Catalyzes the stereospecific condensation of tryptamine with secologanin to form strictosidine, the key intermediate of indole alkaloid biosynthesis. 3alpha(S)-strictosidine + H2O = secologanin + tryptamine Alkaloid biosynthesis; 3alpha(S)-strictosidine biosynthesis; 3alpha(S)-strictosidine from secologanin and tryptamine: step 1/1. Upon potassium (K) starvation. Belongs to the strictosidine synthase family.