Our recent work associated the phenotypic alterations caused by the Ri TL-DNA (root-inducing, left-hand, transferred DNA) from Agrobacterium rhizogenes with a general decrease in the accumulation of polyamines and their derivatives and showed that irreversible and specific inhibition of putrescine biosynthesis causes a phenotype similar to that attributed to the Ri TL-DNA. In this review we report recent data supporting a correlation between the degree of expression of the transformed phenotype due to the root-inducing, left-hand, transferred DNA and inhibition of polyamine accumulation, strongly suggesting that genes carried by the root-inducing, transferred DNA may act through interference with polyamine production via the ornithine pathway.