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

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.

The RING-type E3 ligase RIE1 sustains leaf longevity by specifically targeting AtACS7 to fine-tune ethylene production in Arabidopsis

Ethylene is widely recognized as a positive regulator of leaf senescence. However, how plants coordinate the biosynthesis of ethylene to meet the requirements of senescence progression has not been determined. The rate-limiting enzyme in the ethylene biosynthesis pathway is ACC synthase. AtACS7 was previously considered one of the major contributors to the synthesis of "senescence ethylene" in Arabidopsis. However, the "brake signal" that fine-tunes the expression of AtACS7 to ensure optimal ethylene production during leaf development has yet to be identified. In the present study, the RING-H2 zinc-finger protein RIE1 was found to specifically interact with and ubiquitinate AtACS7, among all functional ACSs in Arabidopsis, to promote its degradation. Overexpression of RIE1 markedly decreased ethylene biosynthesis and delayed leaf senescence, whereas loss of function of RIE1 significantly increased ethylene emission and accelerated leaf senescence. The ethylene-related phenotypes of RIE1 overexpressing or knockout mutants were effectively rescued by the ethylene precursor ACC or the competitive inhibitor of ACS, respectively. In particular, AtACS7-induced precocious leaf senescence was strongly enhanced by the loss of RIE1 but was significantly attenuated by the overexpression of RIE1. The specific regions of interaction between AtACS7 and RIE1, as well as the major ubiquitination sites of AtACS7, were further investigated. All results demonstrated that RIE1 functions as an important modulator of ethylene biosynthesis during leaf development by specifically targeting AtACS7 for degradation, thereby enabling plants to produce the optimal levels of ethylene needed.

Characterization and Transcriptomic Analysis of Sorghum EIN/EIL Family and Identification of Their Roles in Internode Maturation

Ethylene-insensitive 3/Ethylene-insensitive3-like proteins (EIN3/EIL) represent a group of transcription factors critical for the ethylene signaling transduction that manipulate downstream ethylene-responsive genes, thereby regulating plant growth, development, and stress responses. However, the identification, evolution, and divergence of the EIL family remain to be studied in Sorghum bicolor. Here, we identified eight SbEILs, which were expanded due to whole-genome-duplication (WGD) events. Characterization of the protein sequences and expression atlas demonstrates that the WGD-duplicated SbEILs could become divergent due to the differential expression patterns, rather than domain and motif architectures. Comparative expression analysis was performed between the RNA-seq data sets of internodes from several sorghum cultivars to understand the potential roles of SbEIL members in internode elongation and maturation. Our results identified SbEIL3 and 7 (the latter as a homolog of OsEIL7/OsEIL1) to be the highly expressed SbEIL genes in sorghum internodes and revealed a potential functional link between SbEIL7 and internode maturation. The co-expression analysis and comparative expression analysis with ethylene-regulated gene sets found that SbEIL7 was co-regulated with a set of ubiquitin-related protein degradation genes, suggesting possible involvement of SbEIL7 in protein degradation and processing during the post-anthesis stages. Altogether, our findings lay a foundation for future functional studies of ethylene signaling-mediated gene regulation and improvement of sorghum internode development.

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.

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

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

Cultivating potential: Harnessing plant stem cells for agricultural crop improvement

Meristems are stem cell-containing structures that produce all plant organs and are therefore important targets for crop improvement. Developmental regulators control the balance and rate of cell divisions within the meristem. Altering these regulators impacts meristem architecture and, as a consequence, plant form. In this review, we discuss genes involved in regulating the shoot apical meristem, inflorescence meristem, axillary meristem, root apical meristem, and vascular cambium in plants. We highlight several examples showing how crop breeders have manipulated developmental regulators to modify meristem growth and alter crop traits such as inflorescence size and branching patterns. Plant transformation techniques are another innovation related to plant meristem research because they make crop genome engineering possible. We discuss recent advances on plant transformation made possible by studying genes controlling meristem development. Finally, we conclude with discussions about how meristem research can contribute to crop improvement in the coming decades.

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.

Hormonal control of the molecular networks guiding vascular tissue development in the primary root meristem of Arabidopsis

Vascular tissues serve a dual function in plants, both providing physical support and controlling the transport of nutrients, water, hormones, and other small signaling molecules. Xylem tissues transport water from root to shoot; phloem tissues transfer photosynthates from shoot to root; while divisions of the (pro)cambium increase the number of xylem and phloem cells. Although vascular development constitutes a continuous process from primary growth in the early embryo and meristem regions to secondary growth in the mature plant organs, it can be artificially separated into distinct processes including cell type specification, proliferation, patterning, and differentiation. In this review, we focus on how hormonal signals orchestrate the molecular regulation of vascular development in the Arabidopsis primary root meristem. Although auxin and cytokinin have taken center stage in this aspect since their discovery, other hormones including brassinosteroids, abscisic acid, and jasmonic acid also take leading roles during vascular development. All these hormonal cues synergistically or antagonistically participate in the development of vascular tissues, forming a complex hormonal control network.

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.

An allelic variant in the ACS7 gene promotes primary root growth in watermelon

Gene mining in a C. lanatus × C. amarus population revealed one gene, ACS7, linked to primary root elongation in watermelon. Watermelon is a xerophytic crop characterized by a long primary root and robust lateral roots. Therefore, watermelon serves as an excellent model for studying root elongation and development. However, the genetic mechanism underlying the primary root elongation in watermelon remains unknown. Herein, through bulk segregant analysis we identified a genetic locus, qPRL.Chr03, controlling primary root length (PRL) using two different watermelon species (Citrullus lanatus and Citrullus amarus) that differ in their root architecture. Fine mapping revealed that xaa-Pro dipeptidase and 1-aminocyclopropane-1-carboxylate synthase 7 (ACS7) are candidate regulators of the primary root growth. Allelic variation in the delimited region among 193 watermelon accessions indicated that the long-root alleles might only exist in C. amarus. Interestingly, the discrepancy in PRL among the C. amarus accessions was clearly associated with a nonsynonymous single nucleotide polymorphism variant within the ACS7 gene. The ACS7 expression and ethylene levels in the primary root tips suggested that ethylene is a negative regulator of root elongation in watermelon, as supported by the application of 1-aminocyclopropane-1-carboxylate (ACC, the ethylene precursor) or 2-aminoethoxyvinyl glycine (AVG, an ACS inhibitor). To the best of our knowledge, these findings provide the first description of the genetic basis of root elongation in watermelon. The detected markers of the ACS7 gene will facilitate marker-assisted selection for the PRL trait to improve water and nutrient use efficacy in watermelon and beyond.

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.

Feedback loop promotes sucrose accumulation in cotyledons to facilitate sugar-ethylene signaling-mediated, etiolated-seedling greening

De-etiolation is indispensable for seedling survival and development. However, how sugars regulate de-etiolation and how sugars induce ethylene (ET) for seedlings to grow out of soil remain elusive. Here, we reveal how a sucrose (Suc) feedback loop promotes de-etiolation by inducing ET biosynthesis. Under darkness, Suc in germinating seeds preferentially induces 1-amino-cyclopropane-1-carboxylate synthase (ACS7; encoding a key ET biosynthesis enzyme) and associated ET biosynthesis, thereby activating ET core component ETHYLENE-INSENSITIVE3 (EIN3). Activated EIN3 directly inhibits the function of Suc transporter 2 (SUC2; a major Suc transporter) to block Suc export from cotyledons and thereby elevate Suc accumulation of cotyledons to induce ET. Under light, ET-activated EIN3 directly inhibits the function of phytochrome A (phyA; a de-etiolation inhibitor) to promote de-etiolation. We therefore propose that under darkness, the Suc feedback loop (Suc-ACS7-EIN3-|SUC2-Suc) promotes Suc accumulation in cotyledons to guarantee ET biosynthesis, facilitate de-etiolation, and enable seedlings to grow out of soil.

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.

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.

Vascular Cambium: The Source of Wood Formation

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