_______ a crown gall bacterium, is called as 'natural genetic engineer' of plants.

A soil-inhabiting plant bacterium Agrobacterium tumefaciens is a pathogen of several dicot plants. It is able to transfer a piece of DNA known as 'T-DNA' into the plant cells. The T-DNA causes tumours, the tumours are called crown galls. Tumour formation is induced by Ti plasmid (Ti for tumour inducing). As gene transfer occurs without human effort, the bacterium is called natural genetic engineer of plants. Similarly retroviruses in animals including humans are able to change normal cells into cancerous cells. So, the correct answer is 'Agrobacterium tumefaciens'.