Transgenic Cotton Breeding

Natural variation exists within the genus Gossypium L. that affords continuous genetic improvement of cotton in lint yield, fiber quality, and resistance to biotic and abiotic stresses. However, genetic engineering has provided a complementary avenue to introduce novel or desirable genes and traits into cotton. Since 1996, several insect-resistant biotech traits conferred by various Bacillus thuringiensis (Bt) genes or their combinations and herbicide-resistant traits have been commercialized in the United States and major cotton-growing countries, accounting for 90 and 60% of the cotton acreages in the United States and the world, respectively. The sources of transgenes used in commercialization are predominantly from bacteria. With an understanding of the biochemical and molecular basis of plant growth and development based on genomic research, genes from other plants or cotton can also be used in transformation. In this chap ter, major sources of transgenes used to develop commercial genetically engineered (GE) cotton, selectable marker genes and their promoters, and transformation methods are first described. Methods used in assessments of transgenic cotton for regulatory approval are then discussed. These include genetic and molecular characterization and requirements for addressing issues related to agronomic performance, nutrient analysis, and environmental safety. Finally, breeding methods in developing GE cultivars from approved transgenic events are discussed in detail. C otton improvement through classical genetics and breeding targets high yields, better fiber and seed quality, resistance or tolerance to abiotic and biotic stresses, and adaption to production systems where cotton is grown. Con ventional cotton breeding takes advantage of desirable alleles that are existent in the cultivated cotton of the same or a different Gossypium species. Intraspecific breeding within tetraploid Upland (G. hirsutum L.) or Pima cotton ( G. barbadense L.) does not have any reproductive barriers between parental lines, while interspecific breeding between Upland and Pima cotton is faced with the hybrid breakdown problem in F 2 and advanced generations, even though heterotic Abbreviations: 2,4-D, 2,4-dichlorophenoxyacetic acid ; Bt, Bacillus thuringiensis; DMA, dicamba monooxygenase; ELISA, enzyme-linked immunosorbent assay; EPSP, 5-enolpyruvylshikimate-3-phosphate; EPSPS, enolpyruvylshikimate 3-phosphate synthase; GE, genetically engineered; miRNA, microRNA; PAT, phosphinothricin acetyltransferase; PCR, polymerase chain reaction; PEP, phosphoenolpyruvate; QTL, quantitative trait locus; RNAi, RNA interference; TALEN, transcription activator-like effector nuclease; ZFN, zinc-finger nuclease.