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Unique traits not found in alfalfa (Medicago sativa) germplasm can be introduced through genetic engineering for crop improvement. Agrobacterium-mediated transformation by co-cultivation of plant tissue pieces followed by somatic embryogenesis and plant regeneration or transformation of cotyledonary node meristems is now routine for generating transgenic alfalfa plants. Numerous genes have been introduced into alfalfa for understanding gene function, characterizing promoters, and for introducing a wide array of agronomic traits: tolerance to aluminum toxicity, salt, and drought; resistance to herbicides, diseases, and insects; bioremediation of heavy metals; increased plant biomass, improved nutrient uptake, and improved forage quality and nutritional content. Genetically modified alfalfa was also explored for large-scale production of enzymes, biodegradable plastics, and pharmaceuticals including antigens for veterinary applications. The most widely used promoter for transgenic alfalfa research is the CaMV 35S promoter, although expression in alfalfa is lower than in other plants. This chapter presents a summary of constitutive, tissue-specific, and inducible promoters tested in alfalfa and recent advances in developing transgenic alfalfa for desired agronomic traits. The development of the two commercialized traits, glyphosate resistance (Roundup Ready® alfalfa) and reduced lignin (HarvXtra® alfalfa) is detailed including challenges encountered in breeding and deregulation of these genetically modified traits. The emerging use of gene editing will likely have a large impact on alfalfa improvement and commercialization of new traits. |
