Regulation of vascular cambium activity

Proteomic reprogramming underlying anatomical and physiological characteristics of poplar wood in acclimation to changing light and nitrogen availabilities

To explore the proteomic regulation that underlies the physiological, anatomical, and chemical characteristics of wood in acclimation to changing light and nitrogen (N), saplings of Populus × canescens were treated with control or high irradiance in combination with low, control or high N for 4 months. High irradiance led to elevated levels of starch, sucrose, glucose, and fructose, decreased concentrations of ammonium, nitrate, most amino acids and total N, wider xylem, more xylem cell layers, narrower vessel lumina, longer fiber cells, greater fiber wall thickness, and more cellulose and hemicellulose but less lignin deposition in poplar wood. Limiting N resulted in increased levels of starch and sucrose, reduced levels of glucose, fructose, ammonium, nitrate, amino acids and total N, narrower xylem, fewer xylem cell layers, reduced vessel lumen diameter, thicker fiber walls, and less cellulose and more hemicellulose and lignin accumulation, whereas high N had the opposite effects on poplar wood. Correspondingly, numerous differentially abundant proteins, which are related mainly to the metabolism of carbohydrates and amino acids, cell division and expansion, and deposition of secondary cell walls, such as sucrose synthase 6 (SUS6), cell division cycle protein 48 (CDC48) and laccases (LACs), were identified in poplar cambiums in response to changes in light intensity and N availability. These results suggest that proteomic relays play essential roles in regulating the physiological characteristics and anatomical and chemical properties of poplar wood in acclimation to changing light and N availabilities.

Phytohormones involved in vascular cambium activity in woods: current progress and future challenges

Vascular cambium is the continuation of meristem activity at the top of plants, which promotes lateral growth of plants. The vascular cambium evolved as an adaptation for secondary growth, initially in early seed plants, and became more refined in the evolution of gymnosperms and angiosperms. In angiosperms, it is crucial for plant growth and wood formation. The vascular cambium is regulated by a complex interplay of phytohormones, which are chemical messengers that coordinate various aspects of plant growth and development. This paper synthesizes the current knowledge on the regulatory effects of primary plant hormones and peptide signals on the development of the cambium in forest trees, and it outlines the current research status and future directions in this field. Understanding these regulatory mechanisms holds significant potential for enhancing our ability to manage and cultivate forest tree species in changing environmental conditions.

Integrating biochemical and anatomical characterizations with transcriptome analysis to dissect superior stem strength of ZS11 (Brassica napus)

Stem lodging resistance is a serious problem impairing crop yield and quality. ZS11 is an adaptable and stable yielding rapeseed variety with excellent resistance to lodging. However, the mechanism regulating lodging resistance in ZS11 remains unclear. Here, we observed that high stem mechanical strength is the main factor determining the superior lodging resistance of ZS11 through a comparative biology study. Compared with 4D122, ZS11 has higher rind penetrometer resistance (RPR) and stem breaking strength (SBS) at flowering and silique stages. Anatomical analysis shows that ZS11 exhibits thicker xylem layers and denser interfascicular fibrocytes. Analysis of cell wall components suggests that ZS11 possessed more lignin and cellulose during stem secondary development. By comparative transcriptome analysis, we reveal a relatively higher expression of genes required for S-adenosylmethionine (SAM) synthesis, and several key genes (4-COUMATATE-CoA LIGASE, CINNAMOYL-CoA REDUCTASE, CAFFEATE O-METHYLTRANSFERASE, PEROXIDASE) involved in lignin synthesis pathway in ZS11, which support an enhanced lignin biosynthesis ability in the ZS11 stem. Moreover, the difference in cellulose may relate to the significant enrichment of DEGs associated with microtubule-related process and cytoskeleton organization at the flowering stage. Protein interaction network analysis indicate that the preferential expression of several genes, such as LONESOME HIGHWAY (LHW), DNA BINDING WITH ONE FINGERS (DOFs), WUSCHEL HOMEOBOX RELATED 4 (WOX4), are related to vascular development and contribute to denser and thicker lignified cell layers in ZS11. Taken together, our results provide insights into the physiological and molecular regulatory basis for the formation of stem lodging resistance in ZS11, which will greatly promote the application of this superior trait in rapeseed breeding.

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.

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.

Comparative transcriptome analysis of Citrus macrophylla tree infected with Citrus tristeza virus stem pitting mutants provides new insight into the role of phloem regeneration in stem pitting disease

Stem pitting is a complex and economically important virus-associated disease of perennial woody plants. Molecular mechanisms and pathways occurring during virus-plant interaction that result in this phenomenon are still obscure. Previous studies indicated that different Citrus tristeza virus (CTV) mutants induce defined stem pitting phenotypes ranging from mild (CTVΔp13) to severe (CTVΔp33) in Citrus macrophylla trees. In this study, we conducted comparative transcriptome analyses of C. macrophylla trees infected with CTV mutants (CTVΔp13 and CTVΔp33) and a full-length virus in comparison to healthy plants as control. The mild CTV stem pitting mutant had very few differentially expressed genes (DEGs) related to plant defense mechanism and plant growth and development. In contrast, substantial gene expression changes were observed in plants infected with the severe mutant and the full-length virus, indicating that both the p13 and p33 proteins of CTV acted as a regulator of symptom production by activating and modulating plant responses, respectively. The analysis of transcriptome data for CTVΔp33 and the full-length virus suggested that xylem specification has been blocked by detecting several genes encoding xylem, cell wall and lignin degradation, and cell wall loosening enzymes. Furthermore, stem pitting was accompanied by downregulation of transcription factors involved in regulation of xylem differentiation and downregulation of some genes involved in lignin biosynthesis, showing that the xylem differentiation and specification program has been shut off. Upregulation of genes encoding transcription factors associated with phloem and cambium development indicated the activation of this program in stem pitting disease. Furthermore, we detected the induction of several DEGs encoding proteins associated with cell cycle re-entry such as chromatin remodeling factors and cyclin, and histone modification. This kind of expression pattern of genes related to xylem differentiation and specification, phloem and cambium development, and cell cycle re-entry is demonstrated during secondary vascular tissue (SVT) regeneration. The microscopy analysis confirmed that the regeneration of new phloem is associated with stem pitting phenotypes. The findings of this study, thus, provide evidence for the association between stem pitting phenotypes and SVT regeneration, suggesting that the expression of these genes might play important roles in development of stem pitting symptoms. Overall, our findings suggest that phloem regeneration contributes to development of stem pitting symptoms.

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.

The XVP/ NAC003 protein associates with the plasma membrane through KR rich regions and translocates to the nucleus by changing phosphorylation status

Membrane localized transcription factors play essential roles in various plant developmental processes. The XVP/NAC003 protein is a NAC domain transcription factor associated with the plasma membrane and involved in the TDIF-PXY signaling during vascular development. We report here the mechanisms of XVP membrane localization and its nuclear translocation. Using a transient transformation approach, we found that XVP is associated with the plasma membrane through positively charged KR-rich regions. Mutagenesis studies found that the threonine amino acid at position 354 (T354) is critical for XVP translocation to the nucleus. In particular, the threonine to alanine mutation (T354A) resulted in a partial nucleus localization, while threonine to aspartic acid (T354D) mutation showed no effect on protein localization, indicating that dephosphorylation at T354 may serve as a nucleus translocation signal. This research sheds new light on the nucleus partitioning of plasma membrane-associated transcription factors.

Peptide Signaling Pathways Regulate Plant Vascular Development

Plant small peptides, including CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) and Epidermal Patterning Factor-Like (EPFL) peptides, play pivotal roles in coordinating developmental processes through cell-cell communication. Recent studies have revealed that the phloem-derived CLE peptides, CLE41/44 and CLE42, promote (pro-)cambial cell proliferation and inhibit xylem cell differentiation. The endodermis-derived EPFL peptides, EPFL4 and EPFL6, modulate vascular development in the stem. Further, several other peptide ligands CLE9, CLE10, and CLE45 play crucial roles in regulating vascular development in the root. The peptide signaling pathways interact with each other and crosstalk with plant hormone signals. In this mini-review, we summtarize the recent advances on peptides function in vascular development and discuss future perspectives for the research of the CLE and EPFL peptides.

Xylem Parenchyma-Role and Relevance in Wood Functioning in Trees

Woody plants are characterised by a highly complex vascular system, wherein the secondary xylem (wood) is responsible for the axial transport of water and various substances. Previous studies have focused on the dead conductive elements in this heterogeneous tissue. However, the living xylem parenchyma cells, which constitute a significant functional fraction of the wood tissue, have been strongly neglected in studies on tree biology. Although there has recently been increased research interest in xylem parenchyma cells, the mechanisms that operate in these cells are poorly understood. Therefore, the present review focuses on selected roles of xylem parenchyma and its relevance in wood functioning. In addition, to elucidate the importance of xylem parenchyma, we have compiled evidence supporting the hypothesis on the significance of parenchyma cells in tree functioning and identified the key unaddressed questions in the field.

Activation of ACS7 in Arabidopsis affects vascular development and demonstrates a link between ethylene synthesis and cambial activity

Ethylene is a gaseous hormone that affects many processes of plant growth and development. During vascular development, ethylene positively regulates cambial cell division in parallel with tracheary element differentiation inhibitory factor (TDIF) peptide signaling. In this study, we identified an ethylene overproducing mutant, acs7-d, exhibiting enhanced cambial activity and reduced wall development in fiber cells. Using genetic analysis, we found that ethylene signaling is necessary for the phenotypes of enhanced cambial cell division as well as defects in stem elongation and fiber cell wall development. Further, the cambial cell proliferation phenotype of acs7-d depends on WOX4, indicating that the two parallel pathways, ethylene and TDIF signaling, converge at WOX4 in regulating cambium activity. Gene expression analysis showed that ethylene impedes fiber cell wall biosynthesis through a conserved hierarchical transcriptional regulation. These results advance our understanding of the molecular mechanisms of ethylene in regulating vascular meristem activity.

Gene Regulatory Network Guided Investigations and Engineering of Storage Root Development in Root Crops

The plasticity of plant development relies on its ability to balance growth and stress resistance. To do this, plants have established highly coordinated gene regulatory networks (GRNs) of the transcription factors and signaling components involved in developmental processes and stress responses. In root crops, yields of storage roots are mainly determined by secondary growth driven by the vascular cambium. In relation to this, a dynamic yet intricate GRN should operate in the vascular cambium, in coordination with environmental changes. Despite the significance of root crops as food sources, GRNs wired to mediate secondary growth in the storage root have just begun to emerge, specifically with the study of the radish. Gene expression data available with regard to other important root crops are not detailed enough for us directly to infer underlying molecular mechanisms. Thus, in this review, we provide a general overview of the regulatory programs governing the development and functions of the vascular cambium in model systems, and the role of the vascular cambium on the growth and yield potential of the storage roots in root crops. We then undertake a reanalysis of recent gene expression data generated for major root crops and discuss common GRNs involved in the vascular cambium-driven secondary growth in storage roots using the wealth of information available in Arabidopsis. Finally, we propose future engineering schemes for improving root crop yields by modifying potential key nodes in GRNs.