Metabolic engineering of hydroxycinnamic acid amide in nictoiana tabacum
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AbstractFeruloyltyramine and 4-coumaroyltyramine participate in the defense of plants against pathogens through their extracellular peroxidative polymerization, which is thought to reduce cell wall digestibility or otherwise inhibit fungal growth. HydroxycinnamoylCoA:tyramine N-(hydroxycinnamoyl)transferase (THT) and tyrosine decarboxylase (TYDC) are purported to play key roles in the stress-induced regulation of tyraminederived hydroxycinnamic acid amide (HCAAT) biosynthesis. Transgenic tobacco (Nicotiana tabacum cv Xanthi) was engineered to constitutively express tobacco THT. A T₁ plant over-expressing THT was crossbred with T₁ tobacco expressing opium poppy TYDC2 to produce a T₂ line with elevated THT and TYDC activities compared with wild type plants. The effects of an independent increase in TYDC or THT activity, or a dual increase in both TYDC and THT on the cellular pools of HCAAT pathway intermediates and the accumulation of soluble and cell wall-bound feruloyltyrarnine and 4-coumaroyltyramine were examined. In roots, THT and TYDC activities were not substantially altered in transgenic lines beyond their relatively high wild type levels; thus, no changes in HCAAT precursor or product levels were detected. In leaves, increased TYDC activity resulted in a larger cellular pool of tyramine, and lower levels of phenylalanine. In contrast, elevated THT activity reduced tyramine levels. HCAAT levels were low in healthy leaves, but were induced in response to wounding and accumulated around wound sites. Similarly, endogenous THT and TYDC activities were wound-induced. The initial rate of wound-induced HCAAT accumulation was highest in transgenic plants with elevated THT and TYDC activities. These results show that both THT and TYDC exert partial and synergistic control over the flux of intermediates involved in HCAAT biosynthesis under some conditions.
Bibliography: p. 86-101