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We assessed the stability of transgene expression in 79 transgenic lines (i.e., transformation events) of hybrid poplars during several years of field trials. The transgenic lines were comprised of 40 lines of hybrid cottonwoods (P. trichocarpa × P. deltoides) that were grown at three field sites, and 39 lines of hybrid aspens (section Leuce, P. alba × P. tremula) that were grown at a single field site. All the lines were transformed with a binary construct that included two genes that confer tolerance to glyphosate (GOX and CP4), a gene encoding resistance to the antibiotic kanamycin (nptII), and a visible marker gene (GUS). Agrobacterium tumefaciens was used for transformation; callogenesis and organogenesis occurred under kanamycin selection. In addition to repeated applications of herbicide to test stability of transgene expression, for the first time, we challenged ramets of 40 lines that had not previously been tested for herbicide resistance in their fourth season of vegetative growth. We report on the stability of herbicide resistance and GUS expression and evidence for somaclonal variation in growth and leaf morphology.
Ascorbate peroxidase (APX) plays an important role in the metabolism of hydrogen peroxide in higher plants, affording them protection against oxidative stress. We studied the effect of overexpressing a cytosolic ascorbate peroxidase (cAPX) gene—derived from pea (Pisum sativum L.)—in transgenic tomato plants (Lycopersicon esculentum L.). Transformants were selected in vitro using kanamycin resistance and confirmed by polymerase chain reaction (PCR) and northern analyses. An APX native-gel assay indicated that, in the absence of stress, APX activity in transgenic plants was several times greater than that measured in wild-type (WT) plants. Several independently transformed lines were propagated and evaluated for resistance to chilling and salt stress. After placing seeds at 9 °C for 5 weeks, percent germination was greater for seeds obtained from transgenic lines (26% to 37%) compared to the WT (3%). Plants from transgenic lines also had lower electrolyte leakage (20% to 23%) than WT (44%) after exposure to 4 °C. Visual assessment of transgenic and WT lines exposed to salinity stress (200 or 250 mm) confirmed that overexpression of APX minimized leaf damage. Moreover, APX activity was nearly 25- and 10-fold higher in the leaves of transgenic plants in response to chilling and salt stresses, respectively. Our results substantiate that increased levels of APX activity brought about by overexpression of a cytosolic APX gene may play an important role in ameliorating oxidative injury induced by chilling and salt stress.