Reducing nitrogen leaching and nitrous oxide emissions with the goal of more sustainability in ag... more Reducing nitrogen leaching and nitrous oxide emissions with the goal of more sustainability in agriculture implies better identification and characterization of the different patterns in nitrogen use efficiency by crops. However, a change in the ability of varieties to use nitrogen resources could also change the access to nutrient resources for a foliar pathogen such as rice blast and lead to an increase in the susceptibility of these varieties. This study focuses on the pre- and post-floral biomass accumulation and nitrogen uptake and utilization of ten temperate japonica rice genotypes grown in controlled conditions, and the relationship of these traits with molecular markers and susceptibility to rice blast disease. After flowering, the ten varieties displayed diversity in nitrogen uptake and remobilization. Surprisingly, post-floral nitrogen uptake was correlated with higher susceptibility to rice blast, particularly in plants fertilized with nitrogen. This increase in suscepti...
Nitrogen fertilization can affect the susceptibility of Brassica napus to the telluric pathogen P... more Nitrogen fertilization can affect the susceptibility of Brassica napus to the telluric pathogen Plasmodiophora brassicae. Our previous works highlighted that the influence of nitrogen can strongly vary regarding plant cultivar/pathogen strain combinations, but the underlying mechanisms are unknown. The present work aims to explore how nitrogen supply can affect the molecular physiology of P. brassicae through its life epidemiological cycle. A time-course transcriptome experiment was conducted to study the interaction, under two conditions of nitrogen supply, between isolate eH and two B. napus genotypes (Yudal and HD-018), harboring (or not harboring) low nitrogen-conditional resistance toward this isolate (respectively). P. brassicae transcriptional patterns were modulated by nitrogen supply, these modulations being dependent on both host-plant genotype and kinetic time. Functional analysis allowed the identification of P. brassicae genes expressed during the secondary phase of inf...
Etude de l’implication du metabolisme primaire dans la reponse a la hernie des cruciferes chez le... more Etude de l’implication du metabolisme primaire dans la reponse a la hernie des cruciferes chez le colza par une approche de metabotypage.
Clubroot, caused byPlasmodiophora brassicaeWoronin, is one of the most important diseases of oils... more Clubroot, caused byPlasmodiophora brassicaeWoronin, is one of the most important diseases of oilseed rape (Brassica napusL.). The rapid erosion of monogenic resistance in clubroot-resistant (CR) varieties underscores the need to diversify resistance sources controlling disease severity and traits related to pathogen fitness, such as resting spore production. The genetic control of disease index (DI) and resting spores per plant (RSP) was evaluated in a doubled haploid (DH) population consisting of 114 winter oilseed rape lines, obtained from the cross ‘Aviso’ × ‘Montego,’ inoculated withP. brassicaeisolate “eH.” Linkage analysis allowed the identification of three quantitative trait loci (QTLs) controlling DI (PbBn_di_A02, PbBn_di_A04, and PbBn_di_C03). A significant decrease in DI was observed when combining effects of the three resistance alleles at these QTLs. Only one QTL, PbBn_rsp_C03, was found to control RSP, reducing resting spore production by 40%. PbBn_rsp_C03 partially ov...
Clubroot is an important root disease of canola/oilseed rape (Brassica napus L.) caused by Plasmo... more Clubroot is an important root disease of canola/oilseed rape (Brassica napus L.) caused by Plasmodiophora brassicae. To cope with the emergence of new virulent pathotypes of P. brassicae that threaten canola production in western Canada, there is a need to identify novel sources of resistance for breeding programmes. In the present study, a series of nine B. napus genotypes, from different geographic origins, were screened for resistance to Canadian single-spore and field isolates of P. brassicae. The effect of nitrogen fertilization on the resistance level of these genotypes was also evaluated. Under full nitrogen fertilization, several of the tested host genotypes harboured isolate-specific partial resistance to P. brassicae infection, with some of these interactions resulting in disease indices <50. Accessions showing a partial resistance response to the novel pathotype 5X of P. brassicae also were identified. The influence of nitrogen on the level of resistance varied according to the specific plant genotype/pathogen isolate combination. Under low nitrogen fertilization, the genotype 'Darmor-bzh' showed a high level of partial resistance to isolates belonging to pathotypes 2 and 3, and 'Express' and 'Grouse' showed resistance to pathotype 2. The results confirm that nitrogen fertilization is an important factor to consider in combination with the use of resistant varieties for the management of clubroot.
Clubroot caused by the protistPlasmodiophora brassicaeis a major disease affecting cultivatedBras... more Clubroot caused by the protistPlasmodiophora brassicaeis a major disease affecting cultivatedBrassicaceae. Here, we uncover the existence of a natural epigenetic variation that is associated with partial resistance to clubroot inArabidopsis,by using QTL fine mapping followed by extensive DNA sequence and methylation analyses. We show that at QTLPb-At5.2, DNA methylation variation is extensive across accessions and strictly correlates with expression variation of the two neighboring genes At5g47260 and At5g47280, which encode NLR-immune receptors. Moreover, these natural variants are stably inherited and are not consistently associated with any nucleotide variation. These findings suggest a direct role for epigenetic variation in quantitative resistance of plants to pathogen attacks.
Highlight Modifications in glutamine synthetase OsGS1-2 expression and fungal pathogenicity unde... more Highlight Modifications in glutamine synthetase OsGS1-2 expression and fungal pathogenicity underlie nitrogen-induced susceptibility to rice blast. Understanding why nitrogen fertilization increase the impact of many plant diseases is of major importance. The interaction between Magnaporthe oryzae and rice was used as a model for analyzing the molecular mechanisms underlying Nitrogen-Induced Susceptibility (NIS). We show that our experimental system in which nitrogen supply strongly affects rice blast susceptibility only slightly affects plant growth. In order to get insights into the mechanisms of NIS, we conducted a dual RNA-seq experiment on rice infected tissues under two nitrogen fertilization regimes. On the one hand, we show that enhanced susceptibility was visible despite an over-induction of defense gene expression by infection under high nitrogen regime. On the other hand, the fungus expressed to high levels effectors and pathogenicity-related genes in plants under high n...
Nitrogen (N) availability can impact plant resistance to pathogens by the regulation of plant imm... more Nitrogen (N) availability can impact plant resistance to pathogens by the regulation of plant immunity. To better understand the links between N nutrition and plant defence, we analysed the impact of N availability on Medicago truncatula resistance to the root pathogen Aphanomyces euteiches. This oomycete is considered to be the most limiting factor for legume production. Ten plant genotypes were tested in vitro for their resistance to A. euteiches in either complete or nitrate-deficient medium. N deficiency led to enhanced or reduced susceptibility depending on the plant genotype. Focusing on four genotypes displaying contrasting responses, we determined the impact of N deficiency on plant growth and shoot N concentration, and performed expression analyses on N-and defence-related genes, as well as the quantification of soluble phenolics and different amino acids in roots. Our analyses suggest that N modulation of plant resistance is not linked to plant response to N deprivation or to mechanisms previously identified to be involved in plant resistance. Furthermore, our studies highlight a role of glutamine in mediating the susceptibility to A. euteiches in M. truncatula.
The induction of alcohol fermentation in roots is a plant adaptive response to flooding stress an... more The induction of alcohol fermentation in roots is a plant adaptive response to flooding stress and oxygen deprivation. Available transcriptomic data suggest that fermentation-related genes are also frequently induced in roots infected with gall forming pathogens, but the biological significance of this induction is unclear. In this study, we addressed the role of hypoxia responses in Arabidopsis roots during infection by the clubroot agent Plasmodiophora brassicae. The hypoxia-related gene markers PYRUVATE DECARBOXYLASE 1 (PDC1), PYRUVATE DECARBOXYLASE 2 (PDC2) and ALCOHOL DEHYDROGENASE 1 (ADH1) were induced during secondary infection by two isolates of P. brassicae, eH and e2. PDC2 was highly induced as soon as 7 days post inoculation (dpi), i.e., before the development of gall symptoms, and GUS staining revealed that ADH1 induction was localised in infected cortical cells of root galls at 21 dpi. Clubroot symptoms were significantly milder in the pdc1 and pdc2 mutants compared wit...
Clubroot, a disease of Brassicaceae species, is caused by the soilborne pathogen Plasmodiophora b... more Clubroot, a disease of Brassicaceae species, is caused by the soilborne pathogen Plasmodiophora brassicae. High soil water content was previously described to favour the motility of zoospores and their penetration into root cells. In this study, the effect of irrigation regimes on clubroot development during the post-invasive secondary phase of infection was investigated. Three irrigation regimes (low, standard, high) were tested on two Arabidopsis accessions, Col-0 (susceptible) and Bur-0, a partially resistant line. In Col-0, clubroot symptoms and resting spore content were higher under the 'low irrigation' regime than the other two regimes, thus enhancing the phenotypic contrast between the two Arabidopsis accessions. Clubroot severity under high and low irrigation regimes was evaluated in near-isogenic lines derived from a Col-0 9 Bur-0 cross, to assess the effect of soil moisture on the expression of each of four quantitative trait loci (QTL) controlling partial resistance. The presence of the Bur-0 allele at the QTL PbAt5.2 resulted in reduced severity only under low irrigation, whereas the Bur-0 allele at QTL PbAt5.1 was associated with partial resistance only under high irrigation. QTL PbAt4 reduced the number of resting spores in infected roots, but was not associated with reduced clubroot symptoms. The results indicated that soil moisture could have consequences for the secondary phase of clubroot development, depending on plant genotype. Future genetic studies may benefit from using combinations of watering conditions during the secondary stage of infection, thus opening up the possibility of identifying genetic factors expressed under specific environmental conditions.
Plants produce cytokinin (CK) hormones for controlling key developmental processes like source/si... more Plants produce cytokinin (CK) hormones for controlling key developmental processes like source/sink distribution, cell division or programmed cell-death. Some plant pathogens have been shown to produce CKs but the function of this mimicry production by non-tumor inducing pathogens, has yet to be established. Here we identify a gene required for CK biosynthesis, CKS1, in the rice blast fungus Magnaporthe oryzae. The fungal-secreted CKs are likely perceived by the plant during infection since the transcriptional regulation of rice CK-responsive genes is altered in plants infected by the mutants in which CKS1 gene was deleted. Although cks1 mutants showed normal in vitro growth and development, they were severely affected for in planta growth and virulence. Moreover, we showed that the cks1 mutant triggered enhanced induction of plant defenses as manifested by an elevated oxidative burst and expression of defense-related markers. In addition, the contents of sugars and key amino acids for fungal growth were altered in and around the infection site by the cks1 mutant in a different manner than by the control strain. These results suggest that fungal-derived CKs are key effectors required for dampening host defenses and affecting sugar and amino acid distribution in and around the infection site.
The role of salicylic acid (SA) and jasmonic acid (JA) signaling in resistance to root pathogens ... more The role of salicylic acid (SA) and jasmonic acid (JA) signaling in resistance to root pathogens has been poorly documented. We assessed the contribution of SA and JA to basal and partial resistance of Arabidopsis to the biotrophic clubroot agent Plasmodiophora brassicae. SA and JA levels as well as the expression of the SA-responsive genes PR2 and PR5 and the JA-responsive genes ARGAH2 and THI2.1 were monitored in infected roots of the accessions Col-0 (susceptible) and Bur-0 (partially resistant). SA signaling was activated in Bur-0 but not in Col-0. The JA pathway was weakly activated in Bur-0 but was strongly induced in Col-0. The contribution of both pathways to clubroot resistance was then assessed using exogenous phytohormone application and mutants affected in SA or JA signaling. Exogenous SA treatment decreased clubroot symptoms in the two Arabidopsis accessions, whereas JA treatment reduced clubroot symptoms only in Col-0. The cpr5-2 mutant, in which SA responses are constitutively induced, was more resistant to clubroot than the corresponding wild type, and the JA signalingdeficient mutant jar1 was more susceptible. Finally, we showed that the JA-mediated induction of NATA1 drove N(d)-acetylornithine biosynthesis in infected Col-0 roots. The 35S::NATA1 and nata1 lines displayed reduced or enhanced clubroot symptoms, respectively, thus suggesting that in Col-0 this pathway was involved in the JA-mediated basal clubroot resistance. Overall, our data support the idea that, depending on the Arabidopsis accession, both SA and JA signaling can play a role in partial inhibition of clubroot development in compatible interactions with P. brassicae.
Camalexin has been reported to play defensive functions against several pathogens in Arabidopsis.... more Camalexin has been reported to play defensive functions against several pathogens in Arabidopsis. In this study, we investigated the possible role of camalexin accumulation in two Arabidopsis genotypes with different levels of basal resistance to the compatible eH strain of the clubroot agent Plasmodiophora brassicae. Camalexin biosynthesis was induced in infected roots of both Col-0 (susceptible) and Bur-0 (partially resistant) accessions during the secondary phase of infection. However, the level of accumulation was four-to-seven times higher in Bur-0 than Col-0. This was associated with the enhanced transcription of a set of camalexin biosynthetic P450 genes in Bur-0: CYP71A13, CYP71A12, and CYP79B2. This induction correlated with slower P. brassicae growth in Bur-0 compared to Col-0, thus suggesting a relationship between the levels of camalexin biosynthesis and the different levels of resistance. Clubroot-triggered biosynthesis of camalexin may also participate in basal defense in Col-0, as gall symptoms and pathogen development were enhanced in the pad3 mutant (Col-0 genetic background), which is defective in camalexin biosynthesis. Clubroot and camalexin responses were then studied in Heterogeneous Inbred Families (HIF) lines derived from a cross between Bur-0 and Col-0. The Bur/Col allelic substitution in the region of the previously identified clubroot resistance QTL PbAt5.2 (Chromosome 5) was associated with both the enhanced clubroot-triggered induction of camalexin biosynthesis and the reduced P. brassicae development. Altogether, our results suggest that high levels of clubroot-triggered camalexin biosynthesis play a role in the quantitative control of partial resistance of Arabidopsis to clubroot.
The nicotianamine synthase (NAS) enzymes catalyze the formation of nicotianamine (NA), a nonprote... more The nicotianamine synthase (NAS) enzymes catalyze the formation of nicotianamine (NA), a nonproteinogenic amino acid involved in iron homeostasis. We undertook the functional characterization of AtNAS4, the fourth member of the Arabidopsis thaliana NAS gene family. A mutant carrying a T-DNA insertion in AtNAS4 (atnas4), as well as lines overexpressing AtNAS4 both in the atnas4 and the wild-type genetic backgrounds, were used to decipher the role of AtNAS4 in NA synthesis, iron homeostasis and the plant response to iron deficiency or cadmium supply. We showed that AtNAS4 is an important source for NA. Whereas atnas4 had normal growth in iron-sufficient medium, it displayed a reduced accumulation of ferritins and exhibited a hypersensitivity to iron deficiency. This phenotype was rescued in the complemented lines. Under iron deficiency, atnas4 displayed a lower expression of the iron uptake-related genes IRT1 and FRO2 as well as a reduced ferric reductase activity. Atnas4 plants also showed an enhanced sensitivity to cadmium while the transgenic plants overexpressing AtNAS4 were more tolerant. Collectively, our data, together with recent studies, support the hypothesis that AtNAS4 displays an important role in iron distribution and is required for proper response to iron deficiency and to cadmium supply.
Reducing nitrogen leaching and nitrous oxide emissions with the goal of more sustainability in ag... more Reducing nitrogen leaching and nitrous oxide emissions with the goal of more sustainability in agriculture implies better identification and characterization of the different patterns in nitrogen use efficiency by crops. However, a change in the ability of varieties to use nitrogen resources could also change the access to nutrient resources for a foliar pathogen such as rice blast and lead to an increase in the susceptibility of these varieties. This study focuses on the pre- and post-floral biomass accumulation and nitrogen uptake and utilization of ten temperate japonica rice genotypes grown in controlled conditions, and the relationship of these traits with molecular markers and susceptibility to rice blast disease. After flowering, the ten varieties displayed diversity in nitrogen uptake and remobilization. Surprisingly, post-floral nitrogen uptake was correlated with higher susceptibility to rice blast, particularly in plants fertilized with nitrogen. This increase in suscepti...
Nitrogen fertilization can affect the susceptibility of Brassica napus to the telluric pathogen P... more Nitrogen fertilization can affect the susceptibility of Brassica napus to the telluric pathogen Plasmodiophora brassicae. Our previous works highlighted that the influence of nitrogen can strongly vary regarding plant cultivar/pathogen strain combinations, but the underlying mechanisms are unknown. The present work aims to explore how nitrogen supply can affect the molecular physiology of P. brassicae through its life epidemiological cycle. A time-course transcriptome experiment was conducted to study the interaction, under two conditions of nitrogen supply, between isolate eH and two B. napus genotypes (Yudal and HD-018), harboring (or not harboring) low nitrogen-conditional resistance toward this isolate (respectively). P. brassicae transcriptional patterns were modulated by nitrogen supply, these modulations being dependent on both host-plant genotype and kinetic time. Functional analysis allowed the identification of P. brassicae genes expressed during the secondary phase of inf...
Etude de l’implication du metabolisme primaire dans la reponse a la hernie des cruciferes chez le... more Etude de l’implication du metabolisme primaire dans la reponse a la hernie des cruciferes chez le colza par une approche de metabotypage.
Clubroot, caused byPlasmodiophora brassicaeWoronin, is one of the most important diseases of oils... more Clubroot, caused byPlasmodiophora brassicaeWoronin, is one of the most important diseases of oilseed rape (Brassica napusL.). The rapid erosion of monogenic resistance in clubroot-resistant (CR) varieties underscores the need to diversify resistance sources controlling disease severity and traits related to pathogen fitness, such as resting spore production. The genetic control of disease index (DI) and resting spores per plant (RSP) was evaluated in a doubled haploid (DH) population consisting of 114 winter oilseed rape lines, obtained from the cross ‘Aviso’ × ‘Montego,’ inoculated withP. brassicaeisolate “eH.” Linkage analysis allowed the identification of three quantitative trait loci (QTLs) controlling DI (PbBn_di_A02, PbBn_di_A04, and PbBn_di_C03). A significant decrease in DI was observed when combining effects of the three resistance alleles at these QTLs. Only one QTL, PbBn_rsp_C03, was found to control RSP, reducing resting spore production by 40%. PbBn_rsp_C03 partially ov...
Clubroot is an important root disease of canola/oilseed rape (Brassica napus L.) caused by Plasmo... more Clubroot is an important root disease of canola/oilseed rape (Brassica napus L.) caused by Plasmodiophora brassicae. To cope with the emergence of new virulent pathotypes of P. brassicae that threaten canola production in western Canada, there is a need to identify novel sources of resistance for breeding programmes. In the present study, a series of nine B. napus genotypes, from different geographic origins, were screened for resistance to Canadian single-spore and field isolates of P. brassicae. The effect of nitrogen fertilization on the resistance level of these genotypes was also evaluated. Under full nitrogen fertilization, several of the tested host genotypes harboured isolate-specific partial resistance to P. brassicae infection, with some of these interactions resulting in disease indices <50. Accessions showing a partial resistance response to the novel pathotype 5X of P. brassicae also were identified. The influence of nitrogen on the level of resistance varied according to the specific plant genotype/pathogen isolate combination. Under low nitrogen fertilization, the genotype 'Darmor-bzh' showed a high level of partial resistance to isolates belonging to pathotypes 2 and 3, and 'Express' and 'Grouse' showed resistance to pathotype 2. The results confirm that nitrogen fertilization is an important factor to consider in combination with the use of resistant varieties for the management of clubroot.
Clubroot caused by the protistPlasmodiophora brassicaeis a major disease affecting cultivatedBras... more Clubroot caused by the protistPlasmodiophora brassicaeis a major disease affecting cultivatedBrassicaceae. Here, we uncover the existence of a natural epigenetic variation that is associated with partial resistance to clubroot inArabidopsis,by using QTL fine mapping followed by extensive DNA sequence and methylation analyses. We show that at QTLPb-At5.2, DNA methylation variation is extensive across accessions and strictly correlates with expression variation of the two neighboring genes At5g47260 and At5g47280, which encode NLR-immune receptors. Moreover, these natural variants are stably inherited and are not consistently associated with any nucleotide variation. These findings suggest a direct role for epigenetic variation in quantitative resistance of plants to pathogen attacks.
Highlight Modifications in glutamine synthetase OsGS1-2 expression and fungal pathogenicity unde... more Highlight Modifications in glutamine synthetase OsGS1-2 expression and fungal pathogenicity underlie nitrogen-induced susceptibility to rice blast. Understanding why nitrogen fertilization increase the impact of many plant diseases is of major importance. The interaction between Magnaporthe oryzae and rice was used as a model for analyzing the molecular mechanisms underlying Nitrogen-Induced Susceptibility (NIS). We show that our experimental system in which nitrogen supply strongly affects rice blast susceptibility only slightly affects plant growth. In order to get insights into the mechanisms of NIS, we conducted a dual RNA-seq experiment on rice infected tissues under two nitrogen fertilization regimes. On the one hand, we show that enhanced susceptibility was visible despite an over-induction of defense gene expression by infection under high nitrogen regime. On the other hand, the fungus expressed to high levels effectors and pathogenicity-related genes in plants under high n...
Nitrogen (N) availability can impact plant resistance to pathogens by the regulation of plant imm... more Nitrogen (N) availability can impact plant resistance to pathogens by the regulation of plant immunity. To better understand the links between N nutrition and plant defence, we analysed the impact of N availability on Medicago truncatula resistance to the root pathogen Aphanomyces euteiches. This oomycete is considered to be the most limiting factor for legume production. Ten plant genotypes were tested in vitro for their resistance to A. euteiches in either complete or nitrate-deficient medium. N deficiency led to enhanced or reduced susceptibility depending on the plant genotype. Focusing on four genotypes displaying contrasting responses, we determined the impact of N deficiency on plant growth and shoot N concentration, and performed expression analyses on N-and defence-related genes, as well as the quantification of soluble phenolics and different amino acids in roots. Our analyses suggest that N modulation of plant resistance is not linked to plant response to N deprivation or to mechanisms previously identified to be involved in plant resistance. Furthermore, our studies highlight a role of glutamine in mediating the susceptibility to A. euteiches in M. truncatula.
The induction of alcohol fermentation in roots is a plant adaptive response to flooding stress an... more The induction of alcohol fermentation in roots is a plant adaptive response to flooding stress and oxygen deprivation. Available transcriptomic data suggest that fermentation-related genes are also frequently induced in roots infected with gall forming pathogens, but the biological significance of this induction is unclear. In this study, we addressed the role of hypoxia responses in Arabidopsis roots during infection by the clubroot agent Plasmodiophora brassicae. The hypoxia-related gene markers PYRUVATE DECARBOXYLASE 1 (PDC1), PYRUVATE DECARBOXYLASE 2 (PDC2) and ALCOHOL DEHYDROGENASE 1 (ADH1) were induced during secondary infection by two isolates of P. brassicae, eH and e2. PDC2 was highly induced as soon as 7 days post inoculation (dpi), i.e., before the development of gall symptoms, and GUS staining revealed that ADH1 induction was localised in infected cortical cells of root galls at 21 dpi. Clubroot symptoms were significantly milder in the pdc1 and pdc2 mutants compared wit...
Clubroot, a disease of Brassicaceae species, is caused by the soilborne pathogen Plasmodiophora b... more Clubroot, a disease of Brassicaceae species, is caused by the soilborne pathogen Plasmodiophora brassicae. High soil water content was previously described to favour the motility of zoospores and their penetration into root cells. In this study, the effect of irrigation regimes on clubroot development during the post-invasive secondary phase of infection was investigated. Three irrigation regimes (low, standard, high) were tested on two Arabidopsis accessions, Col-0 (susceptible) and Bur-0, a partially resistant line. In Col-0, clubroot symptoms and resting spore content were higher under the 'low irrigation' regime than the other two regimes, thus enhancing the phenotypic contrast between the two Arabidopsis accessions. Clubroot severity under high and low irrigation regimes was evaluated in near-isogenic lines derived from a Col-0 9 Bur-0 cross, to assess the effect of soil moisture on the expression of each of four quantitative trait loci (QTL) controlling partial resistance. The presence of the Bur-0 allele at the QTL PbAt5.2 resulted in reduced severity only under low irrigation, whereas the Bur-0 allele at QTL PbAt5.1 was associated with partial resistance only under high irrigation. QTL PbAt4 reduced the number of resting spores in infected roots, but was not associated with reduced clubroot symptoms. The results indicated that soil moisture could have consequences for the secondary phase of clubroot development, depending on plant genotype. Future genetic studies may benefit from using combinations of watering conditions during the secondary stage of infection, thus opening up the possibility of identifying genetic factors expressed under specific environmental conditions.
Plants produce cytokinin (CK) hormones for controlling key developmental processes like source/si... more Plants produce cytokinin (CK) hormones for controlling key developmental processes like source/sink distribution, cell division or programmed cell-death. Some plant pathogens have been shown to produce CKs but the function of this mimicry production by non-tumor inducing pathogens, has yet to be established. Here we identify a gene required for CK biosynthesis, CKS1, in the rice blast fungus Magnaporthe oryzae. The fungal-secreted CKs are likely perceived by the plant during infection since the transcriptional regulation of rice CK-responsive genes is altered in plants infected by the mutants in which CKS1 gene was deleted. Although cks1 mutants showed normal in vitro growth and development, they were severely affected for in planta growth and virulence. Moreover, we showed that the cks1 mutant triggered enhanced induction of plant defenses as manifested by an elevated oxidative burst and expression of defense-related markers. In addition, the contents of sugars and key amino acids for fungal growth were altered in and around the infection site by the cks1 mutant in a different manner than by the control strain. These results suggest that fungal-derived CKs are key effectors required for dampening host defenses and affecting sugar and amino acid distribution in and around the infection site.
The role of salicylic acid (SA) and jasmonic acid (JA) signaling in resistance to root pathogens ... more The role of salicylic acid (SA) and jasmonic acid (JA) signaling in resistance to root pathogens has been poorly documented. We assessed the contribution of SA and JA to basal and partial resistance of Arabidopsis to the biotrophic clubroot agent Plasmodiophora brassicae. SA and JA levels as well as the expression of the SA-responsive genes PR2 and PR5 and the JA-responsive genes ARGAH2 and THI2.1 were monitored in infected roots of the accessions Col-0 (susceptible) and Bur-0 (partially resistant). SA signaling was activated in Bur-0 but not in Col-0. The JA pathway was weakly activated in Bur-0 but was strongly induced in Col-0. The contribution of both pathways to clubroot resistance was then assessed using exogenous phytohormone application and mutants affected in SA or JA signaling. Exogenous SA treatment decreased clubroot symptoms in the two Arabidopsis accessions, whereas JA treatment reduced clubroot symptoms only in Col-0. The cpr5-2 mutant, in which SA responses are constitutively induced, was more resistant to clubroot than the corresponding wild type, and the JA signalingdeficient mutant jar1 was more susceptible. Finally, we showed that the JA-mediated induction of NATA1 drove N(d)-acetylornithine biosynthesis in infected Col-0 roots. The 35S::NATA1 and nata1 lines displayed reduced or enhanced clubroot symptoms, respectively, thus suggesting that in Col-0 this pathway was involved in the JA-mediated basal clubroot resistance. Overall, our data support the idea that, depending on the Arabidopsis accession, both SA and JA signaling can play a role in partial inhibition of clubroot development in compatible interactions with P. brassicae.
Camalexin has been reported to play defensive functions against several pathogens in Arabidopsis.... more Camalexin has been reported to play defensive functions against several pathogens in Arabidopsis. In this study, we investigated the possible role of camalexin accumulation in two Arabidopsis genotypes with different levels of basal resistance to the compatible eH strain of the clubroot agent Plasmodiophora brassicae. Camalexin biosynthesis was induced in infected roots of both Col-0 (susceptible) and Bur-0 (partially resistant) accessions during the secondary phase of infection. However, the level of accumulation was four-to-seven times higher in Bur-0 than Col-0. This was associated with the enhanced transcription of a set of camalexin biosynthetic P450 genes in Bur-0: CYP71A13, CYP71A12, and CYP79B2. This induction correlated with slower P. brassicae growth in Bur-0 compared to Col-0, thus suggesting a relationship between the levels of camalexin biosynthesis and the different levels of resistance. Clubroot-triggered biosynthesis of camalexin may also participate in basal defense in Col-0, as gall symptoms and pathogen development were enhanced in the pad3 mutant (Col-0 genetic background), which is defective in camalexin biosynthesis. Clubroot and camalexin responses were then studied in Heterogeneous Inbred Families (HIF) lines derived from a cross between Bur-0 and Col-0. The Bur/Col allelic substitution in the region of the previously identified clubroot resistance QTL PbAt5.2 (Chromosome 5) was associated with both the enhanced clubroot-triggered induction of camalexin biosynthesis and the reduced P. brassicae development. Altogether, our results suggest that high levels of clubroot-triggered camalexin biosynthesis play a role in the quantitative control of partial resistance of Arabidopsis to clubroot.
The nicotianamine synthase (NAS) enzymes catalyze the formation of nicotianamine (NA), a nonprote... more The nicotianamine synthase (NAS) enzymes catalyze the formation of nicotianamine (NA), a nonproteinogenic amino acid involved in iron homeostasis. We undertook the functional characterization of AtNAS4, the fourth member of the Arabidopsis thaliana NAS gene family. A mutant carrying a T-DNA insertion in AtNAS4 (atnas4), as well as lines overexpressing AtNAS4 both in the atnas4 and the wild-type genetic backgrounds, were used to decipher the role of AtNAS4 in NA synthesis, iron homeostasis and the plant response to iron deficiency or cadmium supply. We showed that AtNAS4 is an important source for NA. Whereas atnas4 had normal growth in iron-sufficient medium, it displayed a reduced accumulation of ferritins and exhibited a hypersensitivity to iron deficiency. This phenotype was rescued in the complemented lines. Under iron deficiency, atnas4 displayed a lower expression of the iron uptake-related genes IRT1 and FRO2 as well as a reduced ferric reductase activity. Atnas4 plants also showed an enhanced sensitivity to cadmium while the transgenic plants overexpressing AtNAS4 were more tolerant. Collectively, our data, together with recent studies, support the hypothesis that AtNAS4 displays an important role in iron distribution and is required for proper response to iron deficiency and to cadmium supply.
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Papers by Antoine Gravot