tumefaciens YH-2, which contains an ACC Daporinad order deaminase gene, a transformation frequency of 2.9% was obtained. However, compared with using OA medium, the transformation frequencies obtained with
both A. tumefaciens YH-1 and YH-2 strains were significantly lower, which indicates that the presence of ACC deaminase can only partially replace AgNO3 in inhibiting ethylene levels during the regeneration process and promoting regeneration frequency. It has been reported that ethylene synthesized by plants, when challenged with pathogens, may inhibit bacterial growth by triggering the expression of genes involved in the plant defense system such as chitinase, β-1,3-glucanase and pathogen-related gene 1 (PR1) (Deikman, 1997; Glick et al., 2007). For example, bacterial growth in the ethylene-insensitive Arabidopsis mutants ein2 and coi1 was increased about 7–10 times more than that in wild-type Arabidopsis (Norman-Setterblad et al., 2000). Similarly, the growth of the plant pathogen Xanthomonas campestris in the highly ethylene-sensitive tomato plant mutant Ensartinib cell line LeETR4AS was
inhibited about 10-fold more than that in the wild-type tomato plants (Ciardi et al., 2001). In a recent study, it was found that the introduction of ACC deaminase into the virulent A. tumefaciens strain C58 increased the proliferation of Agrobacterium in crown galls. In 5-week-old crown galls of both tomato and castor bean plants, the A. tumefaciens strain with ACC deaminase accumulated to a population that was >20 times that of wild-type A. tumefaciens (Hao et al., 2007). This could be due to two reasons: first, the presence of ACC deaminase reduced the ethylene level synthesized by plants with the concomitant reduction of the expression of plant defense genes. Second, A. tumefaciens with ACC deaminase might use ACC as a nitrogen and carbon source
and thereby survive better and proliferate faster in the tumor than the wild-type strain. On the other hand, using melon cotyledon segments, Nonaka and colleagues reported that inclusion of ACC in the germination and cocultivation medium increased new ethylene evolution by the plant tissue, but did not inhibit A. tumefaciens growth (Nonaka et al., 2008b). To study whether the presence of ACC deaminase also affected Agrobacterium proliferation during the infection and cocultivation process, bacterial populations in the infected canola tissues were estimated 2 days after infection. Both the canola cultivars 4414RR and Hyola 401 yielded similar results. That is, when plants were infected with either an OD600 nm=1 or an OD600 nm=0.1 culture suspension (about 5 × 108 or 5 × 107 per cell mL−1, respectively), after 2 days of cocultivation on MS with 2,4-D (1 mg L−1) medium, both A. tumefaciens YH-1 and A. tumefaciens YH-2 were able to propagate to a population of about 109 CFU g−1 fresh weight of plant tissue.