LIGHT-INDUCED EXPRESSION OF IPT FROM AGROBACTERIUM-TUMEFACIENS RESULTS IN CYTOKININ ACCUMULATION AND OSMOTIC-STRESS SYMPTOMS IN TRANSGENIC TOBACCO

Cytokinins are plant growth regulators that induce shoot formation, in hibit senescence and root growth. Experiments with hydroponically grow n tobacco plants, however, indicated that exogenously applied cytokini n led to the accumulation of proline and osmotin. These responses were also associated with environmental stress reactions, such as salt str ess, in many plant species. To test whether increased endogenous cytok inin accumulation led to NaCl stress symptoms, the gene ipt from Agrob acterium tumefaciens, encoding isopentenyl transferase, was transforme d into Nicotiana tabacum cv. SR-1 under the control of the light-induc ible rbcS-3A promoter from pea. In high light (300 mu mol PPFD m(-2) s (-1)), ipt mRNA was detected and zeatin/zeatin glucoside levels were 1 0-fold higher than in control plants or when transformants were grown in low light (30 mu mol PPFD m(-2) s(-1)). High light treatment was ac companied by increased levels of proline and osmotin when compared to low light grown transformed and untransformed control plants. Elevated in planta cytokinin levels induced responses also stimulated by salt stress, suggesting either common or overlapping signaling pathways are initiated independently by cytokinin and NaCl, setting in motion gene expression normally elicited by developmental processes such as flowe ring or environmental stress.