GROWTH, PHOTOSYNTHESIS, AND HERBICIDE TOLERANCE OF GENETICALLY-MODIFIED HYBRID POPLAR

Hybrid poplar clone NC-5339 (Populus alba X Populus grandidentata cv. Crandon) was genetically modified for glyphosate (N-(phosphonomethyl)g lycine) tolerance by Agrobacterium-mediated transformation with geneti c constructs (pPMG 85/587 and pCGN 1107) that included the mutant aroA gene for 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase (EC 2.5.1 .19) and the neomycin phosphotransferase selectable marker gene. pCGN 1107 also harbored the coding sequence for a chloroplast transit pepti de and the CaMV 35S promoter fused to the mutant aroA gene. Transforma nts were selected for kanamycin tolerance, and integration of the aroA gene was verified by Southern blot analysis. Cuttings of NC-5339 and the derived transformants were rooted and grown in glasshouses at sepa rate locations, with maximum photosynthetic photon flux density of 160 0 and 750 mu mol . m(-2). s(-1). Productivity was assessed by growth s tudies and photosynthesis measurements at both locations. Glyphosate t olerance was tested by (i) measurement of chlorophyll concentration in herbicide-treated leaf discs and (ii) whole-plant spray tests. Plants transformed with construct pCGN 1107 were the most herbicide tolerant . Perhaps high-level expression of the aroA gene by the CaMV 35S promo ter, transport of mutant EPSP synthase into the chloroplasts, or both facilitated glyphosate tolerance. Plants grown at higher photosyntheti c photon flux densities (1600 vs. 750 mu mol . m(-2). s(-1)) had signi ficantly higher maximum net photosynthesis (19.8 vs. 16.2 mu mol . m(- 2). s(-1)) and more biomass accumulation (47.6 vs. 33.7 g). However, t here were no significant differences between NC-5339 and transformants within location for net photosynthesis or any growth parameter. Genet ic modification of hybrid poplar NC-5339 for glyphosate tolerance did not adversely affect plant productivity at either location.