**10. Transgenics**

With the advent of recombinant DNA technology in the 1970s, the genetic manipulation of plants entered a new age. Genes and traits previously unavailable through traditional breeding became available through DNA recombination and with greater specificity than ever before. This modern genetic engineering technology allows the transfer of genetic material across a wide range of species and has removed the traditional limits of crossbreeding. It involves the transfer of desired genes into the plant genome, and then regeneration of a whole plant from the transformed tissue/cell. For successful development of transgenic plants, identification of suitable target tissue and efficient gene transfer protocol are essential. Therefore, understanding the genetic variability of different crop plants and genotypes for in vitro regeneration and optimization of routine regeneration protocol is prerequisite for the utilization of transformation technology in any crop. Currently, the most widely used method for transferring genes into plants is Agrobacteriummediated transformation [151–153] and particle bombardment method [154]. Other methods, such as polyethylene glycol (PEG) mediated transformation [155] and electroporation [156] have also been used to transfer genes into plants. Cotton is a recalcitrant crop to generate from in vitro tissue cultures. Compared with many other crops, it is more difficult to obtain somatic embryogenesis, shoot multiplication and plant regeneration in cotton. The nature of tissue explants, the genetic makeup of the crop plant and presence of different growth hormones have a direct effect on regeneration potential. Genotype dependent genetic transformation is well studied and used commercially in cotton. Coker genotypes, which are amenable for regeneration in vitro by somatic embryogenesis, are widely used in genetic transformation experiments [151–153, 157]. Genotype independent genetic transformation techniques although developed [152, 158] show very low frequency of heritable gene incorporation. In the beginning, the two major goals of genetic engineering in cotton were to confer insect resistance and tolerance to more environmentally acceptable herbicides [159]. To date, 65 plus transgenic cotton events approved in India and all over the world. Continuous exposure of bollworms to BT cotton has led to resistance in them and thereby affecting the efficiency of controlling them. Cotton bollworm P450 monooxygenase gene (CYP6AE14) gene was silenced to impair larval tolerance to gossypol through the plant-mediated RNAi approach [160]. Genetic engineering is a remarkable breakthrough in modern crop improvement. Bt cotton came at the most opportune time when bollworms were causing a lot of destruction to the cotton crop making farmers helpless. Since its release in the USA in 1995, China during 1997 and in India during 2002 the Bt. technology has had a significant impact on bollworm control and reduction in usage of pesticides has been seen.

Acceptance of genetically altered cotton in various regions of the world is offering new opportunities for improvement of cotton yield and quality. Overexpression of GhUGP1 (Cotton uridine diphosphate glucose pyrophosphorylase) in upland cotton improves the fiber quality and reduces fiber sugar content [161]. Overexpression of novel sucrose synthase GhSusA1 gene leads to a considerable increase in biomass and fiber length with a moderate increase in fiber strength [162]. A silkworm fibroin gene was used to improve the fiber structure and quality [163]. The transgenic cotton plants expressing the fiber expansin gene (GhEXPA8) showed a significant improvement in fiber lengths and micronaire values [63]. The fiber quality QTL-associated phytochrome PHYA1 gene was targeted through RNAi to explore the biological roles of PHYA1 and (indirectly) other phytochrome genes in cotton [164]. The elimination of gossypol from cottonseed has been a longstanding goal of geneticists. A cotton variant was obtained using antisense technology against (+)-delta-cadinene gene to suppress terpenoid aldehydes (gossypol) but with lysigenous glands [165]. RNAi-knockdown of delta-cadinene synthase gene(s) was used to engineer plants that produced ultra-low gossypol cottonseed (ULGCS) [166]. Recently, ultra-low gossypol cottonseed (ULGCS) was obtained by using

PTGS and seed-specific promoter (α-globulin) through suppression of CDN genes and these lines are under field evaluations [167]. In the future, increased research investment on biotic and abiotic stresses through a transgenic approach is needed. Much focus is required for exploiting and improving cotton fiber and yield traits with the help of alien gene incorporation. In regards to public acceptance and questions, there is a need to carry on a massive public awareness campaign i.e. benefits, biosafety and risk assessment.
