An Empirical Assessment of Transgene Flow from a Bt Transgenic Poplar Plantation

Editing Metabolism, Sex, and Microbiome: How Can We Help Poplar Resist Pathogens?

Poplar (Populus) is a genus of woody plants of great economic value. Due to the growing economic importance of poplar, there is a need to ensure its stable growth by increasing its resistance to pathogens. Genetic engineering can create organisms with improved traits faster than traditional methods, and with the development of CRISPR/Cas-based genome editing systems, scientists have a new highly effective tool for creating valuable genotypes. In this review, we summarize the latest research data on poplar diseases, the biology of their pathogens and how these plants resist pathogens. In the final section, we propose to plant male or mixed poplar populations; consider the genes of the MLO group, transcription factors of the WRKY and MYB families and defensive proteins BbChit1, LJAMP2, MsrA2 and PtDef as the most promising targets for genetic engineering; and also pay attention to the possibility of microbiome engineering.

Impact of PaGLK transgenic poplar on microbial community and soil enzyme activity in rhizosphere soil

Rhizosphere microorganisms are essential parts in maintaining soil ecological functions. Reforestation using genetically modified trees might have great potential to enhance tree production in biotic and abiotic stress, however, their long-term impact on rhizosphere microorganisms is scant. In this study, we studied soil enzyme activities and composition of rhizosphere microorganisms in 2-year-old transgenic PaGLK overexpression (OE), repressed expression (RE) and wild-type (WT) poplar (P. alba × P.berlinensis). The root exudates of PaGLK transgenic poplar (P.alba × P. berlinensis) were analyzed by liquid chromatography-mass spectrometry (LC-MS). The results showed that there were significant difference for soil sucrase, urease, catalase, neutral protease and cellulase between the transgenic and WT lines at different growth periods. Alpha diversity analysis showed that bacterial community abundance and diversity for RE lines were significantly lower than WT (p < 0.05), while RE lines for fungi were significantly higher than WT lines. At the genus level, Burkholderia was the dominant group of rhizosphere bacterial community, and the relative abundance for RE was significantly higher than WT. Tomentella was the dominant group for fungi community. Serendipita for RE was significantly higher than WT and OE. Main metabolite contents of (S)-ACPA, geniposidic acid, agnuside, hydroquinone and pyranocoumarins were significantly different among transgenic lines. These results suggest that transgenic activities have effects on root exudates, rhizosphere soil enzyme activities and soil microbial community composition, but long term effects need to be further investigated.

Plastid-expressed Bacillus thuringiensis (Bt) cry3Bb confers high mortality to a leaf eating beetle in poplar

The Bacillus thuringiensis (Bt) cry3Bb gene was successfully introduced into poplar plastid genome, leading to transplastomic poplar with high mortality to Plagiodera versicolora. Poplar (Populus L.) is one of the main resource of woody industry, but being damaged by insect pests. The feasibility and efficiency of plastid transformation technology for controlling two lepidopteran caterpillars have been demonstrated previously. Here, we introduced B. thuringiensis (Bt) cry3Bb into poplar plastid genome by biolistic bombardment for controlling P. versicolora, a widely distributed forest pest. Chimeric cry3Bb gene is controlled by the tobacco plastid rRNA operon promoter combined with the 5'UTR from gene10 of bacteriophage T7 (NtPrrn:T7g10) and the 3'UTR from the E. coli ribosomal RNA operon rrnB (TrrnB). The integration of transgene and homoplasmy of transplastomic poplar plants was confirmed by Southern blot analysis. Northern blot analysis indicated that cry3Bb was transcribed to both read through and shorter length transcripts in plastid. The transplastomic poplar expressing Cry3Bb insecticidal protein showed the highest accumulation level in young leaves, which reach up to 16.8 μg/g fresh weight, and comparatively low levels in mature and old leaves. Feeding the young leaves from Bt-Cry3Bb plastid lines to P. versicolora caused 100% mortality in the first-instar larvae after only 1 day, in the second-instar larvae after 2 days, and in the third-instar larvae for 3 days. Thus, we report a successful extension of plastid engineering poplar against the chrysomelid beetle.

Optimization of the cry1Ah1 Sequence Enhances the Hyper-Resistance of Transgenic Poplars to Hyphantria cunea

Increased expression of the insect control protein genes of Bacillus thuringiensis in Populus has been critical to the development of genetically improved plants with agronomically acceptable levels of insect resistance. Bacillus thuringiensis (Cry1Ah1) proteins with highly specific toxicity against Hyphantria cunea were screened using an indoor bioactivity assay to obtain hyper-resistant transgenic poplars. Then, the Cry1Ah1 sequence was optimized and transformed according to the optimal codon in poplar using software of our own design (http://120.79.60.226:8080/u/chen/w/codonpoplar). A vector was constructed to transform poplar NL895. The Cry1Ah1 gene was transformed to poplar NL895 and six transgenic lines were obtained. The expression and insecticidal effect of the Cry1Ah1 gene in transgenic poplar were evaluated by PCR and ELISA, and the specific indoor activity and field insecticidal activity against H. cunea were compared with a control. We concluded that the insecticidal activity of the transgenic NL895 was significantly better against lower instar larvae of H. cunea than against higher instar larvae. The mortality and pupation rates clearly differed among the various instar larvae and between transgenic and non-transgenic poplar. We obtained poplar seedlings with hyper-resistance to H. cunea by screening Bt genes and optimizing their genetic sequence.

Phenotypic Expression and Stability in a Large-Scale Field Study of Genetically Engineered Poplars Containing Sexual Containment Transgenes

Genetic engineering (GE) has the potential to help meet demand for forest products and ecological services. However, high research and development costs, market restrictions, and regulatory obstacles to performing field tests have severely limited the extent and duration of field research. There is a notable paucity of field studies of flowering GE trees due to the time frame required and regulatory constraints. Here we summarize our findings from field testing over 3,300 GE poplar trees and 948 transformation events in a single, 3.6 hectare field trial for seven growing seasons; this trial appears to be the largest field-based scientific study of GE forest trees in the world. The goal was to assess a diversity of approaches for obtaining bisexual sterility by modifying RNA expression or protein function of floral regulatory genes, including LEAFY, AGAMOUS, APETALA1, SHORT VEGETATIVE PHASE, and FLOWERING LOCUS T. Two female and one male clone were transformed with up to 23 different genetic constructs designed to obtain sterile flowers or delay onset of flowering. To prevent gene flow by pollen and facilitate regulatory approval, the test genotypes chosen were incompatible with native poplars in the area. We monitored tree survival, growth, floral onset, floral abundance, pollen production, seed formation and seed viability. Tree survival was above 95%, and variation in site conditions generally had a larger impact on vegetative performance and onset of flowering than did genetic constructs. Floral traits, when modified, were stable over three to five flowering seasons, and we successfully identified RNAi or overexpression constructs that either postponed floral onset or led to sterile flowers. There was an absence of detectable somaclonal variation; no trees were identified that showed vegetative or floral modifications that did not appear to be related to the transgene added. Surveys for seedling and sucker establishment both within and around the plantation identified small numbers of vegetative shoots (root sprouts) but no seedlings, indicative of a lack of establishment of trees via seeds in the area. Overall, this long term study showed that GE containment traits can be obtained which are effective, stable, and not associated with vegetative abnormalities or somaclonal variation.

PHENETIC ANALYSIS OF POPULUS NIGRA, P. LAURIFOLIA AND P. × JRTYSCHENSIS IN NATURAL HYBRIDIZATION ZONE

The wide spread of hybridization in the genus Populus, including spontaneous hybridization, caused by cultivars, requires studying the variability and inheritance of morphological traits by hybrids for initial tracking of these processes. The analysis of endogenous, intra- and inter-population variability was performed on 533 individual poplar trees in seven populations of P. nigra, seven populations of P. laurifolia and four populations of P. × jrtyschensis in the Tom river basin. On each specimen, 15 leaves from short mid-crown branches were collected to determine the shape of the leaf blade, the shape of its tip and base, as well as the branch morphotype. Some biometric indicators were proposed for geometric assessment of the leaf blade shapes of poplar species. The analysis showed that of all the traits examined only the leaf blade shape did not meet the criterion for “phene”, since it is usually represented by several forms in the crown of one and the same tree. In all the species studied, the level of their intra-population diversity was found to be much higher than the inter-population one. According to the increase of intra-population variability of qualitative traits, the taxa could be ranked as P. nigra < P. laurifolia < P. × jrtyschensis. The share of inter-population diversity differed among the species studied, accounting for 21.5% in P. laurifolia, 13.8% in P. nigra and 8.0% in P. × jrtyschensis. The P. laurifolia populations showed the greatest inter-population differentiation, most likely because of a disjunct distribution due to narrow specialization to the montane river environment. The lower differentiation in P. nigra is probably due to the facts that this species dominates the poplar stands of the Tom River basin; its populations are not fragmented and are linked by vast gene flow. In P. nigra, an increase in the diversity of qualitative characteristics and phenotypes was observed in populations confined to hybridization centers. Natural selection is most likely the factor governing the inter-population differentiation in P. × jrtyschensis, leading to the predominance of F1 hybrids in populations and hence to a sharp decrease in inter-population variability.