INHIBITION OF LEAF SENESCENCE BY AUTOREGULATED PRODUCTION OF CYTOKININ

Controlling expression of IPT, a gene encoding isopentenyl transferase (the enzyme that catalyzes the rate-limiting step in cytokinin biosyn thesis), with a senescence-specific promoter results in the suppressio n of leaf senescence. Transgenic tobacco plants expressing this chimer ic gene do not exhibit the developmental abnormalities usually associa ted with IPT expression because the system is autoregulatory. Because sufficient cytokinin is produced to retard senescence, the activity of the senescence-specific promoter is attenuated. Senescence-retarded l eaves exhibit a prolonged, photosynthetically active life-span. This r esult demonstrates that endogenously produced cytokinin can regulate s enescence and provides a system to specifically manipulate the senesce nce program.