CYTOKININ-MEDIATED INSECT RESISTANCE IN NICOTIANA PLANTS TRANSFORMED WITH THE IPT GENE

The bacterial isopentenyl transferase (ipt) gene involved in cytokinin biosynthesis was fused with a promoter from the proteinase inhibitor II (PI-IIK) gene and introduced into Nicotiana plumbaginifolia. Transc ripts of the ipt gene were wound-inducible in leaves of transgenic PI- II-ipt plants. In leaf disks excised from fully expanded leaves, trans cript levels increased 25- to 35-fold within 24 h and by 48 h were red uced by about 50%. In flowering plants, message levels were 2- to 5-fo ld higher than in preflowering plants. These plants were used to test for defensive properties of cytokinins against insects. Manduca sexta larvae consumed up to 70% less of the PI-II-ipt leaf material on flowe ring plants than larvae feeding on controls. Normal development of Myz us persicae nymphs was also delayed. Approximately half as many nymphs reached adulthood on PI-II-ipt leaves than on controls. Zeatin and ze atinriboside levels in leaves remaining on PI-II-ipt plants after horn worm feeding were elevated by about 70-fold and the chlorophyll a/b co ntent was double that of controls. Exogenous applications of zeatin to the PI-II-ipt leaves enhanced the level of resistance to the tobacco hornworm and almost completely inhibited normal development of the gre en peach aphid nymphs. Transcript levels of an acidic chitinase gene w ere low and minimally inducible in PI-II-ipt leaves. The mode of actio n of the cytokinin gene product on enhanced insect resistance is not c lear but may involve the products of secondary metabolic pathways.