Agrobacterium tumefaciens increases cytokinin production in plastids by modifying the biosynthetic pathway in the host plant

Agrobacterium tumefaciens infects plants and induces the formation of tumors called "crown galls" by integrating the transferred-DNA (T-DNA) region of the Ti-plasmid into the plant nuclear genome. Tumors are formed because the T-DNA encodes enzymes that modify the synthesis of two plant growth hormones, auxin and cytokinin (CK). Here, we show that a CK biosynthesis enzyme, Tmr, which is encoded by the Agrobacterium T-DNA region, is targeted to and functions in plastids of infected plant cells, despite having no typical plastid-targeting sequence. Evidence is provided that Tmr is an adenosine phosphate-isopentenyltransferase (IPT) that creates a new CK biosynthesis bypass by using 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (HMBDP) as a substrate. Unlike in the conventional CK biosynthesis pathway in plants, transzeatin-type CKs are produced directly without the requirement for P450 monooxygenase-mediated hydroxylation. Consistent with the plastid localization of Tmr, HMBDP is an intermediate in the methylerythritol phosphate pathway, a plastid-localized biosynthesis route for universal isoprenoid precursors. These results demonstrate that A. tumefaciens modifies CK biosynthesis by sending a key enzyme into plastids of the host plant to promote tumorigenesis.