A HD-ZIP III gene, PtrHB4, is required for interfascicular cambium development in Populus

Genome-wide analysis of HD-Zip genes in Sophora alopecuroides and their role in salt stress response

We aimed to identify HD-Zip (homologous domain leucine zipper) family genes based on the complete Sophora alopecuroides genome sequence. Eighty-six Sophora alopecuroides HD-Zip family (SaHDZ) genes were identified and categorized into four subclasses using phylogenetic analysis. Chromosome localization analysis revealed that these genes were distributed across 18 chromosomes. Gene structure and conserved motif analysis showed high similarity among members of the SaHDZ genes. Prediction analysis revealed 71 cis-acting elements in SaHDZ genes. Transcriptome and quantitative real-time polymerase chain reaction analyses showed that under salt stress, SaHDZ responded positively in S. alopecuroides, and that SaHDZ22 was significantly upregulated afterward. Functional verification experiments revealed that SaHDZ22 overexpression increased the tolerance of Arabidopsis to salt and osmotic stress. Combined with cis-acting element prediction and expression level analysis, HD-Zip family transcription factors may be involved in regulating the balance between plant growth and stress resistance under salt stress by modulating the expression of auxin and abscisic acid signaling pathway genes. The Sophora alopecuroides adenylate kinase protein (SaAKI) and S. alopecuroides tetrapeptide-like repeat protein (SaTPR; pCAMBIA1300-SaTPR-cLUC) expression levels were consistent with those of SaHDZ22, indicating that SaHDZ22 may coordinate with SaAKI and SaTPR to regulate plant salt tolerance. These results lay a foundation in understanding the salt stress response mechanisms of S. alopecuroides and provide a reference for future studies oriented toward exploring plant stress resistance.

Identification and Functional Studies on the Role of PlSPL14 in Herbaceous Peony Stem Development

Stem strength plays a crucial role in the growth and development of plants, as well as in their flowering and fruiting. It not only impacts the lodging resistance of crops, but also influences the ornamental value of ornamental plants. Stem development is closely linked to stem strength; however, the roles of the SPL transcription factors in the stem development of herbaceous peony (Paeonia lactiflora Pall.) are not yet fully elucidated. In this study, we obtained and cloned the full-length sequence of PlSPL14, encoding 1085 amino acids. Quantitative real-time PCR (qRT-PCR) analysis revealed that the expression level of PlSPL14 gradually increased with the stem development of P. lactiflora and was significantly expressed in vascular bundles. Subsequently, utilizing the techniques of virus-induced gene silencing (VIGS) and heterologous overexpression in tobacco (Nicotiana tabacum L.), it was determined that PlSPL14-silenced P. lactiflora had a thinner xylem thickness, a decreased stem diameter, and weakened stem strength, while PlSPL14-overexpressing tobacco resulted in a thicker xylem thickness, an increased stem diameter, and enhanced stem strength. Further screening of the interacting proteins of PlSPL14 using a yeast two-hybrid (Y2H) assay revealed an interactive relationship between PlSPL14 and PlSLR1 protein, which acts as a negative regulator of gibberellin (GA). Additionally, the expression level of PlSLR1 gradually decreased during the stem development of P. lactiflora. The above results suggest that PlSPL14 may play a positive regulatory role in stem development and act in the xylem, making it a potential candidate gene for enhancing stem straightness in plants.

Vascular cambium stem cells: past, present and future

Secondary xylem and phloem originate from a lateral meristem called the vascular cambium that consists of one to several layers of meristematic cells. Recent lineage tracing studies have shown that only one of the cambial cells in each radial cell file functions as the stem cell, capable of producing both secondary xylem and phloem. Here, we first review how phytohormones and signalling peptides regulate vascular cambium formation and activity. We then propose how the stem cell concept, familiar from apical meristems, could be applied to cambium studies. Finally, we discuss how this concept could set the basis for future research.

PagARGOS promotes low-lignin wood formation in poplar

Wood formation, which occurs mainly through secondary xylem development, is important not only for supplying raw material for the 'ligno-chemical' industry but also for driving the storage of carbon. However, the complex mechanisms underlying the promotion of xylem formation remain to be elucidated. Here, we found that overexpression of Auxin-Regulated Gene involved in Organ Size (ARGOS) in hybrid poplar 84 K (Populus alba × Populus tremula var. glandulosa) enlarged organ size. In particular, PagARGOS promoted secondary growth of stems with increased xylem formation. To gain further insight into how PagARGOS regulates xylem development, we further carried out yeast two-hybrid screening and identified that the auxin transporter WALLS ARE THIN1 (WAT1) interacts with PagARGOS. Overexpression of PagARGOS up-regulated WAT1, activating a downstream auxin response promoting cambial cell division and xylem differentiation for wood formation. Moreover, overexpressing PagARGOS caused not only higher wood yield but also lower lignin content compared with wild-type controls. PagARGOS is therefore a potential candidate gene for engineering fast-growing and low-lignin trees with improved biomass production.

Genome-wide characterisation of HD-Zip transcription factors and functional analysis of PbHB24 during stone cell formation in Chinese white pear (Pyrus bretschneideri)

BACKGROUND

The homodomain-leucine zipper (HD-Zip) is a conserved transcription factor family unique to plants that regulate multiple developmental processes including lignificaion. Stone cell content is a key determinant negatively affecting pear fruit quality, which causes a grainy texture of fruit flesh, because of the lignified cell walls.

RESULTS

In this study, a comprehensive bioinformatics analysis of HD-Zip genes in Chinese white pear (Pyrus bretschneideri) (PbHBs) was performed. Genome-wide identification of the PbHB gene family revealed 67 genes encoding PbHB proteins, which could be divided into four subgroups (I, II, III, and IV). For some members, similar intron/exon structural patterns support close evolutionary relationships within the same subgroup. The functions of each subgroup of the PbHB family were predicted through comparative analysis with the HB genes in Arabidopsis and other plants. Cis-element analysis indicated that PbHB genes might be involved in plant hormone signalling and external environmental responses, such as light, stress, and temperature. Furthermore, RNA-sequencing data and quantitative real-time PCR (RT-qPCR) verification revealed the regulatory roles of PbHB genes in pear stone cell formation. Further, co-expression network analysis revealed that the eight PbHB genes could be classified into different clusters of co-expression with lignin-related genes. Besides, the biological function of PbHB24 in promoting stone cell formation has been demonstrated by overexpression in fruitlets.

CONCLUSIONS

This study provided the comprehensive analysis of PbHBs and highlighted the importance of PbHB24 during stone cell development in pear fruits.

Transcription factor PagMYB31 positively regulates cambium activity and negatively regulates xylem development in poplar

Wood formation involves consecutive developmental steps, including cell division of vascular cambium, xylem cell expansion, secondary cell wall (SCW) deposition, and programmed cell death. In this study, we identified PagMYB31 as a coordinator regulating these processes in Populus alba × Populus glandulosa and built a PagMYB31-mediated transcriptional regulatory network. PagMYB31 mutation caused fewer layers of cambial cells, larger fusiform initials, ray initials, vessels, fiber and ray cells, and enhanced xylem cell SCW thickening, showing that PagMYB31 positively regulates cambial cell proliferation and negatively regulates xylem cell expansion and SCW biosynthesis. PagMYB31 repressed xylem cell expansion and SCW thickening through directly inhibiting wall-modifying enzyme genes and the transcription factor genes that activate the whole SCW biosynthetic program, respectively. In cambium, PagMYB31 could promote cambial activity through TRACHEARY ELEMENT DIFFERENTIATION INHIBITORY FACTOR (TDIF)/PHLOEM INTERCALATED WITH XYLEM (PXY) signaling by directly regulating CLAVATA3/ESR-RELATED (CLE) genes, and it could also directly activate WUSCHEL HOMEOBOX RELATED4 (PagWOX4), forming a feedforward regulation. We also observed that PagMYB31 could either promote cell proliferation through the MYB31-MYB72-WOX4 module or inhibit cambial activity through the MYB31-MYB72-VASCULAR CAMBIUM-RELATED MADS2 (VCM2)/PIN-FORMED5 (PIN5) modules, suggesting its role in maintaining the homeostasis of vascular cambium. PagMYB31 could be a potential target to manipulate different developmental stages of wood formation.

CsREV-CsTCP4-CsVND7 module shapes xylem patterns differentially between stem and leaf to enhance tea plant tolerance to drought

Cultivating drought-tolerant tea varieties enhances both yield and quality of tea plants in northern China. However, the mechanisms underlying their drought tolerance remain largely unknown. Here we identified a key regulator called CsREV, which differentially regulates xylem patterns between leaves and stems, thereby conferring drought tolerance in tea plants. When drought occurs, upregulation of CsREV activates the CsVND7a-dependent xylem vessel differentiation. However, when drought persists, the vessel differentiation is hindered as CsVND7a is downregulated by CsTCP4a. This, combined with the CsREV-promoted secondary-cell-wall thickness of xylem vessel, leads to the enhanced curling of leaves, a characteristic closely associated with plant drought tolerance. Notably, this inhibitory effect of CsTCP4a on CsVND7a expression is absent in stems, allowing stem xylem vessels to continuously differentiate. Overall, the CsREV-CsTCP4-CsVND7 module is differentially utilized to shape the xylem patterns in leaves and stems, potentially balancing water transportation and utilization to improve tea plant drought tolerance.

Genome-Wide Identification, Characterization, and Expression Analysis of the HD-Zip Gene Family in Lagerstroemia for Regulating Plant Height

The Homeodomain leucine zipper (HD-Zip) family of transcription factors is crucial in helping plants adapt to environmental changes and promoting their growth and development. Despite research on the HD-Zip family in various plants, studies in Lagerstroemia (crape myrtle) have not been reported. This study aimed to address this gap by comprehensively analyzing the HD-Zip gene family in crape myrtle. This study identified 52 HD-Zip genes in the genome of Lagerstroemia indica, designated as LinHDZ1-LinHDZ52. These genes were distributed across 22 chromosomes and grouped into 4 clusters (HD-Zip I-IV) based on their phylogenetic relationships. Most gene structures and motifs within each cluster were conserved. Analysis of protein properties, gene structure, conserved motifs, and cis-acting regulatory elements revealed diverse roles of LinHDZs in various biological contexts. Examining the expression patterns of these 52 genes in 6 tissues (shoot apical meristem, tender shoot, and mature shoot) of non-dwarf and dwarf crape myrtles revealed that 2 LinHDZs (LinHDZ24 and LinHDZ14) and 2 LinHDZs (LinHDZ9 and LinHDZ35) were respectively upregulated in tender shoot of non-dwarf crape myrtles and tender and mature shoots of dwarf crape myrtles, which suggested the important roles of these genes in regulate the shoot development of Lagerstroemia. In addition, the expression levels of 2 LinHDZs (LinHDZ23 and LinHDZ34) were significantly upregulated in the shoot apical meristem of non-dwarf crape myrtle. These genes were identified as key candidates for regulating Lagerstroemia plant height. This study enhanced the understanding of the functions of HD-Zip family members in the growth and development processes of woody plants and provided a theoretical basis for further studies on the molecular mechanisms underlying Lagerstroemia plant height.

Wood of trees: Cellular structure, molecular formation, and genetic engineering

Wood is an invaluable asset to human society due to its renewable nature, making it suitable for both sustainable energy production and material manufacturing. Additionally, wood derived from forest trees plays a crucial role in sequestering a significant portion of the carbon dioxide fixed during photosynthesis by terrestrial plants. Nevertheless, with the expansion of the global population and ongoing industrialization, forest coverage has been substantially decreased, resulting in significant challenges for wood production and supply. Wood production practices have changed away from natural forests toward plantation forests. Thus, understanding the underlying genetic mechanisms of wood formation is the foundation for developing high-quality, fast-growing plantation trees. Breeding ideal forest trees for wood production using genetic technologies has attracted the interest of many. Tremendous studies have been carried out in recent years on the molecular, genetic, and cell-biological mechanisms of wood formation, and considerable progress and findings have been achieved. These studies and findings indicate enormous possibilities and prospects for tree improvement. This review will outline and assess the cellular and molecular mechanisms of wood formation, as well as studies on genetically improving forest trees, and address future development prospects.

A cytokinin response factor PtCRF1 is involved in the regulation of wood formation in poplar

Wood formation is a complex developmental process under the control of multiple levels of regulatory transcriptional network and hormone signals in trees. It is well known that cytokinin (CK) signaling plays an important role in maintaining the activity of the vascular cambium. The CK response factors (CRFs) encoding a subgroup of AP2 transcription factors have been identified to mediate the CK-dependent regulation in different plant developmental processes. However, the functions of CRFs in wood development remain unclear. Here, we characterized the function of PtCRF1, a CRF transcription factor isolated from poplar, in the process of wood formation. The PtCRF1 is preferentially expressed in secondary vasculature, especially in vascular cambium and secondary phloem, and encodes a transcriptional activator. Overexpression of PtCRF1 in transgenic poplar plants led to a significant reduction in the cell layer number of vascular cambium. The development of wood tissue was largely promoted in the PtCRF1-overexpressing lines, while it was significantly compromised in the CRISPR/Cas9-generated double mutant plants of PtCRF1 and its closest homolog PtCRF2. The RNA sequencing (RNA-seq) and quantitative reverse transcription PCR (RT-qPCR) analyses showed that PtCRF1 repressed the expression of the typical CK-responsive genes. Furthermore, bimolecular fluorescence complementation assays revealed that PtCRF1 competitively inhibits the direct interactions between histidine phosphotransfer proteins and type-B response regulator by binding to PtHP protein. Collectively, these results indicate that PtCRF1 negatively regulates CK signaling and is required for woody cell differentiation in poplar.

Trees need closure too: Wound-induced secondary vascular tissue regeneration

Trees play a pivotal role in terrestrial ecosystems as well as being an important natural resource. These attributes are primarily associated with the capacity of trees to continuously produce woody tissue from the vascular cambium, a ring of stem cells located just beneath the bark. Long-lived trees are exposed to a myriad of biological and environmental stresses that may result in wounding, leading to a loss of bark and the underlying vascular cambium. This affects both wood formation and the quality of timber arising from the tree. In addition, the exposed wound site is a potential entry point for pathogens that cause disease. In response to wounding, trees have the capacity to regenerate lost or damaged tissues at this site. Investigating gene expression changes associated with different stages of wound healing reveals complex and dynamic changes in the activity of transcription factors, signalling pathways and hormone responses. In this review we summarise these data and discuss how they relate to our current understanding of vascular cambium formation and xylem differentiation during secondary growth. Based on this analysis, a model for wound healing that provides the conceptual foundations for future studies aimed at understanding this intriguing process is proposed.

Genes expression profiles in vascular cambium of Eucalyptus urophylla × Eucalyptus grandis at different ages

BACKGROUND

Wood is a secondary xylem generated by vascular cambium. Vascular cambium activities mainly include cambium proliferation and vascular tissue formation through secondary growth, thereby producing new secondary phloem inward and secondary xylem outward and leading to continuous tree thickening and wood formation. Wood formation is a complex biological process, which is strictly regulated by multiple genes. Therefore, molecular level research on the vascular cambium of different tree ages can lead to the identification of both key and related genes involved in wood formation and further explain the molecular regulation mechanism of wood formation.

RESULTS

In the present study, RNA-Seq and Pac-Bio Iso-Seq were used for profiling gene expression changes in Eucalyptus urophylla × Eucalyptus grandis (E. urograndis) vascular cambium at four different ages. A total of 59,770 non-redundant transcripts and 1892 differentially expressed genes (DEGs) were identified. The expression trends of the DEGs related to cell division and differentiation, cell wall biosynthesis, phytohormone, and transcription factors were analyzed. The DEGs encoding expansin, kinesin, cycline, PAL, GRP9, KNOX, C2C2-dof, REV, etc., were highly expressed in E. urograndis at three years old, leading to positive effects on growth and development. Moreover, some gene family members, such as NAC, MYB, HD-ZIP III, RPK, and RAP, play different regulatory roles in wood formation because of their sophisticated transcriptional network and function redundantly.

CONCLUSIONS

These candidate genes are a potential resource to further study wood formation, especially in fast-growing and adaptable eucalyptus. The results may also serve as a basis for further research to unravel the molecular mechanism underlying wood formation.

Combining single-cell RNA sequencing with spatial transcriptome analysis reveals dynamic molecular maps of cambium differentiation in the primary and secondary growth of trees

Primary and secondary growth of the tree stem are responsible for corresponding increases in trunk height and diameter. However, our molecular understanding of the biological processes that underlie these two types of growth is incomplete. In this study, we used single-cell RNA sequencing and spatial transcriptome sequencing to reveal the transcriptional landscapes of primary and secondary growth tissues in the Populus stem. Comparison between the cell atlas and differentiation trajectory of primary and secondary growth revealed different regulatory networks involved in cell differentiation from cambium to xylem precursors and phloem precursors. These regulatory networks may be controlled by auxin accumulation and distribution. Analysis of cell differentiation trajectories suggested that vessel and fiber development followed a sequential pattern of progressive transcriptional regulation. This research provides new insights into the processes of cell identity and differentiation that occur throughout primary and secondary growth of tree stems, increasing our understanding of the cellular differentiation dynamics that occur during stem growth in trees.

Screening genome-editing knockouts reveals the receptor-like kinase ASX role in regulations of secondary xylem development in Populus

In trees, secondary xylem development is essential for the growth of perennial stem increments. Many signals regulate the process of development, but our knowledge of the molecular components involved in signal transduction is still limited. In this study, we identified Attenuation of Secondary Xylem (ASX) knockouts by screening genome-editing knockouts of xylem-expressed receptor-like kinases (RLKs) in Populus. The ASX role in secondary xylem development in Populus was discovered using biochemical, cellular, and genomic analyses. The ASX knockout plants had abnormal secondary stem growth but had little effect on shoot apical primary growth. ASX and SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK)2/4 were co-precipitated in developing xylem. Through their interaction, ASX is phosphorylated by SERK. Transcriptome analysis of developing xylem revealed that ASX deficiency inhibited the transcriptional activity of genes involved in xylem differentiation and secondary cell wall formation. By forming a complex, ASX and SERK may function as a signaling module for signal transduction required in the regulation of secondary xylem development in trees. This study shows that ASX, which encodes a RLKs, is required for secondary xylem development and sheds light on regulatory signals found in tree stem secondary growth.

High-resolution anatomical and spatial transcriptome analyses reveal two types of meristematic cell pools within the secondary vascular tissue of poplar stem

The secondary vascular tissue emanating from meristems is central to understanding how vascular plants such as forest trees evolve, grow, and regulate secondary radial growth. However, the overall molecular characterization of meristem origins and developmental trajectories from primary to secondary vascular tissues in woody tree stems is technically challenging. In this study, we combined high-resolution anatomic analysis with a spatial transcriptome (ST) technique to define features of meristematic cells in a developmental gradient from primary to secondary vascular tissues in poplar stems. The tissue-specific gene expression of meristems and derived vascular tissue types were accordingly mapped to specific anatomical domains. Pseudotime analyses were used to track the origins and changes of meristems throughout the development from primary to secondary vascular tissues. Surprisingly, two types of meristematic-like cell pools within secondary vascular tissues were inferred based on high-resolution microscopy combined with ST, and the results were confirmed by in situ hybridization of, transgenic trees, and single-cell sequencing. The rectangle shape procambium-like (PCL) cells develop from procambium meristematic cells and are located within the phloem domain to produce phloem cells, whereas fusiform shape cambium zone (CZ) meristematic cells develop from fusiform metacambium meristematic cells and are located inside the CZ to produce xylem cells. The gene expression atlas and transcriptional networks spanning the primary transition to secondary vascular tissues generated in this work provide new resources for studying the regulation of meristem activities and the evolution of vascular plants. A web server (https://pgx.zju.edu.cn/stRNAPal/) was also established to facilitate the use of ST RNA-seq data.

Single-cell transcriptomics unveils xylem cell development and evolution

BACKGROUND

Xylem, the most abundant tissue on Earth, is responsible for lateral growth in plants. Typical xylem has a radial system composed of ray parenchyma cells and an axial system of fusiform cells. In most angiosperms, fusiform cells comprise vessel elements for water transportation and libriform fibers for mechanical support, while both functions are performed by tracheids in other vascular plants such as gymnosperms. Little is known about the developmental programs and evolutionary relationships of these xylem cell types.

RESULTS

Through both single-cell and laser capture microdissection transcriptomic profiling, we determine the developmental lineages of ray and fusiform cells in stem-differentiating xylem across four divergent woody angiosperms. Based on cross-species analyses of single-cell clusters and overlapping trajectories, we reveal highly conserved ray, yet variable fusiform, lineages across angiosperms. Core eudicots Populus trichocarpa and Eucalyptus grandis share nearly identical fusiform lineages, whereas the more basal angiosperm Liriodendron chinense has a fusiform lineage distinct from that in core eudicots. The tracheids in the basal eudicot Trochodendron aralioides, an evolutionarily reversed trait, exhibit strong transcriptomic similarity to vessel elements rather than libriform fibers.

CONCLUSIONS

This evo-devo framework provides a comprehensive understanding of the formation of xylem cell lineages across multiple plant species spanning over a hundred million years of evolutionary history.

Cell-type-specific PtrWOX4a and PtrVCS2 form a regulatory nexus with a histone modification system for stem cambium development in Populus trichocarpa

Stem vascular cambium cells in forest trees produce wood for materials and energy. WOX4 affects the proliferation of such cells in Populus. Here we show that PtrWOX4a is the most highly expressed stem vascular-cambium-specific (VCS) gene in P. trichocarpa, and its expression is controlled by the product of the second most highly expressed VCS gene, PtrVCS2, encoding a zinc finger protein. PtrVCS2 binds to the PtrWOX4a promoter as part of a PtrWOX13a-PtrVCS2-PtrGCN5-1-PtrADA2b-3 protein tetramer. PtrVCS2 prevented the interaction between PtrGCN5-1 and PtrADA2b-3, resulting in H3K9, H3K14 and H3K27 hypoacetylation at the PtrWOX4a promoter, which led to fewer cambium cell layers. These effects on cambium cell proliferation were consistent across more than 20 sets of transgenic lines overexpressing individual genes, gene-edited mutants and RNA interference lines in P. trichocarpa. We propose that the tetramer-PtrWOX4a system may coordinate genetic and epigenetic regulation to maintain normal vascular cambium development for wood formation.

The roles of HD-ZIP proteins in plant abiotic stress tolerance

Homeodomain leucine zipper (HD-ZIP) proteins are plant-specific transcription factors that contain a homeodomain (HD) and a leucine zipper (LZ) domain. The highly conserved HD binds specifically to DNA and the LZ mediates homodimer or heterodimer formation. HD-ZIP transcription factors control plant growth, development, and responses to abiotic stress by regulating downstream target genes and hormone regulatory pathways. HD-ZIP proteins are divided into four subclasses (I-IV) according to their sequence conservation and function. The genome-wide identification and expression profile analysis of HD-ZIP proteins in model plants such as Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) have improved our understanding of the functions of the different subclasses. In this review, we mainly summarize and discuss the roles of HD-ZIP proteins in plant response to abiotic stresses such as drought, salinity, low temperature, and harmful metals. HD-ZIP proteins mainly mediate plant stress tolerance by regulating the expression of downstream stress-related genes through abscisic acid (ABA) mediated signaling pathways, and also by regulating plant growth and development. This review provides a basis for understanding the roles of HD-ZIP proteins and potential targets for breeding abiotic stress tolerance in plants.

Ubiquitinated DA1 negatively regulates vascular cambium activity through modulating the stability of WOX4 in Populus

Activity of the vascular cambium gives rise to secondary xylem for wood formation in trees. The transcription factor WUSCHEL-related HOMEOBOX4 (WOX4) is a central regulator downstream of the hormone and peptide signaling pathways that maintain cambial activity. However, the genetic regulatory network underlying WOX4-mediated wood formation at the post-transcriptional level remains to be elucidated. In this study, we identified the ubiquitin receptor PagDA1 in hybrid poplar (Populus alba × Populus glandulosa clone 84K) as a negative regulator of wood formation, which restricts cambial activity during secondary growth. Overexpression of PagDA1 in poplar resulted in a relatively reduced xylem due to decreased cambial cell division. By contrast, mutation of PagDA1 by CRISPR/Cas9 resulted in an increased cambial cell activity and promoted xylem formation. Genetic analysis demonstrated that PagDA1 functions antagonistically in a common pathway as PagWOX4 to regulate cambial activity. We propose that PagDA1 physically associates with PagWOX4 and modulates the degradation of PagWOX4 by the 26S proteasome. Moreover, genetic analysis revealed that PagDA1 exerts its negative effect on cambial development by modulating the stability of PagWOX4 in a ubiquitin-dependent manner mediated by the E3 ubiquitin ligase PagDA2. In sum, we have identified a cambial regulatory protein complex, PagDA1-PagWOX4, as a potential target for wood biomass improvement.

Current Understanding of the Genetics and Molecular Mechanisms Regulating Wood Formation in Plants

Unlike herbaceous plants, woody plants undergo volumetric growth (a.k.a. secondary growth) through wood formation, during which the secondary xylem (i.e., wood) differentiates from the vascular cambium. Wood is the most abundant biomass on Earth and, by absorbing atmospheric carbon dioxide, functions as one of the largest carbon sinks. As a sustainable and eco-friendly energy source, lignocellulosic biomass can help address environmental pollution and the global climate crisis. Studies of Arabidopsis and poplar as model plants using various emerging research tools show that the formation and proliferation of the vascular cambium and the differentiation of xylem cells require the modulation of multiple signals, including plant hormones, transcription factors, and signaling peptides. In this review, we summarize the latest knowledge on the molecular mechanism of wood formation, one of the most important biological processes on Earth.

HD-Zip III Gene Family: Identification and Expression Profiles during Leaf Vein Development in Soybean

Leaf veins constitute the transport network for water and photosynthetic assimilates in vascular plants. The class III homeodomain-leucine zipper (HD-Zip III) gene family is central to the regulation of vascular development. In this research, we performed an overall analysis of the HD-Zip III genes in soybean (Glycine max L. Merr.). Our analysis included the phylogeny, conservation domains and cis-elements in the promoters of these genes. We used the quantitative reverse transcription-polymerase chain reaction to characterize the expression patterns of HD-Zip III genes in leaf vein development and analyze the effects of exogenous hormone treatments. In this study, twelve HD-Zip III genes were identified from the soybean genome and named. All soybean HD-Zip III proteins contained four highly conserved domains. GmHB15-L-1 transcripts showed steadily increasing accumulation during all stages of leaf vein development and were highly expressed in cambium cells. GmREV-L-1 and GmHB14-L-2 had nearly identical expression patterns in soybean leaf vein tissues. GmREV-L-1 and GmHB14-L-2 transcripts remained at stable high levels at all xylem developmental stages. GmREV-L-1 and GmHB14-L-2 were expressed at high levels in the vascular cambium and xylem cells. Overall, GmHB15-L-1 may be an essential regulator that is responsible for the formation or maintenance of soybean vein cambial cells. GmREV-L-1 and GmHB14-L-2 were correlated with xylem differentiation in soybean leaf veins. This study will pave the way for identifying the molecular mechanism of leaf vein development.

PdeHCA2 affects biomass in Populus by regulating plant architecture, the transition from primary to secondary growth, and photosynthesis

PdeHCA2 regulates the transition from primary to secondary growth, plant architecture, and affects photosynthesis by targeting PdeBRC1 and controlling the anatomy of the mesophyll, and intercellular space, respectively. Branching, secondary growth, and photosynthesis are vital developmental processes of woody plants that determine plant architecture and timber yield. However, the mechanisms underlying these processes are unknown. Here, we report that the Populus transcription factor High Cambium Activity 2 (PdeHCA2) plays a role in the transition from primary to secondary growth, vascular development, and branching. In Populus, PdeHCA2 is expressed in undifferentiated provascular cells during primary growth, in phloem cells during secondary growth, and in leaf veins, which is different from the expression pattern of its homolog in Arabidopsis. Overexpression of PdeHCA2 has pleiotropic effects on shoot and leaf development; overexpression lines showed delayed growth of shoots and leaves, reduced photosynthesis, and abnormal shoot branching. In addition, auxin-, cytokinin-, and photosynthesis-related genes were differentially regulated in these lines. Electrophoretic mobility shift assays and transcriptome analysis indicated that PdeHCA2 directly up-regulates the expression of BRANCHED1 and the MADS-box gene PdeAGL9, which regulate plant architecture, by binding to cis-elements in the promoters of these genes. Taken together, our findings suggest that HCA2 regulates several processes in woody plants including vascular development, photosynthesis, and branching by affecting the proliferation and differentiation of parenchyma cells.

The role of senescence-associated gene101 (PagSAG101a) in the regulation of secondary xylem formation in poplar

Wood is produced by the accumulation of secondary xylem via proliferation and differentiation of the cambium cells in woody plants. Identifying the regulators involved in this process remains a challenging task. In this study, we isolated PagSAG101a, the homolog of Arabidopsis thaliana SAG101, from a hybrid poplar (Populus alba × Populus glandulosa) clone 84K and investigated its role in secondary xylem development. PagSAG101a was expressed predominantly in lignified stems and localized in the nucleus. Compared with non-transgenic 84K plants, transgenic plants overexpressing PagSAG101a displayed increased plant height, internode number, stem diameter, xylem width, and secondary cell wall thickness, while opposite phenotypes were observed for PagSAG101a knock-out plants. Transcriptome analyses revealed that differentially expressed genes were enriched for those controlling cambium cell division activity and subsequent secondary cell wall deposition during xylem formation. In addition, the tandem CCCH zinc finger protein PagC3H17, which positively regulates secondary xylem width and secondary wall thickening in poplar, could bind to the promoter of PagSAG101a and mediate the regulation of xylem differentiation. Our results support that PagSAG101a, downstream of PagC3H17, functions in wood development.

The SlHB8 Acts as a Negative Regulator in Stem Development and Lignin Biosynthesis

The stem is an important organ in supporting plant body, transporting nutrients and communicating signals for plant growing. However, studies on the regulation of stem development in tomato are rather limited. In our study, we demonstrated that SlHB8 negatively regulated tomato stem development. SlHB8 belongs to homeo domain-leucine zipper Class III gene family transcription factors and expressed in all the organs examined including root, stem, leaves, flower, and fruit. Among these tissues, SlHB8 showed stable high expression level during tomato stem development. Overexpression of SlHB8 gene decreased stem diameter with inhibited xylem width and xylem cell layers, while loss of function of SlHB8gene increased the stem diameter and xylem width. The contents of lignin were decreased both in leaves and stems of SlHB8 overexpression plants. RNA-seq analysis on the stems of wild type and SlHB8 transgenic plants showed that the 116 DEGs (differential expressed genes) with reversible expression profiles in SlHB8-ox and SlHB8-cr plants were significantly enriched in the phenylpropanoid biosynthesis pathway and plant-pathogen pathway which were related to lignin biosynthesis and disease resistance. Meanwhile, the key genes involved in the lignin biosynthesis pathway such as SlCCR (cinnamoyl-CoA reductase), SlCYP73A14/C4H (cinnamate 4-hydroxylase), SlC3H (coumarate 3-hydroxylase) and SlCAD (cinnamoyl alcohol dehydrogenase) were down-regulated in both stem and leaves of SlHB8 overexpression plants, indicating a negative regulatory role of SlHB8 in the lignin biosynthesis and stem development.

Transcriptional landscape of highly lignified poplar stems at single-cell resolution

BACKGROUND

Plant secondary growth depends on the activity of the vascular cambium, which produces xylem and phloem. Wood derived from xylem is the most abundant form of biomass globally and has played key socio-economic and subsistence roles throughout human history. However, despite intensive study of vascular development, the full diversity of cell types and the gene networks engaged are still poorly understood.

RESULTS

Here, we have applied an optimized protoplast isolation protocol and RNA sequencing to characterize the high-resolution single-cell transcriptional landscape of highly lignified poplar stems. We identify 20 putative cell clusters with a series of novel cluster-specific marker genes and find that these cells are highly heterogeneous based on the transcriptome. Analysis of these marker genes' expression dynamics enables reconstruction of the cell differentiation trajectories involved in phloem and xylem development. We find that different cell clusters exhibit distinct patterns of phytohormone responses and emphasize the use of our data to predict potential gene redundancy and identify candidate genes related to vascular development in trees.

CONCLUSIONS

These findings establish the transcriptional landscape of major cell types of poplar stems at single-cell resolution and provide a valuable resource for investigating basic principles of vascular cell specification and differentiation in trees.

The transcription factor PagLBD3 contributes to the regulation of secondary growth in Populus

LATERAL ORGAN BOUNDARIES DOMAIN (LBD) genes encode plant-specific transcription factors that participate in regulating various developmental processes. In this study, we genetically characterized PagLBD3 encoding an important regulator of secondary growth in poplar (Populus alba × Populus glandulosa). Overexpression of PagLBD3 increased stem secondary growth in Populus with a significantly higher rate of cambial cell differentiation into phloem, while dominant repression of PagLBD3 significantly decreased the rate of cambial cell differentiation into phloem. Furthermore, we identified 1756 PagLBD3 genome-wide putative direct target genes (DTGs) through RNA sequencing (RNA-seq)-coupled DNA affinity purification followed by sequencing (DAP-seq) assays. Gene Ontology analysis revealed that genes regulated by PagLBD3 were enriched in biological pathways regulating meristem development, xylem development, and auxin transport. Several central regulator genes for vascular development, including PHLOEM INTERCALATED WITH XYLEM (PXY), WUSCHEL RELATED HOMEOBOX4 (WOX4), Secondary Wall-Associated NAC Domain 1s (SND1-B2), and Vascular-Related NAC-Domain 6s (VND6-B1), were identified as PagLBD3 DTGs. Together, our results indicate that PagLBD3 and its DTGs form a complex transcriptional network to modulate cambium activity and phloem/xylem differentiation.

Two MADS-box genes regulate vascular cambium activity and secondary growth by modulating auxin homeostasis in Populus

In trees, stem secondary growth depends on vascular cambium proliferation activity and subsequent cell differentiation, in which an auxin concentration gradient across the cambium area plays a crucial role in regulating the process. However, the underlying molecular mechanism for the establishment of auxin concentration is not fully understood. In this study, we identified two function-unknown MADS-box genes, VCM1 and VCM2, which are expressed specifically in the vascular cambium and modulate the subcellular homeostasis of auxin. Simultaneous knockdown of both VCM1 and VCM2 enhanced vascular cambium proliferation activity and subsequent xylem differentiation. Overexpression of VCM1 suppressed vascular cambium activity and wood formation by regulating PIN5 expression, which tuned the soluble auxin concentration in the vascular cambium area. This study reveals the role of VCM1 and VCM2 in regulating the proliferation activity of the vascular cambium and secondary growth by modulating the subcellular auxin homeostasis in Populus.

Vascular Cambium: The Source of Wood Formation

Wood is the most abundant biomass produced by land plants and is mainly used for timber, pulping, and paper making. Wood (secondary xylem) is derived from vascular cambium, and its formation encompasses a series of developmental processes. Extensive studies in Arabidopsis and trees demonstrate that the initiation of vascular stem cells and the proliferation and differentiation of the cambial derivative cells require a coordination of multiple signals, including hormones and peptides. In this mini review, we described the recent discoveries on the regulation of the three developmental processes by several signals, such as auxin, cytokinins, brassinosteroids, gibberellins, ethylene, TDIF peptide, and their cross talk in Arabidopsis and Populus. There exists a similar but more complex regulatory network orchestrating vascular cambium development in Populus than that in Arabidopsis. We end up with a look at the future research prospects of vascular cambium in perennial woody plants, including interfascicular cambium development and vascular stem cell regulation.

The BOP-type co-transcriptional regulator NODULE ROOT1 promotes stem secondary growth of the tropical Cannabaceae tree Parasponia andersonii

Tree stems undergo a massive secondary growth in which secondary xylem and phloem tissues arise from the vascular cambium. Vascular cambium activity is driven by endogenous developmental signalling cues and environmental stimuli. Current knowledge regarding the genetic regulation of cambium activity and secondary growth is still far from complete. The tropical Cannabaceae tree Parasponia andersonii is a non-legume research model of nitrogen-fixing root nodulation. Parasponia andersonii can be transformed efficiently, making it amenable for CRISPR-Cas9-mediated reverse genetics. We considered whether P. andersonii also could be used as a complementary research system to investigate tree-related traits, including secondary growth. We established a developmental map of stem secondary growth in P. andersonii plantlets. Subsequently, we showed that the expression of the co-transcriptional regulator PanNODULE ROOT1 (PanNOOT1) is essential for controlling this process. PanNOOT1 is orthologous to Arabidopsis thaliana BLADE-ON-PETIOLE1 (AtBOP1) and AtBOP2, which are involved in the meristem-to-organ-boundary maintenance. Moreover, in species forming nitrogen-fixing root nodules, NOOT1 is known to function as a key nodule identity gene. Parasponia andersonii CRISPR-Cas9 loss-of-function Pannoot1 mutants are altered in the development of the xylem and phloem tissues without apparent disturbance of the cambium organization and size. Transcriptomic analysis showed that the expression of key secondary growth-related genes is significantly down-regulated in Pannoot1 mutants. This allows us to conclude that PanNOOT1 positively contributes to the regulation of stem secondary growth. Our work also demonstrates that P. andersonii can serve as a tree research system.

Plant Transcription Factors Involved in Drought and Associated Stresses

Transcription factors (TFs) play a significant role in signal transduction networks spanning the perception of a stress signal and the expression of corresponding stress-responsive genes. TFs are multi-functional proteins that may simultaneously control numerous pathways during stresses in plants-this makes them powerful tools for the manipulation of regulatory and stress-responsive pathways. In recent years, the structure-function relationships of numerous plant TFs involved in drought and associated stresses have been defined, which prompted devising practical strategies for engineering plants with enhanced stress tolerance. Vast data have emerged on purposely basic leucine zipper (bZIP), WRKY, homeodomain-leucine zipper (HD-Zip), myeloblastoma (MYB), drought-response elements binding proteins/C-repeat binding factor (DREB/CBF), shine (SHN), and wax production-like (WXPL) TFs that reflect the understanding of their 3D structure and how the structure relates to function. Consequently, this information is useful in the tailored design of variant TFs that enhances our understanding of their functional states, such as oligomerization, post-translational modification patterns, protein-protein interactions, and their abilities to recognize downstream target DNA sequences. Here, we report on the progress of TFs based on their interaction pathway participation in stress-responsive networks, and pinpoint strategies and applications for crops and the impact of these strategies for improving plant stress tolerance.

Betula platyphylla BpHOX2 transcription factor binds to different cis-acting elements and confers osmotic tolerance

The homeodomain-leucine zipper (HD-Zip) proteins play crucial roles in plant developmental and environmental responses. However, how they mediate gene expression to facilitate abiotic stress tolerance remains unknown. In the present study, we characterized BpHOX2 (encoding a HD-Zip I family protein) from birch (Betula platyphylla). BpHOX2 is predominately expressed in mature stems and leaves, but expressed at a low level in apical buds and roots, suggesting that it has tissue-specific characteristics. BpHOX2 expression was highly induced by osmotic and salt, but only slightly induced by abscisic acid. Overexpression of BpHOX2 markedly improved osmotic tolerance, while knockdown of BpHOX2 increased sensitivity to osmotic stress. BpHOX2 could induce the expression of pyrroline-5-carboxylate synthase, peroxidase, and superoxide dismutase genes to improve proline levels and the reactive oxygen species scavenging capability. Chromatin immunoprecipitation sequencing combined with RNA sequencing showed that BpHOX2 could bind to at least four cis-acting elements, including dehydration-responsive element "RCCGAC", Myb-p binding box "CCWACC," and two novel cis-acting elements with the sequences of "AAGAAG" and "TACGTG" (termed HBS1 and HBS2, respectively) to regulate gene expression. Our results suggested that BpHOX2 is a transcription factor that binds to different cis-acting elements to regulate gene expression, ultimately improving osmotic tolerance in birch.

Genome-wide characterization and expression profiling of Eucalyptus grandis HD-Zip gene family in response to salt and temperature stress

BACKGROUND

The HD-Zip transcription factors are unique to plants and play an essential role in plant growth, development and stress responses. The HD-Zip transcription factor family consists of a highly conserved homeodomain (HD) and a leucine zipper domain (LZ) domain. Although the HD-Zip gene family has been extensively studied in many plant species, a systematic study of the Eucalyptus HD-Zip family has not been reported until today. Here, we systematically identified 40 HD-Zip genes in Eucalyptus (Eucalyptus grandis). Besides, we comprehensively analyzed the HD-Zips of Eucalyptus by studying the homology, conserved protein regions, gene structure, 3D structure of the protein, location of the genes on the chromosomes and the expression level of the genes in different tissues.

RESULTS

The HD-Zip family in Eucalyptus has four subfamilies, which is consistent with other plants such as Arabidopsis and rice. Moreover, genes that are in the same group tend to have similar exon-intron structures, motifs, and protein structures. Under salt stress and temperature stress, the Eucalyptus HD-Zip transcription factors show a differential expression pattern.

CONCLUSIONS

Our findings reveal the response of HD-Zip transcription factors under salt and temperature stresses, laying a foundation for future analysis of Eucalyptus HD-Zip transcription factors.

Peptide encoding Populus CLV3/ESR-RELATED 47 (PttCLE47) promotes cambial development and secondary xylem formation in hybrid aspen

The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION (ESR)-RELATED (CLE) peptide ligands in connection with their receptors are important players in cell-to-cell communications in plants. Here, we investigated the function of the Populus CLV3/ESR-RELATED 47 (PttCLE47) gene during secondary growth and wood formation in hybrid aspen (Populus tremula × tremuloides) using an RNA interference (RNAi) approach. Expression of PttCLE47 peaks in the vascular cambium. Silencing of the PttCLE47 gene expression affected lateral expansion of stems and decreased apical height growth and leaf size. In particular, PttCLE47 RNAi trees exhibited a narrower secondary xylem zone with less xylem cells/cell file. The reduced radial growth phenotype also correlated with a reduced number of cambial cell layers. In agreement with these results, expression of several cambial regulator genes was downregulated in the stems of the transgenic trees in comparison with controls. Altogether, these results suggest that the PttCLE47 gene is a major positive regulator of cambial activity in hybrid aspen, mainly promoting the production of secondary xylem. Furthermore, in contrast to previously characterized CLE genes expressed in the wood-forming zone, PttCLE47 appears to be active at its site of expression.

Water lily (Nymphaea thermarum) genome reveals variable genomic signatures of ancient vascular cambium losses

For more than 225 million y, all seed plants were woody trees, shrubs, or vines. Shortly after the origin of angiosperms ∼140 million y ago (MYA), the Nymphaeales (water lilies) became one of the first lineages to deviate from their ancestral, woody habit by losing the vascular cambium, the meristematic population of cells that produces secondary xylem (wood) and phloem. Many of the genes and gene families that regulate differentiation of secondary tissues also regulate the differentiation of primary xylem and phloem, which are produced by apical meristems and retained in nearly all seed plants. Here, we sequenced and assembled a draft genome of the water lily Nymphaea thermarum, an emerging system for the study of early flowering plant evolution, and compared it to genomes from other cambium-bearing and cambium-less lineages (e.g., monocots and Nelumbo). This revealed lineage-specific patterns of gene loss and divergence. Nymphaea is characterized by a significant contraction of the HD-ZIP III transcription factors, specifically loss of REVOLUTA, which influences cambial activity in other angiosperms. We also found the Nymphaea and monocot copies of cambium-associated CLE signaling peptides display unique substitutions at otherwise highly conserved amino acids. Nelumbo displays no obvious divergence in cambium-associated genes. The divergent genomic signatures of convergent loss of vascular cambium reveals that even pleiotropic genes can exhibit unique divergence patterns in association with independent events of trait loss. Our results shed light on the evolution of herbaceousness-one of the key biological innovations associated with the earliest phases of angiosperm evolution.

Dual regulation of xylem formation by an auxin-mediated PaC3H17-PaMYB199 module in Populus

Wood (secondary xylem) formation in tree species is dependent on auxin-mediated vascular cambium activity in stems. However, the complex regulatory networks underlying xylem formation remain elusive. Xylem development in Populus was characterized based on microscopic observations of stem sections in transgenic plants. Transcriptomic, quantitative real-time PCR, chromatin immunoprecipitation PCR, and electrophoretic mobility shift assay analyses were conducted to identify target genes involved in xylem development. Yeast two-hybrid, pull-down, bimolecular fluorescence complementation, and co-immunoprecipitation assays were used to validate protein-protein interactions. PaC3H17 and its target PaMYB199 were found to be predominantly expressed in the vascular cambium and developing secondary xylem in Populus stems and play opposite roles in controlling cambial cell proliferation and secondary cell wall thickening through an overlapping pathway. Further, PaC3H17 interacts with PaMYB199 to form a complex, attenuating PaMYB199-driven suppression of its xylem targets. Exogenous auxin application enhances the dual control of the PaC3H17-PaMYB199 module during cambium division, thereby promoting secondary cell wall deposition. Dual regulation of xylem formation by an auxin-mediated PaC3H17-PaMYB199 module represents a novel regulatory mechanism in Populus, increasing our understanding of the regulatory networks involved in wood formation.

A xylem-produced peptide PtrCLE20 inhibits vascular cambium activity in Populus

In trees, lateral growth of the stem occurs through cell divisions in the vascular cambium. Vascular cambium activity is regulated by endogenous developmental programmes and environmental cues. However, the underlying mechanisms that regulate cambium activity are largely unknown. Genomic, biochemical and genetic approaches were used here to elucidate the role of PtrCLE20, a CLAVATA3 (CLV3)/embryo surrounding region (ESR)-related peptide gene, in the regulation of lateral growth in Populus. Fifty-two peptides encoded by CLE genes were identified in the genome of Populus trichocarpa. Among them PtrCLE20 transcripts were detected in developing xylem while the PtrCLE20 peptide was mainly localized in vascular cambium cells. PtrCLE20 acted in repressing vascular cambium activity indicated by that upregulation of PtrCLE20 resulted in fewer layers of vascular cambium cells with repressed expression of the genes related to cell dividing activity. PtrCLE20 peptide also showed a repression effect on the root growth of Populus and Arabidopsis, likely through inhibiting meristematic cell dividing activity. Together, the results suggest that PtrCLE20 peptide, produced from developing xylem cells, plays a role in regulating lateral growth by repression of cambium activity in trees.

Genome-wide identification, characterization and expression analysis of the HD-Zip gene family in the stem development of the woody plant Prunus mume

The homeodomain-leucine zipper (HD-Zip) gene family, a group of plant-specific transcriptional factors (TFs), participates in regulating growth, development, and environmental responses. However, the characteristics and biological functions of HD-Zip genes in Prunus mume, which blooms in late winter or early spring, have not been reported. In this study, 32 HD-Zip genes, named PmHB1-PmHB32 based on their chromosomal positions, were identified in the genome of P. mume. These genes are distributed among seven chromosomes and are phylogenetically clustered into four major groups. Gene structure and motif composition were mostly conserved in each group. The Ka/Ks ratios showed that purifying selection has played a leading role in the long-term evolution of the genes, which maintained the function of this family. MicroRNA target site prediction indicated that the genes of the HD-Zip III subfamily may be regulated by miR165/166. Expression pattern analysis showed that the 32 genes were differentially expressed across five different tissues (leaf, flower bud, stem, fruit, and root) and at different stages of stem and leaf-bud development, suggesting that 10 of the genes may play important roles in stem development. Protein-protein interaction predictions showed that the subfamily III genes may regulate vascular development and shoot apical meristem (SAM) maintenance. Promoter analysis showed that the HD-Zip III genes might be involved in responses to light, hormones, and abiotic stressors and stem development. Taken together, our results provide an overview of the HD-Zip family in P. mume and lay the foundation for the molecular breeding of woody ornamental plants.

A Physic Nut Stress-Responsive HD-Zip Transcription Factor, JcHDZ07, Confers Enhanced Sensitivity to Salinity Stress in Transgenic Arabidopsis

Homeodomain-leucine zipper (HD-Zip) transcription factors are reported to play crucial roles in the growth, development, and stress responses of plants. However, there is little knowledge of the molecular mechanisms involved in physic nut's stress tolerance generally, or the functions of its HD-Zip genes. In the present study, a HD-Zip family transcription factor, designated JcHDZ07, was isolated from physic nut. Expression profile analysis showed that salinity stress inhibited the expression of JcHDZ07. Transient expression of JcHDZ07-YFP in Arabidopsis protoplast cells revealed that JcHDZ07 was a nuclear-localized protein. Additionally, no obvious difference in growth and development between wild-type and JcHDZ07-overexpressing plants was observed in the absence of stress. Our results further indicated that JcHDZ07 overexpressing transgenic plants had lower proline contents, lower survival rates, and activities of catalase and superoxide dismutase, but higher relative electrical leakage and malonaldehyde contents compared with wild-type plants under salinity stress conditions, suggesting that overexpression of JcHDZ07 confers enhanced sensitivity to salinity stress in transgenic Arabidopsis. Expression of salt stress-responsive genes were upregulated in leaves of transgenic plants under salinity stress, but less strongly than in wild-type plants. Collectively, our results suggest that JcHDZ07 functions as an important regulator during the process of plant responses to salinity stress.

Genome-wide identification and expression profile of HD-ZIP genes in physic nut and functional analysis of the JcHDZ16 gene in transgenic rice

BACKGROUND

Homeodomain-leucine zipper (HD-ZIP) transcription factors play important roles in the growth, development and stress responses of plants, including (presumably) physic nut (Jatropha curcas), which has high drought and salinity tolerance. However, although physic nut's genome has been released, there is little knowledge of the functions, expression profiles and evolutionary histories of the species' HD-ZIP genes.

RESULTS

In this study, 32 HD-ZIP genes were identified in the physic nut genome (JcHDZs) and divided into four groups (I-IV) based on phylogenetic analysis with homologs from rice, maize and Arabidopsis. The analysis also showed that most of the JcHDZ genes were closer to members from Arabidopsis than to members from rice and maize. Of the 32 JcHDZ genes, most showed differential expression patterns among four tissues (root, stem cortex, leaf, and seed). Expression profile analysis based on RNA-seq data indicated that 15 of the JcHDZ genes respond to at least one abiotic stressor (drought and/or salinity) in leaves at least at one time point. Transient expression of a JcHDZ16-YFP fusion protein in Arabidopsis protoplasts cells showed that JcHDZ16 is localized in the nucleus. In addition, rice seedlings transgenically expressing JcHDZ16 had lower proline contents and activities of antioxidant enzymes (catalase and superoxide dismutase) together with higher relative electrolyte leakage and malondialdehyde contents under salt stress conditions (indicating higher sensitivity) than wild-type plants. The transgenic seedlings also showed increased sensitivity to exogenous ABA, and increases in the transcriptional abundance of several salt stress-responsive genes were impaired in their responses to salt stress. Further data on JcHDZ16-overexpressing plants subjected to salt stress treatment verified the putative role of JcHDZ genes in salt stress responses.

CONCLUSION

Our results may provide foundations for further investigation of functions of JcHDZ genes in responses to abiotic stress, and promote application of JcHDZ genes in physic nut breeding.

Mosaic modularity: an updated perspective and research agenda for the evolution of vascular cambial growth

Secondary growth from a vascular cambium, present today only in seed plants and isoetalean lycophytes, has a 400-million-yr evolutionary history that involves considerably broader taxonomic diversity, most of it hidden in the fossil record. Approaching vascular cambial growth as a complex developmental process, we review data from living plants and fossils that reveal diverse modes of secondary growth. These are consistent with a modular nature of secondary growth, when considered as a tracheophyte-wide structural feature. This modular perspective identifies putative constituent developmental modules of cambial growth, for which we review developmental anatomy and regulation. Based on these data, we propose a hypothesis that explains the sources of diversity of secondary growth, considered across the entire tracheophyte clade, and opens up new avenues for exploring the origin of secondary growth. In this hypothesis, various modes of secondary growth reflect a mosaic pattern of expression of different developmental-regulatory modules among different lineages. We outline an approach that queries three information systems (living seed plants, living seed-free plants, and fossils) and integrates data on developmental regulation, anatomy, gene evolution and phylogeny to test the mosaic modularity hypothesis and its implications, and to inform efforts aimed at understanding the evolution of secondary growth.