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Background: High concentrations of petroleum hydrocarbon (PHC) pollution can be hazardous to human health and leave soils incapable of supporting agricultural crops. A cheap solution, which can help restore biodiversity and bring land back to productivity, is cultivation of high biomass yielding willow trees. However, the genetic mechanisms which allow these fast-growing trees to tolerate PHCs are as yet unclear. Methods: Salix purpurea 'Fish Creek' trees were pot-grown in soil from a former petroleum refinery, either lacking or enriched with C10-C50 PHCs. De novo assembled transcriptomes were compared between tree organs and impartially annotated without a priori constraint to any organism. Results: Over 45 % of differentially expressed genes originated from foreign organisms, the majority from the two-spotted spidermite, Tetranychus urticae. Over 99 % of T. urticae transcripts were differentially expressed with greater abundance in non-contaminated trees. Plant transcripts involved in the polypropanoid pathway, including phenylalanine ammonia-lyase (PAL), had greater expression in contaminated trees whereas most resistance genes showed higher expression in non-contaminated trees. Conclusions: The impartial approach to annotation of the de novo transcriptomes, allowing for the possibility for multiple species identification, was essential for interpretation of the crop's response treatment. The meta-transcriptomic pattern of expression suggests a cross-tolerance mechanism whereby abiotic stress resistance systems provide improved biotic resistance. These findings highlight a valuable but complex biotic and abiotic stress response to real-world, multidimensional contamination which could, in part, help explain why crops such as willow can produce uniquely high biomass yields on challenging marginal land.

Key message: Highly tolerant and productive willow cultivars have been identified as potential candidates for phytoremediation of contaminated soil with petroleum hydrocarbons (PHs) in southern Quebec, Canada. Abstract: Tolerance of Salix to various organic and inorganic contaminants has been well documented and can vary widely among genotypes. In this study, we compared the responses and tolerances of 11 willow cultivars grown in soil contaminated with petroleum hydrocarbons. The study aimed to identify cultivars with the best performance and capacity to establish under these conditions. Using cuttings of the selected willow genotypes, a high-density field experiment was set up on a former industrial site encompassing two distinct sectors: one contaminated (900 m2), and the other uncontaminated (2,400 m2), which was used as control following a randomized block design. Trees were monitored over two growing seasons by recording a series of growth parameters and physiological measurements (leaf area, chlorophyll concentration, stomatal conductance and nutrient concentration). Genotypic differences explained most of the responses of cultivars observed to soil contamination. S. miyabeana (SX67 and SX61) achieved the highest biomass production, while S. nigra (S05) and S. acutifolia (S54) had the highest photosynthetic capacity. While the cultivars S. × dasyclados (SV1), S. purpurea (‘Fish Creek’) and S. caprea (S365) appeared to be negatively affected by the presence of contaminants, the establishment and development of S05, S. eriocephala (S25) and S. purpurea × S. miyabeana (‘Millbrook’) was much less influenced by soil pollution. Our results will help to guide cultivar selection in future phytoremediation projects.

We investigated the short-term response of poplar roots to low and high nitrogen availability in order to elucidate the mechanisms involved in nutrient acquisition. After 28 days of fertilization with low versus high ammonium nitrate, an increase in aboveground biomass was observed accompanied by a decrease in root biomass, reducing the root: shoot ratio after 28 days. These changes in biomass allocation were accompanied by changes in root architecture and altered gene expression. The gene expression response was evaluated after 7 days using a custom cDNA micrarray following transfer to low and high nitrogen supply. We found that 56 sequences were differentially expressed in poplar roots. Many of these 56 genes could be associated with putative roles in development or response to biotic and abiotic stress. A time course analysis of selected cell wall-related genes by RT-qPCR confirmed the expression patterns obtained by microarray and also showed the timing of this differential response. Our results show that patterns of transcript accumulation in roots of poplars are influenced by nitrogen supply, providing evidence of unique nitrogen-adaptative mechanisms.