Poplars are among the most important deciduous tree species in China. China is replete with natural resources of poplars. Poplars have a number of good characteristics, including fast growth rate, high yield, many uses, easiness of tissue culture and small gene group that make them well suited as a model system for the application of genetic engineering in forest trees. In the last decade, much progress has been made in genetic improvement of poplar species in China. Modern biotechnology is an important tool for genetic improvement in forest trees, and its applications to genetic improvement in poplars, which covers genetic transformation, gene expression, construction of genetic linkage map, QTLs (quantitative trait loci) identification and molecular assisted selection are reviewed in this paper. At the same time, the existing problems and outlook about the application of modern biotechnology to genetic improvement in forest trees are also discussed.
Both non-transgenic hybrid triploid poplars [ (Populus tomentosa×P.bolleana)×P.tomentosa ] and transgenic ones expressing cowpea trypsin inhibitor were cut at the base of the stem to produce auxoblasts, and used as source of leaves for insect feeding trials performed on 3 major insect species of poplar, the forest tent caterpillar ( Malacosoma disstria L.), gypsy moth ( Lymantria dispar L.) and willow moth ( Stilpnotia candida Staudinger). The height and basal diameter of trees were measured by the end of that year (2000). The results indicated that the growth elements of transgenic poplars were not interfered by the incorporation of the CpTI gene. Intriguingly, the height and basal diameter of the clone TG04 were much greater than that of the control. The transgenic foliage consumed by insects induced the increase of larval mortality, and decrease of larval wet weight gain, faecal output, pupal weight and egg deposition. Among them 3 transgenic clones, TG04, TG07 and TG71 received special attention for their outstanding insect resistance compared with other transgenic clones, which showed that the CpTI gene in them was expressed more actively and stably than in others.
The contents of total soluble protein and cowpea trypsin inhibitor (CpTI) in the browse and metaphylla of transgenic hybrid triploid poplars [ (Populus tomentosa×P.bolleana)×P.tomentosa ] transformed with CpTI gene were determined in order to study the products of CpTI gene expression at protein level. The results indicated that the amount of total soluble protein was greater in transgenic poplars than in non transgenic poplars, but was more in the metaphylla than in browse. The expression of CpTI gene resulted in an obvious increase in CpTI content, whereas CpTI was not detected in non transgenic poplars. It was found that there were high amount of total soluble protein and CpTI in 3 clones of TG07, TG04 and TG71 compared with other transgenic clones. In addition, the analysis of protein by SDS PAGE showed that a specific protein band of about 11.3?kD corresponding to the 80 amino acids encoded by the CpTI gene was observed in transgenic poplars on the gel of protein, which was not detected in non transgenic poplars.
Populus tomentosa cuttings were treated with 1mmol·L -1 , 5mmol·L -1 , 10mmol·L -1 or 15mmol·L -1 of CaCl 2 for 1\|7 d, respectively, for studying the effects of different concentrations of CaCl 2 on freezing resistance. Results indicated that 10?mmol·L -1 of CaCl 2 has greater effect than other concentrations on the enhancement of freezing resistance, and the optimum time of pretreatment was 5?d. In addition, cuttings used for cold acclimation at -3℃ were pretreated with or without 10?mmol·L -1 of CaCl 2, 3?mmol·L -1 of Ca 2+ chelator EGTA, 0 05?mmol·L -1 of CaM antagonist CPZ or 0 1?mmol·L -1 of Ca 2+ channel inhibitor LaCl 3 The changes in CaM and freezing resistance of all cuttings were investigated. The results showed that cold acclimation at -3℃ increased CaM content and decreased the minimum temperature for 100% survival. The CaCl 2 pretreatment enhanced the effect of cold acclimation and obviously increased CaM content and decreased the minimum temperature for 100% survival, but this effect was strongly inhibited by the EGTA, CPZ or LaCl 3 It is concluded that the effect of CaCl 2 on freezing resistance is associated with its concentration and time of pretreatment, Ca 2+ CaM may be involved in the induction of freezing resistance of the cuttings.
