The Gram-negative bacterium Agrobacterium tumefaciens causes crown gall disease on a large number of plants. The disease is characterized by the formation of tumor-like structures on stems and other organs. Tumors develop by the integration of a bacterial DNA fragment, the T-DNA, into the plant genome and by the expression of encoded genes involved in biosynthesis of plant hormones. This unique property of interkingdom gene transfer has been used for plant genetic engineering, leading to the first genetically modified plants. Despite the enormous economic importance, the exact mechanisms of T-DNA transfer still await to be unraveled. Here, we describe the agrobacteria, including A. rhizogenes, the causal agent of hairy root disease. We summarize the general properties of the tumor-inducing (Ti) and root-inducing (Ri) megaplasmids that contain the T-DNA and virulence genes implicated in T-DNA transfer. The use of agrobacteria for plant biotechnology is discussed from a historical perspective together with current improvements and their use for CRISPR/Cas9 gene editing. Finally, we report how research on Agrobacterium has led to new insights relevant to other systems, including on the type IV secretion system. We anticipate that we are only at the start of understanding Agrobacterium-related plant disease and of the development of Agrobacterium-based tools for plant genetic engineering.
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