Flavonoids are key secondary metabolites that are biologically active and perform diverse functions in plants such as stress defense against abiotic and biotic stress. In addition to its importance, no comprehensive information has been available about the secondary metabolic response of Populus tree, especially the genes that encode key enzymes involved in flavonoid biosynthesis under drought stress. In this study, the quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the expression of flavonoid biosynthesis genes (PtPAL, Pt4-CL, PtCHS, PtFLS-1, PtF3H, PtDFR, and PtANS) gradually increased in the leaves of hybrid poplar (P. tremula × P. alba), corresponding to the drought stress duration. In addition, the activity and capacity of antioxidants have also increased, which is positively correlated with the increment of phenolic, flavonoid, anthocyanin, and carotenoid compounds under drought stress. As the drought stress prolonged, the level of reactive oxygen species such as hydrogen peroxide (H₂O₂) and singlet oxygen (O₂⁻) too increased. The concentration of phytohormone salicylic acid (SA) also increased significantly in the stressed poplar leaves. Our research concluded that drought stress significantly induced the expression of flavonoid biosynthesis genes in hybrid poplar plants and enhanced the accumulation of phenolic and flavonoid compounds with resilient antioxidant activity.

Somatic embryogenesis is a notable illustration of cell totipotency, by which somatic cells undergo dedifferentiation and then differentiate into somatic embryos. Our previous work demonstrated that pretreatment of immature zygotic embryos with 0.5 M sucrose solution for 72 h efficiently induced somatic embryo initiation in camphor tree. To better understand the molecular basis of somatic embryogenesis induced by osmotic stress, de novo transcriptome sequencing of three tissues of camphor tree (immature zygotic embryos, sucrose-pretreated immature zygotic embryos, and somatic embryos induced from sucrose-pretreated zygotic embryos) were conducted using Illumina Hiseq 2000 platform. A total of 30.70 G high quality clean reads were obtained from cDNA libraries of the three samples. The overall de novo assembly of cDNA sequence data generated 205592 transcripts, with an average length of 998 bp. 114229 unigenes (55.56 % of all transcripts) with an average length of 680 bp were annotated with gene descriptions, gene ontology terms or metabolic pathways based on Blastx search against Nr, Nt, Swissprot, GO, COG/KOG, and KEGG databases. CEGMA software identified 237 out of 248 ultra-conserved core proteins as ‘complete’ in the transcriptome assembly, showing a completeness of 95.6 %. A total of 897 genes previously annotated to be potentially involved in somatic embryogenesis were identified. Comparative transcriptome analysis showed that a total of 3335 genes were differentially expressed in the three samples. The differentially expressed genes were divided into six groups based on K-means clustering. Expression level analysis of 52 somatic embryogenesis-related genes indicated a high correlation between RNA-seq and qRT-PCR data. Gene enrichment analysis showed significantly differential expression of genes responding to stress and stimulus. The present work reported a de novo transcriptome assembly and global analysis focused on gene expression changes during initiation and formation of somatic embryos in camphor tree. Differential expression of somatic embryogenesis-related genes indicates that sucrose induced somatic embryogenesis may share or partly share the mechanisms of somatic embryogenesis induced by plant hormones. This study provides comprehensive transcript information and gene expression data for camphor tree. It could also serve as an important platform resource for further functional studies in plant embryogenesis.

Background: The CLV3/ESR-RELATED (CLE) gene family encodes small secreted peptides (SSPs) and plays vital roles in plant growth and development by promoting cell-to-cell communication. The prediction and classification of CLE genes is challenging because of their low sequence similarity. Results: We developed a machine learning-aided method for predicting CLE genes by using a CLE motif-specific residual score matrix and a novel clustering method based on the Euclidean distance of 12 amino acid residues from the CLE motif in a site-weight dependent manner. In total, 2156 CLE candidates—including 627 novel candidates—were predicted from 69 plant species. The results from our CLE motif-based clustering are consistent with previous reports using the entire pre-propeptide. Characterization of CLE candidates provided systematic statistics on protein lengths, signal peptides, relative motif positions, amino acid compositions of different parts of the CLE precursor proteins, and decisive factors of CLE prediction. The approach taken here provides information on the evolution of the CLE gene family and provides evidence that the CLE and IDA/IDL genes share a common ancestor. Conclusions: Our new approach is applicable to SSPs or other proteins with short conserved domains and hence, provides a useful tool for gene prediction, classification and evolutionary analysis.

MicroRNA390 (miR390) is involved in plant growth and development by down-regulating the expression of the downstream genes trans-acting short interfering RNA3 (TAS3) and AUXIN RESPONSE FACTORs (ARFs). There is a scarcity of research on the involvement of the miR390-TAS3-ARFs pathway in the stem development of Populus. Here, differentially expressed miRNAs during poplar stem development were screened by small RNA sequencing analysis, and a novel function of miR390b in stem development was revealed. Overexpression of miR390b (OE-miR390b) resulted in a large increase in the number of xylem fiber cells and a slight decrease in the cell length at the longitudinal axis. Overall increases in stem elongation and plant height were observed in the OE-miR390b plants. According to transcriptome sequencing results and transient co-expression analysis, TAS3.1 and TAS3.2 were identified as the target genes of miR390 in poplar and were negatively regulated by miR390 in the apex. The transcription levels of ARF3.2 and ARF4 were significantly repressed in OE-miR390b plants and strongly negatively correlated with the number of xylem fiber cells along the longitudinal axis. These findings indicate that the conserved miR390-TAS3-ARFs pathway in poplar is involved in stem elongation and plant height growth.

Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch ( Betula pendula ). Using a candidate gene approach, we identified a bushy kanttarelli variant with a loss-of-function mutation in the BpMAX1 gene required for strigolactone (SL) biosynthesis. While kanttarelli is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied kanttarelli in transgenic BpMAX1::RNAi birch lines. SL profiling confirmed that both kanttarelli and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and BpMAX1::RNAi . In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture.