Transformation of kiwifruit using the <Emphasis Type="Italic">ipt</Emphasis> gene alters tree architecture

To manipulate the architecture of woody plants by controlling endogenous cytokinin levels, the isopentenyl transferase gene (ipt) from Agrobacterim tumefaciens was introduced to kiwifruit using stable transformation. Consequently, eight transgenic lines were obtained. Transgenic shoots harboring the ipt gene were recalcitrant to rooting under tissue-culture conditions; thus, their in vitro-cultivated shoots were directly grafted onto potted wild-type kiwifruit seedlings to evaluate their morphological features, and three lines (tmr2-4, tmr2-G, tmr3-C) were successfully grafted. The grafted transgenic plants had dwarfing and branching phenotypes, both of which are typical features of cytokinin overproduction. In addition, the number of buds increased and internode length was shorter in the grafted transgenic plants. The content of a precursor, trans-zeatin riboside, and an active cytokinin, trans-zeatin, increased in one transgenic line, in which the level of ipt gene expression was high, indicating that morphological changes were related to expression levels of the ipt gene and cytokinin content. Possibilities for potential utilization of the ipt gene in manipulating tree shape are discussed.