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Proc Natl Acad Sci U S A. 2005 Jul 12;102(28):9972-7. Epub 2005 Jul 5.

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

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Laboratories for Communication Mechanisms and Cellular Growth and Development, Plant Science Center, RIKEN, Suehiro 1-7-22, Tsurumi, Yokohama 230-0045, Japan.


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.

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