Over-expression of transcription factor ARK1 gene leads to down-regulation of lignin synthesis related genes in hybrid poplar '717'

Functional Diversification and the Plant Secondary Cell Wall

Much evidence exists suggesting the presence of genetic functional diversification in plants, though literature associated with the role of functional diversification in the evolution of the plant secondary cell wall (SCW) has sparsely been compiled and reviewed in a recent context. This review aims to elucidate, through the examination of gene phylogenies associated with its biosynthesis and maintenance, the role of functional diversification in shaping the critical, dynamic, and characteristic organelle, the secondary cell wall. It will be asserted that gene families resulting from gene duplication and subsequent functional divergence are present and are heavily involved in SCW biosynthesis and maintenance. Furthermore, diversification will be presented as a significant driver behind the evolution of the many functional characteristics of the SCW. The structure and function of the plant cell wall and its constituents will first be explored, followed by a discussion on the phenomenon of gene duplication and the resulting genetic functional divergence that can emerge. Finally, the major constituents of the SCW and their individual relationships with duplication and divergence will be reviewed to the extent of current knowledge on the subject.

PtrHAT22, as a higher hierarchy regulator, coordinately regulates secondary cell wall component biosynthesis in Populus trichocarpa

Homeodomain-leucine zipper (HD-Zip) II transcription factors (TFs) have been reported to play vital roles in diverse biological processes of plants. However, it remains unclear whether HD-Zip II TFs regulate secondary cell wall (SCW) in woody plants. In this study, we performed the functional characterization of a Populus trichocarpa HD-Zip II TF, PtrHAT22, which encodes a nuclear localized transcription repressor predominantly expressing in secondary developing tissues. Overexpression of PtrHAT22 showed arrested growths, including reduced heights and diameters above the ground, small leaves, and decreased biomass. Meanwhile, the contents of lignin, cellulose, and thickness of SCW significantly decreased, whilst the content of hemicellulose obviously increased in PtrHAT22 transgenic poplar. The expressions of some wood-associated TFs and structural genes significantly changed accordingly with the alternations of SCW characteristics in PtrHAT22 transgenic poplar. Furthermore, PtrHAT22 directly repressed the promoter activities of PtrMYB20, PtrMYB28, and PtrCOMT2, and bind two cis-acting elements that were specifically enriched in their promoter regions. Taken together, our results suggested that PtrHAT22, as a higher hierarchy TF like PtrWNDs, exerted coordination regulation of poplar SCW component biosynthesis through directly and indirectly regulating structural genes and different hierarchy TFs of SCW formation network.

Transcriptome analysis of diploid and triploid Populus tomentosa

Triploid Chinese white poplar (Populus tomentosa Carr., Salicaceae) has stronger advantages in growth and better stress resistance and wood quality than diploid P. tomentosa. Using transcriptome sequencing technology to identify candidate transcriptome-based markers for growth vigor in young tree tissue is of great significance for the breeding of P. tomentosa varieties in the future. In this study, the cuttings of diploid and triploid P. tomentosa were used as plant materials, transcriptome sequencing was carried out, and their tissue culture materials were used for RT-qPCR verification of the expression of genes. The results showed that 12,240 differentially expressed genes in diploid and triploid P. tomentosa transcripts were annotated and enriched into 135 metabolic pathways. The top six pathways that enriched the most significantly different genes were plant-pathogen interaction, phenylpropanoid biosynthesis, MAPK signalling pathway-plant, ascorbate and aldarate metabolism, diterpenoid biosynthesis, and the betalain biosynthesis pathway. Ten growth-related genes were selected from pathways of plant hormone signal transduction and carbon fixation in photosynthetic organisms for RT-qPCR verification. The expression levels of MDH and CYCD3 in tissue-cultured and greenhouse planted triploid P. tomentosa were higher than those in tissue-cultured diploid P. tomentosa, which was consist ent with the TMM values calculated by transcriptome.

Overexpression of PtrMYB121 Positively Regulates the Formation of Secondary Cell Wall in Arabidopsis thaliana

MYB transcription factors have a wide range of functions in plant growth, hormone signaling, salt, and drought tolerances. In this study, two homologous transcription factors, PtrMYB55 and PtrMYB121, were isolated and their functions were elucidated. Tissue expression analysis revealed that PtrMYB55 and PtrMYB121 had a similar expression pattern, which had the highest expression in stems. Their expression continuously increased with the growth of poplar, and the expression of PtrMYB121 was significantly upregulated in the process. The full length of PtrMYB121 was 1395 bp, and encoded protein contained 464 amino acids including conserved R2 and R3 MYB domains. We overexpressed PtrMYB121 in Arabidopsis thaliana, and the transgenic lines had the wider xylem as compared with wild-type Arabidopsis. The contents of cellulose and lignin were obviously higher than those in wild-type materials, but there was no significant change in hemicellulose. Quantitative real-time PCR demonstrated that the key enzyme genes regulating the synthesis of lignin and cellulose were significantly upregulated in the transgenic lines. Furthermore, the effector-reporter experiment confirmed that PtrMYB121 bound directly to the promoters of genes relating to the synthesis of lignin and cellulose. These results suggest that PtrMYB121 may positively regulate the formation of secondary cell wall by promoting the synthesis of lignin and cellulose.

Enhancing the expression of ARK1 genes in poplar leads to multiple branches and transcriptomic changes

The ARBORKNOX1 (ARK1) gene is an important gene for regulating plant growth and development; however, transcriptomic responses of enhancing expression of ARK1 gene in poplar are poorly investigated. To provide insight into the gene function of the ARK1 gene in poplar, the ARK1 transgenic poplar '717' and '84 K' lines were obtained, the morphology of transgenic plants was observed, and transcriptome profiles were compared. The results showed that there were multiple branches in ARK1 transgenic seedlings compared with non-transgenic seedlings. The results of transcriptome analysis showed that there were significant differences in transcriptome profiles between the transgenic lines of '717' and '84 K', and between non-transgenic lines (CK) and transgenic plants. The real-time quantitative polymerase chain reaction (RT-qPCR) analysis confirmed the expression levels of the genes involved in the pathway of zeatin biosynthesis and brassinosteroid biosynthesis. The increase in expression levels of AHP and CYCD3 was related to multiple branches. Enhancing the expression of the ARK1 gene in poplar seedlings leads to multiple branches and transcriptomic changes.