The secondary cell wall is a defining feature of xylem cells and allows them to resist both gravitational forces and the tension forces associated with the transpirational pull on their internal columns of water. Secondary walls also constitute the majority of plant biomass. Formation of secondary walls requires co-ordinated transcriptional regulation of the genes involved in the biosynthesis of cellulose, hemicellulose and lignin. This co-ordinated control appears to involve a multifaceted and multilayered transcriptional regulatory programme. Transcription factor MYB46 (At5g12870) has been shown to function as a master regulator in secondary wall formation in Arabidopsis thaliana. Recent studies show that MYB46 not only regulates the transcription factors but also the biosynthesis genes for all of the three major components (i.e. cellulose, hemicellulose and lignin) of secondary walls. This review considers our current understanding of the MYB46-mediated transcriptional regulatory network, including upstream regulators, downstream targets and negative regulators of MYB46. MYB46 is a unique transcription factor in that it directly regulates the biosynthesis genes for all of the three major components of the secondary wall as well as the transcription factors in the biosynthesis pathway. As such, MYB46 may offer a useful means for pathway-specific manipulation of secondary wall biosynthesis. However, realization of this potential requires additional information on the ‘MYB46-mediated transcriptional regulatory programme’, such as downstream direct targets, upstream regulators and interacting partners of MYB46.

Anthocyanins are a group of colorful and bioactive natural pigments with important physiological and ecological functions in plants. We found an MYB transcription factor (PtrMYB119) from Populus trichocarpa that positively regulates anthocyanin production when expressed under the control of the CaMV 35S promoter in transgenic Arabidopsis. Amino acid sequence analysis revealed that PtrMYB119 is highly homologous to Arabidopsis PAP1 (PRODUCTION OF ANTHOCYANIN PIGMENT1), a well-known transcriptional activator of anthocyanin biosynthesis. Independently produced transgenic poplars overexpressing PtrMYB119 or PtrMYB120 (a paralogous gene to PtrMYB119) (i.e., 35S::PtrMYB119 and 35S::PtrMYB120, respectively) showed elevated accumulation of anthocyanins in the whole plants, including leaf, stem and even root tissues. Using a reverse-phase high-performance liquid chromatography, we confirmed that the majority of the accumulated anthocyanin in our transgenic poplar is cyanidin-3-O-glucoside. Gene expression analyses revealed that most of the genes involved in the anthocyanin biosynthetic pathway were highly upregulated in 35S::PtrMYB119 poplars compared with the nontransformed control poplar. Among these genes, expression of PtrCHS1 (Chalcone Synthase1) and PtrANS2 (Anthocyanin Synthase2), which catalyze the initial and last steps of anthocyanin biosynthesis, respectively, was upregulated by up to 350-fold. Subsequent transient activation assays confirmed that PtrMYB119 activated the transcription of both PtrCHS1 and PtrANS2. Interestingly, expression of MYB182, a repressor of both anthocyanin and proanthocyanidin (PA) biosynthesis, was largely suppressed in 35S::PtrMYB119 poplars, while expression of MYB134, an activator of PA biosynthesis, was not changed significantly. More interestingly, high-level accumulation of anthocyanins in 35S::PtrMYB119 poplars did not have an adverse effect on plant growth. Taken together, our results demonstrate that PtrMYB119 and PtrMYB120 function as transcriptional activators of anthocyanin accumulation in both Arabidopsis and poplar.

Summary: With the exponential growth of the human population and industrial developments, research on renewable energy resources is required to alleviate environmental and economic impacts caused by the consumption of fossil fuels. In this study, we present a synthetic biological application of a wood forming tissue‐specific bicistronic gene expression system to improve both the quantity and quality of woody biomass to minimize undesirable growth penalties. Our transgenic poplars, designed to express both PdGA20ox1 (a GA20‐oxidase from Pinus densiflora producing bioactive gibberellin, GA) and PtrMYB221 (a MYB transcription factor negatively regulating lignin biosynthesis) under the developing xylem (DX) tissue‐specific promoter (i.e., DX15::PdGA20ox1‐2A‐PtrMYB221 poplar), resulted in a 2‐fold increase in biomass quantity compared to wild‐type (WT), without undesirable growth defects. A similar phenotype was observed in transgenic Arabidopsis plants harboring the same gene constructs. These phenotypic consequences were further verified in the field experiments. Importantly, our transgenic poplars exhibited an improved quality of biomass with reduced lignin content (~16.0 wt%) but increased holocellulose content (~6.6 wt%). Furthermore, the saccharification efficiency of our transgenic poplar increased significantly by up to 8%. Our results demonstrate that the controlled production of both GA and a secondary wall modifying regulator in the same spatio‐temporal manner can be utilized as an efficient biotechnological tool for producing the desired multi‐purpose woody biomass.

Pattern-recognition receptor (PRR)-triggered immunity (PTI) wards off a wide range of pathogenic microbes, playing a pivotal role in angiosperms. The model liverwort Marchantia polymorpha triggers defense-related gene expression upon sensing components of bacterial and fungal extracts, suggesting the existence of PTI in this plant model. However, the molecular components of the putative PTI in M. polymorpha and the significance of PTI in bryophytes have not yet been described. We here show that M. polymorpha has four lysin motif (LysM)-domain-containing receptor homologs, two of which, LysM-receptor-like kinase (LYK) MpLYK1 and LYK-related (LYR) MpLYR, are responsible for sensing chitin and peptidoglycan fragments, triggering a series of characteristic immune responses. Comprehensive phosphoproteomic analysis of M. polymorpha in response to chitin treatment identified regulatory proteins that potentially shape LysM-mediated PTI. The identified proteins included homologs of well-described PTI components in angiosperms as well as proteins whose roles in PTI are not yet determined, including the blue-light receptor phototropin MpPHOT. We revealed that MpPHOT is required for negative feedback of defense-related gene expression during PTI. Taken together, this study outlines the basic framework of LysM-mediated PTI in M. polymorpha and highlights conserved elements and new aspects of pattern-triggered immunity in land plants.

Wood, the most abundant biomass on Earth, is composed of secondary xylem differentiated from vascular cambium. However, the underlying molecular mechanisms of wood formation remain largely unclear. To gain insight into wood formation, we performed a series of wood-forming tissue-specific transcriptome analyses from a hybrid poplar (Populus alba × P. glandulosa, clone BH) using RNA-seq. Together with shoot apex and leaf tissue, cambium and xylem tissues were isolated from vertical stem segments representing a gradient of secondary growth developmental stages (i.e., immature, intermediate, and mature stem). In a comparative transcriptome analysis of the ‘developing xylem’ and ‘leaf’ tissue, we could identify critical players catalyzing each biosynthetic step of secondary wall components (e.g., cellulose, xylan, and lignin). Several candidate genes involved in the initiation of vascular cambium formation were found via a co-expression network analysis using abundantly expressed genes in the ‘intermediate stem-derived cambium’ tissue. We found that transgenic Arabidopsis plants overexpressing the PtrHAM4-1, a GRAS family transcription factor, resulted in a significant increase of vascular cambium development. This phenotype was successfully reproduced in the transgenic poplars overexpressing the PtrHAM4-1. Taken together, our results may serve as a springboard for further research to unravel the molecular mechanism of wood formation, one of the most important biological processes on this planet.