STUDIES OF CYTOKININ ACTION AND METABOLISM USING TOBACCO PLANTS EXPRESSING EITHER THE IPT OR THE GUS GENE CONTROLLED BY A CHALCONE SYNTHASEPROMOTER .1. DEVELOPMENTAL FEATURES OF THE TRANSGENIC PLANTS

A chimaeric cytokinin biosynthetic gene was constructed by placing the coding region of the bacterial ipt gene under the control of a chalco ne synthase (chs) promoter (P-CHS) from Antirrhinum majus. The P-CHS-i pt 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 beta-glucuronidase gene fusion was also in troduced 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 flower ing and retardation of flower development. These effects, which were q uantified in P-CHS-ipt plants, have previously been associated with ex pression of ipt genes controlled by heat shock or other promoters. Add itional effects of ipt gene expression characterised in P-CHS-ipt plan ts included growth of leafy shoots from the primary root, change in le af shape with the production of broader and larger leaves, induction o f expansion of excised leaf discs and development of leaves with an en larged midrib and enlarged veins. A particularly striking effect of th e expression of the P-CHS-ipt gene was development of thicker stems du e mainly to increase of pith tissue caused by an enhancement of both c ell division and cell enlargement. Node number per primary stem was al so increased. Endogenous cytokinin and applied auxin interacted antago nistically to affect both root and stem development in plants cultured in vitro. The leaves of P-CHS-ipt transformed plants exhibited increa sed transpiration rates and reduced diffusion resistance associated wi th increased number of stomata and modified stomatal dimensions. The a bove changes, which were associated with elevated endogenous cytokinin levels, are discussed in relation to previous studies with ipt gene t ransformed plants and to some aspects of normal plant development.