Abstract
Agrobacterium tumefaciens infection induces tumor formation by integrating the T-DNA region of Ti-plasmid into the plant nuclear genome. Tumors are formed because the T-DNA encodes enzymes that synthesize auxin and cytokinin. We had demonstrated that a cytokinin biosynthesis enzyme isopentenyltransferase (IPT), Tmr, which is encoded by the T-DNA region, is targeted to and functions in plastids of infected plant. Tmr creates a metabolic bypass for direct synthesis of tZ-type cytokinin by using HMBDP. To reveal biological importance of the Tmr function for gall formation, we studied biochemical properties of Tmr with comparing to Tzs, which functions in the bacterial cell. We generated transgenic Arabidopsis overexpressing Tmr, Tzs, or transit peptide-fused Tzs (TP-Tzs) under the control of DEX-inducible promoter. DEX-treatment induced the accumulation of tZ-type cytokinin in Tmr-overexpressing Arabidopsis, whereas it induced that of both tZ and iP in Tzs and TP-Tzs-overexpressing plants. Tmr and Tzs show similar substrate specificity in vitro. Thus, it is strongly suggest that Tmr has a specialized function to predominantly use HMBDP for tZ biosynthesis in the host plant plastids.
