Studies of Cytokinin Action and Metabolism Using Tobacco Plants Expressing either the ipt or the GUS Gene Controlled by a Chalcone Synthase Promoter. I. Developmental Features of the Transgenic Plants

Abstract

A chimaeric cytokinin biosynthetic gene was constructed by placing the coding region of the bacterial ipt gene under the control of a chalcone synthase (chs) promoter (P_CHS) from Antirrhinum majus. The P_CHS-ipt gene was transferred to tobacco (Nicotiana tabacum L.). To provide control plants for studies of the effect of expression of this gene on plant development, a P_CHS β-glucuronidase gene fusion was also introduced into tobacco. Expression of the P_CHS-ipt gene caused release of axillary buds, inhibition of root development, retardation of leaf senescence, elevation of chlorophyll levels, delay in onset of flowering and retardation of flower development. These effects, which were quantified in P_CHS- ipt plants, have previously been associated with expression of ipt genes controlled by heat shock or other promoters. Additional effects of ipt gene expression characterised in P_CHS-ipt plants included growth of leafy shoots from the primary root, change in leaf shape with the production of broader and larger leaves, induction of expansion of excised leaf discs and development of leaves with an enlarged midrib and enlarged veins. A particularly striking effect of the expression of the P_CHS-ipt gene was development of thicker stems due mainly to increase of pith tissue caused by an enhancement of both cell division and cell enlargement. Node number per primary stem was also increased. Endogenous cytokinin and applied auxin interacted antagonistically to affect both root and stem development in plants cultured in vitro. The leaves of P_CHS -ipt transformed plants exhibited increased transpiration rates and reduced diffusion resistance associated with increased number of stomata and modified stomatal dimensions. The above changes, which were associated with elevated endogenous cytokinin levels, are discussed in relation to previous studies with ipt gene transformed plants and to some aspects of normal plant development.