Minisymposium 11: Secondary Metabolites

Abs # 23001: Phosphoenolpyruvate levels in the plastid stroma control the pattern and the quantity of vacuolar phenylpropanoids

Presenter:	Voll, Lars M, lars.voll@smail.rrz.uni-koeln.de
Authors	Voll, Lars M (A) (B)  Häusler, Rainer E (B)   Löttgert, Tanja  (B)   Hecker, Rolf  (B)   Weissenböck, Gottfried  (B)   Waffenschmidt, Sabine  (B)   Fiene, Gabi  (B)   Flügge, Ulf-Ingo  (B)   Weber, Andreas  (A) (B)
Affiliations:	(A): Michigan State University (B): University of Cologne

Erythrose-4-phosphate and phosphoenolpyruvate (PEP) act as precursors for the biosynthesis of aromatic amino acids and secondary plant products, like e.g. phenylpropanoids, via the shikimate pathway. The first steps of the shikimate pathway up to chorismate are restricted to the stroma. In chloroplasts of C₃plants, PEP cannot be generated glycolytically from Calvin cycle intermediates and has to be imported from the cytosol by the phosphoenolpyruvate/ phosphate translocator (PPT). The Arabidopsis cue1 mutant is defective in the AtPPT1 and displays a reticulate leaf phenotype characterized by a dark green vasculature and a light green, chlorotic mesophyll. Phytochrome signalling and the development of mesophyll chloroplasts are impaired in the cue1 mutant. The photosynthetic performance of cue1 was not severely diminished compared to the controls, but carbohydrate accumulation in the mutants was substantially reduced. The reticulate leaf phenotype could be cured by constitutive overexpression of a heterologous PPT or a plastid-targeted C₄-type pyruvate, orthophosphate dikinase (PPDK). PPDK overexpression in the cue1 background bypasses the PPT in providing PEP for the shikmate pathway from stromal pyruvate. However, PPDK overexpression did not alleviate all of the restrictions in cue1 primary metabolism and even imposed some more metabolic constraints. Feeding of aromatic amino acids abrogated the reticulate cue1 leaf phenotype indicating a limitation of the shikimate pathway in cue1. Cue1 phenylalanine contents were reduced by 50% compared to the controls suggesting a substrate limitation of phenylalanine-ammonia lyase (PAL), the entry site of the phenylpropanoid pathway. The accumulation of many phenylpropanoids could be induced by various stresses in the wild-type, e.g. root anaerobiosis or UV-light. While the contents and the induction of most phenylpropanoids by UV-stress was dampened in the cue1 mutant compared to the wild-type, the induction of other UV-protective phenylpropanoid compounds remained unaffected or was even enhanced. In contrast, the induction of anthocyanins by anaerobiosis or UV-light was clearly suppressed in the cue1 mutant. PPT or PPDK overexpression in the cue1 background restored the wild-type response of the analysed phenylpropanoids upon stress or even accelerated their accumulation under UV-stress. Taken together, our results suggest that PEP levels in the stroma trigger the flux into the phenylpropanoid pathway.