Cambium Reactivation Is Closely Related to the Cell-Cycle Gene Configuration in Larix kaempferi

Dormancy release and reactivation in temperate trees are mainly controlled by temperature and are affected by age, but the underlying molecular mechanisms are still unclear. In this study, we explored the effects of low temperatures in winter and warm temperatures in spring on dormancy release and reactivation in Larix kaempferi. Further, we established the relationships between cell-cycle genes and cambium cell division. The results showed that chilling accelerated L. kaempferi bud break overall, and the longer the duration of chilling is, the shorter the bud break time is. After dormancy release, warm temperatures induced cell-cycle gene expression; when the configuration value of the cell-cycle genes reached 4.97, the cambium cells divided and L. kaempferi reactivated. This study helps to predict the impact of climate change on wood production and provides technical support for seedling cultivation in greenhouses.

Plasmodesmata callose binding protein 2 contributes to the regulation of cambium/phloem formation and auxin response during the tissue reunion process in incised Arabidopsis stem

Plants are exposed to a variety of biotic and abiotic stresses, including wounding at the stem. The healing process (tissue reunion) begins immediately after stem wounding. The plant hormone auxin plays an important role during tissue reunion. In decapitated stems, auxin transport from the shoot apex is reduced and tissue reunion does not occur but is restored by application of indole-3-acetic acid (IAA). In this study, we found that plasmodesmata callose binding protein 2 (PDCB2) affects the expansion of the cambium/phloem region via changes in auxin response during the process of tissue reunion. PDCB2 was expressed in the cortex and endodermis on the incised side of stems 1-3 days after incision. PDCB2-knockout plants showed reduced callose deposition at plasmodesmata and DR5::GUS activity in the endodermis/cortex in the upper region of the incision accompanied by an increase in size of the cambium/phloem region during tissue reunion. In addition, PIN(PIN-FORMED)3, which is involved in lateral auxin transport, was induced by auxin in the cambium/phloem and endodermis/cortex in the upper part of the incision in wild type, but its expression of PIN3 was decreased in pdcb2 mutant. Our results suggest that PDCB2 contributes to the regulation of cambium/phloem development via auxin response.

Mosaic assembly of regulatory programs for vascular cambial growth: a view from the Early Devonian

Evidence for secondary growth extends into the Early Devonian, 407 million years ago, raising questions about tempo and mode of origination of this key developmental feature. To address such questions, we analyze anatomy in the four oldest fossil plants with well-characterized woody tissues; one of these represents a new genus, described here formally. The new fossil is documented using the cellulose acetate peel technique and associated methods. We use the paradigm of structural fingerprints to identify developmental components of cambial growth based on fossil anatomy. We integrate developmental inferences within a theoretical framework of modular regulation of secondary growth. The fossils possess structural fingerprints consistent with four different combinations of regulatory mechanisms (modules) acting in cambial growth, representing four distinct modes of secondary growth. The different modes of secondary growth demonstrate that cambial growth is an assemblage of regulatory modules whose deployment followed a mosaic pattern across woody plants, which may represent ancestors of younger lineages that exhibit woody growth. The diverse modes of wood development occupy a wide morphospace in the anatomy of wood in the Early Devonian, suggesting that the origins of secondary growth and of its modular components pre-date this interval.

Single-cell resolution analysis reveals the preparation for reprogramming the fate of stem cell niche in cotton lateral meristem

BACKGROUND

Somatic embryogenesis is a major process for plant regeneration. However, cell communication and the gene regulatory network responsible for cell reprogramming during somatic embryogenesis are still largely unclear. Recent advances in single-cell technologies enable us to explore the mechanism of plant regeneration at single-cell resolution.

RESULTS

We generate a high-resolution single-cell transcriptomic landscape of hypocotyl tissue from the highly regenerable cotton genotype Jin668 and the recalcitrant TM-1. We identify nine putative cell clusters and 23 cluster-specific marker genes for both cultivars. We find that the primary vascular cell is the major cell type that undergoes cell fate transition in response to external stimulation. Further developmental trajectory and gene regulatory network analysis of these cell clusters reveals that a total of 41 hormone response-related genes, including LAX2, LAX1, and LOX3, exhibit different expression patterns in the primary xylem and cambium region of Jin668 and TM-1. We also identify novel genes, including CSEF, PIS1, AFB2, ATHB2, PLC2, and PLT3, that are involved in regeneration. We demonstrate that LAX2, LAX1 and LOX3 play important roles in callus proliferation and plant regeneration by CRISPR/Cas9 editing and overexpression assay.

CONCLUSIONS

This study provides novel insights on the role of the regulatory network in cell fate transition and reprogramming during plant regeneration driven by somatic embryogenesis.

Function and Characteristic Analysis of Candidate PEAR Proteins in Populus yunnanensis

PEAR proteins are a type of plant-specific DNA binding with one finger (Dof) transcription factors that play a key role in the regulation of plant growth, especially during phloem cell growth and seed germination in Arabidopsis. However, the identification, characteristics and function of PEAR proteins, particularly in woody plants, need to be further studied. In the present study, 43 candidate PEAR proteins harboring the conserved Zf-Dof domain were obtained in Populus yunnanensis. Based on phylogenetic and structural analysis, 10 representative PEAR candidates were selected, belonging to different phylogenetic groups. The functions of PEAR proteins in the stress response, signal transduction, and growth regulation of stem cambium and roots undergoing vigorous cell division in Arabidopsis were revealed based on their expression patterns as characterized by qRT-PCR analysis, in accordance with the results of cis-element analysis. In vitro experiments showed that the interaction of transcription factor (E2F) and cyclin indirectly reflects the growth regulation function of PEAR through light signaling and cell-cycle regulation. Therefore, our results provide new insight into the identity of PEAR proteins and their function in stress resistance and vigorous cell division regulation of tissues in P. yunnanensis, which may serve as a basis for further investigation of the functions and characteristics of PEAR proteins in other plants.

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.

Investigation of chemical linkages between lignin and carbohydrates in cultured poplar cambium tissues via double isotope labeling

Lignin precursor labeled with ¹³C (coniferin-¹³Cα), carbohydrate precursor labeled with D (6,6-D₂-glucose) were put into cambium tissue stripped from a growing poplar. The tissue was further cultured in vitro for 18d. Then, the isotopic abundance was determined. The results showed that the labeled precursors could be normally involved in the formation of new xylem. The labeled new xylem tissue was fractionated by ionic liquid DMSO/TBAH system to obtain two components: glucan-lignin complex (GL) and xylan-lignin complex (XL). The X-ray diffraction (XRD) results indicated that the crystalline form of cellulose in the GL component was transformed from type I to type II after the ionic liquid separation. Then the GL and XL were purified and modified by enzymatic and chemical methods, and their structures were elucidated by nuclear magnetic resonance (NMR) spectroscopy. The results showed that lignin subunits in the cultured tissues were mainly connected by β-5 and β-O-4 linkages, of which the β-O-4 substructure unit predominated. Lignin and carbohydrates were mainly connected by acetal bonds, ether bonds, and ester bonds. Combined with the carbohydrate composition and XRD analysis results, the GL components also confirmed the existence of acetal bonds, ester bonds and ether bonds between lignin and cellulose.

Inducible, Tissue-Specific Gene Expression in Arabidopsis Using GR-LhG4-Mediated Trans-Activation

Inducible, tissue-specific gene expression is a potent tool to study gene regulatory networks as it allows spatially and temporally controlled genetic perturbations. To this end, we generated a toolkit that covers many cell types in the three main meristems: the root apical meristem, the shoot apical meristem, and the vascular cambium. The system is based on an extensive set of driver lines expressing a synthetic transcription factor under cell type-specific promoters. Induction leads to nuclear translocation of the transcription factor and expression of response elements under control of a cognate synthetic promoter. In addition, a fluorescent reporter incorporated in driver lines allows to monitor induction. All previously generated driver lines are available from the Nottingham Arabidopsis Stock Center. This protocol describes how users can create their own constructs compatible with the existing set of lines and as well as induction and imaging procedures.

Upscaling xylem phenology: sample size matters

BACKGROUND AND AIMS

Upscaling carbon allocation requires knowledge of the variability at the scales at which data are collected and applied. Trees exhibit different growth rates and timings of wood formation. However, the factors explaining these differences remain undetermined, making samplings and estimations of the growth dynamics a complicated task, habitually based on technical rather than statistical reasons. This study explored the variability in xylem phenology among 159 balsam firs [Abies balsamea (L.) Mill.].

METHODS

Wood microcores were collected weekly from April to October 2018 in a natural stand in Quebec, Canada, to detect cambial activity and wood formation timings. We tested spatial autocorrelation, tree size and cell production rates as explanatory variables of xylem phenology. We assessed sample size and margin of error for wood phenology assessment at different confidence levels.

KEY RESULTS

Xylem formation lasted between 40 and 110 d, producing between 12 and 93 cells. No effect of spatial proximity or size of individuals was detected on the timings of xylem phenology. Trees with larger cell production rates showed a longer growing season, starting xylem differentiation earlier and ending later. A sample size of 23 trees produced estimates of xylem phenology at a confidence level of 95 % with a margin of error of 1 week.

CONCLUSIONS

This study highlighted the high variability in the timings of wood formation among trees within an area of 1 km2. The correlation between the number of new xylem cells and the growing season length suggests a close connection between the processes of wood formation and carbon sequestration. However, the causes of the observed differences in xylem phenology remain partially unresolved. We point out the need to carefully consider sample size when assessing xylem phenology to explore the reasons underlying this variability and to allow reliable upscaling of carbon allocation in forests.

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.

Genome-wide analysis of long non-coding RNAs in shoot apical meristem and vascular cambium in Populus tomentosa

Shoot apical and lateral meristems play essential roles in the formation and development of primary and secondary growth in plants. A delicate regulatory mechanism is needed to maintain homeostatic balance between the primary and secondary growth, as well as the self-renewal of meristems with the rate of cell division and differentiation of new meristems. However, little is known about the roles of long non-coding RNAs (lncRNAs) in the regulation of maintenance and differentiation of primary and secondary growth in Populus, especially in the cambium division and differentiation into secondary xylem. Here, 1298 lncRNAs were identified both in the apical meristem and vascular cambium, with 80 lncRNAs being expressed only in shoot apical meristem and 45 only in vascular cambium. There are 410 differentially expressed lncRNAs in shoot apical meristem and vascular cambium, among which 271 lncRNAs were up-regulated and 139 were down-regulated in cambium. The GO enrichment analysis revealed that differentially expressed lncRNAs mainly influenced the expression of lncRNAs related to the ribosome pathway, plant hormone signal pathway and photosynthesis pathway. The differentially expressed lncRNAs mainly target mRNA through cis-regulation in the vascular cambium. In addition, six key lncRNAs and also their significantly upregulated target genes were identified. Theses target genes are involved in plant secondary metabolites, cellulose and lignin synthesis, hormone and signal transduction. In addition, six key lncRNAs were identified, their significantly upregulated target genes are related to plant secondary metabolites, cellulose and lignin synthesis, hormone and signal transduction. Investigating lncRNA-mRNA interactions, we further found some genes that may be related to the development of vascular cambium, such as domain-containing transcription factors, cellulose synthesis genes, calcium dependent protein kinase 2, cytokinin receptor 1, glycosyl transferase and polyphenol oxidase. Our findings provide new insights into the lncRNA-mRNA networks in the development of vascular cambium of secondary growth in Populus.

Localized stem heating from the rest to growth phase induces latewood-like cell formation and slower stem radial growth in Norway spruce saplings

Recent climate projections predict a more rapid increase of winter temperature than summer and global temperature averages in temperate and cold environments. As there is relatively little experimental knowledge on the effect of winter warming on cambium phenology and stem growth in species growing in cold environments, the setting of manipulative experiments is considered of primary importance, and they can help to decipher the effect of reduced winter chilling and increased forcing temperatures on cambium reactivation, growth and xylem traits. In this study, localized stem heating was applied to investigate the effect of warming from the rest to the growth phase on cambium phenology, intra-annual stem growth dynamics and ring wood features in Picea abies (L.) H.Karst. We hypothesized that reduced winter chilling induces a postponed cambium dormancy release and decrease of stem growth, while high temperature during cell wall lignification determines an enrichment of latewood-like cells. The heating device was designed to maintain a +5 °C temperature delta with respect to air temperature, thus allowing an authentic scenario of warming. Continuous stem heating from the rest (November) to the growing phase determined, at the beginning of radial growth, a reduction of the number of cell layers in the cambium, higher number of cell layers in the wall thickening phase and an asynchronous stem radial growth when comparing heated and ambient saplings. Nevertheless, heating did not induce changes in the number of produced cell layers at the end of the growing season. The analyses of two-photon fluorescence images showed that woody rings formed during heating were enriched with latewood-like cells. Our results showed that an increase of 5 °C of temperature applied to the stem from the rest to growth might not influence, as generally reported, onset of cambial activity, but it could affect xylem morphology of Norway spruce in mountain environments.

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.

The Making of Plant Armor: The Periderm

The periderm acts as armor protecting the plant's inner tissues from biotic and abiotic stress. It forms during the radial thickening of plant organs such as stems and roots and replaces the function of primary protective tissues such as the epidermis and the endodermis. A wound periderm also forms to heal and protect injured tissues. The periderm comprises a meristematic tissue called the phellogen, or cork cambium, and its derivatives: the lignosuberized phellem and the phelloderm. Research on the periderm has mainly focused on the chemical composition of the phellem due to its relevance as a raw material for industrial processes. Today, there is increasing interest in the regulatory network underlying periderm development as a novel breeding trait to improve plant resilience and to sequester CO₂. Here, we discuss our current understanding of periderm formation, focusing on aspects of periderm evolution, mechanisms of periderm ontogenesis, regulatory networks underlying phellogen initiation and cork differentiation, and future challenges of periderm research.

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.

Seasonal changes in cambium activity from active to dormant stage affect the formation of secondary xylem in Pinus tabulaeformis Carr

Understanding the changing patterns of vascular cambium during seasonal cycles is crucial to reveal the mechanisms that control cambium activity and wood formation, but this area has been underexplored, especially in conifers. Here, we quantified the changing cellular morphology patterns of cambial zones during the active, transition and dormant stages. With the help of toluidine blue and periodic acid-Schiff staining to visualize cell walls and identify their constituents, we observed decreasing cambial cell layers, thickening of newly formed xylem cell walls and increased polysaccharide granules in phloem from June to the following March over the course of our collecting period. Pectin immunofluorescence showed that dormant-stage cambium can produce highly abundant de-esterified homogalacturonan and (1-4)-β-d-galactan epitopes, whereas active cambium can strong accumulate high methylesterified homogalacturonan. Calcofluor white staining and confocal Raman spectroscopy analysis revealed regular changes in the chemical composition of cell walls, such as relative lower cellulose deposition in transition stage in vascular cambium, and higher lignin accumulation was found in dormant stage in secondary xylem. Moreover, real-time quantitative polymerase chain reaction analysis suggested that various IAA (Aux/IAA protein), CesA, CslA and HDZ genes, as well as NAC, PME3 and PME4, may be involved in cambium activities and secondary xylem formation. Taken together, these findings provide new information about cambium activity and cell differentiation in the formation, structure and chemistry in conifers during the active-dormant transition.

Parasitic plant, from inside out: endophytic development in Lathrophytum peckoltii (Balanophoraceae) in host liana roots from tribe Paullineae (Sapindaceae)

BACKGROUND AND AIMS

Balanophoraceae is one of the most bizarre and biologically interesting plant clades. It groups species with peculiar features that offers an opportunity for investigating several aspects of parasite plant development and morphogenesis. We analysed the development and the mature vegetative body of Lathrophytum peckoltii Eichler, focusing on the formation of the host-parasite interface. Additionally, we analysed how this parasitic interaction causes modifications to the anatomy of Paullinia uloptera Radlk and Serjania clematidifolia Cambess host roots.

METHODS

Vegetative bodies of the parasite at different developmental stages were collected while infesting the roots of Sapindaceae vines. Non-parasitized host roots were also collected for comparison. Light, epifluorescence, confocal and scanning electron microscopy were used for the analysis.

KEY RESULTS

The initial cells of the vegetative axis divide repeatedly, originating a parenchymatous matrix, which occupies the space from the cortex to the vascular cylinder of the host's root. In the peripheral layers of the matrix, located near the xylem of the host's roots, a few cells initiate the process of wall lignification, originating the parasitic bundle. The vascular cambium of the host's root changes the division plane and becomes composed of fusiform initials, forming the vascular bundle. The vegetative axis presents a dermal tissue similar to a phellem, a parenchymatous matrix and a vascular system with different origins.

CONCLUSION

The parasite reproduces by endophytic development, in a manner similar to that observed for endoparasites. The strategy of late cell differentiation could aid the parasite in avoiding early detection and triggering of defence responses by the host. This development causes changes to the host root cambial activity, leading to the establishment of direct, vessel to vessel connection between host and parasite. We associate these changes with the cambium modularity and an influx of parasite-derived hormones into the host cambium.

Auxin concentration and xylem production of Pinus massoniana in a subtropical forest in south China

Auxin is involved in various developmental processes of plants, including cell division in cambium and xylem differentiation. However, most studies linking auxin and xylem cell production are performed in environments with a strong seasonality (i.e., temperate and boreal climates). The temporal dynamics of auxin and cambial activity of subtropical trees remain basically unknown. In this study, we sampled four microcores weekly in three individuals of Chinese red pine (Pinus massoniana Lamb.) from February to December 2015-16 to compare xylem formation with auxin concentration in subtropical China. During the entire period of sampling, the number of cambial cells varied from 2 to 7, while the number of cells in the enlarging zone ranged from 1 to 4 and from 1 to 5 in the wall-thickening zone. In 2015, the average auxin concentration was 3.46 ng g-1, with 33 xylem cells being produced at the end of the year. In 2016, a lower auxin concentration (2.59 ng g-1) corresponded to a reduced annual xylem production (13.7 cells). No significant relationship between auxin concentration and number of xylem cells in differentiation was found at the weekly scale. Unlike in boreal and temperate forests, the lack of wood formation seasonality in subtropical forests makes it more difficult to reveal the relationship between auxin concentration and number of xylem cells in differentiation at the intra-annual scale. The frequent and repeated samplings might have reduced auxin concentration in the developing cambium and xylem, resulting in a lower xylem cell production.

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.

Climatic regulation of leaf and cambial phenology in Quercus pubescens: Their interlinkage and impact on xylem and phloem conduits

Increased frequency and severity of stressful events affects the growth patterns and functioning of trees which adjust their phenology to given conditions. Here, we analysed environmental effects (temperature, precipitation, VPD and SWC) on the timing of leaf phenology, seasonal stem radial growth patterns, and xylem and phloem anatomy of Quercus pubescens in the sub-Mediterranean in the period 2014-2019, when various adverse weather events occurred, i.e. spring drought in 2015, summer fire in 2016 and summer drought in 2017. Results showed that the timings of leaf and cambium phenology do not occur simultaneously in Q. pubescens, reflecting different environmental and internal constraints. Although year-to-year variability in the timings of leaf and cambial phenology exists, their chronological sequence is fairly fixed. Different effects of weather conditions on different stages of leaf development in spring were observed. Common climatic drivers (i.e., negative effect of hot and dry summers and a positive effect of increasing moisture availability in winter and summer) were found to affect the widths of xylem and phloem increments with more pronounced effect on late formed parts. A legacy effect of the timing of leaf and cambial phenology of the previous growing season on the timing of phenology of the following spring was confirmed. Rarely available phloem data permitted a comprehensive insight into the interlinkage of the timing of cambium and leaf phenology and adjustment strategies of vascular tissues in Mediterranean pubescent oak to various environmental constraints, including frequent extreme events (drought, fire). Our results suggest that predicted changes in autumn/winter and spring climatic conditions for this area could affect the timings of leaf and stem cambial phenology of Q. pubescens in the coming years, which would affect stem xylem and phloem structure and hydraulic properties, and ultimately its performance.

How Do Trees Grow in Girth? Controversy on the Role of Cellular Events in the Vascular Cambium

Radial growth has long been a subject of interest in tree biology research. Recent studies have brought a significant change in the understanding of some basic processes characteristic to the vascular cambium, a meristem that produces secondary vascular tissues (phloem and xylem) in woody plants. A new hypothesis regarding the mechanism of intrusive growth of the cambial initials, which has been ratified by studies of the arrangement of cambial cells, negates the influence of this apical cell growth on the expansion of the cambial circumference. Instead, it suggests that the tip of the elongating cambial initial intrudes between the tangential (periclinal) walls, rather than the radial (anticlinal) walls, of the initial(s) and its(their) derivative(s) lying ahead of the elongating cell tip. The new concept also explains the hitherto obscure mechanism of the cell event called ‘elimination of initials’. This article evaluates these new concepts of the cambial cell dynamics and offers a new interpretation for some curious events occurring in the cambial meristem in relation to the radial growth in woody plants.

Ontogeny, anatomical structure and function of lobed stems in the evolution of the climbing growth form in Malvaceae (Byttneria Loefl.)

BACKGROUND AND AIMS

Byttneria is one of the few climbing genera in Malvaceae. Some Byttneria are known for their lobed stems. We explore the development of these stems, how they have evolved within the group and their relevance in the evolution of the climbing growth form in Malvaceae.

METHODS

We combine developmental anatomical work with phylogenetic comparative methods. We use Byttneria divaricata and B. filipes as models in the anatomical work, a review of herbarium vouchers, and the most recent phylogeny of Byttneria and allies to elucidate how these stems evolved within the clade under maximum-likelihood and Bayesian approaches. We use Pagel94 tests to analyse the correlated evolution of lobed stems and prickles.

KEY RESULTS

Each lobe coincides with one of the five vascular bundles. By augmented activity of the fascicular cambium in the lobes coupled with reduced activity of the interfascicular cambium in the interlobes, secondary growth increases the lobulation already present during primary growth. Within Byttneria and allies, lobed young stems appeared at least three times, once in Ayenia and twice in the paraphyletic Byttneria. Lobed adult stems were conserved in Byttneria s.s., where lobed adult stems in combination with prickles were shown to have evolved as a climbing mechanism within the group; prickles were lost once within Byttneria s.s., in a shrubby subclade. Byttneria Clade 2 comprises climbers with twining cylindrical adult stems and no prickles, which constitutes a different climbing mechanism in the group.

CONCLUSIONS

We provide evidence of one of the few cambial variants known whose secondary body reflects the primary body vasculature and show that lobed adult stems and prickles in Byttneria could be used in the new delimitation of genera in the group. Lobed stems independently appeared in climbing Grewia, suggesting a convergence favouring the climbing growth form.

Beginnings of a plant parasite: early development of Rafflesia consueloae inside its Tetrastigma host

Extensive histology of host organs revealed the early events in the vegetative growth of Rafflesia consueloae including initial infection site, endophyte distribution, and other developmental events prior to bud emergence. The early events in the vegetative development of the holoparasite Rafflesia have long remained a mystery. Because its entire vegetative growth occurs within the host body, very little is known about the developmental events prior to emergence of the floral shoot. The goal of this study was to describe the events that occur during the vegetative growth of R. consueloae, particularly in the early stages of infection. We performed extensive microtome sectioning of multiple root and stem segments from different Tetrastigma host individuals to examine the cytology, distribution, and development of the R. consueloae endophyte within the host tissues. We found that R. consueloae infection is restricted to the roots of its host. Infection begins within the vascular cambium where the endophyte appears to initially reside prior to their radial spread to the vascular tissues. The tissues obtained from different host individuals had varying degrees of infection alluding to a possible role of host resistance mechanisms and/or varying levels of parasite infectiousness. Endophyte presence in host vines without external manifestations of infection indicates that the parasite may dwell within the host tissues for prolonged periods as small cell clusters without transitioning to the reproductive stage. Furthermore, we found that floral shoots may develop in scarcely infected host tissues indicating that extensive endophyte growth within the host is not a prerequisite to the onset of reproductive development. Overall, our study describes for the first time the developmental events prior to emergence of R. consueloae buds from its host.

The formation of interlocked grain in African mahogany (Khaya spp.) analysed by X-ray computed microtomography

Interlocked grain occurs when the orientation of xylem fibres oscillates, alternating between left- and right-handed spirals in successive wood layers. The cellular mechanisms giving rise to interlocked grain, thought to involve the slow rotation of fusiform initials within the vascular cambium, remain unclear. We suggest that observations of wood structure at the cellular level, but over large areas, might reveal these mechanisms. We assayed timber from several commercially important tropical angiosperms from the genus Khaya (African mahogany) that exhibit interlocked grain using X-ray computed microtomography followed by orthogonal slicing and image processing in ImageJ. Reconstructed tangential longitudinal sections were processed with the ImageJ directionality plug-in to directly measure fibre orientation and showed grain deviations of more than 10° from vertical in both left- and right-handed directions. Grain changed at locally constant rates, separated by locations where the direction of grain change sharply reversed. Image thresholding and segmentation conducted on reconstructed cross sections allowed the identification of vessels and measurement of their location, with vessel orientations then calculated in Matlab and, independently, in recalculated tangential longitudinal sections with the directionality plug-in. Vessel orientations varied more than fibre orientations, and on average deviated further from vertical than fibres at the locations where the direction of grain change reversed. Moreover, the reversal location for vessels was shifted ~400 μm towards the pith compared with the fibres, despite both cell types arising from the same fusiform initials within the vascular cambium. We propose a simple model to explain these distinct grain patterns. Were an auxin signal to control both the reorientation of cambial initials, as well as coordinating the end-on-end differentiation and linkage of xylem vessel elements, then it would be possible for fibres and vessels to run at subtly different angles, and to show different grain reversal locations.

Revisiting the anatomy of the monocot cambium, a novel meristem

The monocot cambium is semi-storied, and its cells do not undergo rearrangement. The monocot cambium is a lateral meristem responsible for secondary growth in some monocotyledons of Asparagales. It is an unusual meristem, not homologous with the vascular cambia of gymnosperms and non-monocotyledonous angiosperms. Owing to the limited information available on the characteristics of this meristem, the aim of this study was to survey the structure of the monocot cambium in order to clarify the similarities and dissimilarities of this lateral meristem to the vascular cambium of trees. Using the serial sectioning analysis, we have studied the monocot cambium of three species of arborescent monocotyledons, i.e., Quiver Tree Aloe dichotoma, Dragon Tree Dracaena draco, and Joshua Tree Yucca brevifolia, native to different parts of the world. Data showed that in contrast to the vascular cambium, the monocot cambium is composed of a single type of short initials that vary in shape, and in tangential view display a semi-storied pattern. Furthermore, the cells of the monocot cambium do not undergo rearrangement. The criteria used in identifying monocot cambium initial cell are also discussed.

Xylem versus phloem in secondary growth: a balancing act mediated by gibberellins

This article comments on: Ben-Targem M, Ripper D, Bayer M, Ragni L. 2021. Auxin and gibberellin signaling cross-talk promotes hypocotyl xylem expansion and cambium homeostasis. Journal of Experimental Botany 72, 3647–3660.

Brassinosteroid-BZR1/2-WAT1 module determines the high level of auxin signalling in vascular cambium during wood formation

The tight regulation of local auxin homeostasis and signalling maxima in xylem precursor cells specifies the organising activity of the vascular cambium and consequently promotes xylem differentiation and wood formation. However, the molecular mechanisms underlying the local auxin signalling maxima in the vascular cambium are largely unknown. Here, we reveal that brassinosteroid (BR)-activated WALLS ARE THIN1 (WAT1) facilitates wood formation by enhancing local auxin signalling in the vascular cambium in Solanum lycopersicum. Growth defects and low auxin signalling readouts in the BR-deficient tomato cultivar, Micro-Tom, were associated with a novel recessive allele, Slwat1-copi, created by the insertion of a retrotransposon in the last exon of the SlWAT1 locus. Molecular and genetic studies by generating the gain-of-function and loss-of-function tomato mutants revealed that SlWAT1 is a critical regulator for fine tuning local auxin homeostasis and signalling outputs in vascular cambium to facilitate secondary growth. Finally, we discovered that BR-regulated SlBZR1/2 directly activated downstream auxin responses by SlWAT1 upregulation in xylem precursor cells to facilitate xylem differentiation and subsequent wood formation. Our data suggest that the BR-SlBZR1/2-WAT1 signalling network contributes to the high level of auxin signalling in the vascular cambium for secondary growth.

Turgor - a limiting factor for radial growth in mature conifers along an elevational gradient

A valid representation of intra-annual wood formation processes in global vegetation models is vital for assessing climate change impacts on the forest carbon stock. Yet, wood formation is generally modelled with photosynthesis, despite mounting evidence that cambial activity is rather directly constrained by limiting environmental factors. Here, we apply a state-of-the-art turgor-driven growth model to simulate 4 yr of hourly stem radial increment from Picea abies (L.) Karst. and Larix decidua Mill. growing along an elevational gradient. For the first time, wood formation observations were used to validate weekly to annual stem radial increment simulations, while environmental measurements were used to assess the climatic constraints on turgor-driven growth. Model simulations matched the observed timing and dynamics of wood formation. Using the detailed model outputs, we identified a strict environmental regulation on stem growth (air temperature > 2°C and soil water potential > -0.6 MPa). Warmer and drier summers reduced the growth rate as a result of turgor limitation despite warmer temperatures being favourable for cambial activity. These findings suggest that turgor is a central driver of the forest carbon sink and should be considered in next-generation vegetation models, particularly in the context of global warming and increasing frequency of droughts.

A metabolite roadmap of the wood-forming tissue in Populus tremula

Wood, or secondary xylem, is the product of xylogenesis, a developmental process that begins with the proliferation of cambial derivatives and ends with mature xylem fibers and vessels with lignified secondary cell walls. Fully mature xylem has undergone a series of cellular processes, including cell division, cell expansion, secondary wall formation, lignification and programmed cell death. A complex network of interactions between transcriptional regulators and signal transduction pathways controls wood formation. However, the role of metabolites during this developmental process has not been comprehensively characterized. To evaluate the role of metabolites during wood formation, we performed a high spatial resolution metabolomics study of the wood-forming zone of Populus tremula, including laser dissected aspen ray and fiber cells. We show that metabolites show specific patterns within the wood-forming zone, following the differentiation process from cell division to cell death. The data from profiled laser dissected aspen ray and fiber cells suggests that these two cell types host distinctly different metabolic processes. Furthermore, by integrating previously published transcriptomic and proteomic profiles generated from the same trees, we provide an integrative picture of molecular processes, for example, deamination of phenylalanine during lignification is of critical importance for nitrogen metabolism during wood formation.

MiR319a-targeted PtoTCP20 regulates secondary growth via interactions with PtoWOX4 and PtoWND6 in Populus tomentosa

Secondary growth is a key characteristic of trees, which requires the coordination of multiple regulatory mechanisms including transcriptional regulators and microRNAs (miRNAs). However, the roles of microRNAs in the regulation of secondary growth need to be explored in depth. Here, the role of miR319a and its target, PtoTCP20, in the secondary growth of Populus tomentosa stem was investigated using genetic and molecular analyses. The expression level of miR319a gradually decreased from primary to secondary growth in P. tomentosa, while that of PtoTCP20 gradually increased. MiR319a overexpression in seedlings resulted in delayed secondary growth and decreased xylem production, while miR319a knockdown and PtoTCP20 overexpression promoted secondary growth and increased xylem production. Further analysis showed that PtoTCP20 interacted with PtoWOX4a and activated PtoWND6 transcription in vitro and in vivo. Our data show that PtoTCP20 controls vascular cambium proliferation by binding to PtoWOX4a, and promotes secondary xylem differentiation by activating PtoWND6 transcription, thereby regulating secondary growth in P. tomentosa. Our findings provide insight into the molecular mechanisms underlying secondary growth in trees.

Transcriptional networks regulating root vascular development

Vascular development involves multiple processes, including the establishment of vascular stem cells (e.g. procambium/cambium cells), stem cell divisions, and cell specification. A number of key transcription factors regulating vascular development have been identified, and the molecular mechanisms underlying these regulators have been keenly investigated. These studies uncovered that transcriptional regulation and phytohormone signaling have central roles in proceeding vascular developmental processes. Recent research approaches contributed to identify key transcription factors and their downstream genes, which enhanced our understanding of vascular development. This review discusses some research approaches and emerging molecular mechanisms that mediate the activation of transcriptional networks regulating root vascular development.

Long-Lived Trees Are Not Immortal

Separating out the different effects of ageing on long-lived trees remains challenging. Herein current approaches used to explore senescence in millennial trees are highlighted. Molecular and biochemical analyses of the vascular cambium provide novel insight into the extent to which millennial trees can withstand the wear and tear of ageing.

Temperature and water potential co-limit stem cambial activity along a steep elevational gradient

Efforts to develop mechanistic tree growth models are hindered by the uncertainty of whether and when tree growth responses to environmental factors are driven by carbon assimilation or by biophysical limitations of wood formation. In this study, we used multiannual weekly wood-formation monitoring of two conifer species (Larix decidua and Picea abies) along a 900 m elevational gradient in the Swiss Alps to assess the biophysical effect of temperature and water potential on wood formation. To this end, we developed a model that simulates the effect of water potential on turgor-driven cambial division, modulated by the effect of temperature on enzymatic activity. The model reproduced the observed phenology of tracheid production, as well as intra- and interannual tracheid production dynamics of both species along the elevational gradient, although interannual model performance was lower. We found that temperature alone explains the onset of tracheid production, yet water potential appears necessary to predict the ending and the total amount of tracheids produced annually. We conclude that intra-annual cambial activity is strongly constrained by both temperature and water potential at all elevations, independently of carbon assimilation. At the interannual scale, biophysical constraints likely interact with other factors.

The development of the periderm: the final frontier between a plant and its environment

The periderm acts as the first line of defence for a plant, protecting wood and phloem from abiotic and biotic stresses. During secondary growth, through the increase in girth of plant organs, the periderm replaces the epidermis as the outermost tissue. The phellogen, a bifacial post-embryonic meristem, forms the phelloderm inwards (toward the vasculature) and the suberized phellem outwards (toward the environment). These three tissues are collectively referred to as the periderm. Here, we summarize recent findings on the molecular mechanisms of periderm development by describing periderm formation in connection to the fate of the surrounding tissues, by discussing common regulatory hubs between the vascular cambium and the phellogen, and by highlighting transcription factors (TFs) controlling phellem differentiation.

Climatic and physiological regulation of the bimodal xylem formation pattern in Pinus pinaster saplings

Seasonality in tree cambial activity and xylem formation encompass large variation in environmental conditions. Abiotic stressors such as warming or drought also modulate plant behavior at species and individual level. Despite xylem formation susceptibility to carbon (C) and water availability, it is still unknown which are the key physiological variables that regulate xylogenesis, and to what extent plant performance contributes to further explain the number of cells in the different phases of xylem development. Xylogenesis and physiological behavior was monitored in saplings of Pinus pinaster Aiton, a bimodal growth pattern species, distributed in different irrigation regimes. Xylogenesis and plant physiological behavior were compared between treatments and the relationship between climate, physiology and the number of cells in the cambium, enlargement and cell-wall thickening phases was evaluated. Xylogenesis regulation shifted from physiological to climatic control as cell differentiation advanced to mature tracheids. The number of cells in the cambium increased with assimilation rates and decreased with the water potential gradient through the plant. Enlargement was the most susceptible phase to plant relative water content, whereas no physiological variable contributed to explain the number of cells in the wall thickening phase, which declined as temperatures increased. All treatments showed a bimodal growth pattern with a second growth period starting when primary growth was completed and after plants had experienced the highest summer hydraulic losses. Our study demonstrates the importance of including physiological responses and not only climate to fully understand xylogenesis, with special attention to the enlargement phase. This is critical when studying species with a bimodal growth pattern because the second growth peak responds to internal shifts of C allocation and may strongly depend on plant hydraulic responses and not on a fine tuning of cambial activity with soil water availability.

Fire damage to cambium affects localized xylem anatomy and hydraulics: the case of Nothofagus pumilio in Patagonia

PREMISE

Fire scars on trees are created by excessive heat from a fire that kills the vascular cambium. Although, fires are one of the most important forest disturbances in Patagonia, the effects of fire on tree physiology and wood anatomy are still unknown. In this study, we hypothesized that abnormal functioning of the cambium after a fire will induce anatomical changes in the wood. We also assumed that these anatomical changes would affect xylem safety transport.

METHODS

We quantified wood anatomical traits in Nothofagus pumilio, the dominant subalpine tree species of Patagonia, using two approaches: time and distance. In the first, anatomical changes in tree rings were compared before, during, and after fire occurrence. In the second, the spatial extent of these changes was evaluated with respect to the wound by measuring anatomical traits in sampling bands in two directions (0° and 45°) with respect to the onset of healing.

RESULTS

Reductions in lumen diameter and vessel number were the most conspicuous changes associated with fire damage and observed in the fire ring and subsequent post-fire rings. In addition, the fire ring had more rays than in control rings. In terms of distance, anatomical changes were only restricted to short distances from the wound.

CONCLUSIONS

Post-fire changes in wood anatomical traits were confined close to the wound margins. These changes might be associated with a defense strategy related to the compartmentalization of the wound and safety of water transport.

How to organize bidirectional tissue production?

The cambium is a plant-borne stem cell system producing wood and bast, two distinct types of vascular tissues, in strictly opposite directions. Thereby, the cambium contributes substantially to terrestrial biomass accumulation and represents the basis for the formation of large plant bodies. Although the bidirectional mode of tissue production by a common stem cell pool holds interesting implications for developmental biology, functional domains of the cambium, and their interaction remained poorly defined for decades. Here, we summarize recent findings on domain organization of the cambium and discuss potential mechanisms important for its bipartite organization. By highlighting the conceptual implication for stem cell biology, we integrate our understanding of cambium regulation into a larger context.

Asynchronous leaf and cambial phenology in a tree species of the Congo Basin requires space-time conversion of wood traits

BACKGROUND AND AIMS

Wood traits are increasingly being used to document tree performance. In the Congo Basin, however, weaker seasonality causes asynchrony of wood traits between trees. Here, we monitor growth and phenology data to date the formation of traits.

METHODS

For two seasons, leaf and cambial phenology were monitored on four Terminalia superba trees (Mayombe) using cameras, cambial pinning and dendrometers. Subsequently, vessel lumen and parenchyma fractions as well as high-resolution isotopes (δ13C/δ18O) were quantified on the formed rings. All traits were dated and related to weather data.

KEY RESULTS

We observed between-tree differences in green-up of 45 d, with trees flushing before and after the rainy season. The lag between green-up and onset of xylem formation was 59 ± 21 d. The xylem growing season lasted 159 ± 17 d with between-tree differences of up to 53 d. Synchronized vessel, parenchyma and δ13C profiles were related to each other. Only parenchyma fraction and δ13C were correlated to weather variables, whereas the δ18O pattern showed no trend.

CONCLUSIONS

Asynchrony of leaf and cambial phenology complicates correct interpretation of environmental information recorded in wood. An integrated approach including high-resolution measurements of growth, stable isotopes and anatomical features allows exact dating of the formation of traits. This methodology offers a means to explore the asynchrony of growth in a rainforest and contribute to understanding this aspect of forest resilience.

Effects of phloem on canopy dieback, tested with manipulations and a canker pathogen in the Corylus avellana/Anisogramma anomala host/pathogen system

Canker pathogens cause necrosis of the phloem, but in many host/pathogen systems, they also cause canopy dieback, which implicates xylem, not phloem dysfunction. We hypothesize that this dieback distal to the canker is caused by water stress resulting from the lack of a phloem-to-xylem connection, which in a healthy plant would allow delivery of nonstructural carbohydrates (NSCs) and water inward to aid in xylem embolism refilling. We tested several components of this hypothesis in the host/pathogen system Corylus avellana L./Anisogramma anomala (Peck) E. Müll (Eastern filbert blight). Cankers were non-girdling and usually ≥0.1 m long. As expected, healthy controls had higher specific conductivity (Ks) than diseased stems, but unexpectedly, had similar moisture content (m.c.), showing that the lower Ks did not result from more embolisms in the diseased stems. Moreover, manipulations that removed cambium and phloem to simulate a canker, or that shaded stems to lower NSCs, did not result in lower Ks or m.c. than controls. The outer millimeter of xylem adjacent to a canker had infrequent tyloses and/or fungal hyphae in many but not all samples, and dye studies showed little xylem water transport in that region, but the incidence of these blockages was insufficient to cause the observed 19% decrease in Ks. Healthy stems had higher m.c. than diseased stems above the canker (or analogous) location and were longer for the same leaf weight, suggestive of water stress in the upper portion of diseased stems. These results suggest that dieback distal to cankers in this system results from the bottleneck in water transport in the region adjacent to a canker, but did not find evidence to support the requirement of a phloem-to-xylem connection for continued water transport.

The Dynamics of Cambial Stem Cell Activity

Stem cell populations in meristematic tissues at distinct locations in the plant body provide the potency of continuous plant growth. Primary meristems, at the apices of the plant body, contribute mainly to the elongation of the main plant axes, whereas secondary meristems in lateral positions are responsible for the thickening of these axes. The stem cells of the vascular cambium-a secondary lateral meristem-produce the secondary phloem (bast) and secondary xylem (wood). The sites of primary and secondary growth are spatially separated, and mobile signals are expected to coordinate growth rates between apical and lateral stem cell populations. Although the underlying mechanisms have not yet been uncovered, it seems likely that hormones, peptides, and mechanical cues orchestrate primary and secondary growth. In this review, we highlight the current knowledge and recent discoveries of how cambial stem cell activity is regulated, with a focus on mobile signals and the response of cambial activity to environmental and stress factors.