The high infection rate observed after 2 min infection time with the Agrobacterium suspension might be due to the presence of phenolic compounds in guayule callus ( Trautmann and Visser, 1991), which can play an important role in enhancing infection rate ( Bhattacharya et al., 2010). During the selection period (two weeks) in presence of high kanamycin concentration, some in vitro calli bleached, suggesting the elimination of non-transformed tissue ( Fig. 3D).The transformed calli remained green. Selected green calli were further subcultured on fresh SIM supplemented with 50 mg/l kanamycin and 400 mg/l cefotaxime for another two weeks ( Fig. 3E), and the average transformation efficiency was 72 ± 5.5% ( Table 2).