Tree age affects wood formation and yield. However, the underlying mechanisms are poorly understood, particularly at the molecular level. In this study, we investigated the transcriptomic changes of the uppermost main stems of Larix kaempferi in an entire rotation period using the RNA-Seq method. In total, ∼151 million reads were obtained from the stems of 1-, 2-, 5-, 10-, 25-, and 50-year-old L. kaempferi trees. Combining these with the published Illumina sequencing reads, 299,637 assembled transcripts were generated, of which 161,232 were annotated. Time series expression profiling identified 12,927 transcripts as differentially expressed genes (DEGs); function enrichment analysis of these DEGs showed that 459 gene ontology terms in the biological process category were enriched. These terms were associated with the processes of wood formation, such as cell differentiation, growth and death, and its hormonal regulation. Based on the expression patterns of L. kaempferi homologues of genes associated with ethylene, calcium, and cell wall expansion and synthesis, the regulatory network of tracheid growth was outlined. Altogether, the comparative transcriptomic analysis reported here demonstrated the molecular aspects of aging effects on L. kaempferi wood formation. The identification of genes involved in the regulatory network of tracheid growth provides a means of investigating the regulation of wood formation in gymnosperm trees and also offers potential targets for genetic manipulation to improve the properties of xylem fibers.

We studied the parental taxa and the interspecific reciprocal hybrids between Larix leptolepis with Larix gmelinii, using classical cytogenetic methods, as well as fluorescence in situ hybridization (FISH) and genomic in situ hybridization. A high frequency (>90%) of complete bivalent formation was observed in reciprocal hybrids. Less than 10% of pollen mother cells exhibited abnormalities. The most frequent abnormalities were bridges. Multivalent chromosome associations were also observed in both reciprocal hybrids, suggesting that some chromosome interchange events did occur, and introgressions from one to the other species were possible. Intergenomic recombination indicates that genes might be readily introgressed into one species from the other in the genus Larix. Interspecific hybridization may be a potential method for genetic improvement in larch. FISH markers documented that the recombinant genomes of reciprocal hybrids were strictly additive and stable, indicating that FISH also might be a useful tool in Larix breeding.

The Inter-Simple Sequence Repeat (ISSR) was used in this study for genetic fingerprinting and identification of 28 important Populus L. (poplar) cultivars (varieties/ clones), and determination of the genetic relationships among these cultivars. These 28 cultivars belonged to sections Aigeiros, Tacahamaca, Leuce, Turanga, and hybrids between sections Aigeiros and Tacahamaca. Out of 27 ISSR primers tested, eight primers generated clear multiplex profiles. The best three primers produced 154 easily detectable fragments, 129 (84%) of which were polymorphic among the cultivars. Each of these 3 primers produced fingerprint profiles unique to each of the accessions studied, and thus could be solely used for their identification. Twenty-five markers, unique to 10 of the cultivars studied, were detected. These markers may be converted into cultivar-specific probes for identification purposes. Genetic relationships among the cultivars were evaluated by generating a similarity matrix based on the simple matching coefficient and the unweighted pair group method with arithmetic average (UPGMA) dendrogram. The results showed a clear-cut separation of cultivars among different sections of poplar, and were in agreement with the genealogy of the sampled cultivars. The present study shows that ISSR markers could generate abundant polymorphism, are reproducible, and are quick for characterization of poplar cultivars. In the future, the markers used in this study, in combination with other molecular techniques, could provide a useful panel of ISSR markers for largescale DNA fingerprinting of poplar cultivars and determination of the genetic relationships among these cultivars.