Long-term fitness effects of abiotic stress tolerance transgenes in Arabidopsis thaliana populations under competitive conditions

The demands on agricultural lands from a growing world population will rise at the same time that climate change is predicted to increase the abiotic stresses (e.g. drought, heat-waves, frosts and salinity) that decrease crop yield today. Efforts to increase crop abiotic stress tolerance are ongoing, including via transgenic approaches. However, unlike past transgenes, abiotic stress tolerance genes function through indirect alterations to regulatory, signaling and metabolic pathways, increasing the possibility of complex secondary effects. Concerns have been raised that such genes could alter crop persistence and ferality and, through gene flow, the invasiveness and ecological range of recipient interfertile wild or weedy relatives. These possible ecological risks are influenced by fitness effects conferred by the transgene. Despite the link between competitive fitness and long-term environmental risks, competitive fitness has been rarely empirically determined in ecological risk assessment. In this study three transgenes, which increase salinity tolerance in Arabidopsis thaliana in growth chamber studies, were examined for impacts on plant fitness: (I) the abiotic stress response transcription factor C-repeat binding factor 3/drought responsive element binding factor 1a ( CBF3/DREB1a), (II) the plasma membrane Na+/H+ antiporter Salt Overly-Sensitive 1 (SOS1), and (III) the mannitol biosynthetic enzyme mannose-6-phosphate reductase (M6PR). Transgene fitness impacts were examined across six field seasons and in the presence and absence of competition with the wild-type parental genotype at planting densities (2600/m2) chosen to replicate conditions observed in wild populations. Fourteen replicate competitive populations, initially 1:1 transgenic:wild-type mixes, of each transgenic line (2-3 lines/transgene) were maintained separately for six generations to allow transgene frequencies to fluctuate according to genetic drift and field selective pressures. Transgene frequencies were monitored each generation via phenotypic screening of progeny seed for presence of the co-integrated kanamycin resistance trait; low frequency populations were verified by qPCR analysis to rule out artifacts due to possible gene silencing. The fitness effects observed in competition with WT differed from relative fitness in pure populations. In pure populations, CBF3 lines showed moderately negative fitness relative to wild-type, but decreased to near extinction in direct competition. SOS1 lines performed equivalently to wild-type in pure populations but decreased in frequency by 50% in competition. The fitness of both M6PR lines was enhanced relative to wild-type in field pure populations, but in competition one line exhibited a competitive advantage while the other was selectively neutral and exhibited random drift. Selection and drift modeling, incorporating short-term non-competitive and competitive transgene fitness measurements, determined that only models which utilized competitive fitness values yielded long-term transgene frequency patterns comparable to trends observed in the field. Significant relative fitness gains were observed from all three transgenes under salt stress in the growth chamber, but from only SOS1 and M6PR lines in the greenhouse. In competition with wild-type plants no advantage was observed, indicating that like the field, competition reduced observed transgene fitness. The implications of these findings, together with prior transcriptomic analysis of the three transgenes, were examined in the context of environmental risk assessment (ERA) practices. Together these results indicate the important role competition has on the success or failure of a transgene to establish and support the use of competitive field assessments in estimating the risk of transgene establishment.