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).