Asymmetrical natural hybridization between Populus deltoides and P. balsamifera (Salicaceae)This note is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Natural hybridization has long been recognized as a means for gene flow between species and has important evolutionary consequences. Although hybridization is generally considered to be symmetrical, with both hybridizing species being equally likely to be the male or female parent, several studies have demonstrated the presence of asymmetrical hybridization and introgression from one species to the other. We investigated the direction of natural hybridization between two sympatric forest tree species in North America ( Populus deltoides Bartr. ex Marsh. and Populus balsamifera L.) using species-specific single nucleotide polymorphism (SNP) markers in both the nuclear and chloroplast genomes. All natural hybrid individuals, identified from morphological traits, had nuclear alleles corresponding to both parental species, while the chloroplast genotypes showed similarity to P. deltoides, indicating asymmetrical hybridization with P. deltoides as the maternal and P. balsamifera as the paternal donor species. This observed asymmetrical hybridization may be attributable to cytonuclear interactions.

Cryptosporiopsis species isolated from the roots of aspen in central Alberta: identification, morphology, and interactions with the host, in vitroThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Cryptosporiopsis Bubák & Kabát isolates were obtained for the first time from roots of apparently healthy aspen seedlings in Alberta. These isolates were similar in all the major morphological features previously used to separate Cryptosporiopsis species, but sequencing data of the ITS1-5.8S-ITS2 region indicated that they were separated into two groups, one belonging to Cryptosporiopsis ericae Sigler and the other to Cryptosporiopsis radicicola Kowalski & Bartnik. Scanning electron microscopy of ex-type cultures and selected isolates from aspen roots revealed that C. ericae and C. radicicola differed in morphogenesis and structure of conidiomata: those of C. ericae were either synnematous or sporodochial, whereas those of C. radicicola possessed a peridium-like mycelial envelope bearing amorphous adhesive material. Phialides in the hymenium of C. radicicola were also embedded in amorphous matrix material but such material was absent in C. ericae. Microscopic examination of artificially inoculated aspen roots indicated that both species are endophytes of the host. Hyphal penetration by C. ericae was only occasional and confined to the host epidermis, whereas C. radicicola was more aggressive and its hyphal ingress extended to the cortical region.

Trees of the people: the growing science of poplars in Canada and worldwideThis commentary is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Poplars are trees of the angiosperm genus Populus , a group of generally diploid, dioecious, and deciduous pioneers that are widespread in forests around the Northern Hemisphere. Canada is a centre of poplar abundance and diversity, with 7 of the 29 global Populus species occurring naturally, and other species being introduced. Numerous interspecific hybrids also occur, including native hybrids that create centres of biodiversity, and artificial hybrid poplars that are planted for wood and fiber production, as well as for environmental and landscape applications. This paper introduces poplars and their distribution across Canada and explores Canadian and international contributions to poplar research. It also discusses papers of the special issue Poplar Research in Canada in the November 2007 and December 2007 issues of the Canadian Journal of Botany, which contains a collection of 19 contributions from Canadian institutions that report on a diverse range of topics including genomics and molecular biology, physiology, ecology, population genetics, and systematics. Many of these contributions address issues pertinent to silviculture and other applications, and one article chronicles poplar research and poplar utilization in Canada. These studies that appear in this special issue reflect the growing international emphasis on poplar, not only as an important and widespread group of trees, but also as a model that provides broader insights relevant to fundamental and applied sciences involving trees and forest systems.

Shoot–root defense signaling and activation of root defense by leaf damage in poplarThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Shoot–root systemic defense signaling of hybrid poplar (Populus trichocarpa Torr. & A. Gray × Populus deltoides Bartr. ex Marsh.) was investigated with molecular techniques to extend existing knowledge of poplar defense. Treatment of roots with methyl jasmonate demonstrated that transcripts of PtdTI3, a poplar trypsin inhibitor and marker of poplar defense responses, can be induced in poplar roots as well as leaves. Moreover, simulated herbivory of poplar leaves with methyl jasmonate treatment or wounding with pliers also induced PtdTI3 mRNA in roots, which implies downward, or basipetal, systemic signaling from shoots to roots. In addition, the inducible root-defense response comprised both increased PtdTI3 protein levels and trypsin-inhibitor activity. The inducible systemic response was further investigated with comparative macroarray analyses which indicated that in addition to PtdTI3, other genes respond in roots after wounding and methyl jasmonate treatment of leaves. The majority of the 17 genes encode previously identified leaf herbivory defense genes; however, some genes strongly up-regulated in leaves were not induced in roots. The identification of multiple defense genes that are inducible in roots following leaf damage is clear evidence of a systemic defense response in roots and the presence of basipetal shoot–root defense signaling.

Induction of xylem and fiber differentiation in Populus tremuloidesThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Vascular tissues are of particular importance to terrestrial plants as they allow long-distance transport within the plant and also provide support for upright growth. Nowhere are these traits more obvious than in tree species. Here we have evaluated the role of auxin transport in the differentiation of primary and secondary vascular tissues in a tree species, trembling aspen ( Populus tremuloides Michx). We found that a partial inhibition of auxin transport resulted in increased width and numbers of veins in leaves. A similar vascular overgrowth was observed during early secondary vascular differentiation of stems. This stem overgrowth consisted almost entirely of early differentiation of metaxylem and fibers. We hypothesize that the early differentiation of metaxylem and fibers results from inhibitor-induced accumulation of auxin in stems and that the differentiation of these tissues requires higher levels of auxin exposure than protoxylem. The controlled conditions used in this study also provide a framework for reverse genetics approaches to identify genes involved in vascular differentiation based on elevated expression in tissues developing vascular overgrowth.

Poplar defense against insect herbivoresThis review is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

The availability of a poplar ( Populus trichocarpa Torr & A. Gray, black cottonwood) genome sequence is enabling new research approaches in angiosperm tree biology. Much of the recent genomics research in poplars has been on wood formation, growth and development, resistance to abiotic stress and pathogens, motivated, at least in part, by the fact that poplars provide an important system for large-scale, short-rotation plantation forestry in the Northern Hemisphere. To sustain productivity and ecosystem health of natural and planted poplar forests it is of critical importance to also develop a better understanding of the molecular mechanisms of defense and resistance of poplars against insect pests. Previous research has established a solid foundation of the chemical ecology of poplar defense against insects. This review summarizes some of the relevant literature on defense against insect herbivores in poplars with an emphasis on molecular, biochemical, and emerging genomic research in this important field within forest biotechnology and chemical ecology. Following a general introduction, we provide a brief overview of some of the most relevant insect pests of poplars; we then describe some of the general defense strategies of poplars along with selected examples of their activities. We conclude with a summary of emerging results and perspectives from recent advances in genomics research on poplar defense against insects.

Genome-wide analyses of phenylpropanoid-related genes in Populus trichocarpa, Arabidopsis thaliana, and Oryza sativa: the Populus lignin toolbox and conservation and diversification of angiosperm gene familiesThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

The completion of the Populus trichocarpa (Torr. & A. Gray) (poplar) genome sequence offers an opportunity to study complete genome families in a third fully sequenced angiosperm (after Arabidopsis and rice) and to conduct comparative genomics studies of angiosperm gene family evolution. We focussed on gene families encoding phenylpropanoid and phenylpropanoid-like enzymes, and identified and annotated the full set of genes encoding these and related enzymes in the poplar genome. We used a similar approach to identify an analogous set of genes from the rice genome and generated phylogenetic trees for nine phenylpropanoid gene families from aligned poplar, Arabidopsis, and rice predicted protein sequences. This enabled us to identify the likely full set of bona fide poplar lignin-related phenylpropanoid genes (poplar “lignification toolbox”) apparent within well-defined clades in all phylogenetic trees. Analysis of expression data for poplar genes confirmed and refined annotations of lignin-related genes, which generally showed high expression in wood-forming tissues. Expression data from both poplar and Arabidopsis were used to make inferences regarding biochemical and biological functions of phenylpropanoid-like genes with unknown functions. The comparative approach also provided insights into the evolution of angiosperm phenylpropanoid-like gene families, illustrating lineage-specific clades as well as ancient clades containing genes with apparent conserved function.