An ipt gene under control of the senescence-specific SAG12 promoter from Arabidopsis (P SAG12-IPT) significantly delayed developmental and postharvest leaf senescence in mature heads of transgenic lettuce (Lactuca sativa L. cv Evola) homozygous for the transgene. Apart from retardation of leaf senescence, mature, 60-d-old plants exhibited normal morphology with no significant differences in head diameter or fresh weight of leaves and roots. Induction of senescence by nitrogen starvation rapidly reduced total nitrogen, nitrate, and growth of transgenic and azygous (control) plants, but chlorophyll was retained in the lower (outer) leaves of transgenic plants. Harvested P SAG12-IPT heads also retained chlorophyll in their lower leaves. During later development (bolting and preflowering) of transgenic plants, the decrease in chlorophyll, total protein, and Rubisco content in leaves was abolished, resulting in a uniform distribution of these components throughout the plants. Homozygous P SAG12-IPT lettuce plants showed a slight delay in bolting (4-6 d), a severe delay in flowering (4-8 weeks), and premature senescence of their upper leaves. These changes correlated with significantly elevated concentrations of cytokinin and hexoses in the upper leaves of transgenic plants during later stages of development, implicating a relationship between cytokinin and hexose concentrations in senescence.

Inheritance of resistance to herbicide (300 mg/l glufosinate ammonium) up to the third (T3) seed generation was compared in two populations of transgenic lettuce (Lactuca sativa L. cv 'Evola') harbouring a T-DNA containing the bar gene, linked to either the Cauliflower Mosaic Virus (CaMV) 35S promoter, or a -784-bp plastocyanin promoter from pea (petE). Only 2.5% (4/163) of CaMV 35S-bar plants, selected by their kanamycin resistance(T0 generation), transmitted herbicide resistance at high frequency to their T3 seed generation compared with 97% (29/30) for kanamycin resistant petE-bar plants. In the case of 35S-bar transformants, only 16% (341/2,150) of the first seed generation (T1) plants, 22% (426/1,935) T2 plants and 11% (1,235/10,949) T3 plants were herbicide-resistant. In contrast, 63% (190/300) T1 plants, 83% (2,370/2,845) T2 plants and 99% (122/123) T3 petE-bar transformed plants were resistant to glufosinate ammonium. The T-DNAs carrying the petE-bar and CaMV 35S-bar genes also contained a CaMV 35S-neomycin phosphotransferase (nptII) gene. ELISA showed that NPTII protein was absent in 29% (45/156) of the herbicide-resistant T2 plants from 8/19 herbicide-resistant petE-bar lines. This indicated specific inactivation of the CaMV 35S promoter on the same T-DNA locus as an active petE promoter. The choice of promoter and T-DNA construct are crucial for long-term expression of transgenes in lettuce.

No abstract

Cauliflower (Brassica oleracea var. botrytis) was transformed, using Agrobacterium tumefaciens, with an autoregulated isopentenyl transferase (ipt) gene under the control of a senescence-associated gene promoter, pSAG 12 , isolated from Arabidopsis thaliana. The effect of introducing this chimeric construct on cytokinin (CK) content, chlorophyll retention, and plant morphology and development were investigated. A range of CK and chlorophyll contents was found among the individual primary transformants. Progeny were studied from one of the primary transformed lines that did not have elevated cytokinin content and was phenotypically similar to the parent line but displayed delayed leaf senescence. The pSAG 12 :ipt gene was inherited in a Mendelian manner, and the effect of this gene on senescence-related parameters was observed in a number of the progeny. While the pSAG 12 :ipt progeny did exhibit delayed leaf senescence, it was accompanied by undesirable agronomic traits, including less synchronous curd initiation, smaller curd size, and greater susceptibility to fungal infection.