Abstract
Main conclusion
The use of a VIGS approach to silence the newly characterized apple tree SQS isoforms points out the biological function of phytosterols in plastid pigmentation and leaf development.
Triterpenoids are beneficial health compounds highly accumulated in apple; however, their metabolic regulation is poorly understood. Squalene synthase (SQS) is a key branch point enzyme involved in both phytosterol and triterpene biosynthesis. In this study, two SQS isoforms were identified in apple tree genome. Both isoforms are located at the endoplasmic reticulum surface and were demonstrated to be functional SQS enzymes using an in vitro activity assay. MdSQS1 and MdSQS2 display specificities in their expression profiles with respect to plant organs and environmental constraints. This indicates a possible preferential involvement of each isoform in phytosterol and/or triterpene metabolic pathways as further argued using RNAseq meta-transcriptomic analyses. Finally, a virus-induced gene silencing (VIGS) approach was used to silence MdSQS1 and MdSQS2. The concomitant down-regulation of both MdSQS isoforms strongly affected phytosterol synthesis without alteration in triterpene accumulation, since triterpene-specific oxidosqualene synthases were found to be up-regulated to compensate metabolic flux reduction. Phytosterol deficiencies in silenced plants clearly disturbed chloroplast pigmentation and led to abnormal development impacting leaf division rather than elongation or differentiation. In conclusion, beyond the characterization of two SQS isoforms in apple tree, this work brings clues for a specific involvement of each isoform in phytosterol and triterpene pathways and emphasizes the biological function of phytosterols in development and chloroplast integrity. Our report also opens the door to metabolism studies in Malus domestica using the apple latent spherical virus-based VIGS method.
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Abbreviations
- ALSV:
-
Apple latent spherical virus
- CAS:
-
Cycloartenol synthase
- SQS:
-
Squalene synthase
- FPP:
-
Farnesyl diphosphate
- OSC:
-
Oxidosqualene cyclase
- VIGS:
-
Virus-induced gene silencing
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Acknowledgements
The authors wish to thank Dr. Marc Fischer (INRA Colmar) for providing yeast erg9 strain and Dr. Marie-Noëlle Brisset (INRA Angers) for providing apple tree seeds. We gratefully acknowledge support from the Région Centre-Val de Loire (France) for financing SNG and CEP. DD was financed by a doctoral fellowship from the Région Centre-Val de Loire (France) and the Ministère de l’Enseignement Supérieur et de la Recherche (France).
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Navarro Gallón, S.M., Elejalde-Palmett, C., Daudu, D. et al. Virus-induced gene silencing of the two squalene synthase isoforms of apple tree (Malus × domestica L.) negatively impacts phytosterol biosynthesis, plastid pigmentation and leaf growth. Planta 246, 45–60 (2017). https://doi.org/10.1007/s00425-017-2681-0
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DOI: https://doi.org/10.1007/s00425-017-2681-0