Genetic Engineering of Crops

Substantial loss of yield worldwide is due to abiotic stress. In fact, more than 50% of the yield is lost to abiotic stress. For instance, in wheat, the record yield (average yield + yield without loss to diseases, pests, and unfavouable environments) in 1975 was 14500 kg/hectares while the average yield (with losses) was 1880 kg/hectares. Translated, this would mean a loss of 87% to diseases, pests, and unfavourable physicochemical environments. Given the increase in the world’s population, much has been attempted to increase yield, or production capacity, and with the advent of biotechnology-an alternative to traditional breeding, this process has seen many successes.

By altering the genetic code of a plant, one can modify said plant to tolerate a variety of stresses. In the majority of cases, crop improvement is the goal. The latter is typically achieved by delivering a particular gene that can confer an advantage to the plant using agrobacterium mediated transformation or the shotgun method, selecting those cells that possesses the transgene using a selectable marker, and subsequently using cell culture to regenerate the plant. In such cases, one must also consider the number of inserts (copies present), the expression levels of the transgene (mRNA, and protein), and location of expression. For instance, with the addition of a stress inducible transcription factor DREBIA (rd29A:DREBIA transgene), plants that can grow under salt stress have been generated; in addition, the use of a stress inducible promoter has permitted both high yield, and tolerance to stress (using the constitutive promoter 35S CaMV promoter - 35S CaMV:DREBIA transgene, one would generate a plant that is tolerant to salt stress but that does not grow well). Alternatively, one can modify pathways by rerouting the carbon source for the production of a target compound; the latter can be utilized to increase, or decrease the production of a target compound. Golden rice – rich in vitamin A - is the best known example of said modification; by providing the plant with genes necessary to complete the beta-carotene pathway, one can trigger the plant to produce vitamin A. Yet in another instance, the insertion of 1-aminocyclopropane-1-carboxyllic acid (ACC) synthase has been shown to decrease ethylene accumulation thereby delaying ripening of fruits. In other cases, the task is to optimize the production of secondary metabolites. Brassica napus (canola) is a valued crop that requires secondary metabolite optimization. For instance, an increase in carotenoids by overexpressing the CRTB gene resulted in a 50-fold increase in carotenoid levels but the embryos were orange. Yet in another instance, a tomato plant that was conferred the AtNHX1 transgene to permit its overexpression was found to grow in a high salt solution. In other words, a transgenic plant had the ability to grow without a decrease in yield or accumulation of salt in fruits. Given the environmental conditions around the world, the ability to grow crops under a variety of harsh conditions such as cold temperatures, freezing conditions, salty soils, and waters amongst others is a large step forward.

Table 1: Examples of crop improvement using biotechnology. 

Stress

Transgene

Effect

References

Salt Stress

rd29A:DREBIA transgene

Replacing a constitutive promoter with a stress-inducible promoter results in transgenic Solanum tuberosum plants that are both highly productive, and resistant

Kasuga et al. 1999

Salt Stress

Rd29A:AtCBF transgene

Replacing a constitutive promoter with a stress-inducible promoter results in transgenic Solanum tuberosum plants that are both highly productive, and resistant

Pino et al. 2007

Freezing stress

PLDδ overexpression

Overexpression of transgene increases freezing tolerance, underexpression decreases freezing tolerance

Li et al., 2004

Salt Stress

AtNHX1 overexpression

Tomato plants can be grown in a high salt solution without decreasing yield, or salt accumulation in fruits

Zhang and Blumwal, 2001

Optimization

CRTB overexpression

50-fold increase in carotenoid levels but the embryos were orange

Hannoufa et al., 2013

References

Hannoufa, A., Pillai, B., and Chellamma, S. (2013). Genetic enhancement of Brassica napus seed quality. Transgenic Res 23, 39-52.

Kasuga, M., Liu, Q., Miura, S., Yamaguchi-Shinozaki, K., and Shinozaki, K. (1999). Improving plant drought, salt, and freezing tolerance by gene transfer of a single stress-inducible transcription factor. Nature Biotechnology 17, 287-291.

Li, W., Li, M., Zhang, W., Welti, R., and Wang, X. (2004). The plasma membrane–bound phospholipase Dδ enhances freezing tolerance in Arabidopsis thaliana. Nat Biotechnol 22, 427-433.

Pino, M., Park, E., Jeknic, Z., and Hayes, P. (2007). Use of a stress inducible promoter to drive ectopic AtCBF expression improves potato freezing tolerance while minimizing negative effects on tuber yield. Plant Biotechnology Journal 5, 591–604.

Zhang, H., and Blumwal, E. (2001). Transgenic salt-tolerant tomato plants accumulate salt in foliage but not in fruit. Nature Biotechnology 19, 765–768.