Populus alba is widely distributed and cultivated in Europe and Asia. This species has been used for diverse studies. In this study, we assembled a de novo genome sequence of P. alba var. pyramidalis (= P. bolleana) and confirmed its high transformation efficiency and short transformation time by experiments. Through a process of hybrid genome assembly, a total of 464 M of the genome was assembled. Annotation analyses predicted 37 901 protein-coding genes. This genome is highly collinear to that of P. trichocarpa, with most genes having orthologs in the two species. We found a marked expansion of gene families related to histone and the hormone auxin but loss of disease resistance genes in P. alba if compared with the closely related P. trichocarpa. The genome sequence presented here represents a valuable resource for further molecular functional analyses of this species as a new tree model, poplar breeding practices and comparative genomic analyses across different poplars.

High salinity, one of the most widespread abiotic stresses, inhibits photosynthesis, reduces vegetation growth, blocks respiration and disrupts metabolism in plants. In order to survive their long-term lifecycle, trees, such as Populus species, recruit the abscisic acid (ABA) signaling pathway to adapt to a saline environment. However, the molecular mechanism behind the ABA-mediated salt stress response in woody plants remains elusive. We have isolated a WRKY transcription factor gene, PalWRKY77, from Populus alba var. pyramidalis (poplar), the expression of which is repressed by salt stress. PalWRKY77 decreases salt tolerance in poplar. Furthermore, PalWRKY77 negatively regulated ABA-responsive genes and relieved ABAmediated growth inhibition, indicating that PalWRKY77 is a repressor of the ABA response. In vivo and in vitro assays revealed that PalWRKY77 targets the ABA-and salt-induced PalNAC002 and PalRD26 genes by binding to the W-boxes in their promoters. In addition, overexpression of both PalNAC002 and PalRD26 could elevate salt tolerance in transgenic poplars. These findings reveal a novel negative regulation mechanism for the ABA signaling pathway mediated by PalWRKY77 that results in more sensitivity to salt stress in poplar. This deepens our understanding of the complex responses of woody species to salt stress.

Long noncoding RNAs (lncRNAs) are involved in various biological regulatory processes, but their roles in plants resistance to salt stress remain largely unknown. To systematically explore the characteristics of lncRNAs and their roles in plant salt responses, we conducted strand-specific RNA-sequencing of four tissue types with salt treatments in two closely related poplars (Populus euphratica and Populus alba var. pyramidalis), and a total of 10,646 and 10,531 lncRNAs were identified, respectively. These lncRNAs showed significantly lower values in terms of length, expression, and expression correction than with mRNA. We further found that about 40% and 60% of these identified lncRNAs responded to salt stress with tissue-specific expression patterns across the two poplars. Furthermore, lncRNAs showed weak evolutionary conservation in sequences and exhibited diverse regulatory styles; in particular, tissue- and species-specific responses to salt stress varied greatly in two poplars, for example, 322 lncRNAs were found highly expressed in P. euphratica but not in P. alba var. pyramidalis and 3,425 lncRNAs were identified to be species-specific in P. euphratica in response to salt stress. Moreover, tissue-specific expression of lncRNAs in two poplars were identified with predicted target genes included Aux/IAA, NAC, MYB, involved in regulating plant growth and the plant stress response. Taken together, the systematic analysis of lncRNAs between sister species enhances our understanding of the characteristics of lncRNAs and their roles in plant growth and salt response.

Secondary metabolites of the flavonoid pathway participate in plant defense, and bHLH and MYB transcription factors regulate the synthesis of these metabolites. Here, we define the regulatory mechanisms in response to pathogens. Two transcription factors from Populus alba var. pyramidalis, PalbHLH1 and PalMYB90, were overexpressed together in poplar, and transcriptome analysis revealed differences in response to pathogen infection. The transgenic plants showed elevated levels of several key flavonoid pathway components: total phenols, proanthocyanidins (PAs), and anthocyanins and intermediates quercetin and kaempferol. Furthermore, PalbHLH1 and PalMYB90 overexpression in poplar enhanced antioxidase activities and H 2 O 2 release and also increased resistance to Botrytis cinerea and Dothiorella gregaria infection. Gene expression profile analysis showed most genes involved in the flavonoid biosynthesis pathway or antioxidant response to be upregulated in MYB90/bHLH1-OE poplar, but significant differential expression occurred in response to pathogen infection. Specifically, expression of PalF3H (flavanone 3-hydroxylase), PalDFR (dihydroflavonol 4-seductase), PalANS (anthocyanin synthase), and PalANR (anthocyanin reductase), which function in initial, middle, and final steps of anthocyanin and PA biosynthesis, respectively, was significantly upregulated in D. gregaria-infected MYB90/ bHLH1-OE poplar. Our results highlight that PalbHLH1 and PalMYB90 function as transcriptional activators of flavonoid pathway secondary-metabolite synthesis genes, with differential mechanisms in response to bacterial or fungal infection.

Herein, electrochemical water oxidation catalyzed by a binuclear copper complex [Cu2(MePy3P)2] (1, MePy3PH2 = N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarbox-amide) with helical structure is reported. Oxygen evolution occurs with low onset overpotential of 483 mV,...