Transcriptome sequencing-based analysis of primary vein development in Betula pendula 'Dalecarlica'.

Keymessage The study revealed the major biological processes occurred at three developmental stages and identified candidate genes involved in primary vein development of birch plants. Vascular tissues usually mirror the surrounding leaf shape and its development plays a fundamental role in plant performance. However, the information of vascular development in birch trees, especially primary vein development, remains unclear. Therefore, we conducted the anatomical observation on primary veins from leaves at different development stages in Betula pendula 'Dalecarlica'. With the development of primary vein, dynamic changes in mechanical tissue thickness and primary vein diameter were consistent with each other, and the sum of phloem, xylem and cambium thickness was significantly varied. Transcriptome analysis indicated that primary vein development could be divided into three stages, namely Stage I, II and III, which were in aggreement with anatomical observation. Expression of marker genes associated with vascular tissues revealed that pro-vasculature development occurred at Stage I and II, and phloem development occurred at Stage III. GO enrichment analysis of differentially expressed genes (DEGs) showed that shared DEGs at Stage II were mainly engaged in cell division and cell cycle, and shared DEGs at Stage III were mainly engaged in phosphorylation. Decreased cell division and cell cycle as well as activation of lignin biosynthesis might contribute to primary vein development. Combining phenotypic traits, we performed weighted gene co-expression network analysis and identified a cytochrome P450 84A (CYP84A) family gene (BpF5H1). Based on analyses of gene families, expression patterns and yeast-two hybrid assay results, we proposed a potential electron transfer pathway involving BpF5H1 and three cytochrome b5 proteins during primary vein development in B. pendula 'Dalecarlica'. These results could shed some light on which biological processes occurred during primary vein formation and provide some valuable clues for vascular morphogenesis in woody plants.

Ectopic expression of Populus MYB10 promotes secondary cell wall thickening and inhibits anthocyanin accumulation.

Secondary cell wall (SCW) formation is regulated by a multilevel transcriptional regulatory network, in which MYB transcription factors (TFs) play key roles. In woody plants, hundreds of MYB TFs have been identified, most of which have unknown functions in wood SCW biosynthesis. Here, we characterized the function of a Populus MYB gene, PtoMYB10. PtoMYB10 was found to encode an R2R3-MYB TF and exhibit dominant expression in xylem tissues. PtoMYB10 was determined to be located in the nucleus with the ability to activate transcription. Overexpression of PtoMYB10 in Populus resulted in a drastic increase in SCW thickening in xylem fiber cells as well as ectopic deposition of lignin in cortex cells. The expression of genes associated with lignin biosynthesis was induced in PtoMYB10 overexpressing plants, whereas repressed gene expression was found with the anthocyanin biosynthesis pathway. Lignin and anthocyanin are both produced from metabolites of the phenylpropanoid pathway. Accordingly, the anthocyanin content of Populus overexpressing PtoMYB10 decreased by more than 68%. These results indicate that PtoMYB10 can positively regulate xylary fiber SCW thickening, accompanied by the reprogramming of phenylpropanoid metabolism, which redirects metabolic flux from anthocyanin biosynthesis to monolignol biosynthesis.

Overexpression of BpCUC2 Influences Leaf Shape and Internode Development in Betula pendula.

The CUP-SHAPED COTYLEDON 2 (CUC2) gene, which is negatively regulated by microRNA164 (miR164), has been specifically linked to the regulation of leaf margin serration and the maintenance of phyllotaxy in model plants. However, few studies have investigated these effects in woody plants. In this study, we integrated genomic, transcriptomic, and physiology approaches to explore the function of BpCUC2 gene in Betula pendula growth and development. Our results showed that Betula pendula plants overexpressing BpCUC2, which is targeted by BpmiR164, exhibit shortened internodes and abnormal leaf shapes. Subsequent analysis indicated that the short internodes of BpCUC2 overexpressed transgenic lines and were due to decreased epidermal cell size. Moreover, transcriptome analysis, yeast one-hybrid assays, and ChIP-PCR suggested that BpCUC2 directly binds to the LTRECOREATCOR15 (CCGAC), CAREOSREP1 (CAACTC), and BIHD1OS (TGTCA) motifs of a series of IAA-related and cyclin-related genes to regulate expression. These results may be useful to our understanding of the functional role and genetic regulation of BpCUC2.

Comparative analysis of growth and photosynthetic characteristics of (Populus simonii × P. nigra) × (P. nigra × P. simonii) hybrid clones of different ploidides.

To evaluate differences among poplar clones of various ploidies, 12 hybrid poplar clones (P. simonii × P. nigra) × (P. nigra × P. simonii) with different ploidies were used to study phenotypic variation in growth traits and photosynthetic characteristics. Analysis of variance showed remarkable differences for each of the investigated traits among these clones (P < 0.01). Coefficients of phenotypic variation (PCV) ranged from 2.38% to 56.71%, and repeatability ranged from 0.656 to 0.987. The Pn (photosynthetic rate) photosynthetic photon flux density (PPFD) curves of the 12 clones were S-shaped, but the Pn-ambient CO2 (Ca) curves were shaped like an inverted "V". The stomatal conductance (Gs)-PPFD and transpiration rate (Tr)-PPFD curves had an upward tendency; however, with increasing PFFD, the intercellular CO2 concentration (Ci)-PPFD curves had a downward tendency in all of the clones. The Pn-PPFD and Pn-Ca curves followed the pattern of a quadratic equation. The average light saturation point and light compensation point of the triploid clones were the highest and lowest, respectively, among the three types of clones. For Pn-Ca curves, diploid clones had a higher average CO2 saturation point and average CO2 compensation point compared with triploid and tetraploid clones. Correlation analyses indicated that all investigated traits were strongly correlated with each other. In future studies, molecular methods should be used to analyze poplar clones of different ploidies to improve our understanding of the growth and development mechanisms of polyploidy.