Agrobacterium rhizogenes is a natural plant genetic engineer. It is a gram-negative soil bacterium that induces hairy root formation. Success has been obtained in exploring the molecular mechanisms of transferred DNA (T-DNA) transfer, interaction with host plant proteins, plant defense signaling and integration to plant genome for successful plant genetic transformation. T-DNA and corresponding expression of rol genes alter morphology and plant host secondary metabolism. During transformation, there is a differential loss of a few T-DNA genes. Loss of a few ORFs drastically affect the growth and morphological patterns of hairy roots, expression pattern of biosynthetic pathway genes and accumulation of specific secondary metabolites.