3.2. Utilization of ACC to support the growth of cadmium-resistant PGPR
Both Micrococcus sp. and Klebsiella sp. were able to grow in DF salt minimal medium supplemented with ACC. The growth of Micrococcus sp. MU1 in DF salt minimal medium supplemented either with ACC or (NH4)2SO4 was lower than that of Klebsiella sp. (Fig. 2). Moreover, Micrococcus sp. grew 105% more in DF salt minimal medium amended with ACC than in DF salt minimal med- ium amended with (NH4)2SO4. Klebsiella sp. grew 74% lower density in a medium supplemented with ACC than in a medium supplemented with (NH4)2SO4. This finding suggests that Micrococcus sp. has a high ability to use ACC as a sole nitrogen source. Generally, bacteria that use ACC as a sole nitrogen source are able to produce ACC deaminase, an enzyme that can hydrolyze ACC as the plant ethylene precursor and help to reduce the level of stress from ethylene gas generated by the plant in the presence of salts and heavy metals (Belimov et al., 2005; Zahir et al., 2009). Endophytic PGPR have the ability to protect Brassica napus against lead toxicity (Rajkumar and Freitas, 2008; Sheng et al., 2008b) found that Pseudomonas sp. PsM6 and Pseudomonas jessenii Pjm15 could grow in DF salts minimal medium supplemented with ACC. Achromobacter xylosoxidans Ax10, a copper-resistant bacte- rium, was able to use ACC as a sole nitrogen source to produce