoding detoxifying enzymes like glutathione S-transferase AtGSTU11 (At1g69930, group V). Lacomme and Roby  demonstrated that expression of AtGSTU11 is induced in response to salicylic acid and methyl jasmonate and in response to avirulent pathogens causing a hypersensitive reaction. At5g02780 (group V) encodes an In2-1 protein that is induced in response to iron treatment . However, analysis of At5g02780 sequence shows the presence of two conserved domains corresponding, respectively, to the N- and C-terminal domains of glutathione S-transferase, thus suggesting a role of this gene in detoxification mechanisms. A PDC gene (At4g33070, group V) encoding a pyruvate decarboxylase was induced by sucrose and to a much more higher level by the presence of sucrose plus atrazine. This At4g33070 locus is highly-expressed during anoxia , and exogenous sucrose, which enhances anoxia tolerance, correlatively increases At4g33070 transcript accumulation. Since induction of PDC genes has been described as a response to situations of abiotic stress leading to mitochondrial impairment , the present induction of the At4g33070 PDC gene may contribute to promote a back-up fermentative pathway that compensates mitochondrial impairment. Indeed, we have shown above that atrazine injury was associated with downregulation of a mitochondrial NADH-cytochrome-b5 reductase (At5g20080, group III), while a return to the basal level was observed under tolerance condition (Table 3). The mitochondrial AOX1A (At3g22370, group V) is known to use excess reductant capacity of the cytochrome c oxidase pathway, thus preventing ROS formation from an over-reduced ubiquinone pool . AOX1A is induced by sucrose alone and much more by atrazine plus sucrose, which may thus increase potential antioxidative properties of this detoxifying enzyme through the glyoxylate pathway.
The sucrose plus atrazine treatment induced more than 22-fold the expression of a cytochrome P450 CYP710A1 (At2g34500, group V), responsible for a C22 desaturation reaction which produces stimasterol , which may thus contribute to maintain proper sterol composition of membranes and associated cell functions.
A number of genes involved in abiotic stress response were upregulated by the sucrose-atrazine combination (Figure 2C). Two genes encoding AAA Type ATPases (At3g50930, group V, At2g18193, group V) presented an important induction. This large protein family is involved, via chaperone-like activity, in numerous cellular activities including membrane fusion, proteolysis, DNA replication and repair, protein folding, and cytoskeletal regulation . They were identified as highly upregulated after genotoxin application , and may thus contribute to defence/stress response or cell cycle control. Sucrose-atrazine treatment also activated a zinc finger TF (PMZ, At3g28210, group V) that is induced during senescence, pathogen infection and in the ozone-sensitive vtc1 mutant [53, 54]. The embryo-abundant protein (At2g41380, group V) induced by sucrose-atrazine treatment has been described as induced by UV-B irradiation and ozone fumigation . The At5g54100 (group V) and At5g09570 (group V) genes, of unknown function, have been shown to be induced by cesium treatment in roots .
Differential expression of specific transcription factors during sucrose-induced atrazine tolerance
Transcription factors potentially involved in sucrose-induced atrazine tolerance
Zinc finger (C3HC4-type RING finger) family protein
AP2 domain-containing transcription factor family protein
bZIP transcription factor family protein (AtbZIP60)
AP2 domain-containing transcription factor, putative
WRKY family transcription factor
WRKY family transcription factor
AP2 domain-containing transcription factor, putative (DRE2B)
Zinc finger (B-box type) family protein
Zinc finger (AN1-like) family protein (PMZ)
AP2 domain-containing transcription factor, putative
SIB1, sigma factor binding protein
AP2 domain-containing transcription factor, putative
WRKY family transcription factor
Zinc finger (C2H2 type) family protein (Zat12)
The potential involvement of these TFs was further investigated by the research of cis-acting regulatory elements in promoters of genes presenting high induction by the sucrose-atrazine combination (Group V, Figure 3). Identification of cis-acting regulatory elements was carried out using the AtcisDB database . The most repeated and frequent motifs found were cis-acting regulatory elements corresponding to the WRKY, bZIP, MYB, and LFY TFs families (Additional file 11). Thus, the selected genes from group V (Figure 3), highly-responsive to sucrose-atrazine combination and potentially important for tolerance mechanisms, presented promoters with identified cis-acting regulatory elements corresponding to two of the TF families of Table 5, the WRKY and bZIP families.
In contrast, the MYB and LFY cis-acting regulatory elements did not correspond to TFs described in Table 5, thus suggesting that these MYB and LFY cis-elements may be regulated by TFs showing lower level of induction or expressed at earlier or later steps of the response. However, it was surprising that the identification of AP2 and Zinc finger TFs in Table 5 did not correspond to any cis-element in the promoters of group V genes. This may be due to the partial information contained in AtcisDB database, where the binding sites of the seven AP2 and Zinc finger genes of Table 5 are not described, thus suggesting that these specific binding sites are significantly different from consensus cis-elements.
Time-course of induction of transcription factors during sucrose-dependent atrazine protection
Using Expression Angler with the AtGenExpress Stress Set , we selected, from Table 5, TFs which presented an expression pattern that was highly correlated (Pearson correlation coefficient between 0.78 and 0.96) with that of genes potentially related to sucrose-induced atrazine protection, such as AOX1A (At3g22370, group V) or AAA Type ATPases (At3g50930, group V). This analysis highlighted the potential importance of four genes encoding different families of TFs: Zinc finger protein Zat12 (At5g59820, group V), bZIP family protein AtbZIP60 (At1g42990, group V), Sigma factor binding protein SIB1 (At3g56710, group V) and AP2 domain-containing TF (At3g61630, group V).
Gene regulation was shown to be affected on a large scale by atrazine treatment, prior to the development of atrazine injury, in accordance with previous studies of exposure to other toxicants such as 2,4-D  or explosives . Moreover, a number of these gene expression effects (Figure 2), such as repression of ROS defence mechanisms or repression of protein translation, were found to be potential components of atrazine sensitivity. This is, to our knowledge, the first report that atrazine can cause such large-scale primary and deleterious effects at the gene expression level.
Interestingly, atrazine alone and the combined presence of sucrose and atrazine resulted in derepression of the sucrose-repressed AtbZIP1 transcription factor. The differences between the sucrose-atrazine and the individual sucrose transcriptomes were therefore strongly reflected at the level of TF gene expression. It thus seemed that development of tolerance response depended on sets of original TFs integrating signals from sucrose and from atrazine. The ability of atrazine to generate ROS  and the existence of ROS-signalling pathways  could suggest that atrazine-related signalling may involve ROS signalling. Moreover, our results (Table 4) strongly suggest interactions with salicylate, ABA and jasmonate signalling pathways. Since abiotic stress situations seem to involve signalling interactions between sugar and ethylene , it is therefore clear that the analysis of signalling pathways involved in sucrose-atrazine effects should use a panel of mutants affected not only in sugar signalling, but also in ethylene, salicylate, ABA and jasmonate signalling.
Since the initial ROS generated by atrazine is 1O2 , it may have been expected to observe similar gene expression modifications between atrazine treatment and the effects of the flu mutation, which results in higher 1O2 production upon dark/light shifts . However, important differences were identified. For instance, the allene oxide synthase gene (At5g42650, group III) (Table 2), which is upregulated in the flu mutant, was strongly downregulated (7-fold repression) in the atrazine treatment. These differences could be ascribed to differences of ROS production and dynamics in terms of the chemical nature of generated ROS, in terms of time-course of accumulation, or in terms of signal intensity, between the atrazine treatment and the flu mutation. However, since the overall effect of the atrazine treatment seemed to result in partial induction of some ROS defence genes and in unexpected repression of other important ROS defence genes [5, 31], such as the cytosolic APX1 (At1g07890, group III) and allene oxide synthase (At5g42650, group III), it was tempting to speculate that a xenobiotic signalling-pathway may exist. Even though xenobiotic-related signalling pathways are strongly suspected to exist in plants [65, 66, 73], none of them has yet been identified, in contrast with the detailed identification of xenobiotic receptors in animal cells. It would thus be interesting to investigate whether atrazine and other xenobiotics also develop their toxicity through signalling effects leading to the silencing or overcoming of ROS stress defences. Thus, the cytosolic APX1 (At1g07890, group III), which is a central component of the ROS gene network according to Davletova et al. , was the most significantly atrazine-repressed gene among the six genes of the APX family.
In contrast, under the situation of protection combining sucrose and atrazine, the most expressed gene was At2g21640 (group V), which corresponds to an expressed protein of unknown function, that has been found to be upregulated in most experiments of oxidative stress . Moreover, TFs, which are strongly induced by atrazine plus sucrose, such as At3g50260 (group V, AP2 domain-containing factor), At2g30250 (group V, WRKY family), and At5g13080 (group V, WRKY family), have been described as common ROS-upregulated TFs . These three TFs were also significantly, but to a lesser extent, upregulated by sucrose alone. In other words, sucrose appeared to be useful in re-establishing the expression of TFs that may be important for ROS defence. Sucrose lifting of atrazine-mediated TF repression was apparently not sufficient to establish tolerance to atrazine, since other TFs were significantly enhanced by the combination of sucrose and atrazine. Such is particularly the case for the At5g59820 (group V) and At1g42990 (group V) genes encoding respectively the zinc finger protein Zat12 and AtbZIP60 (Table 5). Both these TFs are likely to be important for atrazine tolerance, since Zat12 and AtbZIP60 have been respectively involved in the response to oxidative stress  and in the regulation of endoplasmic stress response . Moreover, our results show that these genes were sequentially expressed in the course of the sucrose plus atrazine treatment (Figure 4), with Zat12 induction preceding that of AtbZIP60.
The set of TFs that is induced by the sucrose-atrazine treatment results in typical differential expression of multigene families that are involved in oxidative and xenobiotic stress responses, such as the GST multigene family. Besides the effects on ROS defence, sucrose treatment also allows plants to accumulate very high levels of atrazine in root and shoot tissues . Whereas complete mineralization of atrazine does not seem to occur in sucrose-treated plants, radiolabelling experiments strongly suggest that conjugation processes are likely to occur . According to the different mechanisms described in higher plants, these processes could involve conjugation to glutathione, glucose or macromolecular cell wall components . Among the different pathways that may be involved in conjugation, the strongest differential effects of sucrose-atrazine treatment affected the P450 and GST multigene families, thus emphasising the importance of further work on the possibility of sucrose-induced atrazine-glutathione conjugation in Arabidopsis. These mechanisms, which depend on gene induction, would facilitate atrazine accumulation, while hampering binding of free atrazine to PSII, thus resulting in the observed maintenance of photosynthetic efficiency .
The comparison of the atrazine-induced stress situation and of the sucrose-atrazine protection situation through the transcriptomic approach therefore sheds a new light on xenobiotic-signalling pathways, on xenobiotic tolerance pathways, and in identifying novel xenobiotic tolerance pathways, that would have otherwise, in the presence of lethal concentrations of atrazine, remained cryptic.
The study of early TFs and target genes involved in sucrose-induced tolerance will therefore be useful to investigate whether sugar-induced tolerance towards other xenobiotics involves the same general mechanism. Preliminary results indicate that sucrose can induce tolerance towards other xenobiotics.
Plant material and growth conditions
Seeds of Arabidopsis thaliana (ecotype Colombia, Col0) were surfaced-sterilized in bayrochlore/ethanol (1/1, v/v), rinsed in absolute ethanol and dried overnight. Germination and growth were carried out under axenic conditions. After seeds were sowed, they were placed at 4°C for 48 h in order to break dormancy and homogenize germination, and transferred at 22°C under a 16 h light period regime at 4500 lux until plantlets reached the 1.02 development stage . Growth medium consisted of 0.8% agar in Murashige and Skoog basal salt mix (Sigma, St Louis, MO, USA) adjusted to pH 5.7. After cultivation, plantlets were transferred to fresh medium containing sucrose (80 mM) or mannitol (80 mM) in the absence or presence of 10 μM atrazine. Sucrose or mannitol were directly added during preparation of Murashige and Skoog agar media prior to sterilisation. Atrazine was sterilized by microfiltration through 0.2 μm cellulose acetate filters (Polylabo, Strasbourg, France) and then axenically added to melted Murashige and Skoog agar medium at a concentration of 10 μM.
Measurement of seedling growth and development
Primary root length of plantlets was measured on vertical plates. Pigments were extracted by pounding aerial parts of plantlets in 80% acetone, and absorbance of the resulting extracts was measured at 663 nm, 646 nm and 470 nm. Chlorophyll and total carotenoid (xanthophylls and carotenes) levels in these extracts were determined from the equations given by Lichtenthaler and Wellburn .
Measurement of photosynthesis parameters
Chlorophyll fluorescence and maximum PSII efficiency (Fv/Fm) were measured with a PAM-210 chlorophyll fluorometer system (Heinz Walz, Effeltrich, Germany). After dark adaptation during 30 min, minimum fluorescence (F0) was determined under weak red light. Maximum fluorescence of dark-adapted leaf (Fm) was measured under a subsequent saturating pulse of red light, and variable fluorescence (Fv = Fm - F0) was determined .
RNA isolation and microarray analysis
For the transcriptome studies, microarray analysis was performed with the CATMA array , which is especially dedicated to Arabidopsis thaliana and contains 24576 gene-specific tags corresponding to 22 089 genes plus 516 chloroplastic and mitochondrial probes . The GST amplicons were purified on Multiscreen plates (Millipore, Bedford, MA, USA) and resuspended in TE-DMSO at 100 ng μl-1. The purified probes were transferred to 1536-well plates with a Genesis workstation (TECAN, Männedorf, Switzerland) and spotted on UltraGAPS slides (Corning, New York, NY, USA) using a Microgrid II (Genomic Solution, Huntingdon, UK).
The transcriptome analysis compared RNA of plantlets transferred on treatment medium during 24 h. Treatment media were: 80 mM mannitol (M, osmotic control and reference), 80 mM mannitol and 10 μM atrazine (MA, herbicide treatment), 80 mM sucrose (S, control medium) and 80 mM sucrose and 10 μM atrazine (SA, tolerance medium). The four conditions (M, S, MA, SA) were compared pairwise, so that the complete analysis consisted of 6 comparisons (Additional file 5).
Each treatment was repeated three times in independent experiments. In each experiment, 60 plantlets corresponding to a given treatment were harvested, frozen in liquid nitrogen and extracted for RNA. Total RNA was extracted using TRI Reagent® (Sigma, St. Louis, MO, USA) following the manufacturer's protocol. Quantification, high quality and integrity of each RNA sample were verified by spectrophotometry and with the Agilent Bioanalyser (Waldbroon, Germany). For each treatment, RNAs from three independent biological experiments were pooled. cRNAs were produced from 2 μg of total RNA from each pool with the 'MessageAmp™ aRNA' kit (Ambion, Austin, TX, USA). Then, 5 μg of cRNAs was reverse transcribed in the presence of 200 U of SuperScript™ II (Invitrogen, Carlsbad, CA, USA), cy3-dUTP and cy5-dUTP (NEN, Boston, MA, USA). Samples were combined, purified and concentrated with YM30 Microcon columns (Millipore). For each of the six comparisons, two technical replicates with fluorochrome reversal were performed for each pool of RNA (i.e one dye swap per comparison). Each pool of three RNA samples corresponding to a given treatment was therefore hybridised six times. Slides were pre-hybridised for 1 h and hybridised overnight at 42°C in 25% formamide, as described by Lurin et al.. The arrays were scanned on a GenePix 4000 A scanner (Axon Instruments, Foster City, CA, USA) and images were analysed by GenePix Pro 3.0 (Axon Instruments).
Statistical analysis of microarray data
Statistical analysis, which was carried out by the statistics group of the Unité de Recherche en Génomique Végétale (URGV, Evry), followed the same design as described in several previous publications, such as Lurin et al.. The statistical analysis was based on one dye-swap per comparison. For each array, the raw data comprised the logarithm of median feature pixel intensity at wavelengths 635 nm (red) and 532 nm (green). No background was subtracted. In the following description, log ratio refers to the differential expression between the different treatments. It is either log2(red/green) or log2(green/red) according to the experimental design. An array-by-array normalisation was performed to remove systematic biases. First, we excluded spots that were considered to show badly formed features. Then, we performed a global intensity-dependent normalisation using the loess procedure  to correct the dye bias. Finally, on each block, the log-ratio median was subtracted from each value of the log-ratio of the block to correct any print-tip effect on each metablock. In order to determine differentially expressed genes, we performed a paired t-test on the log-ratios, assuming that the variance of the log-ratios is the same for all genes. Spots displaying extremes of specific variance (too small or too large) were excluded. The raw P-values were adjusted by the Bonferroni method, which controls the Family Wise Error Rate (FWER) . We considered as being differentially expressed the genes with a Bonferroni P-value ≤ 0.05 as described in Lurin et al. . The data were deposited in ArrayExpress  (E-MEXP-411) according to the MIAME standards proposed by the Microarray Gene Expression Data (MGED) Society .
Microarray data validation and qRT-PCR experiment
qRT-PCR experiments were carried out with cDNA synthesised (Iscript™ cDNA Synthesis kit, Bio-Rad, Hercules, CA, USA) from the cRNA used in the microarray analysis and from total RNA isolated from independent experiments, where plantlets were transferred as indicated on mannitol (80 mM) (M), mannitol (80 mM) plus atrazine (10 μM) (MA) and sucrose (80 mM) plus atrazine (10 μM) (SA). Resulting cDNAs were used to determine expression profiles according to the different treatments. Quantitative PCR was performed using iQ™ SYBR Green Supermix (Bio-Rad, Hercules, CA, USA). Conditions were as follows: 95°C 3 min, and 40 (95°C 15 sec, 60°C 45 sec) cycles. All reactions were performed in triplicate. Specific primers for each gene selected for analysis were designed using Beacon Designer 5.0 software (Additional file 12). The results of the analysis were treated with Gene Expression version 1.1 software. For real-time RT-PCR validation, the log2(relative expression to the control sample M) of the qRT-PCR was compared with the log2(intensity ratio) of the array analysis. The real-time RT-PCR time-course experiment takes as references appropriate controls (Mannitol-treated plants harvested at the same time point).
Analysis of microarray data
Functional categories of differentially expressed genes were adapted from the categories defined by the Munich Information Center for Protein Sequences (MIPS) . For analysis of gene-expression data across the different experiments, hierarchical clustering was performed with the Genesis software  using Euclidian distance for the similarity distance and the average linkage clustering for the linkage rule. The AtcisDB database  was used for analysis of cis-acting regulatory elements in gene promoters.
reactive oxygen species
quantitative real-time reverse transcription-polymerase chain reaction
This work was supported in part by the programme Environnement-Vie-Société (CNRS, France) and by Rennes Métropole (France) local council.
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