Transformation of sweetgum via microprojectile bombardment of nodule cultures

A sweetgum (Liquidambar styraciflua) nodule culture system was developed and integrated with genetic transformation by microprojectile bombardment. Nodule cultures were established from seedling hypocotyls and proliferated in liquid medium containing 0.1 mg (0.45 µM) thidiazuron (TDZ) per 1 and 0.01 mg (0.045 µM) 2,4-dichlorophenoxyacetic acid (2,4-D) per 1. Shoots differentiated from the nodules in liquid media containing (per 1) 1 mg (4.4 µM) benzyladenine (BA), 0.5 mg (2.2 µM) BA, and 0.01 mg (0.054 µM) naphthaleneacetic acid (NAA), or 0.5 mg BA, 0.01 mg NAA, and 0.05 mg (0.23 µM) TDZ under the light. Differentiating shoots required 4 wk of dark treatment for further development on semisolid medium containing 1 mg BA per 1. Elongated shoots were harvested and the basal ends were soaked in a solution containing 10 mg (49.2 µM) indole-3-butyric acid (IBA) per 1 before being planted in potting mix for ex vitro rooting. Roots formed and leaves expanded in 2 wk. Sweetgum nodules were stably transformed by microprojectile bombardment with a 7.4-kb plasmid, pTRA 140, harboring CaMV 35S-HPH and CaMV 35S-GUS. Evidence that nodules growing in the presence of hygromycin B were stably transformed was provided by polymerase chain reaction analysis and β-glucuronidase activity. Sweetgum shoots differentiated in liquid medium in the presence of hygromycin B. Shoots transferred to solid medium lacking hygromycin B elongated and displayed β-glucuronidase activity in their expanding leaves and stems. Southern analysis confirmed the presence of the GUS gene in nodules and shoots. Transgenic shoots initiated roots and showed leaf expansion 2 wk after being planted in potting mix.