Some work was also done on the creation of disease-tolerant trees. It was, however, on a much smaller scale than the work on insect tolerance. The Forestry University in Beijing has carried out work in breeding Eucalyptus urophylla ST Blake lines with an increased tolerance for Pseudomonas solanacearum. The basis for that transformation was the introduction of the cecropin D gene which codes for an antibacterial peptide into the plant (Shao et al. 2002). Such transgenic plants were less affected by inoculation with Pseudomonas solanacearum compared to control plants of the wildtype.
Work is also underway to transform poplar plants with genes which can cause resistance to fungal attack. A research group of Shandong Agricultural University developed a regeneration system and transformed Populus deltoides clone G1 with the chitinase 5B gene (CH5B-from French bean Phaseolus vulgaris). The integration of the gene into the plant genome was confirmed (Meng et al. 2004). With the same poplar clone (G1) a glucanase gene (P-1,3-glucanase; BG2 was also introduced and the integration of the gene into the plant genome has already been confirmed (Han et al. 2004).
Investigations aimed at reducing lignin content in poplar wood were carried out at the Beijing Agro-Biotechnology Research Center. The antisense gene of 4-coumarate CoA ligase (4CL) was transformed using Agrobacterium tumefaciens into a triploid white poplar (Populus tomentosa Carr.; Jia et al. 2004). The integration of the gene was confirmed by PCR and Southern blotting. RT-PCR and Western blot showed the expression of the gene. Transgenic poplars which were obtained had a reduced lignin content, but were unchanged in their holocellulose content. The wildtype clone used for this work is a rapid growing tree that has been bred for afforestation and as raw material for the pulp and paper industry. The growth rate was unchanged in transgenic individuals.
A Taiwanese publication reports a successful transformation of Eucalyptus camaldulensis with the reporter genes GUS (g-glucuronidase) and NPT (neomycin phosphotransferase) by Agrobacterium tumefaciens transformation (Ho et al. 1998). This kind of work was the necessary precondition for the transfer of the cinnamate 4-hydroxylase gene (C4H) from Populus tremu-loides L. (Chen et al. 2001). This gene was inserted in sense or antisense direction. In comparison with control shoots and transgenic shoots with the 6-glucuronidase gene the transgenic shoots with C4H proliferated and elongated faster. More than 100 plants were produced for further characterization concerning a possible alteration of lignin composition or lignin content. A group of researchers from the Chinese Academy of Sciences (Wei et al. 2001) transformed a hybrid of Populus tremula x P. alba with an antisense gene of caffeoyl CoA O-methyltransferase (CCoAOMT) from the Chinese white poplar (P. tomentosa). The measurement of the lignin content in five- to six-month-old transgenic plants showed that one transgenic line had a lower lignin content (17.9% less) as compared to the control. Although the authors highlighted the benefits they also addressed possible consequences for the stability and resistance of the plants as well, given that lignin is central to wood structure and plays a role in plant defence.
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