Cytokinin and Auxin Display Distinct but Interconnected Distribution and Signaling Profiles to Stimulate Cambial Activity

Plant hormone profiling of scion and rootstock incision sites and intra- and inter-family graft junctions in Nicotiana benthamiana

Many previous studies have suggested that various plant hormones play essential roles in the grafting process. In this study, to understand the plant hormones that accumulate in the graft junctions, whether these are supplied from the scion or rootstock, and how these hormones play a role in the grafting process, we performed a hormonome analysis that accumulated in the incision site of the upper plants from the incision as "ungrafted scion" and lower plants from the incision as "ungrafted rootstock" in Nicotiana benthamiana. The results revealed that indole-3-acetic acid (IAA) and gibberellic acid (GA), which regulate cell division; abscisic acid (ABA) and jasmonic acid (JA), which regulate xylem formation; cytokinin (CK), which regulates callus formation, show different accumulation patterns in the incision sites of the ungrafted scion and rootstock. In addition, to try discussing the differences in the degree and speed of each event during the grafting process between intra- and inter-family grafting by determining the concentration and accumulation timing of plant hormones in the graft junctions, we performed hormonome analysis of graft junctions of intra-family grafted plants with N. benthamiana as scion and Solanum lycopersicum as rootstock (Nb/Sl) and inter-family grafted plants with N. benthamiana as scion and Arabidopsis thaliana as rootstock (Nb/At), using the ability of Nicotiana species to graft with many plant species. The results revealed that ABA and CK showed different accumulation timings; IAA, JA, and salicylic acid (SA) showed similar accumulation timings, while different accumulated concentrations in the graft junctions of Nb/Sl and Nb/At. This information is important for understanding the molecular mechanisms of plant hormones in the grafting process and the differences in molecular mechanisms between intra- and inter-family grafting.

Spatio-temporal plant hormonomics: from tissue to subcellular resolution

Due to technological advances in mass spectrometry, significant progress has been achieved recently in plant hormone research. Nowadays, plant hormonomics is well established as a fully integrated scientific field focused on the analysis of phytohormones, mainly on their isolation, identification, and spatiotemporal quantification in plants. This review represents a comprehensive meta-study of the advances in the phytohormone analysis by mass spectrometry over the past decade. To address current trends and future perspectives, Web of Science data were systematically collected and key features such as mass spectrometry-based analyses were evaluated using multivariate data analysis methods. Our findings showed that plant hormonomics is currently divided into targeted and untargeted approaches. Both aim to miniaturize the sample, allowing high-resolution quantification to be covered in plant organs as well as subcellular compartments. Therefore, we can study plant hormone biosynthesis, metabolism, and signalling at a spatio-temporal resolution. Moreover, this trend has recently been accelerated by technological advances such as fluorescence-activated cell sorting or mass spectrometry imaging.

The transcription factor PagLBD4 represses cell differentiation and secondary cell wall biosynthesis in Populus

LBD (LATERAL ORGAN BOUNDARIES DOMAIN) transcription factors are key regulators of plant growth and development. In this study, we functionally characterized the PagLBD4 gene in Populus (Populus alba × Populus glandulosa). Overexpression of PagLBD4 (PagLBD4OE) significantly repressed secondary xylem differentiation and secondary cell wall (SCW) deposition, while CRISPR/Cas9-mediated PagLBD4 knockout (PagLBD4KO) significantly increased secondary xylem differentiation and SCW deposition. Consistent with the functional analysis, gene expression analysis revealed that SCW biosynthesis pathways were significantly down-regulated in PagLBD4OE plants but up-regulated in PagLBD4KO plants. We also performed DNA affinity purification followed by sequencing (DAP-seq) to identify genes bound by PagLBD4. Integration of RNA sequencing (RNA-seq) and DAP-seq data identified 263 putative direct target genes (DTGs) of PagLBD4, including important regulatory genes for SCW biosynthesis, such as PagMYB103 and PagIRX12. Together, our results demonstrated that PagLBD4 is a repressor of secondary xylem differentiation and SCW biosynthesis in Populus, which possibly lead to the dramatic growth repression in PagLBD4OE plants.

Providing biological context for GWAS results using eQTL regulatory and co-expression networks in Populus

Our study utilized genome-wide association studies (GWAS) to link nucleotide variants to traits in Populus trichocarpa, a species with rapid linkage disequilibrium decay. The aim was to overcome the challenge of interpreting statistical associations at individual loci without sufficient biological context, which often leads to reliance solely on gene annotations from unrelated model organisms. We employed an integrative approach that included GWAS targeting multiple traits using three individual techniques for lignocellulose phenotyping, expression quantitative trait loci (eQTL) analysis to construct transcriptional regulatory networks around each candidate locus and co-expression analysis to provide biological context for these networks, using lignocellulose biosynthesis in Populus trichocarpa as a case study. The research identified three candidate genes potentially involved in lignocellulose formation, including one previously recognized gene (Potri.005G116800/VND1, a critical regulator of secondary cell wall formation) and two genes (Potri.012G130000/AtSAP9 and Potri.004G202900/BIC1) with newly identified putative roles in lignocellulose biosynthesis. Our integrative approach offers a framework for providing biological context to loci associated with trait variation, facilitating the discovery of new genes and regulatory networks.

PagKNAT2/6b regulates tension wood formation and gravitropism by targeting cytokinin metabolism

Tension wood is a specialized xylem tissue associated with gravitropism in angiosperm trees. However, few regulators of tension wood formation have been identified. The molecular mechanisms underpinning tension wood formation remain elusive. Here, we report that a Populus KNOTTED-like homeobox gene, PagKNAT2/6b, is involved in tension wood formation and gravity response. Transgenic poplar plants overexpressing PagKNAT2/6b displayed more sensitive gravitropism than controls, as indicated by increased stem curvature. Microscopic examination revealed greater abundance of fibre cells with a gelatinous cell wall layer (G-layer) and asymmetric growth of secondary xylem in PagKNAT2/6b overexpression lines. Conversely, PagKNAT2/6b dominant repression plants exhibited decreased tension wood formation and reduced response to gravity stimulation. Moreover, sensitivity to gravity stimulation showed a negative relationship with development stage. Expression of genes related to growth and senescence was affected in PagKNAT2/6b transgenic plants. More importantly, transcription activation and electrophoretic mobility shift assays suggested that PagKNAT2/6b promotes the expression of cytokinin metabolism genes. Consistently, cytokinin content was increased in PagKNAT2/6b overexpression plants. Therefore, PagKNAT2/6b is involved in gravitropism and tension wood formation, likely via modulation of cytokinin metabolism.

PagJAZ5 regulates cambium activity through coordinately modulating cytokinin concentration and signaling in poplar

Wood is resulted from the radial growth paced by the division and differentiation of vascular cambium cells in woody plants, and phytohormones play important roles in cambium activity. Here, we identified that PagJAZ5, a key negative regulator of jasmonate (JA) signaling, plays important roles in enhancing cambium cell division and differentiation by mediating cytokinin signaling in poplar 84K (Populus alba × Populus glandulosa). PagJAZ5 is preferentially expressed in developing phloem and cambium, weakly in developing xylem cells. Overexpression (OE) of PagJAZ5m (insensitive to JA) increased cambium activity and xylem differentiation, while jaz mutants showed opposite results. Transcriptome analyses revealed that cytokinin oxidase/dehydrogenase (CKXs) and type-A response regulators (RRs) were downregulated in PagJAZ5m OE plants. The bioactive cytokinins were significantly increased in PagJAZ5m overexpressing plants and decreased in jaz5 mutants, compared with that in 84K plants. The PagJAZ5 directly interact with PagMYC2a/b and PagWOX4b. Further, we found that the PagRR5 is regulated by PagMYC2a and PagWOX4b and involved in the regulation of xylem development. Our results showed that PagJAZ5 can increase cambium activity and promote xylem differentiation through modulating cytokinin level and type-A RR during wood formation in poplar.

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.

Stem Cells and Differentiation in Vascular Tissues

Plant vascular tissues are crucial for the long-distance transport of water, nutrients, and a multitude of signal molecules throughout the plant body and, therefore, central to plant growth and development. The intricate development of vascular tissues is orchestrated by unique populations of dedicated stem cells integrating endogenous as well as environmental cues. This review summarizes our current understanding of vascular-related stem cell biology and of vascular tissue differentiation. We present an overview of the molecular and cellular mechanisms governing the maintenance and fate determination of vascular stem cells and highlight the interplay between intrinsic and external cues. In this context, we emphasize the role of transcription factors, hormonal signaling, and epigenetic modifications. We also discuss emerging technologies and the large repertoire of cell types associated with vascular tissues, which have the potential to provide unprecedented insights into cellular specialization and anatomical adaptations to distinct ecological niches.

Is calcium deficiency the real cause of bitter pit? A review

Bitter pit is a disorder affecting the appearance of apples. Susceptibility is genetically controlled by both the cultivar and rootstock, with both environmental and horticultural factors affecting its severity and proportional incidence. Symptoms appear more frequently at the calyx end of the fruit and consist of circular necrotic spots, which take on a "corky" appearance visible through the peel. Bitter pit may develop before harvest, or after harvest, reducing the proportions of marketable fruit. In this review, current knowledge of the factors associated with the occurrence of bitter pit in apples is summarized and discussed along with their interactions with Ca uptake and distribution to fruit. This disorder has been previously linked with localized Ca deficiencies in fruit during its development. However, these relationships are not always clear. Even with over a century of research, the precise mechanisms involved in its development are still not fully understood. Additional factors also contribute to bitter pit development, like imbalances of mineral nutrients, low concentration of auxins, high concentration of gibberellins, changes in xylem functionality, or physiological responses to abiotic stress. Bitter pit remains a complex disorder with multiple factors contributing to its development including changes at whole plant and cellular scales. Apple growers must carefully navigate these complex interactions between genetics, environment, and management decisions to minimize bitter pit in susceptible cultivars. Accordingly, management of plant nutrition, fruit crop load, and tree vigor still stands as the most important contribution to reducing bitter pit development. Even so, there will be situations where the occurrence of bitter pit will be inevitable due to cultivar and/or abiotic stress conditions.

Phytochrome-interacting factors play shared and distinct roles in regulating shade avoidance responses in Populus trees

Plants adjust their growth and development in response to changing light caused by canopy shade. The molecular mechanisms underlying shade avoidance responses have been widely studied in Arabidopsis and annual crop species, yet the shade avoidance signalling in woody perennial trees remains poorly understood. Here, we first showed that PtophyB1/2 photoreceptors serve conserved roles in attenuating the shade avoidance syndrome (SAS) in poplars. Next, we conducted a systematic identification and characterization of eight PtoPIF genes in Populus tomentosa. Knocking out different PtoPIFs led to attenuated shade responses to varying extents, whereas overexpression of PtoPIFs, particularly PtoPIF3.1 and PtoPIF3.2, led to constitutive SAS phenotypes under normal light and enhanced SAS responses under simulated shade. Notably, our results revealed that distinct from Arabidopsis PIF4 and PIF5, which are major regulators of SAS, the Populus homologues PtoPIF4.1 and PtoPIF4.2 seem to play a minor role in controlling shade responses. Moreover, we showed that PtoPIF3.1/3.2 could directly activate the expression of the auxin biosynthetic gene PtoYUC8 in response to shade, suggesting a conserved PIF-YUC-auxin pathway in modulating SAS in tree. Overall, our study provides insights into shared and divergent functions of PtoPIF members in regulating various aspects of the SAS in Populus.

Transcriptome and metabolome profiling reveal the effects of hormones on current-year shoot growth in Chinese 'Cuiguan' pear grafted onto vigorous rootstock 'Duli' and dwarf rootstock 'Quince A'

BACKGROUND

Dwarf rootstocks have important practical significance for high-density planting in pear orchards. The shoots of 'Cuiguan' grafted onto the dwarf rootstock were shorter than those grafted onto the vigorous rootstock. However, the mechanism of shorter shoot formation is not clear.

RESULTS

In this study, the current-year shoot transcriptomes and phytohormone contents of 'CG‒QA' ('Cuiguan' was grafted onto 'Quince A', and 'Hardy' was used as interstock) and 'CG‒DL' ('Cuiguan' was grafted onto 'Duli', and 'Hardy' was used as interstock) were compared. The transcriptome results showed that a total of 452 differentially expressed genes (DEGs) were identified, including 248 downregulated genes and 204 upregulated genes; the plant hormone signal transduction and zeatin biosynthesis pathways were significantly enriched in the top 20 KEGG enrichment terms. Abscisic acid (ABA) was the most abundant hormone in 'CG‒QA' and 'CG‒DL'; auxin and cytokinin (CTK) were the most diverse hormones; additionally, the contents of ABA, auxin, and CTK in 'CG‒DL' were higher than those in 'CG‒QA', while the fresh shoot of 'CG‒QA' accumulated more gibberellin (GA) and salicylic acid (SA). Metabolome and transcriptome co-analysis identified three key hormone-related DEGs, of which two (Aldehyde dehydrogenase gene ALDH3F1 and YUCCA2) were upregulated and one (Cytokinin oxidase/dehydrogenase gene CKX3) was downregulated.

CONCLUSIONS

Based on the results of transcriptomic and metabolomic analysis, we found that auxin and CTK mainly regulated the shoot differences of 'CG-QA' and 'CG-DL', and other hormones such as ABA, GA, and SA synergistically regulated this process. Three hormone-related genes ALDH3F1, YUCCA2, and CKX3 were the key genes contributing to the difference in shoot growth between 'CG-QA' and 'CG-DL' pear. This research provides new insight into the molecular mechanism underlying shoot shortening after grafted onto dwarf rootstocks.

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.

Ectopic Expression of PtrLBD39 Retarded Primary and Secondary Growth in Populus trichocarpa

Primary and secondary growth of trees are needed for increments in plant height and stem diameter, respectively, affecting the production of woody biomass for applications in timber, pulp/paper, and related biomaterials. These two types of growth are believed to be both regulated by distinct transcription factor (TF)-mediated regulatory pathways. Notably, we identified PtrLBD39, a highly stem phloem-specific TF in Populus trichocarpa and found that the ectopic expression of PtrLBD39 in P. trichocarpa markedly retarded both primary and secondary growth. In these overexpressing plants, the RNA-seq, ChIP-seq, and weighted gene co-expression network analysis (WGCNA) revealed that PtrLBD39 directly or indirectly regulates TFs governing vascular tissue development, wood formation, hormonal signaling pathways, and enzymes responsible for wood components. This regulation led to growth inhibition, decreased fibrocyte secondary cell wall thickness, and reduced wood production. Therefore, our study indicates that, following ectopic expression in P. trichocarpa, PtrLBD39 functions as a repressor influencing both primary and secondary growth.

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.

The microRNA7833-AUX6 module plays a critical role in wood development by modulating cellular auxin influx in Populus tomentosa

The critical role of auxin on secondary vascular development in woody plants has been demonstrated. The concentration gradient of endogenous indole-3-acetic acid and the cellular and molecular pathways contributing to the auxin-directed vascular organization and wood growth have been uncovered in recent decades. However, our understanding of the roles and regulations of auxin influx in wood formation in trees remains limited. Here, we reported that a microRNA, miR7833, participates in the negative regulation of stem cambial cell division and secondary xylem development in Populus tomentosa. The miR7833 is mainly expressed in the vascular cambium during stem radical growth and specifically targets and represses two AUX/LAX family auxin influx carriers, AUX5 and AUX6, in poplar. We further revealed that poplar AUX6, the most abundant miR7833 target in the stem, is preferentially enriched in the developing xylem and is a positive regulator for cell division and differentiation events during wood formation. Moreover, inhibition of auxin influx carriers by 1-naphthoxyacetic acids abolished the regulatory effects of miR7833 and AUX6 on secondary xylem formation in poplar. Our results revealed the essential roles of the miR7833-AUX6 module in regulating cellular events in secondary xylem development and demonstrated an auxin influx-dependent mechanism for wood formation 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.

Network Analysis of Metabolome and Transcriptome Revealed Regulation of Different Nitrogen Concentrations on Hybrid Poplar Cambium Development

Secondary development is a key biological characteristic of woody plants and the basis of wood formation. Exogenous nitrogen can affect the secondary growth of poplar, and some regulatory mechanisms have been found in the secondary xylem. However, the effect of nitrogen on cambium has not been reported. Herein, we investigated the effects of different nitrogen concentrations on cambium development using combined transcriptome and metabolome analysis. The results show that, compared with 1 mM NH4NO3 (M), the layers of hybrid poplar cambium cells decreased under the 0.15 mM NH4NO3 (L) and 0.3 mM NH4NO3 (LM) treatments. However, there was no difference in the layers of hybrid poplar cambium cells under the 3 mM NH4NO3 (HM) and 5 mM NH4NO3 (H) treatments. Totals of 2365, 824, 649 and 398 DEGs were identified in the M versus (vs.) L, M vs. LM, M vs. HM and M vs. H groups, respectively. Expression profile analysis of the DEGs showed that exogenous nitrogen affected the gene expression involved in plant hormone signal transduction, phenylpropanoid biosynthesis, the starch and sucrose metabolism pathway and the ubiquitin-mediated proteolysis pathway. In M vs. L, M vs. LM, M vs. HM and M vs. H, differential metabolites were enriched in flavonoids, lignans, coumarins and saccharides. The combined analysis of the transcriptome and metabolome showed that some genes and metabolites in plant hormone signal transduction, phenylpropanoid biosynthesis and starch and sucrose metabolism pathways may be involved in nitrogen regulation in cambium development, whose functions need to be verified. In this study, from the point of view that nitrogen influences cambium development to regulate wood formation, the network analysis of the transcriptome and metabolomics of cambium under different nitrogen supply levels was studied for the first time, revealing the potential regulatory and metabolic mechanisms involved in this process and providing new insights into the effects of nitrogen on wood development.

The effect of nitrogen source and levels on hybrid aspen tree physiology and wood formation

Nitrogen can be taken up by trees in the form of nitrate, ammonium and amino acids, but the influence of the different forms on tree growth and development is poorly understood in angiosperm species like Populus. We studied the effects of both organic and inorganic forms of nitrogen on growth and wood formation of hybrid aspen trees in experimental conditions that allowed growth under four distinct steady-state nitrogen levels. Increased nitrogen availability had a positive influence on biomass accumulation and the radial dimensions of both xylem vessels and fibers, and a negative influence on wood density. An optimal level of nitrogen availability was identified where increases in biomass accumulation outweighed decreases in wood density. None of these responses depended on the source of nitrogen except for shoot biomass accumulation, which was stimulated more by treatments complemented with nitrate than by ammonium alone or the organic source arginine. The most striking difference between the nitrogen sources was the effect on lignin composition, whereby the abundance of H-type lignin increased only in the presence of nitrate. The differential effect of nitrate is possibly related to the well-known role of nitrate as a signaling compound. RNA-sequencing revealed that while the lignin-biosynthetic genes did not significantly (FDR <0.01) respond to added NO3 - , the expression of several laccases, catalysing lignin polymerization, was dependent on N-availability. These results reveal a unique role of nitrate in wood formation and contribute to the knowledge basis for decision-making in utilizing hybrid aspen as a bioresource.

Gibberellin promotes cambium reestablishment during secondary vascular tissue regeneration after girdling in an auxin-dependent manner in Populus

Secondary vascular tissue (SVT) development and regeneration are regulated by phytohormones. In this study, we used an in vitro SVT regeneration system to demonstrate that gibberellin (GA) treatment significantly promotes auxin-induced cambium reestablishment. Altering GA content by overexpressing or knocking down ent-kaurene synthase (KS) affected secondary growth and SVT regeneration in poplar. The poplar DELLA gene GIBBERELLIC ACID INSENSITIVE (PtoGAI) is expressed in a specific pattern during secondary growth and cambium regeneration after girdling. Overexpression of PtoGAI disrupted poplar growth and inhibited cambium regeneration, and the inhibition of cambium regeneration could be partially restored by GA application. Further analysis of the PtaDR5:GUS transgenic plants, the localization of PIN-FORMED 1 (PIN1) and the expression of auxin-related genes found that an additional GA treatment could enhance the auxin response as well as the expression of PIN1, which mediates auxin transport during SVT regeneration. Taken together, these findings suggest that GA promotes cambium regeneration by stimulating auxin signal transduction.

Woody plant cell walls: Fundamentals and utilization

Cell walls in plants, particularly forest trees, are the major carbon sink of the terrestrial ecosystem. Chemical and biosynthetic features of plant cell walls were revealed early on, focusing mostly on herbaceous model species. Recent developments in genomics, transcriptomics, epigenomics, transgenesis, and associated analytical techniques are enabling novel insights into formation of woody cell walls. Here, we review multilevel regulation of cell wall biosynthesis in forest tree species. We highlight current approaches to engineering cell walls as potential feedstock for materials and energy and survey reported field tests of such engineered transgenic trees. We outline opportunities and challenges in future research to better understand cell type biogenesis for more efficient wood cell wall modification and utilization for biomaterials or for enhanced carbon capture and storage.

Transcription factor VviWOX13C regulates fruit set by directly activating VviEXPA37/38/39 in grape (Vitis vinifera L)

KEY MESSAGE

VviWOX13C plays a key regulatory role in the expansin during fruit set. Expansins as a type of non-enzymatic cell wall proteins, are responsible for the loosening and extension in cell walls leading to the enlargement of the plant cells. However, the current studies are still lacking in expansin genes associated with promoting fruit set. Here, 29 members of the expansin gene family were identified in the whole genome of grapes (Vitis vinifera L.), and the functional prediction of expansins was based on the gene annotated information. Results showed that the 29 members of grape expansin gene family could be mainly divided into four subfamilies (EXPA, EXPB, LIKE A, and LIKE B), distributed on 16 chromosomes. Replication analysis showed that there were four segmental duplications and two tandem duplications. Each expansins sequence contained two conserved domain features of grape EXPs (DPBB_1 and Expansin_C) through protein sequence analysis. The transcriptome sequencing results revealed that VviEXPA37, VviEXPA38, and VviEXPA39 were induced and upregulated by CPPU. Furthermore, transcriptional regulatory prediction network indicated that VviWOX13C targeted regulates VviEXPA37, VviEXPA38, and VviEXPA39 simultaneously. EMSA and dual luciferase assays demonstrated that VviWOX13C directly activated the expression of VviEXPA37, VviEXPA38, and VviEXPA39 by directly binding to its promoter. These results provide a basis for further studies on the function and regulatory mechanisms of expansin genes in fruit set.

Transcriptome and miRNAs Profiles Reveal Regulatory Network and Key Regulators of Secondary Xylem Formation in "84K" Poplar

Secondary xylem produced by stem secondary growth is the main source of tree biomass and possesses great economic and ecological value in papermaking, construction, biofuels, and the global carbon cycle. The secondary xylem formation is a complex developmental process, and the underlying regulatory networks and potential mechanisms are still under exploration. In this study, using hybrid poplar (Populus alba × Populus glandulosa clone 84K) as a model system, we first ascertained three representative stages of stem secondary growth and then investigated the regulatory network of secondary xylem formation by joint analysis of transcriptome and miRNAs. Notably, 7507 differentially expressed genes (DEGs) and 55 differentially expressed miRNAs (DEMs) were identified from stage 1 without initiating secondary growth to stage 2 with just initiating secondary growth, which was much more than those identified from stage 2 to stage 3 with obvious secondary growth. DEGs encoding transcription factors and lignin biosynthetic enzymes and those associated with plant hormones were found to participate in the secondary xylem formation. MiRNA-target analysis revealed that a total of 85 DEMs were predicted to have 2948 putative targets. Among them, PagmiR396d-PagGRFs, PagmiR395c-PagGA2ox1/PagLHW/PagSULTR2/PagPolyubiquitin 1, PagmiR482d-PagLAC4, PagmiR167e-PagbHLH62, and PagmiR167f/g/h-PagbHLH110 modules were involved in the regulating cambial activity and its differentiation into secondary xylem, cell expansion, secondary cell wall deposition, and programmed cell death. Our results give new insights into the regulatory network and mechanism of secondary xylem formation.

Mismatch between primary and secondary growth and its consequences on wood formation in Qinghai spruce

The connections between the primary and secondary growth of trees allows better understanding of the dynamics of carbon sequestration in forest ecosystems. The relationship between primary and secondary growth of trees could change due to the diverging responses of meristems to climate warming. In this study, the bud phenology and radial growth dynamics of Qinghai spruce (Picea crassifolia) in arid and semi-arid areas of China in 2019 and 2020 were weekly monitored to analyze their response to different weather conditions and their links with carbon sink. Xylem anatomical traits (i.e. lumen radial diameter and cell wall thickness) were quantified along cell radial files after the end of xylem lignification to calculate the early-to-latewood transition date. Winter and early spring (January-March) were warmer in 2020 with a colder April compared with 2019. Precipitation in April-June was lower in 2020 than in 2019. In 2019, bud phenology occurred earlier, while the onset of xylem formation and the early-to-latewood transition date were delayed. The duration from the beginning of split bud and exposed shoot to the early-to-latewood transition date was positively correlated with the radial width of earlywood (accounting for ~80% of xylem width) and total xylem width. The longer duration of xylem cell division did not increase xylem cell production and radial width. Moreover, the duration from bud burst to the early-to-latewood transition date in 2020 was negatively linked with early phloem cell production as compared with 2019. Our findings suggest that warm conditions in winter and early spring promote the xylogenesis of Qinghai spruce, but might delay bud burst. However, the xylem width increments largely depend on the duration from bud burst to the start of latewood cell division rather than on the earlier xylogenesis and longer duration of xylem cell differentiation induced by warm conditions.

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.

VS-Cambium-Developer: A New Predictive Model of Cambium Functioning under the Influence of Environmental Factors

Climate changes influence seasonal tree-ring formation. The result is a specific cell structure dependent on internal processes and external environmental factors. One way to investigate and analyze these relationships is to apply diverse simulation models of tree-ring growth. Here, we have proposed a new version of the VS-Cambium-Developer model (VS-CD model), which simulates the cambial activity process in conifers. The VS-CD model does not require the manual year-to-year calibration of parameters over a long-term cell production reconstruction or forecast. Instead, it estimates cell production and simulates the dynamics of radial cell development within the growing seasons. Thus, a new software based on R programming technology, able to efficiently adapt to the VS model online platform, has been developed. The model was tested on indirect observations of the cambium functioning in Larix sibirica trees from southern Siberia, namely on the measured annual cell production from 1963 to 2011. The VS-CD model proves to simulate cell production accurately. The results highlighted the efficiency of the presented model and contributed to filling the gap in the simulations of cambial activity, which is critical to predicting the potential impacts of changing environmental conditions on tree growth.

From procambium patterning to cambium activation and maintenance in the Arabidopsis root

In addition to primary growth, which elongates the plant body, many plant species also undergo secondary growth to thicken their body. During primary vascular development, a subset of the vascular cells, called procambium and pericycle, remain undifferentiated to later gain vascular cambium and cork cambium identity, respectively. These two cambia are the lateral meristems providing secondary growth. The vascular cambium produces secondary xylem and phloem, which give plants mechanical support and transport capacity. Cork cambium produces a protective layer called cork. In this review, we focus on recent advances in understanding the formation of procambium and its gradual maturation to active cambium in the Arabidopsis thaliana root.

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.

The central role of stem cells in determining plant longevity variation

Vascular plants display a huge variety of longevity patterns, from a few weeks for several annual species up to thousands of years for some perennial species. Understanding how longevity variation is structured has long been considered a fundamental aspect of the life sciences in view of evolution, species distribution, and adaptation to diverse environments. Unlike animals, whose organs are typically formed during embryogenesis, vascular plants manage to extend their life by continuously producing new tissues and organs in apical and lateral directions via proliferation of stem cells located within specialized tissues called meristems. Stem cells are the main source of plant longevity. Variation in plant longevity is highly dependent on the activity and fate identity of stem cells. Multiple developmental factors determine how stem cells contribute to variation in plant longevity. In this review, we provide an overview of the genetic mechanisms, hormonal signaling, and environmental factors involved in controlling plant longevity through long-term maintenance of stem cell fate identity.

Hormonal crosstalk controls cell death induced by kinetin in roots of Vicia faba ssp. minor seedlings

Studies of vitality/mortality of cortex cells, as well as of the concentrations of ethylene (ETH), gibberellins (GAs), indolic compounds/auxins (ICs/AUXs) and cytokinins (CKs), were undertaken to explain the hormonal background of kinetin (Kin)-regulated cell death (RCD), which is induced in the cortex of the apical parts of roots of faba bean (Vicia faba ssp. minor) seedlings. Quantification was carried out with fluorescence microscopy, ETH sensors, spectrophotometry and ultrahigh-performance liquid chromatography tandem mass spectrometry (UHPLC‒MS/MS). The results indicated that Kin was metabolized to the transport form, i.e., kinetin-9-glucoside (Kin9G) and kinetin riboside (KinR). KinR was then converted to cis-zeatin (cZ) in apical parts of roots with meristems, to cis-zeatin riboside (cZR) in apical parts of roots without meristems and finally to cis-zeatin riboside 5'-monophosphate (cZR5'MP), which is indicated to be a ligand of cytokinin-dependent receptors inducing CD. The process may be enhanced by an increase in the amount of dihydrozeatin riboside (DHZR) as a byproduct of the pathway of zeatin metabolism. It seems that crosstalk of ETH, ICs/AUXs, GAs and CKs with the cZR5'MP, the cis-zeatin-dependent pathway, but not the trans-zeatin-dependent pathway, is responsible for Kin-RCD, indicating that the process is very specific and offers a useful model for studies of CD hallmarks in plants.

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.

Gibberellins promote polar auxin transport to regulate stem cell fate decisions in cambium

Vascular cambium contains bifacial stem cells, which produce secondary xylem to one side and secondary phloem to the other. However, how these fate decisions are regulated is unknown. Here we show that the positioning of an auxin signalling maximum within the cambium determines the fate of stem cell daughters. The position is modulated by gibberellin-regulated, PIN1-dependent polar auxin transport. Gibberellin treatment broadens auxin maximum from the xylem side of the cambium towards the phloem. As a result, xylem-side stem cell daughter preferentially differentiates into xylem, while phloem-side daughter retains stem cell identity. Occasionally, this broadening leads to direct specification of both daughters as xylem, and consequently, adjacent phloem-identity cell reverts to being stem cell. Conversely, reduced gibberellin levels favour specification of phloem-side stem cell daughter as phloem. Together, our data provide a mechanism by which gibberellin regulates the ratio of xylem and phloem production.

Physiological and Transcriptomic Responses of Growth in Neolamarckia cadamba Stimulated by Exogenous Gibberellins

(1) The phytohormones gibberellins (GAs) play a crucial role in plant growth and development, such as seed germination, flowering, fruiting, and stem elongation. Although many biological roles of GAs have been studied intensively, the molecular mechanisms of GAs in woody plants are still unclear. (2) In this study, we investigated the effects of exogenous application of GAs on Neolamarckia cadamba. (3) The height and biomass of N. cadamba increased after 7 days of GA treatment, especially on the second internode. Transcriptome analysis showed that although the majority of genes involved in the GA signaling pathway were up-regulated, the expression of GA20 oxidase (GA20ox) and GA3 oxidase (GA3ox) was down-regulated in the 3 days GA-treated group compared to the CK group. The expression of the cell elongation-related basic helix-loop-helix genes bHLH74 and bHLH49 was up-regulated in the GA-treated group compared with the CK group. Transcriptional expression levels of transcription factors involved in hormone signaling were changed, mainly including bHLH, ethylene response factor (ERF), and WRKY families. In addition, the transcriptional expression level of the key enzymes engaged in the phenylalanine pathway was downregulated after GA treatment. (4) In brief, our findings reveal the physiological and molecular mechanisms of exogenous GA treatment stimulation in N. cadamba.

Transcriptomic analysis and physiological characteristics of exogenous naphthylacetic acid application to regulate the healing process of oriental melon grafted onto squash

The plant graft healing process is an intricate development influenced by numerous endogenous and environmental factors. This process involves the histological changes, physiological and biochemical reactions, signal transduction, and hormone exchanges in the grafting junction. Studies have shown that applying exogenous plant growth regulators can effectively promote the graft healing process and improve the quality of grafted plantlets. However, the physiological and molecular mechanism of graft healing formation remains unclear. In our present study, transcriptome changes in the melon and cucurbita genomes were analyzed between control and NAA treatment, and we provided the first view of complex networks to regulate graft healing under exogenous NAA application. The results showed that the exogenous NAA application could accelerate the graft healing process of oriental melon scion grafted onto squash rootstock through histological observation, increase the SOD, POD, PAL, and PPO activities during graft union development and enhance the contents of IAA, GA₃, and ZR except for the IL stage. The DEGs were identified in the plant hormone signal-transduction, phenylpropanoid biosynthesis, and phenylalanine metabolism through transcriptome analysis of CK vs. NAA at the IL, CA, and VB stage by KEGG pathway enrichment analysis. Moreover, the exogenous NAA application significantly promoted the expression of genes involved in the hormone signal-transduction pathway, ROS scavenging system, and vascular bundle formation.

Comparative transcriptome analysis during tuberous stem formation in Kohlrabi (B. oleracea var. gongylodes) at early growth periods (seedling stages)

Tuberous stem of kohlrabi is an important agronomic trait, however, the molecular basis of tuberization is poorly understood. To elucidate the tuberization mechanism, we conducted a comparative transcriptomic analysis between kohlrabi and broccoli at 10 and 20 days after germination (DAG) as tuberous stem initiated between these time points. A total of 5580 and 2866 differentially expressed transcripts (DETs) were identified between genotypes (kohlrabi vs. broccoli) and growth stages (10 DAG vs. 20 DAG), respectively, and most of the DETs were down-regulated in kohlrabi. Gene ontology (GO) and KEGG pathway enrichment analyses showed that the DETs between genotypes are involved in cell wall loosening and expansion, cell cycle and division, carbohydrate metabolism, hormone transport, hormone signal transduction and in several transcription factors. The DETs identified in those categories may directly/indirectly relate to the initiation and development of tuberous stem in kohlrabi. In addition, the expression pattern of the hormone synthesis related DETs coincided with the endogenous IAA, IAAsp, GA, ABA, and tZ profiles in kohlrabi and broccoli seedlings, that were revealed in our phytohormone analysis. This is the first report on comparative transcriptome analysis for tuberous stem formation in kohlrabi at early growth periods. The resulting data could provide significant insights into the molecular mechanism underlying tuberous stem development in kohlrabi as well as in other tuberous organ forming crops.

PagDET2 promotes cambium cell division and xylem differentiation in poplar stem

Secondary growth of the woody tree stem is governed by meristematic cell division and differentiation in the vascular cambium. Multiple hormonal signals and endogenous developmental programs regulate vascular cambium activity. Brassinosteroids (BRs) significantly promote secondary stem growth and wood formation in poplar trees. However, the underlying regulatory mechanisms of BRs within the vascular tissue remain unclear. Genetic and anatomical approaches were used here to elucidate the role of PagDET2, the rate-limiting enzyme for BRs biosynthesis, in regulating secondary vascular cambium activity in Populus. This study showed that the elevated endogenous castasterone (CS) levels in tree stems through overexpressing PagDET2 could enhance cambium meristem cell activity and xylem (XY) differentiation to promote secondary stem growth. RNA-seq analysis revealed that genes involved in BRs response, vascular cambium cell division, XY differentiation, and secondary cell wall synthesis were up-regulated.

A Constitutively Active Cytokinin Receptor Variant Increases Cambial Activity and Stem Growth in Poplar

The cambial meristem is responsible for bark and wood formation in woody plants. The activity of the cambial meristem is controlled by various factors; one of them is the plant hormone cytokinin. Here, we have explored different approaches to genetically engineering cambial activity in poplar plants by the ectopic expression of a cytokinin biosynthesis gene with enhanced activity (named ROCK4) or of a gene encoding a constitutively active cytokinin receptor variant (ROCK3). Both genes are derived from Arabidopsis thaliana and were expressed in poplar trees under the control of their own promoter or the cambium-specific pHB8 promoter. pIPT3:ROCK4- and pHB8:ROCK4-expressing plants were smaller than wild-type plants and formed more lateral branches; pHB8:ROCK4 transgenic plants additionally showed an increased stem diameter. In contrast, pAHK3:ROCK3- and pHB8:ROCK3-expressing plants grew taller than wild type without an altered branching pattern and formed more cambial cells, leading to increased radial stem growth. The effectivity of ROCK3 when expressed in either secondary phloem cells or in cambial cells is consistent with a dual, tissue-autonomous and non-autonomous activity of cytokinin in regulating cambial activity. We propose ROCK3 as a novel gene to enhance biomass formation in woody plants.

AUXIN RESPONSE FACTOR7 integrates gibberellin and auxin signaling via interactions between DELLA and AUX/IAA proteins to regulate cambial activity in poplar

Cambial development in the stems of perennial woody species is rigorously regulated by phytohormones. Auxin and gibberellin (GA) play crucial roles in stimulating cambial activity in poplar (Populus spp.). In this study, we show that the DELLA protein REPRESSOR of ga1-3 Like 1 (RGL1), AUXIN RESPONSE FACTOR 7 (ARF7), and Aux/INDOLE-3-ACETIC ACID 9 (IAA9) form a ternary complex that mediates crosstalk between the auxin and GA signaling pathways in poplar stems during cambial development. Biochemical analysis revealed that ARF7 physically interacts with RGL1 and IAA9 through distinct domains. The arf7 loss-of-function mutant showed markedly attenuated responses to auxin and GA, whereas transgenic poplar plants overexpressing ARF7 displayed strongly improved cambial activity. ARF7 directly binds to the promoter region of the cambial stem cell regulator WOX4 to modulate its expression, thus integrating auxin and GA signaling to regulate cambial activity. Furthermore, the direct activation of PIN-FORMED 1 expression by ARF7 in the RGL1-ARF7-IAA9 module increased GA-dependent cambial activity via polar auxin transport. Collectively, these findings reveal that the crosstalk between auxin and GA signaling mediated by the RGL1-ARF7-IAA9 module is crucial for the precise regulation of cambial development in poplar.

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.

Spatiotemporal development of suberized barriers in cork oak taproots

The longevity and high activity of the cork cambium (or phellogen) from Quercus suber L. (cork oak) are the cornerstones for the sustainable exploitation of a unique raw material. Cork oak is a symbolic model to study cork development and cell wall suberization, yet most genetic and molecular studies on these topics have targeted other model plants. In this study, we explored the potential of taproots as a model system to study phellem development and suberization in cork oak, thereby avoiding the time constraints imposed when studying whole plants. In roots, suberin deposition is found in mature endodermis cells during primary development and in phellem cells during secondary development. By investigating the spatiotemporal characteristics of both endodermis and phellem suberization in young seedling taproots, we demonstrated that secondary growth and phellogen activity are initiated very early in cork oak taproots (approx. 8 days after sowing). We further compared the transcriptomic profile of root segments undergoing primary (PD) and secondary development (SD) and identified multiple candidate genes with predicted roles in cell wall modifications, mainly lignification and suberization, in addition to several regulatory genes, particularly transcription factor- and hormone-related genes. Our results indicate that the molecular regulation of suberization and secondary development in cork oak roots is relatively conserved with other species. The provided morphological characterization creates new opportunities to allow a faster assessment of phellogen activity (as compared with studies using stem tissues) and to tackle fundamental questions regarding its regulation.

Cambial meristematic cell culture: a sustainable technology toward in vitro specialized metabolites production

Cambial meristematic cells (CMCs) culture has received a fair share of scientific and industrial attention among the trending topics of plant cell culture, especially their potential toward secondary metabolites production. However, the conventional plant cell culture is often not commercially feasible because of difficulties associated with culture dedifferentiated cells. Several reports have been published to culture CMCs and bypass the dedifferentiation process in plant cell culture. Numerous mitochondria, multiple vacuoles, genetic stability, self-renewal, higher biomass, and stable metabolites accumulation are the characteristics features of CMCs compared with dedifferentiated cells (DDCs) culture. The CMCs culture has a broader application to produce large-scale natural compounds for: pharmaceuticals, food, and cosmetic industries. Cutting-edge progress in plant cellular and molecular biology has allowed unprecedented insights into cambial stem cell culture and its fundamental processes. Therefore, regarding sustainability and natural compound production, cambial cell culture ranks among the most vital biotechnological interventions for industrial and economic perspectives. This review highlights the recent advances in plant stem cell culture and understands the cambial cells induction and culture mechanisms that affect the growth and natural compounds production.

Comparative Transcriptomics Suggests Early Modifications by Vintec® in Grapevine Trunk of Hormonal Signaling and Secondary Metabolism Biosynthesis in Response to Phaeomoniella chlamydospora and Phaeoacremonium minimum

Given their well-known antifungal abilities, species of the genus Trichoderma are of significant interest in modern agriculture. Recent studies have shown that Trichoderma species can induce plant resistance against different phytopathogens. To further extend this line of investigation, we investigate herein the transcriptomic response of grapevine trunk to Vintec^®, which is a Trichoderma atroviride SC1-based commercial formulation for biological control of grapevine trunk diseases and which reduces wood colonization. The aim of the study is to understand whether the biocontrol agent Vintec^® modifies the trunk response to Phaeoacremonium minimum and Phaeomoniella chlamydospora, which are two esca-associated fungal pathogens. An analysis of transcriptional regulation identifies clusters of co-regulated genes whose transcriptomic reprogramming in response to infection depends on the absence or presence of Vintec^®. On one hand, the results show that Vintec^® differentially modulates the expression of putative genes involved in hormonal signaling, especially those involved in auxin signaling. On the other hand, most significant gene expression modifications occur among secondary-metabolism-related genes, especially regarding phenylpropanoid metabolism and stilbene biosynthesis. Taken together, these results suggest that the biocontrol agent Vintec^® induces wood responses that counteract disease development.

Infection by Moniliophthora perniciosa reprograms tomato Micro-Tom physiology, establishes a sink, and increases secondary cell wall synthesis

Witches' broom disease of cacao is caused by the pathogenic fungus Moniliophthora perniciosa. By using tomato (Solanum lycopersicum) cultivar Micro-Tom (MT) as a model system, we investigated the physiological and metabolic consequences of M. perniciosa infection to determine whether symptoms result from sink establishment during infection. Infection of MT by M. perniciosa caused reductions in root biomass and fruit yield, a decrease in leaf gas exchange, and down-regulation of photosynthesis-related genes. The total leaf area and water potential decreased, while ABA levels, water conductance/conductivity, and ABA-related gene expression increased. Genes related to sugar metabolism and those involved in secondary cell wall deposition were up-regulated upon infection, and the concentrations of sugars, fumarate, and amino acids increased. 14C-glucose was mobilized towards infected MT stems, but not in inoculated stems of the MT line overexpressing CYTOKININ OXIDASE-2 (35S::AtCKX2), suggesting a role for cytokinin in establishing a sugar sink. The up-regulation of genes involved in cell wall deposition and phenylpropanoid metabolism in infected MT, but not in 35S::AtCKX2 plants, suggests establishment of a cytokinin-mediated sink that promotes tissue overgrowth with an increase in lignin. Possibly, M. perniciosa could benefit from the accumulation of secondary cell walls during its saprotrophic phase of infection.

Comparative Transcriptome Analysis Reveals Candidate Genes and Pathways for Potential Branch Growth in Elm (Ulmus pumila) Cultivars

Wood plays a vital role in human life. It is important to study the thickening mechanism of tree branches and explore the mechanism of wood formation. Elm (Ulmus pumila) is a strong essential wood, and it is widely used in cabinets, sculptures, and ship making. In the present study, phenotypic and comparative transcriptomic analyses were performed in U. pumila fast- (UGu17 and UZuantian) and slow-growing cultivars (U81-07 and U82-39). Phenotypic observation showed that the thickness of secondary xylem of 2-year-old fast-growing branches was greater compared with slow-growing cultivars. A total of 9367 (up = 4363, down = 5004), 7159 (3413/3746), 7436 (3566/3870), and 5707 (2719/2988) differentially expressed genes (DEGs) were identified between fast- and slow-growing cultivars. Moreover, GO and KEGG enrichment analyses predicted that many pathways were involved in vascular development and transcriptional regulation in elm, such as "plant-type secondary cell wall biogenesis", "cell wall thickening", and "phenylpropanoid biosynthesis". NAC domain transcriptional factors (TFs) and their master regulators (VND1/MYB26), cellulose synthase catalytic subunits (CESAs) (such as IRX5/IRX3/IRX1), xylan synthesis, and secondary wall thickness (such as IRX9/IRX10/IRX8) were supposed to function in the thickening mechanism of elm branches. Our results indicated that the general phenylpropanoid pathway (such as PAL/C4H/4CL) and lignin metabolism (such as HCL/CSE/CCoAOMT/CCR/F5H) had vital functions in the growth of elm branches. Our transcriptome data were consistent with molecular results for branch thickening in elm cultivars.

Cell Fate Decisions Within the Vascular Cambium-Initiating Wood and Bast Formation

Precise coordination of cell fate decisions is a hallmark of multicellular organisms. Especially in tissues with non-stereotypic anatomies, dynamic communication between developing cells is vital for ensuring functional tissue organization. Radial plant growth is driven by a plant stem cell niche known as vascular cambium, usually strictly producing secondary xylem (wood) inward and secondary phloem (bast) outward, two important structures serving as much-needed CO₂ depositories and building materials. Because of its bidirectional nature and its developmental plasticity, the vascular cambium serves as an instructive paradigm for investigating principles of tissue patterning. Although genes and hormones involved in xylem and phloem formation have been identified, we have a yet incomplete picture of the initial steps of cell fate transitions of stem cell daughters into xylem and phloem progenitors. In this mini-review perspective, we describe two possible scenarios of cell fate decisions based on the current knowledge about gene regulatory networks and how cellular environments are established. In addition, we point out further possible research directions.

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.

Wood Formation Modeling - A Research Review and Future Perspectives

Wood formation has received considerable attention across various research fields as a key process to model. Historical and contemporary models of wood formation from various disciplines have encapsulated hypotheses such as the influence of external (e.g., climatic) or internal (e.g., hormonal) factors on the successive stages of wood cell differentiation. This review covers 17 wood formation models from three different disciplines, the earliest from 1968 and the latest from 2020. The described processes, as well as their external and internal drivers and their level of complexity, are discussed. This work is the first systematic cataloging, characterization, and process-focused review of wood formation models. Remaining open questions concerning wood formation processes are identified, and relate to: (1) the extent of hormonal influence on the final tree ring structure; (2) the mechanism underlying the transition from earlywood to latewood in extratropical regions; and (3) the extent to which carbon plays a role as "active" driver or "passive" substrate for growth. We conclude by arguing that wood formation models remain to be fully exploited, with the potential to contribute to studies concerning individual tree carbon sequestration-storage dynamics and regional to global carbon sequestration dynamics in terrestrial vegetation models.

Control of cambium initiation and activity in Arabidopsis by the transcriptional regulator AHL15

Plant secondary growth, which is the basis of wood formation, includes the production of secondary xylem, which is derived from meristematic cambium cells embedded in vascular tissue. Here, we identified an important role for the Arabidopsis thaliana (Arabidopsis) AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED 15 (AHL15) transcriptional regulator in controlling vascular cambium activity. The limited secondary xylem development in inflorescence stems of herbaceous Arabidopsis plants was significantly reduced in ahl15 loss-of-function mutants, whereas constitutive or vascular meristem-specific AHL15 overexpression produced woody inflorescence stems. AHL15 was required for enhanced secondary xylem formation in the woody suppressor of overexpression of constans 1 (soc1) fruitfull (ful) double loss-of-function mutant. Moreover, we found that AHL15 induces vascular cambium activity downstream of the repressing SOC1 and FUL transcription factors, most likely similar to how it enhances lateral branching by promoting biosynthesis of the hormone cytokinin. Our results uncover a novel pathway driving cambium development, in which AHL15 expression levels act in parallel to and are dependent on the well-established TDIF-PXY-WOX pathway to differentiate between herbaceous and woody stem growth.

Spatiotemporal plant hormone analysis from cryosections using laser microdissection-liquid chromatography-mass spectrometry

Laser microdissection (LMD) is used for isolating specific regions or single cells from a wide variety of tissue samples under direct microscopic observation. The LMD method enables the harvest of the cells of interest in a region or specific cells for several analyses, such as DNA/RNA analysis, proteomics, metabolomics, and other molecular analyses. Currently, LMD is used to study various biological events at the tissue or cellular level; it has been used in a wide range of research fields. In this report, we describe techniques for isolating different tissues/specific cells from cryosections of incised Arabidopsis flowering stems by LMD for spatiotemporal quantitative plant hormone analysis. The endogenous indole-3-acetic acid levels in the epidermis/cortex, vascular bundles, and pith of Arabidopsis flowering stems were approximately 19.0 pg mm^-3, 33.5 pg mm^-3, and 3.32 pg mm^-3, respectively, and these endogenous levels were altered spatiotemporally after incision. We also analyzed jasmonic acid from LMD-isolated cells and showed that the endogenous levels increased in the range of approximately 200-3,500 pg mm^-3 depending on the tissue and region at 1 h after incision and then decreased to less than 100 pg mm^-3 or undetectable levels at 24 h after incision. Quantitative analyses of phytohormones, including jasmonic acid-related molecules, gibberellin, abscisic acid, and cytokinins, could also be performed using the same cell samples. These results showed that spatiotemporal changes in plant hormones could be quantitatively and simultaneously analyzed by LMD-isolated cells from cryosections with positional information. The combination of quantitative analysis by liquid chromatography-mass spectrometry (LC-MS) and sampling by the LMD method provides a comprehensive and quantitative understanding of spatiotemporal changes in plant hormones in a region- and tissue-specific manner. Therefore, LMD-LC-MS methods will contribute to our understanding of the physiological events that control the process of plant growth and development.

Laying it on thick: a study in secondary growth

The development of secondary vascular tissue enhances the transport capacity and mechanical strength of plant bodies, while contributing a huge proportion of the world's biomass in the form of wood. Cell divisions in the cambium, which constitutes the vascular meristem, provide progenitors from which conductive xylem and phloem are derived. The cambium is a somewhat unusual stem cell population in two respects, making it an interesting subject for developmental research. Firstly, it arises post-germination, and thus represents a model for understanding stem cell initiation beyond embryogenesis. Secondly, xylem and phloem differentiate on opposing sides of cambial stem cells, making them bifacial in nature. Recent discoveries in Arabidopsis thaliana have provided insight into the molecular mechanisms that regulate the initiation, patterning, and maintenance of the cambium. In this review, the roles of intercellular signalling via mobile transcription factors, peptide-receptor modules, and phytohormones are described. Crosstalk between these regulatory pathways is becoming increasingly apparent, yet the underlying mechanisms are not fully understood. Future study of the interaction between multiple independently identified regulators, as well as the functions of their orthologues in trees, will deepen our understanding of radial growth in plants.