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Unbalanced activation of glutathione metabolic pathways suggests potential involvement in plant defense against the gall midge Mayetiola destructor in wheat.

Liu X, Zhang S, Whitworth RJ, Stuart JJ, Chen MS - Sci Rep (2015)

Bottom Line: However, the enzymatic activity and transcript abundance of glutathione reductases, which convert GSSG back to GSH, did not change.Our data suggest the possibility that GSSG is transported from cytosol to apoplast to serve as an oxidant for class III peroxidases to generate reactive oxygen species for plant defense against Hessian fly larvae.Our results provide a foundation for elucidating the molecular processes involved in glutathione-mediated plant resistance to Hessian fly and potentially other pests as well.

View Article: PubMed Central - PubMed

Affiliation: Department of Entomology, Kansas State University, 123 Waters Hall, Manhattan, KS 66506.

ABSTRACT
Glutathione, γ-glutamylcysteinylglycine, exists abundantly in nearly all organisms. Glutathione participates in various physiological processes involved in redox reactions by serving as an electron donor/acceptor. We found that the abundance of total glutathione increased up to 60% in resistant wheat plants within 72 hours following attack by the gall midge Mayetiola destructor, the Hessian fly. The increase in total glutathione abundance, however, is coupled with an unbalanced activation of glutathione metabolic pathways. The activity and transcript abundance of glutathione peroxidases, which convert reduced glutathione (GSH) to oxidized glutathione (GSSG), increased in infested resistant plants. However, the enzymatic activity and transcript abundance of glutathione reductases, which convert GSSG back to GSH, did not change. This unbalanced regulation of the glutathione oxidation/reduction cycle indicates the existence of an alternative pathway to regenerate GSH from GSSG to maintain a stable GSSG/GSH ratio. Our data suggest the possibility that GSSG is transported from cytosol to apoplast to serve as an oxidant for class III peroxidases to generate reactive oxygen species for plant defense against Hessian fly larvae. Our results provide a foundation for elucidating the molecular processes involved in glutathione-mediated plant resistance to Hessian fly and potentially other pests as well.

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Metabolic pathways of glutathione synthesis, recycling, detoxification, and degradation; and a model for glutathione to serve as an oxidant for class III peroxidases during generation of ROS such as hydrogen peroxides.Glu, Cys, γ-EC, Gly, and Cys-Gly represent glutamate, cysteine, γ-glutamylcysteine, glycine, and cysteinylglycine, respectively. γ-GCS, GS, GT, TPA, GST, GR, GPx, and III-Px represent γ-glutamylcysteine synthetases, glutathione synthetases, γ-glutamyltransferases, tripeptide aminopeptidases, glutathione S-transferases, glutathione reductases, glutathione peroxidases, and class III peroxidases, respectively. Blue arrows indicate direction of metabolite flow, whereas yellow arrows indicate transport of GSSG and GSH between cytosol and apoplast, where GSSG serves as an oxidant for class III peroxidases during ROS generation.
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f6: Metabolic pathways of glutathione synthesis, recycling, detoxification, and degradation; and a model for glutathione to serve as an oxidant for class III peroxidases during generation of ROS such as hydrogen peroxides.Glu, Cys, γ-EC, Gly, and Cys-Gly represent glutamate, cysteine, γ-glutamylcysteine, glycine, and cysteinylglycine, respectively. γ-GCS, GS, GT, TPA, GST, GR, GPx, and III-Px represent γ-glutamylcysteine synthetases, glutathione synthetases, γ-glutamyltransferases, tripeptide aminopeptidases, glutathione S-transferases, glutathione reductases, glutathione peroxidases, and class III peroxidases, respectively. Blue arrows indicate direction of metabolite flow, whereas yellow arrows indicate transport of GSSG and GSH between cytosol and apoplast, where GSSG serves as an oxidant for class III peroxidases during ROS generation.

Mentions: The pathways for glutathione synthesis, recycling, and metabolism are well established (Figure 6). Glutathione synthesis is a two-step process involving the enzymes γ-glutamylcysteine synthetase (γ-GCS) and glutathione synthetase (GS). In this study, we found that the two enzymes for glutathione synthesis were unevenly regulated in resistant plants after Hessian fly infestation. Specifically, the enzymatic activity and transcript abundance of glutathione synthetase, the enzyme for the second reaction, was upregulated rapidly and significantly in infested resistant plants. However, no significant changes were detected for the enzymatic activity and transcript abundance of γ-glutamylcysteine synthetase, the enzyme for the first reaction, in infested resistant plants under the same condition. γ-Glutamylcysteine synthetase was reported to be the rate-limiting step for the overall glutathione biosynthesis process129. This unbalanced regulation of the glutathione synthetic pathway makes it hard to explain the rapid and steady increase of glutathione abundance in infested resistant plants. The finding that there is essentially no change in activity of the rate-limiting enzyme would suggest that there will be no increase in glutathione synthesis. However, the up to 60% increase in total glutathione in infested resistant plants could not be simply explained by increased stability of glutathione, since Hessian fly did not downregulate genes encoding the putative glutathione degradation enzymes (Figure 5). Further research will have to be carried out to explain this dilemma. The unbalanced regulation of the glutathione synthesis coupled with the rapid and steady increase of glutathione abundance in the wheat tissue of the infested resistant plants suggested that the γ-glutamylcysteine synthetase might be either inhibited by a negative feedback from GSH, or it is not the rate-limiting enzyme in the glutathione synthetic pathway in wheat plants under our experimental conditions. In mammalian cells, γ-glutamylcysteine synthetase is effectively inhibited by a GSH feedback, whereas glutathione synthetase is not subject to the negative feedback930. It has also been reported that the γ-glutamylcysteine synthetase is not the limiting step in plants under heavy metal stress conditions, and overexpression of a bacterial glutathione synthetase gene alone in Indian mustard results in higher concentrations of glutathione in the transgenic plant31.


Unbalanced activation of glutathione metabolic pathways suggests potential involvement in plant defense against the gall midge Mayetiola destructor in wheat.

Liu X, Zhang S, Whitworth RJ, Stuart JJ, Chen MS - Sci Rep (2015)

Metabolic pathways of glutathione synthesis, recycling, detoxification, and degradation; and a model for glutathione to serve as an oxidant for class III peroxidases during generation of ROS such as hydrogen peroxides.Glu, Cys, γ-EC, Gly, and Cys-Gly represent glutamate, cysteine, γ-glutamylcysteine, glycine, and cysteinylglycine, respectively. γ-GCS, GS, GT, TPA, GST, GR, GPx, and III-Px represent γ-glutamylcysteine synthetases, glutathione synthetases, γ-glutamyltransferases, tripeptide aminopeptidases, glutathione S-transferases, glutathione reductases, glutathione peroxidases, and class III peroxidases, respectively. Blue arrows indicate direction of metabolite flow, whereas yellow arrows indicate transport of GSSG and GSH between cytosol and apoplast, where GSSG serves as an oxidant for class III peroxidases during ROS generation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4308708&req=5

f6: Metabolic pathways of glutathione synthesis, recycling, detoxification, and degradation; and a model for glutathione to serve as an oxidant for class III peroxidases during generation of ROS such as hydrogen peroxides.Glu, Cys, γ-EC, Gly, and Cys-Gly represent glutamate, cysteine, γ-glutamylcysteine, glycine, and cysteinylglycine, respectively. γ-GCS, GS, GT, TPA, GST, GR, GPx, and III-Px represent γ-glutamylcysteine synthetases, glutathione synthetases, γ-glutamyltransferases, tripeptide aminopeptidases, glutathione S-transferases, glutathione reductases, glutathione peroxidases, and class III peroxidases, respectively. Blue arrows indicate direction of metabolite flow, whereas yellow arrows indicate transport of GSSG and GSH between cytosol and apoplast, where GSSG serves as an oxidant for class III peroxidases during ROS generation.
Mentions: The pathways for glutathione synthesis, recycling, and metabolism are well established (Figure 6). Glutathione synthesis is a two-step process involving the enzymes γ-glutamylcysteine synthetase (γ-GCS) and glutathione synthetase (GS). In this study, we found that the two enzymes for glutathione synthesis were unevenly regulated in resistant plants after Hessian fly infestation. Specifically, the enzymatic activity and transcript abundance of glutathione synthetase, the enzyme for the second reaction, was upregulated rapidly and significantly in infested resistant plants. However, no significant changes were detected for the enzymatic activity and transcript abundance of γ-glutamylcysteine synthetase, the enzyme for the first reaction, in infested resistant plants under the same condition. γ-Glutamylcysteine synthetase was reported to be the rate-limiting step for the overall glutathione biosynthesis process129. This unbalanced regulation of the glutathione synthetic pathway makes it hard to explain the rapid and steady increase of glutathione abundance in infested resistant plants. The finding that there is essentially no change in activity of the rate-limiting enzyme would suggest that there will be no increase in glutathione synthesis. However, the up to 60% increase in total glutathione in infested resistant plants could not be simply explained by increased stability of glutathione, since Hessian fly did not downregulate genes encoding the putative glutathione degradation enzymes (Figure 5). Further research will have to be carried out to explain this dilemma. The unbalanced regulation of the glutathione synthesis coupled with the rapid and steady increase of glutathione abundance in the wheat tissue of the infested resistant plants suggested that the γ-glutamylcysteine synthetase might be either inhibited by a negative feedback from GSH, or it is not the rate-limiting enzyme in the glutathione synthetic pathway in wheat plants under our experimental conditions. In mammalian cells, γ-glutamylcysteine synthetase is effectively inhibited by a GSH feedback, whereas glutathione synthetase is not subject to the negative feedback930. It has also been reported that the γ-glutamylcysteine synthetase is not the limiting step in plants under heavy metal stress conditions, and overexpression of a bacterial glutathione synthetase gene alone in Indian mustard results in higher concentrations of glutathione in the transgenic plant31.

Bottom Line: However, the enzymatic activity and transcript abundance of glutathione reductases, which convert GSSG back to GSH, did not change.Our data suggest the possibility that GSSG is transported from cytosol to apoplast to serve as an oxidant for class III peroxidases to generate reactive oxygen species for plant defense against Hessian fly larvae.Our results provide a foundation for elucidating the molecular processes involved in glutathione-mediated plant resistance to Hessian fly and potentially other pests as well.

View Article: PubMed Central - PubMed

Affiliation: Department of Entomology, Kansas State University, 123 Waters Hall, Manhattan, KS 66506.

ABSTRACT
Glutathione, γ-glutamylcysteinylglycine, exists abundantly in nearly all organisms. Glutathione participates in various physiological processes involved in redox reactions by serving as an electron donor/acceptor. We found that the abundance of total glutathione increased up to 60% in resistant wheat plants within 72 hours following attack by the gall midge Mayetiola destructor, the Hessian fly. The increase in total glutathione abundance, however, is coupled with an unbalanced activation of glutathione metabolic pathways. The activity and transcript abundance of glutathione peroxidases, which convert reduced glutathione (GSH) to oxidized glutathione (GSSG), increased in infested resistant plants. However, the enzymatic activity and transcript abundance of glutathione reductases, which convert GSSG back to GSH, did not change. This unbalanced regulation of the glutathione oxidation/reduction cycle indicates the existence of an alternative pathway to regenerate GSH from GSSG to maintain a stable GSSG/GSH ratio. Our data suggest the possibility that GSSG is transported from cytosol to apoplast to serve as an oxidant for class III peroxidases to generate reactive oxygen species for plant defense against Hessian fly larvae. Our results provide a foundation for elucidating the molecular processes involved in glutathione-mediated plant resistance to Hessian fly and potentially other pests as well.

Show MeSH
Related in: MedlinePlus