Successful wheat transformation is stdyrongly dependent on a number of factors such as the genotype, medium composition, explants resources and transformation protocols. In this study, immature embryo, mature embryo and immature inflorescence of Chinese elite spring wheat genotypes were chosen as transgenic explant resources via in vitro culture. Only a few calli could be induced from immature inflorescences and the callus induction frequency was very low, while higher callus induction frequencies were obtained from immature inflorescences by Dudits et al. (1975) and Liu et al. (2001). We also found that the frequency of callus induction from immature embryos was higher than that of mature embryos and immature inflorescences. It suggested that immature embryos as transgenic explants were superior to mature embryos and immature inflorescences. The similar results had been reported by Altpeter et al. (1996), Chen et al. (1998) and Liu et al. (2002).
The frequency of stable wheat genetic transformations via particle bombardment is also strongly dependent on genotypes (Iser et al. 1999). The application of this technology had been made in genetic transformation of some European and Japanese elite wheat varieties with acceptable agronomic characters (Takumi and Shimada 1997; Iser et al. 1999). Our experiment found that Chuannong 16 was a good genotype for genetic transformation, and MS2 medium was superior to other media for the callus induction. Chuannong 16 is a new breakthrough cultivar with high steady yield, large number of spikes per hectare and resistant to disease. It was registered by National Variety Identify Committee of China, and was released by our research group in 2002 (Zheng et al. 2002). Thus, this elite cultivar could be chosen as an ideal transformation genotype for further Chinese spring wheat genetic transformation. Several reports have focused on the examinations of medium parameters (Perl et al. 1992; Rasco-Gaunt et al. 199dy9; Qin and He 2001). General MS is main salt formulation used for cereal species from the Triticeae. Wu et al. (2003a, b) reported that modified N6 medium is the suitable medium for the wheat commercial cultivars, landraces and breeding lines in Sichuan, China. However, our experiment indicated that the MS with double macro salt was superior to other media and had higher calli induction frequency than-that of general MS and N6 media.
The transient expression of GUS indicated there was a strong GUS positive response and this optimal tissue culture system was available for Chinese spring wheat genetic transformation via particle bombardment. Now, this tissue culture system has been used in the genetic transformation of high molecular-weight (HMW) glutenin subunits gene derived from the wild relative genus of wheat by our research group (Yan et al. 2002; Liu et al. 2003) in order to improve the wheat quality of southwest China.
Acknowledgments
Authors are grateful to Prof. P.W. Wang for kindly providing the plasmid pDM8O3. We thank L.M.Wu, P.P. Zhuang and X.L. Peng for technical assistance. This research was financially supported by the Key Science Foundation of the Education Committee and the Scientific & Technological Committee in Sichuan Province.