Phenotype Modification and Enhanced Tolerance to Insect Pests by Regulated Expression of a Cytokinin Biosynthesis Gene

Phytohormones have long been recognized as modulators of growth and differentiation in plants. Early studies with tobacco (Nicotiana tabacum L.) callus culture (Skoog and Miller, 1957) indicated that treatments with auxins and cytokinins, two major classes of phytohormones, could be used to manipulate morphogenesis. The involvement of cytokinins in a variety of plant developmental processes, such as control of senescence, promotion of cell proliferation, chloroplast development, flowering, and plant defense responses, have since been reported (for review, see Mok and Mok, 1994; Smigocki et al., 1993). Most studies have used exogenous applications to gain a better understanding of the events that lead to the induction of physiological responses by cytokinin. The interpretation of such studies is hampered by problems associated with cytokinin uptake, transport, and metabolism. Therefore, several laboratories have exploited the use of a gene (ipt) from Agrobacterium tumefaciens that induces synthesis of endogenous cytokinins in transformed plant cells (Ainley et al., 1993; Li et al., 1992; Martineau et al., 1994; Medford et al., 1989; Smart et al., 1991; Smigocki, 1991; Smigocki and Hammerschlag, 1991). Isopentenyl transferase, encoded by the ipt gene of the Ti plasmid, catalyzes the condensation of AMP and isopentenyl pyrophosphate to form isopentenyl AMP, a precursor of most other cytokinins (Akiyoshi et al., 1983). Enzymatic activity similar to that encoded by the ipt gene has been observed in plant cytokinin biosynthesis (Blackwell and Horgan, 1994; Chen and Melitz, 1979). However, the plant enzyme has only been partially purified and the corresponding plant gene has yet to be identified and cloned.