Catalyzes the transfer of an aminobutyl unit from spermidine onto putrescine. The resulting polyamine homospermidine is a precursor in the biosynthesis of pyrrolizidine alkaloids. putrescine + spermidine = propane-1,3-diamine + sym-homospermidine Alkaloid biosynthesis; pyrrolizidine alkaloid biosynthesis. Homotetramer. Expressed in roots. The N-terminus is blocked. Belongs to the deoxyhypusine synthase family.