Abstract
The Ti-plasmid of Agrobacterium tumefaciens has IPT gene, Tmr, on the T-DNA region, which is integrated into the genome of host plants after infection. To understand the cytokinin biosynthesis pathway in Agrobacterium-infected cells, we studied biochemical properties and subcellular localization of Tmr in comparison to Arabidopsis IPTs (AtIPTs). Overexpression of Tmr in Arabidopsis caused predominant accumulation of trans-zeatin (tZ)-type cytokinins, whereas those of AtIPT1, 3, 4, 5 and 7 resulted in accumulation of isopentenyladenine-type ones. Recombinant Tmr was able to utilize both DMAPP and HMBDP as a substrate with similar Km values in vitro, whereas DMAPP was a much better substrate for AtIPTs. Unexpectedly, we have found that Tmr is localized in plastids in the host plant cells in spite of the absence of an apparent transit peptide. These results demonstrate that, after Agrobacterium infection, Tmr is targeted to plastids to produce tZ directly using HMBDP in the MEP pathway.
