Over-expression of Tryptophan Decarboxylase Gene in Poplar and its Possible Role in Resistance Against Malacosoma disstria

Amines and their derivatives are known to influence insect behavior involved in feeding and reproduction. In order to examine the feasibility of improving the resistance of poplar to insect pests by the introduction of a plant-derived amine-generating transgene, explants from the hybrid poplar clone ‘INRA 717 1B4’ (P. tremula ×P. albo) were transformed with a Camptotheca acuminata tryptophan decarboxylase (TDC, EC 4.1.1.28) cDNA driven by the CaMV 35S promoter. The enzyme TDC catalyzes the decarboxylation of tryptophan to tryptamine, which, in addition to being a bioactive amine itself, is known to act as a precursor of various other indole derivatives. Putative transgenic lines were confirmed by PCR for the TDC1 gene sequence and by the expression analysis of the transgene mRNA and encoded protein. No visible phenotypic changes were associated with ectopic TDC1 expression. Chemical and radiotracer analyses of the transgenic plants revealed tryptamine accumulation as high as 4 mM in leaf tissue, and suggested that the tryptamine produced by ectopically expressed TDC was not further metabolized. Insect bioassays with the TDC transgenic plants showed that the tryptamine accumulation was consistently associated with adverse effects on feeding potential and physiology of Malacosoma disstria (forest tent caterpillar).