Factors affecting transformation efficiency of shoot apices of Gossypium arboreum and Gossypium hirsutum cultivars with Agrobacterium tumefaciens

Introduction Cotton (Gossypium spp.) is an excellent natural source of textile fiber and is a high value commercial crop. Over 180 million people are associated with the fiber industry that produces 20 to 30 billion dollars worth of raw cotton (Ozyigit and Gozukirmizi, 2009). Both diploid (Gossypium arboreum) and tetraploid (G. hirsutum) cultivars are cultivated in different regions of India and are considered as important crop plants of the country. Because of its high economic value considerable attention has been paid to improve cotton plant by conventional plant breeding methods, it is time-consuming and commercialization of new cotton varieties often takes 6 to 10 years (Sheidai et al., 2008). Additionally, indiscriminate use of pesticides to control insect-pest has led to series of consequences like, insecticide resistance, pest resurgence, outbreak of secondary pests, harmful residual effects, imbalances in natural ecosystem and higher production costs (Sanghera et al., 2011). Besides, genetic improvement of cotton through conventional means is limited due to many factors like absence of necessary variation, especially resistance against pests and diseases and its compatibility associated with pest resistant wild species. Plant tissue culture techniques provide an alternative means of improvement obtaining somaclones, induced variants, somatic hybridization and doubled haploids to develop inbred lines or for introducing genes of interest against insects and different diseases from unrelated sources such as microbes through genetic engineering (Zhang and Zhao, 1997, Kumar, 2003, Sanghera et al., 2009a). Genetic transformation of cotton has been a challenge due to genetic variability of cultivars and differences in their responses to the in vitro culture and transformation procedures Use of Agrobacterium as a vector is technically simple and gene transfer are often low copy, permanent and heritable. Therefore, Agrobacterium mediated transformation is the most widely used method for genetic transformation in plants. Introduction of foreign genes in elite genotypes is limited by the genotype specific nature of gene transfer in cotton (Jin et al., 2006). Cocker genotypes, which are amenable for regeneration in vitro by somatic embryogenesis, are widely used in genetic transformation experiments (Finer and Mcmullen, 1990). However, alternate procedures to transform non-cocker genotypes have been reported (Satyavathi et al., 2002, Trolinder et al., 2006, Kategri et al., 2007, Nandeshwar et al., 2009). In many cases, the production of transgenic plants is prevented due to the inability to regenerate plants from those tissues susceptible to transformation. So, the present study was undertaken to optimize the factors influencing Agrobacterium mediated transformation of two newly developed Indian cotton LD 694 (Gossypium arboreum) and LH 2076 (Gossypium hirsutum) cultivars using shoot apical meristem explants. Materials and Methods Seed germination and cultivation of sterile seedlings Seeds of two cotton cultivars viz. LD 694 (G. arboreum L.) and LH 2076 (G. hirsutum L.) used as source material were obtained from Cotton Section, Department of Plant Breeding and Genetics, and were delinted by using concentrated commercial H2SO4. The seeds were continuously stirred in H2SO4 by wooden rod for 5~10 min until the shiny surface of seeds appeared. Some water was then added and stirred for a