The Effect of Environment On Transgene Evaluation

Many genetically engineered crops now in development aim to combat abiotic stresses that reduce yield worldwide. Numerous kinds of genes, including regulatory and metabolic genes, have potential for increasing abiotic stress tolerance. These genes confer more complex traits than currently commercialized transgenes and are more likely to increase secondary effects of transgene expression. To better understand secondary effects important for risk assessment, we studied three transgenes that increase salinity tolerance by differing mechanisms in the model species Arabidopsis thaliana. Salt Overly-Sensitive 1 (SOS1) encodes a plasma membrane Na+/H+ antiporter, mannose-6-phosphate reductase (M6PR) encodes a mannitol biosynthesis enzyme, and C-repeat binding factor 3/drought responsive element binding factor 1a (CBF3/DREB1a) encodes a transcription factor for abiotic stress gene regulation. Transgenic plants were grown in the field in spring, summer and fall seasons for multiple generations and monitored for survival, seed yield and viability, traits vital to long term population survival and competition. Relative transgene fitness effects were calculated by comparisons of transgenic versus wild-type (WT) frequency by selectable marker screening of successive progeny from fourteen replicate mixed populations per transgenic line. Seed yield comparisons also were made between pure populations of WT and transgenic lines (5 reps/line). Regardless of season CBF3 expression delayed development, while M6PR and SOS1 lines were unaffected (P<0.013, P>0.8). The fall season slowed development for all lines (all P<0.001). Plant survival to seed set was unaffected by transgene expression, but was influenced by season, lowest in fall and highest in spring (P>0.7, P<0.02). Both seed yield and biomass were affected by season, highest in spring and lowest in fall (P<0.02). Averaged across seasons, harvest index was unaffected by CBF3 and SOS1, but was increased by M6PR (P>0.16, P<0.03). M6PR lines had the highest overall fitness relative to WT (P<0.001). M6PR and CBF3 fitness was highest in fall, while SOS1 remained constant across seasons (P<0.001, P<0.001, P>0.99). M6PR and CBF3 had genotype x environment interactions, with M6PR fitness above wild-type summer and fall but equal to WT in spring, while CBF3 fitness was well below WT in spring and summer, and equal to WT in the fall (P<0.01, P<0.02). Thus, transgenic plant productivity and fitness varied by environment with a variety of interactions between different transgenes and the environment. These results indicate the importance of considering environmental influences during the risk assessment process for transgenic crops altered to increase abiotic stress tolerance.