Populus euphratica is mainly distributed in desert environments with dry and hot climate in summer and cold in winter. Compared with other poplars, P. euphratica is more resistant to salt stress. It is critical to investigate the transcriptome and molecular basis of salt tolerance in order to uncover stress-related genes. In this study, salt-tolerant treatment of P. euphratica resulted in an increase in osmo-regulatory substances and recovery of antioxidant enzymes. To improve the mining efficiency of candidate genes, the analysis combining both the transcriptome WGCNA and the former GWAS results was selected, and a range of key regulatory factors with salt resistance were found. The PeERF1 gene was highly connected in the turquoise modules with significant differences in salt stress traits, and the expression levels were significantly different in each treatment. For further functional verification of PeERF1, we obtained stable overexpression and dominant suppression transgenic lines by transforming into Populus alba × Populusglandulosa. The growth and physiological characteristics of the PeERF1 overexpressed plants were better than that of the wild type under salt stress. Transcriptome analysis of leaves of transgenic lines and WT revealed that highly enriched GO terms in DEGs were associated with stress responses, including abiotic stimuli responses, chemical responses, and oxidative stress responses. The result is helpful for in-depth analysis of the salt tolerance mechanism of poplar. This work provides important genes for poplar breeding with salt tolerance.

The plant leaf, the main organ of photosynthesis, is an important regulator of growth. To explore the difference between leaf size of Populusdeltoides ‘Danhong’ (Pd) and Populus simonii ‘Tongliao1’ (Ps), we investigated the leaf length, leaf width, leaf thickness, leaf area, leaf mass per area (LMA), and cell size of leaves from two genotypes and profiled the transcriptome-wide gene expression patterns through RNA sequencing. Our results show that the leaf area of Pd was significantly larger than that of Ps, but the epidermal cell area was significantly smaller than that of Ps. The difference of leaf size was caused by cell numbers. Transcriptome analysis also revealed that genes related to chromosome replication and DNA repair were highly expressed in Pd, while genes such as the EXPANSIN (EXPA) family which promoted cell expansion were highly expressed in Ps. Further, we revealed that the growth-regulating factors (GRFs) played a key role in the difference of leaf size between two genotypes through regulation of cell proliferation. These data provide a valuable resource for understanding the leaf development of the Populus genus.

Light is an important environmental factor for plant growth, and in higher plants, phytochrome A (phyA) is the predominant far-red photoreceptor, involved in various photoresponses. The FAR1/FHY3 transcription factor family, derived from transposases, is able to regulate plant development in response to multiple photosensitizers phytochrome. In total, 51 PtrFRSs were identified in the poplar genome, and were divided into 4 subfamilies. Among them, 47 PtrFRSs are located on 17 chromosomes. Upstream cis-acting elements of the PtrFRS genes were classified into three categories: growth and metabolism, stress and hormone, and the hormone and stress categories contained most of the cis-acting elements. Analysis of the regulatory networks and expression patterns showed that most PtrFRSs responded to changes in light intensity and were involved in the regulation of phytochromes. In this study, 51 PtrFRSs were identified and comprehensively bioinformatically analyzed, and preliminary functional analysis and prediction of PtrFRSs was carried out.

Salt is an important environmental stress factor, which seriously affects the growth, development and distribution of plants. Chlorophyllase plays an important role in stress response. Nevertheless, little is known about the physiological and molecular mechanism of chlorophyll (Chlase, CLH) genes in plants. We cloned PeCLH2 from Populus euphratica and found that PeCLH2 was differentially expressed in different tissues, especially in the leaves of P. euphratica. To further study the role of PeCLH2 in salt tolerance, PeCLH2 overexpression and RNA interference transgenic lines were established in Populus alba × Populus glandulosa and used for salt stress treatment and physiologic indexes studies. Overexpressing lines significantly improved tolerance to salt treatment and reduced reactive oxygen species production. RNA interference lines showed the opposite. Transcriptome analysis was performed on leaves of control and transgenic lines under normal growth conditions and salt stress to predict genes regulated during salt stress. This provides a basis for elucidating the molecular regulation mechanism of PeCLH2 in response to salt stress and improving the tolerance of poplar under salt stress.

Poplars have a strong response to light, and shade is one of the main environmental factors that limits the growth and development of poplars. Exploring the response mechanism of the developing xylem of poplar under shady conditions is of great reference significance for improving wood yields. In this study, three excellent hybrids of poplar (Populus euramericana ‘Zhonglin46’ (Pe), Populus deltoides ‘27-17’ (Pd), Populus × ‘Wq156’ (Pw) were studied under shady conditions. Based on the phenotypic data and developing a xylem transcriptome analysis, the molecular mechanism of poplars’ response to shade was preliminarily revealed, and the core regulatory genes responding to shade were identified by a weighted co-expression network analysis (WGCNA). The results showed that Pw growth was significantly affected by shade, while Pe growth was slightly affected by shade. An enrichment analysis of 13,675 differentially expressed genes (DEGs) found that shade affected the expression of genes related to the glutathione metabolic pathway. The WGCNA analysis identified two modules (“Brown” and “Purple”) related to the shade response and discovered seven hub genes. These hub genes were related to xylem development, vascular cambium division, stomatal development, and phytochrome A signal transduction. These results provide important basic information for gaining insight into the molecular response to shade in different poplar hybrids.