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, trans-zeatin-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.