Transcriptomic Time-Series Analyses of Gene Expression Profile During Zygotic Embryo Development in Picea mongolica

Zygotic embryogenesis is a critical process during seed development in gymnosperms. However, knowledge on the genome-wide transcriptional activation that guides this process in conifers is limited, especially in Picea mongolica. This tree species is endemic to semiarid habitats of Inner Mongolia in China. To extend what is known about the molecular events underpinning its zygotic embryogenesis, comparative transcriptomic analyses of gene expression in zygotic embryos were performed by RNA sequencing in P. mongolica. Our results showed that most changes in transcript levels occurred in the early embryonic pattering determination and formation of mature embryos. Transcripts related to embryogenic competence, cell division pattern, hormones, and stress response genes were identified during embryogenesis. Auxin is essential for early embryo patterning and pre-cotyledon embryonic formation. However, ABA is a major regulator of embryo maturation. Moreover, we found that methylation-related gene expression is associated with activation of early-stage embryos, late embryogenesis abundant proteins, and storage/energy-related genes with late and mature embryos. Furthermore, network analysis revealed stage-specific and multistage gene expression clusters during embryogenesis. WOX, MYB, AP2, and HLH transcription factors seem more relevant to embryogenesis in different stages. Our results provide large-scale and comprehensive transcriptome data for embryo development in P. mongolica. These data will lay a foundation for the protection and utilization of P. mongolica resources.

Multiple analyses of various factors affecting the plantlet regeneration of Picea mongolica (H. Q. Wu) W.D. Xu from somatic embryos

Picea mongolica, a native species with excellent industrial wood quality and strong sand-fixing capacity, may be utilized in construction of urban green spaces in arid areas in China. However, now the sustainability of the ecosystems where this species grows is at serious risk due to a lack of natural regeneration. In this study, we developed an efficient regeneration system and comprehensively analyzed various factors affecting somatic embryogenesis (SE) using zygotic embryos as explants. We identified the optimal plant growth regulators (PGRs) performance and the best donor trees (k81) for the generation of somatic embryos (SEMs). Additionally, we confirmed that the positive developmental window of SEMs initiation was at the end of July to early August, which is when zygotic embryos was at the late embryogeny. In this time period, specific transcripts associated with the regulation of epigenetic modifications, plant hormone-related genes, and embryonic development-related transcription factors play important roles for early SEMs initiation. These results may provide a valuable resource for vegetative propagation of Picea mongolica. Our results may help to establish a reliable protocol for plantlet regeneration, which may facilitate urban greening applications and industrialization in arid areas.

Growth Responses of Picea Mongolica Saplings to Goat Browsing

Picea mongolicaW. D. Xu is an endemic and endangered species that is only found in semiarid areas in north China. It has also been widely used as the forestation tree in China's "Three-North" shelterbelts construction. Although a National Natural Reserve has been set up for conserving the rare species, it always suffers from goat browsing, especially during the winter. Dendrochronology analysis is applied to understand the consequences of goat browsing onPicea mongolica's growth. Two sites with different types of trees were chosen side by side to compare the differences between shape, radial growth, height growth, and age of spruce saplings: (1) stunted and heavily browsed spruce, shorter than the browsing limit and (2) escaped spruce that were taller than the browsing limit but still browsed in their lower part. Under repeated and intense browsing, the shape in stunted spruce was compact and heavily ramified, and the same phenomenon was shown below the browsing limit in escaped spruce (1.05 ± 0.06 m), distinctly different from the shape of escaped spruce above the browsing limit. The results showed that the release of browsing pressure, once the tree reached the browsing limit, was characterized by an abrupt increase in radial growth. Before release, trees showed a growth stagnation characterized by annual rings (0.5 mm per year) and annual height (<5 cm per year) increments. After release, the increments of rings and height were 2 mm and 14 cm each year, respectively. We use this pattern to estimate the release ages and their possible variation over time. Age differences between stunted and escaped spruce were highly significant and indicate that, despite browsing, most, if not all, trees will ultimately reach the browsing limit and escape. Heavy goat pressure delayed spruce sapling recruitment by about 10 years. This delay varied in relation to site quality and seemed to have increased over time, suggesting an increase in browsing pressure.