Fruit-specific expression of the A. tumefaciens isopentenyl transferase gene in tomato: effects on fruit ripening and defense-related gene expression in leaves

This paper describes the analysis of tomato plants transformed with a chimeric gene consisting of the promoter region of a fruit specifically expressed tomato gene linked to the ipt gene coding sequences from the Ti plasmid of Agrobacterium tumefaciens. The pattern of expression of this chimeric gene was found to be consistent with the expression of the endogenous fruit-specific gene and consequently, plants expressing the chimeric gene were phenotypically normal until fruit maturation and ripening. A dramatically altered fruit phenotype, islands of green pericarp tissue remaining on otherwise deep red ripe fruit, was then evident in many of the transformed plants. Cytokinin levels in transformed plant fruit tissues were 10 to 100-fold higher than in control fruit. In the leaves of a fruit-bearing transformant, despite a lack of detectable ipt mRNA accumulation, approximately fourfold higher than control leaf levels of cytokinin were detected. It is suggested that cytokinin produced in fruit is being transported to the leaves since accumulation in leaves of PR-1 and chitinase mRNAs, which encode defense-related proteins known to be induced by cytokinin, occurred only when the transformant was reproductively active. Effects of elevated cytokinin levels on tomato fruit gene expression and cellular differentiation processes are also described.