[Frontiers in Bioscience E2, 1196-1210 June 1, 2010]

Induced endogenous autotoxicity in Camptotheca

Shiyou Li, Ping Wang, Wei Yuan

National Center for Pharmaceutical Crops, Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University, Nacogdoches, TX 75962, USA

TABLE OF CONTENTS

1. Abstract
2. Introduction
3. Materials and methods
3.1. Decapitation pruning
3.2. Exogenous application of CPT
3.3. External application of auxin
3.4. Isolation of compounds
3.5. Cytotoxicity assays
3.6. DNA TOP1 assays
3.7. Statistical analysis
4. Results
4.1. Abnormal morphogenesis and elevated CPT levels following decapitation
4.2. Abnormal morphogenesis following exogenous application of CPT
4.3. Cytotoxicity and DNA TOP1 activity of isolated compounds
5. Discussion
5.1. Endogenous autotoxicity induced by elevated CPT levels
5.2. Auxin reduction as a triggering factor for induced in vivo biosynthesis of CPTs
5.2.1. Pruning reducing endogenous auxin levels increased CPT contents in plants
5.2.2. Exogenous application of auxin restored auxin-deficient dwarf mutations
5.2.3. Exogenous application of auxin decreased CPT levels in normal plants
5.2.4. Autotoxicity induced by auxin deficiency rather than auxin homeostasis
5.3. Defense may play limited roles in induced biosynthesis of CPTs
5.4. Detoxification and autotoxicity mechanisms in Camptotheca
6. Acknowledgements
7. References

1. ABSTRACT

Are plants always immune to their endogenous toxic secondary metabolites? Without disturbance, fast-growing Camptotheca plants can avoid poison by its endogenous camptothecin (DNA topoisomerase I inhibitor) at more than 10 times higher than the fatal concentration of exogenous application to the plant. Pruning has been long known to promote lateral growth; however, here we report that auxin-reducing pruning can induce endogenous autotoxicity in Camptotheca: dramatic deviations from normal morphogenesis, including serrated or lobed leaves, disturbed phyllotaxis, and fasciated stems. The abnormal morphogenesis appears correlatively with the elevated camptothecin contents following decapitation pruning. Plants resume their normal morphogenesis when camptothecin is reduced to natural levels after stress discontinuation. Exogenously applied auxin restores a dwarf mutant of high camptothecin yield to its parent type of lower yield, suggesting that auxin may be a triggering factor for the observed autotoxicity. Autotoxicity induced by endogenous compounds has not been reported before.