Effects of P-SAG12-IPT gene expression on development and senescence in transgenic lettuce

An ipt gene under control of the senescence-specific SAG12 promoter from Ar abidopsis (P-SAG12-IPT) significantly delayed developmental and postharvest leaf senescence in mature heads of transgenic lettuce (Lactuca sativa L. c v Evola) homozygous for the transgene. Apart from retardation of leaf senes cence, mature, 60-d-old plants exhibited normal morphology with no signific ant differences in head diameter or fresh weight of leaves and roots. Induc tion of senescence by nitrogen starvation rapidly reduced total nitrogen, n itrate, and growth of transgenic and azygous (control) plants, but chloroph yll was retained in the lower (outer) leaves of transgenic plants. Harveste d P-SAG12-IPT heads also retained chlorophyll in their lower leaves. During later development (bolting and preflowering) of transgenic plants, the dec rease in chlorophyll, total protein, and Rubisco content in leaves was abol ished, resulting in a uniform distribution of these components throughout t he plants. Homozygous P-SAG12-IPT lettuce plants showed a slight delay in b olting (4-6 d), a severe delay in flowering (4-8 weeks), and premature sene scence of their upper leaves. These changes correlated with significantly e levated concentrations of cytokinin and hexoses in the upper leaves of tran sgenic plants during later stages of development, implicating a relationshi p between cytokinin and hexose concentrations in senescence.