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The timing of phase change from vegetative to reproductive development during aging of forest trees is important for wood and seed production. In a previous study, we investigated the effects of aging on wood formation by measuring the transcriptomic changes in the uppermost main stems of 1-, 2-, 5-, 10-, 25-, and 50-year-old Larix kaempferi. Based on the published transcriptome data, here we investigated the transcriptomic differences between the juvenile vegetative (1- and 2-year-old) and adult reproductive (25- and 50-year-old) phases to determine the molecular mechanisms underlying the phase change. In total, 12,789 transcripts were identified as differentially expressed genes, including 573 transcription factors. Further analysis showed that 27 transcription factors belonging to 8 families were common to all four comparisons between old and young life stage categories: (I) 25 vs 1 year old; (II) 50 vs 1 year old; (III) 25 vs 2 years old; and (IV) 50 vs 2 years old. The analysis of their expression patterns in six age categories showed that members of the AP2 and Dof families were expressed highly in 1- and 2-year-old trees, weakly in 25- and 50-year-old trees, while members of the C3H, G2-like, GRAS, MYB-related, and MADS families had the opposite patterns. Notably, one member of the MADS family was only detected in 25- and 50-year-old trees. These results suggest that the phase change might (1) occur in the early stage of the L. kaempferi lifetime and (2) be controlled by a complex regulatory network of different transcription factors, some of which are known to play roles in the phase change in model plants. These findings not only provide molecular markers to distinguish different stages of tree growth and development and potential targets for genetic manipulation to improve the reproductive traits of trees, but also improve our understanding of the phase change with aging in trees.

Hydrogen peroxide (H₂O₂) content and transcript levels of genes encoding superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) antioxidant enzymes were investigated during different stages of somatic embryogenesis in Larix leptolepis. H₂O₂ content was lowest on day 0 when embryogenic callus was incubated on Murashige and Skoog (MS) medium supplemented with polyethylene glycol (PEG). This content began to increase when callus was transferred to the same medium but containing abscisic acid (ABA), and reached a peak at day 3 following incubation. The level of H₂O₂ dropped from day 7 to day 10, peaked at day 21, then dropped again at day 24 and day 35, but increased when somatic embryos reached maturity at day 45. Transcript levels of SOD, CAT, and APX were lowest when somatic embryos were cultured on callus induction medium without ABA. When calli were transferred onto somatic embryo maturation medium, expression patterns of SOD, CAT, and APX varied, and transcript levels at all stages were higher than those at d 0. SOD expression was highest at day 3. Whereas, CAT expression levels were low during early stages of somatic embryogenesis, but increased at day 21, declined at days 24 and 35, and then began to increase again at day 45. APX gene expression patterns were highest at days 3, 21, and 45. These results suggested that ABA was essential for promoting somatic embryogenesis of L. leptolepis. Moreover, ABA induced production of H₂O₂ and other active oxygen species (AOS), and mediated CAT, SOD, and APX gene expression in somatic embryogenesis of L. leptolepis.

During the process of subculture of embryogenic cultures, sometimes they may become non-embryogenic, which is not desirable. However, this offers an opportunity to explore the mechanisms underlying cell fate determination and the maintenance of embryogenic potential of explants during the process of somatic embryogenesis. In a previous study, differential expression of microRNAs (miRNAs) has been detected between embryogenic and non-embryogenic cultures as well as during somatic embryo maturation of Larix kaempferi (Lamb.) Carr. However, little is known about the target genes of these miRNAs during these cellular differentiation processes. In this study, full-length cDNA of the MYB homologue from L. kaempferi, LaMYB33, was cloned. Sequence analysis showed that the miR159 target sequence is present in LaMYB33. The isolation of the miRNA-guided cleavage products of LaMYB33 further suggested that this gene is regulated by miRNA. LaMYB33 transcript levels between embryogenic and non-embryogenic cultures and during the late stage of somatic embryo maturation were measured and the results showed opposite patterns in the expression of LaMYB33 and mature miR159. Based on the relationships between the expression patterns of LaMYB33 and mature miR159, we concluded that the post-transcriptional regulation of LaMYB33 by miR159 participates in the maintenance of embryogenic or non-embryogenic potential and somatic embryo maturation, providing new insights into the regulatory mechanisms of somatic embryogenesis.

Aims Larix kaempferi planting with broad-leaved trees can increase production and alleviate soil degradation; however, the mechanism is unclear, especially on large scales and in different climate regions. This paper attempts to explain the underlying mechanism in a temperate region and northern subtropical alpine region of China. Methods Litter decomposition rates, nutrient release, enzyme activity and microbial community composition were determined in conifer litter, broad-leaved litter and mixed litter by the litterbag method during about 500 days in the two regions. Results Decomposition rate was increased by 4.00% in temperate region and 3.38% in the subtropical region after litter mixing and the synergistic effect increased with decomposition progress. However, nutrients release was not always enhanced, and enzyme activities were enhanced at the most of decomposition time and higher in the subtropical region. After litter mixing, a few new dominant taxa emerged, and the microbial gene quantities were increased in the temperate region, but not in the subtropical region. The dominant bacterial taxa were similar, while most of the dominant fungal taxa shifted with decomposition. In addition, the dominant fungi completely differed between the two regions. The environmental variables with greater influence on microbial community were similar in the two regions, including C:N, C:P and pH. Conclusions After mixing with broad-leaved litter, the decomposition of Larix kaempferi was enhanced by increasing enzyme activities and microbial quantities in the temperate region, but the mechanisms were not clear in the subtropical region.

Background The fungus Marssonina brunnea is a causal pathogen of Marssonina leaf spot that devastates poplar plantations by defoliating susceptible trees before normal fall leaf drop. Results We sequence the genome of M. brunnea with a size of 52 Mb assembled into 89 scaffolds, representing the first sequenced Dermateaceae genome. By inoculating this fungus onto a poplar hybrid clone, we investigate how M. brunnea interacts and co-evolves with its host to colonize poplar leaves. While a handful of virulence genes in M. brunnea , mostly from the LysM family, are detected to up-regulate during infection, the poplar down-regulates its resistance genes, such as nucleotide binding site domains and leucine rich repeats, in response to infection. From 10,027 predicted proteins of M. brunnea in a comparison with those from poplar, we identify four poplar transferases that stimulate the host to resist M. brunnea . These transferas-encoding genes may have driven the co-evolution of M. brunnea and Populus during the process of infection and anti-infection. Conclusions Our results from the draft sequence of the M. brunnea genome provide evidence for genome-genome interactions that play an important role in poplar-pathogen co-evolution. This knowledge could help to design effective strategies for controlling Marssonina leaf spot in poplar.

Low levels of abscisic acid (ABA) are associated with embryo asynchronism during somatic embryogenesis. Small non-coding RNAs (sncRNAs) are emerging as key regulators of embryogenesis. To investigate the roles of sncRNAs in regulating the synchronism of somatic embryogenesis in Larix leptolepis, we examined the endogenous “sncRNAome” in synchronous and asynchronous embryos. Non-redundant sRNAs 24 nt in length were overrepresented in both synchronous and asynchronous embryos, while the majority of redundant sRNAs were 24 nt long in synchronous embryos and 21 nt long in asynchronous embryos. Many more non-redundant unknown sRNAs were expressed in synchronous embryos than in asynchronous embryos, indicating that many sRNAs might participate in the regulation of synchronism during larch somatic embryogenesis. However, the proportion of sRNAs that were miRNAs was higher in asynchronous embryos than in synchronous embryos, and their expression was much higher in asynchronous embryos. Although the proportion of sRNAs that matched repeat regions (ra-siRNA) was higher in asynchronous embryos than in synchronous embryos, the ratio of 24-nt ra-siRNAs to total sRNAs was much lower in asynchronous embryos than in synchronous embryos. Further analysis suggested that miRNAs exemplified by miR156 might participate in the regulation of embryo synchronism in larch, which causes a large number of transcripts encoding differentiation-promoting factors to be down regulated. Therefore, we speculate that sncRNAs induced by ABA inhibit the precocious expression of differentiation-promoting factors, thereby regulating the synchronism of somatic embryogenesis.