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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2005-07, Vol.102 (28), p.9972-9977
• Main Authors: Sakakibara, H, Kasahara, H, Ueda, N, Kojima, M, Takei, K, Hishiyama, S, Asami, T, Okada, K, Kamiya, Y, Yamaya, T
• Format: Article
• Language: English
• Subjects: Adenine Nucleotides - metabolism; Aerial parts; Agrobacterium; Agrobacterium tumefaciens; Agrobacterium tumefaciens - enzymology; alkyl (aryl) transferases; Alkyl and Aryl Transferases - metabolism; Arabidopsis - microbiology; Arabidopsis thaliana; Bacteria; Bacterial Proteins - metabolism; Biological Sciences; Biosynthesis; Blotting, Western; Cell lines; crown galls; Cytokines; cytokinins; Cytokinins - biosynthesis; Enzymes; Flowers & plants; Genetic Vectors; Green Fluorescent Proteins; host-pathogen relationships; isopentenyltransferase; Oncogene Proteins - metabolism; Organophosphates - metabolism; Plant cells; Plant gall; Plant pathology; Plant Tumor-Inducing Plasmids - genetics; Plant Tumors - genetics; Plant Tumors - microbiology; Plastids; Seedlings; Terpenoids; transgenic plants; Tumors
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0500793102

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.