Agronomic performance and pollen viability of transgenic tall fescue plants have been studied under field conditions (Wang et al., 2004a, 2003b). Tissue culture-derived (untransformed) R0 regenerants, T0 primary transgenic plants, and seed-derived plants were transferred to the field and evaluated for 2 yr. Progenies of these three classes of plants were obtained and evaluated together with seed-derived plants in a second field experiment (Wang et al., 2003b). The following agronomic traits were evaluated: heading date, anthesis date, height, growth habit, number of reproductive tillers, seed yield, and biomass.
Field performance of the T0 plants and R0 regenerants was inferior to that of the seed-derived plants, probably as a result of carry-over effects of tissue culture. No major differences between the progenies of transgenic plants and the progenies of seed-derived plants were found for the agronomic traits evaluated (Wang et al., 2003b). In addition, no significant difference in pollen viability was detected between progenies of transgenic plants and those of seed-derived plants (Wang et al., 2004a). The results indicated that once seeds are obtained from the T0 primary transgenic plants, normal pollen viability and agronomic performance of the progenies can be expected. The studies provided evidence that outcrossing grass plants generated through transgenic approaches are suitable materials for incorporation into breeding programs.
Pollen is an important vector of gene flow in outcrossing grasses. To assess effectively the in vitro viability and longevity of tall fescue pollen, a pollen germination medium (0.8 M sucrose, 1.28 mM boric acid, and 1.27 mM calcium nitrate) was optimized (Wang et al., 2004a). Progenies of transgenic plants (T1 and T2) showed pollen viability similar to seed-derived plants, although T0 primary transgenics had various levels of pollen viability in individual plants (Wang et al., 2004a). Pollen from transgenic progenies and nontransgenic control plants survived up to 22 h under controlled conditions in a growth chamber. However, under sunny atmospheric conditions, viability of transgenic and nontransgenic pollen declined to 5% in 30 min, with a complete loss of viability in 90 min. Under cloudy atmospheric conditions, pollen viability declined to 5% after 150 min, with complete loss of viability in 240 min (Wang et al., 2004a).
Since wind-pollinated grass species have a high potential to pass their genes to adjacent plants, information regarding pollen flow has become an important aspect of risk assessment (Wang et al., 2004b). Biosafety evaluation of transgenic grasses will likely focus on their environmental or ecological impacts. A small-scale experiment designed to examine pollen dispersal from transgenic tall fescue was performed using a central transgenic plot surrounded by exclosures containing recipient plants, up to a distance of 200 m from the central source plants in eight directions (Wang et al., 2004b). Seeds were collected from the recipient plants and germinated seedlings were analyzed by PCR. Transgenes were detected in recipient plants at up to 150 m from the central transgenic plot. The highest transgene frequencies (5% at 50 m, 4.12% at 100 m, and 0.96% at 150 m) were observed in the prevailing wind direction (Wang et al., 2004b). Obviously, large-scale gene flow experiments are needed to better assess how far tall fescue pollen can disperse and remain viable.
Gene flow within and among populations is an essential feature of outcrossing grasses because of their natural self-incompatibility. The addition of transgenes to wind-pollinated crops has intensified the scrutiny of the process. A primary focus in risk assessment research should be placed on the consequences of transgene flow. The costs of meeting regulatory requirements and market restrictions are substantial impediments to the commercialization of transgenic crops. The phenotypes of transgenic plants and their safety in the environment, rather than the method used to produce them, should be the main focus of risk analyses and regulatory concern (Bradford et al., 2005).