Induction of serotonin biosynthesis is uncoupled from the coordinated induction of tryptophan biosynthesis in pepper fruits (Capsicum annuum) upon pathogen infection

Planta. 2009 Nov;230(6):1197-206. doi: 10.1007/s00425-009-1015-2. Epub 2009 Sep 17.


It has been suggested that serotonin biosynthesis is regulated by tryptophan decarboxylase (TDC) in plants. To determine if TDC plays a rate-limiting role in serotonin biosynthesis, two TDC genes, PepTDC1 and PepTDC2, were cloned from pepper (Capsicum annuum L.) fruits infected with anthracnose fungus and their expression was then examined in various organs, including fruit that had been treated with the fungus or various chemicals. PepTDC1 expression was highly induced in pepper fruits after treatment with fungus and ethylene, while PepTDC2 was constitutively expressed at low levels in all pepper tissues. Additionally, predominant induction of PepTDC1 mRNA and TDC enzyme activity was detected in the unripe-green fruit, but not in the ripe-red fruit upon pathogen infection. Higher expression of TDC in unripe-green fruit was closely associated with increased levels of tryptamine, serotonin, and serotonin derivatives. However, unlike the enhanced serotonin synthesis, tryptophan levels responded unchanged when challenged with the pathogen in both the unripe-green fruit and the ripe-red fruit. Expression of two key tryptophan biosynthetic genes, anthranilate synthase (ASalpha) and tryptophan synthase (TSbeta), remained unchanged in response to treatment. Also, anthranilate synthase enzyme activity remained steady regardless of pathogen infection. Taken together, these results suggest that the synthesis of serotonin was regulated by the induction of TDC without a simultaneous increase in tryptophan levels in pepper fruits.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Aromatic-L-Amino-Acid Decarboxylases / genetics
  • Aromatic-L-Amino-Acid Decarboxylases / metabolism
  • Biosynthetic Pathways
  • Blotting, Northern
  • Blotting, Western
  • Capsicum / genetics
  • Capsicum / metabolism*
  • Capsicum / microbiology
  • Colletotrichum / physiology
  • Fruit / genetics
  • Fruit / metabolism*
  • Fruit / microbiology
  • Gene Expression Regulation, Enzymologic
  • Gene Expression Regulation, Plant
  • Host-Pathogen Interactions
  • Isoenzymes / genetics
  • Isoenzymes / metabolism
  • Molecular Sequence Data
  • Molecular Structure
  • Plant Diseases / genetics
  • Plant Diseases / microbiology
  • Plant Proteins / genetics
  • Plant Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Serotonin / biosynthesis*
  • Serotonin / chemistry
  • Tryptophan / biosynthesis*
  • Tryptophan / chemistry


  • Isoenzymes
  • Plant Proteins
  • Serotonin
  • Tryptophan
  • Aromatic-L-Amino-Acid Decarboxylases