Involved in serotonin biosynthesis. Catalyzes the decarboxylation of L-tryptophan to tryptamine, which is converted to serotonin by tryptamine 5-hydroxylase (PubMed:17763868). May play a minor role in serotonin biosynthetis during senescence. Accumulation of serotonin attenuates leaf senescence (PubMed:19439571). H(+) + L-tryptophan = CO2 + tryptamine Belongs to the group II decarboxylase family.