Secondary growth as a determinant of plant shape and form

Tuber, or not tuber: Molecular and morphological basis of underground storage organ development

Underground storage organs occur in phylogenetically diverse plant taxa and arise from multiple tissue types including roots and stems. Thickening growth allows underground storage organs to accommodate carbohydrates and other nutrients and requires proliferation at various lateral meristems followed by cell expansion. The WOX-CLE module regulates thickening growth via the vascular cambium in several eudicot systems, but the molecular mechanisms of proliferation at other lateral meristems are not well understood. In potato, onion, and other systems, members of the phosphatidylethanolamine-binding protein (PEBP) gene family induce underground storage organ development in response to photoperiod cues. While molecular mechanisms of tuber development in potato are well understood, we lack detailed mechanistic knowledge for the extensive morphological and taxonomic diversity of underground storage organs in plants.

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

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

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

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

Polyploid Genome Assembly Provides Insights into Morphological Development and Ascorbic Acid Accumulation of Sauropus androgynus

Sauropus androgynus (S. androgynus) (2n = 4x = 52) is one of the most popular functional leafy vegetables in South and Southeast Asia. With its rich nutritional and pharmaceutical values, it has traditionally had widespread use for dietary and herbal purposes. Here, the genome of S. androgynus was sequenced and assembled, revealing a genome size of 1.55 Gb with 26 pseudo-chromosomes. Phylogenetic analysis traced back the divergence of Sauropus from Phyllanthus to approximately 29.67 million years ago (Mya). Genome analysis revealed that S. androgynus polyploidized around 20.51 Mya and shared a γ event about 132.95 Mya. Gene function analysis suggested that the expansion of pathways related to phloem development, lignin biosynthesis, and photosynthesis tended to result in the morphological differences among species within the Phyllanthaceae family, characterized by varying ploidy levels. The high accumulation of ascorbic acid in S. androgynus was attributed to the high expression of genes associated with the L-galactose pathway and recycling pathway. Moreover, the expanded gene families of S. androgynus exhibited multiple biochemical pathways associated with its comprehensive pharmacological activity, geographic adaptation and distinctive pleasurable flavor. Altogether, our findings represent a crucial genomic asset for S. androgynus, casting light on the intricate ploidy within the Phyllanthaceae family.

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.

Molecular phylogeny of Urvillea (Paullinieae, Sapindaceae) and its implications in stem vascular diversity

BACKGROUND AND AIMS

The tribe Paullinieae has the highest diversity of vascular variants among the seed plants. The developmental diversity is better understood in the species-rich genera Paullinia and Serjania; however, the phylogeny and diversity of vascular variants in the smaller genera of Paullinieae remain understudied. Here we investigate the evolution of development of stem vasculatures in the small genus Urvillea.

METHODS

We generate the first molecular phylogeny of Urvillea derived from 11 markers using a maximum likelihood and Bayesian approach. In combination with phylogenetic reconstruction, stochastic character mapping is used to assess evolutionary changes in stem ontogenies, determined from developmental anatomy of stems collected in the field or from herbarium and wood collections.

KEY RESULTS

Urvillea is supported as a monophyletic group and sister to Serjania. There are five stem ontogenies in Urvillea, including typical growth and four different vascular variants. Most stem ontogenies initiate with lobed stems in primary growth. Lobed stems in secondary growth are ancestral in Urvillea, but this ontogeny was lost multiple times. A reversal to typical growth occurred in non-climbing species. Phloem wedges, fissured stems, and ectopic cambia each evolved once independently. Phloem wedges is an intermediate developmental stage in the formation of fissured stems, which is characterized by a continuous fragmentation of vascular tissues. Lobed stems may generate constriction zones and lobes may split or not.

CONCLUSIONS

Urvillea is the third most diverse genus (after Serjania and Paullinia) with respect to the number of vascular variants within Paullinieae. One ontogeny (fissured stems) is exclusive to the genus. Differential cambial activity and ectopic cambia are the main ontogenetic processes generating stem diversity. The evolutionary history of vascular variants demonstrates the large developmental plasticity of the cambium in such a small genus and further demonstrates that complex anatomies have repeatedly evolved within Paullinieae lianas.

A genotype-specific architectural and physiological profile is involved in the flowering regularity of apple trees

In polycarpic plants, meristem fate varies within individuals in a given year. In perennials, the proportion of floral induction (FI) in meristems also varies between consecutive years and among genotypes of a given species. Previous studies have suggested that FI of meristems could be determined by the within-plant competition for carbohydrates and by hormone signaling as key components of the flowering pathway. At the genotypic level, variability in FI was also associated with variability in architectural traits. However, the part of genotype-dependent variability in FI that can be explained by either tree architecture or tree physiology is still not fully understood. This study aimed at deciphering the respective effect of architectural and physiological traits on FI variability within apple trees by comparing six genotypes with contrasted architectures. Shoot type demography as well as the flowering and fruit production patterns were followed over 6 years and characterized by different indexes. Architectural morphotypes were then defined based on architectural traits using a clustering approach. For two successive years, non-structural starch content in leaf, stem and meristems, and hormonal contents (gibberellins, cytokinins, auxin and abscisic acid) in meristems were quantified and correlated to FI within-tree proportions. Based on a multi-step regression analysis, cytokinins and gibberellins content in meristem, starch content in leaves and the proportion of long shoots in tree annual growth were shown to contribute to FI. Although the predictive linear model of FI was common to all genotypes, each of the explicative variables had a different weight in FI determination, depending on the genotype. Our results therefore suggest both a common determination model and a genotype-specific architectural and physiological profile linked to its flowering behavior.

bHLH heterodimer complex variations regulate cell proliferation activity in the meristems of Arabidopsis thaliana

Root, shoot, and lateral meristems are the main regions of cell proliferation in plants. It has been proposed that meristems might have evolved dedicated transcriptional networks to balance cell proliferation. Here, we show that basic helix-loop-helix (bHLH) transcription factor heterodimers formed by members of the TARGET OF MONOPTEROS5 (TMO5) and LONESOME HIGHWAY (LHW) subclades are general regulators of cell proliferation in all meristems. Yet, genetics and expression analyses suggest specific functions of these transcription factors in distinct meristems, possibly due to their expression domains determining heterodimer complex variations within meristems, and to a certain extent to the absence of some of them in a given meristem. Target gene specificity analysis for heterodimer complexes focusing on the LONELY GUY gene targets further suggests differences in transcriptional responses through heterodimer diversification that could allow a common bHLH heterodimer complex module to contribute to cell proliferation control in multiple meristems.

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Expression of TMO5 and LHW bHLH clade members varies in distinct meristems

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Single mutant analyses reveal functional specificity in meristems

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Variations in TMO5/LHW heterodimer complexes affect target gene regulation

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TMO5/LHW complexes are regulators of cell proliferation in all plant meristems

Solving the regulation puzzle of periderm development using advances in fruit skin

Periderm protects enlarged organs of most dicots and gymnosperms as a barrier to water loss and disease invasion during their secondary growth. Its development undergoes a complex process with genetically controlled and environmental stress-induced characters. Different development of periderm makes the full and partial russet of fruit skin, which diverges in inheritance with qualitative and quantitative characters, respectively, in pear pome. In addition to its specific genetics, fruit periderm has similar development and structure as that of stem and other organs, making it an appropriate material for periderm research. Recently, progress in histochemical as well as transcriptome and proteome analyses, and quantitative trait locus (QTL) mapping have revealed the regulatory molecular mechanism in the periderm based on the identification of switch genes. In this review, we concentrate on the periderm development, propose the conservation of periderm regulation between fruit and other plant organs based on their morphological and molecular characteristics, and summarize a regulatory network with the elicitors and repressors for the tissue development. Spontaneous programmed-cell death (PCD) or environmental stress produces the original signal that triggers the development of periderm. Spatio-temporal specific PCD produced by PyPPCD1 gene and its homologs can play a key role in the coordinated regulation of cell death related tissue development.

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.

Eucalyptus grandis AUX/INDOLE-3-ACETIC ACID 13 (EgrIAA13) is a novel transcriptional regulator of xylogenesis

Our Induced Somatic Sector Analysis and protein-protein interaction experiments demonstrate that Eucalyptus grandis IAA13 regulates xylem fibre and vessel development, potentially via EgrIAA13 modules involving ARF2, ARF5, ARF6 and ARF19. Auxin is a crucial phytohormone regulating multiple aspects of plant growth and differentiation, including regulation of vascular cambium activity, xylogenesis and its responsiveness towards gravitropic stress. Although the regulation of these biological processes greatly depends on auxin and regulators of the auxin signalling pathway, many of their specific functions remain unclear. Therefore, the present study aims to functionally characterise Eucalyptus grandis AUX/INDOLE-3-ACETIC ACID 13 (EgrIAA13), a member of the auxin signalling pathway. In Eucalyptus and Populus, EgrIAA13 and its orthologs are preferentially expressed in the xylogenic tissues and downregulated in tension wood. Therefore, to further investigate EgrIAA13 and its function during xylogenesis, we conducted subcellular localisation and Induced Somatic Sector Analysis experiments using overexpression and RNAi knockdown constructs of EgrIAA13 to create transgenic tissue sectors on growing stems of Eucalyptus and Populus. Since Aux/IAAs interact with Auxin Responsive Factors (ARFs), in silico predictions of IAA13-ARF interactions were explored and experimentally validated via yeast-2-hybrid experiments. Our results demonstrate that EgrIAA13 localises to the nucleus and that downregulation of EgrIAA13 impedes Eucalyptus xylem fibre and vessel development. We also observed that EgrIAA13 interacts with Eucalyptus ARF2, ARF5, ARF6 and ARF19A. Based on these results, we conclude that EgrIAA13 is a regulator of Eucalyptus xylogenesis and postulate that the observed phenotypes are likely to result from alterations in the auxin-responsive transcriptome via IAA13-ARF modules such as EgrIAA13-EgrARF5. Our results provide the first insights into the regulatory role of EgrIAA13 during xylogenesis.

Translational profile of developing phellem cells in Arabidopsis thaliana roots

The phellem is a specialized boundary tissue providing the first line of defense against abiotic and biotic stresses in organs undergoing secondary growth. Phellem cells undergo several differentiation steps, which include cell wall suberization, cell expansion, and programmed cell death. Yet, the molecular players acting particularly in phellem cell differentiation remain poorly described, particularly in the widely used model plant Arabidopsis thaliana. Using specific marker lines we followed the onset and progression of phellem differentiation in A. thaliana roots and further targeted the translatome of newly developed phellem cells using translating ribosome affinity purification followed by mRNA sequencing (TRAP-SEQ). We showed that phellem suberization is initiated early after phellogen (cork cambium) division. The specific translational landscape was organized in three main domains related to energy production, synthesis and transport of cell wall components, and response to stimulus. Novel players in phellem differentiation related to suberin monomer transport and assembly as well as novel transcription regulators were identified. This strategy provided an unprecedented resolution of the translatome of developing phellem cells, giving a detailed and specific view on the molecular mechanisms acting on cell differentiation in periderm tissues of the model plant Arabidopsis.

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

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

The shaping of plant axes and crowns through tropisms and elasticity: an example of morphogenetic plasticity beyond the shoot apical meristem

Shoot morphogenetic plasticity is crucial to the adaptation of plants to their fluctuating environments. Major insights into shoot morphogenesis have been compiled studying meristems, especially the shoot apical meristem (SAM), through a methodological effort in multiscale systems biology and biophysics. However, morphogenesis at the SAM is robust to environmental changes. Plasticity emerges later on during post-SAM development. The purpose of this review is to show that multiscale systems biology and biophysics is insightful for the shaping of the whole plant as well. More specifically, we review the shaping of axes and crowns through tropisms and elasticity, combining the recent advances in morphogenetic control using physical cues and by genes. We focus mostly on land angiosperms, but with growth habits ranging from small herbs to big trees. We show that generic (universal) morphogenetic processes have been identified, revealing feedforward and feedback effects of global shape on the local morphogenetic process. In parallel, major advances have been made in the analysis of the major genes involved in shaping axes and crowns, revealing conserved genic networks among angiosperms. Then, we show that these two approaches are now starting to converge, revealing exciting perspectives.

Delving deeper into the link between sugar transport, sugar signaling, and vascular system development

Plant growth and development rely on the transport and use of sugars produced during photosynthesis. Sugars have a dual function as nutrients and signal molecules in the cell. Many factors maintaining sugar homeostasis and signaling are now identified, but our understanding of the mechanisms involved in coordinating intracellular and intercellular sugar translocation is still limited. We also know little about the interplay between sugar transport and signaling and the formation of the vascular system, which controls long-distance sugar translocation. Sugar signaling has been proposed to play a role; however, evidence to support this hypothesis is still limited. Here, we exploited recent transcriptomics datasets produced in aerial organs of Arabidopsis to identify genes coding for sugar transporters or signaling components expressed in the vascular cells. We identified genes belonging to sugar transport and signaling for which no information is available regarding a role in vasculature development. In addition, the transcriptomics datasets obtained from sugar-treated Arabidopsis seedlings were used to assess the sugar-responsiveness of known genes involved in vascular differentiation. Interestingly, several key regulators of vascular development were found to be regulated by either sucrose or glucose. Especially CLE41, which controls the procambial cell fate, was oppositely regulated by sucrose or glucose in these datasets. Even if more experimental data are necessary to confirm these findings, this survey supports a link between sugar transport/signaling and vascular system development.

An ARF1-binding factor triggering programmed cell death and periderm development in pear russet fruit skin

Plants have a cuticular membrane (CM) and periderm membrane (PM), which act as barriers to terrestrial stresses. The CM covers primary organs with a continuous hydrophobic layer of waxes embedded in cutin, while the PM stacks with suberized cells outermost to the secondary tissues. The formation of native periderm is regulated by a postembryonic meristem phellogen that produces suberized phellem (cork) outwardly. However, the mechanism controlling phellogen differentiation to phellem remains to be clarified. Here, map-based cloning in a pear F1 population with segregation for periderm development in fruit skin facilitated the identification of an aspartic acid repeat deletion in Pyrus Periderm Programmed Cell Death 1.1 (PyPPCD1.1) that triggers phellogen activity for cork formation in pear russet fruit skin. PyPPCD1.1 showed preferential expression in pear fruit skin, and the encoded protein shares a structural similarity to that of the viral capsid proteins. Asp deletion in PyPPCD1.1 weakened its nuclear localization but increased its accumulation in the chloroplast. Both PyPPCD1.1 and its recessive allele directly interact with ADP-ribosylation factor 1 (ARF1). PyPPCD1.1 triggered PCD in an ARF1-dependent manner. Thus, this study identified the switch gene for PCD and periderm development and provided a new molecular regulatory mechanism underlying the development of this trait.

Visualizing polymeric components that define distinct root barriers across plant lineages

Hydrophobic cell wall depositions in roots play a key role in plant development and interaction with the soil environment, as they generate barriers that regulate bidirectional nutrient flux. Techniques to label the respective polymers are emerging, but are efficient only in thin roots or sections. Moreover, simultaneous imaging of the barrier constituents lignin and suberin remains problematic owing to their similar chemical compositions. Here, we describe a staining method compatible with single- and multiphoton confocal microscopy that allows for concurrent visualization of primary cell walls and distinct secondary depositions in one workflow. This protocol permits efficient separation of suberin- and lignin-specific signals with high resolution, enabling precise dissection of barrier constituents. Our approach is compatible with imaging of fluorescent proteins, and can thus complement genetic markers or aid the dissection of barriers in biotic root interactions. We further demonstrate applicability in deep root tissues of plant models and crops across phylogenetic lineages. Our optimized toolset will significantly advance our understanding of root barrier dynamics and function, and of their role in plant interactions with the rhizospheric environment.

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.

Tuber and Tuberous Root Development

Root and tuber crops have been an important part of human nutrition since the early days of humanity, providing us with essential carbohydrates, proteins, and vitamins. Today, they are especially important in tropical and subtropical regions of the world, where they help to feed an ever-growing population. Early induction and storage organ size are important agricultural traits, as they determine yield over time. During potato tuberization, environmental and metabolic status are sensed, ensuring proper timing of tuberization mediated by phloem-mobile signals. Coordinated cellular restructuring and expansion growth, as well as controlled storage metabolism in the tuber, are executed. This review summarizes our current understanding of potato tuber development and highlights similarities and differences to important tuberous root crop species like sweetpotato and cassava. Finally, we point out knowledge gaps that need to be filled before a complete picture of storage organ development can emerge.

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

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

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.

Auxin signaling and vascular cambium formation enable storage metabolism in cassava tuberous roots

Cassava storage roots are among the most important root crops worldwide, and represent one of the most consumed staple foods in sub-Saharan Africa. The vegetatively propagated tropical shrub can form many starchy tuberous roots from its stem. These storage roots are formed through the activation of secondary root growth processes. However, the underlying genetic regulation of storage root development is largely unknown. Here we report distinct structural and transcriptional changes occurring during the early phases of storage root development. A pronounced increase in auxin-related transcripts and the transcriptional activation of secondary growth factors, as well as a decrease in gibberellin-related transcripts were observed during the early stages of secondary root growth. This was accompanied by increased cell wall biosynthesis, most notably increased during the initial xylem expansion within the root vasculature. Starch storage metabolism was activated only after the formation of the vascular cambium. The formation of non-lignified xylem parenchyma cells and the activation of starch storage metabolism coincided with increased expression of the KNOX/BEL genes KNAT1, PENNYWISE, and POUND-FOOLISH, indicating their importance for proper xylem parenchyma function.

Auxin and gibberellin signaling cross-talk promotes hypocotyl xylem expansion and cambium homeostasis

During secondary growth, the thickening of plant organs, wood (xylem) and bast (phloem) is continuously produced by the vascular cambium. In Arabidopsis hypocotyl and root, we can distinguish two phases of secondary growth based on cell morphology and production rate. The first phase, in which xylem and phloem are equally produced, precedes the xylem expansion phase in which xylem formation is enhanced and xylem fibers differentiate. It is known that gibberellins (GA) trigger this developmental transition via degradation of DELLA proteins and that the cambium master regulator BREVIPEDICELLUS/KNAT1 (BP/KNAT1) and receptor like kinases ERECTA and ERL1 regulate this process downstream of GA. However, our understanding of the regulatory network underlying GA-mediated secondary growth is still limited. Here, we demonstrate that DELLA-mediated xylem expansion in Arabidopsis hypocotyl is mainly achieved through DELLA family members RGA and GAI, which promote cambium senescence. We further show that AUXIN RESPONSE FACTOR 6 (ARF6) and ARF8, which physically interact with DELLAs, specifically repress phloem proliferation and induce cambium senescence during the xylem expansion phase. Moreover, the inactivation of BP in arf6 arf8 background revealed an essential role for ARF6 and ARF8 in cambium establishment and maintenance. Overall, our results shed light on a pivotal hormone cross-talk between GA and auxin in the context of plant secondary growth.

Modelling the spatial crosstalk between two biochemical signals explains wood formation dynamics and tree-ring structure

In conifers, xylogenesis during a growing season produces a very characteristic tree-ring structure: large, thin-walled earlywood cells followed by narrow, thick-walled latewood cells. Although many factors influence the dynamics of differentiation and the final dimensions of xylem cells, the associated patterns of variation remain very stable from one year to the next. While radial growth is characterized by an S-shaped curve, the widths of xylem differentiation zones exhibit characteristic skewed bell-shaped curves. These elements suggest a strong internal control of xylogenesis. It has long been hypothesized that much of this regulation relies on a morphogenetic gradient of auxin. However, recent modelling studies have shown that while this hypothesis could account for the dynamics of stem radial growth and the zonation of the developing xylem, it failed to reproduce the characteristic tree-ring structure. Here, we investigated the hypothesis of regulation by a crosstalk between auxin and a second biochemical signal, by using computational morphodynamics. We found that, in conifers, such a crosstalk is sufficient to simulate the characteristic features of wood formation dynamics, as well as the resulting tree-ring structure. In this model, auxin controls cell enlargement rates while another signal (e.g. cytokinin, tracheary element differentiation inhibitory factor) drives cell division and auxin polar transport.

Developmental phenotypes of Arabidopsis plants expressing phosphovariants of AtMYB75

The Arabidopsis transcription factor Myeloblastosis protein 75 (MYB75, AT1G56650) is a well-established transcriptional activator of genes required for anthocyanin and flavonoid production, and a repressor of lignin and other secondary cell wall biosynthesis genes. MYB75 is itself tightly regulated at the transcriptional, translational and post-translational levels, including protein phosphorylation by Arabidopsis MAP kinases Examination of the behavior of different phosphovariant versions of MYB75 in vitro and in vivo revealed that overexpression of the MYB75^T131E phosphovariant had a particularly marked effect on global changes in gene expression suggesting that phosphorylated MYB75 could be involved in a broader range of functions than previously recognized. Here, we describe a range of distinct developmental phenotypes observed among Arabidopsis lines expressing various phosphovariant forms of MYB75. Expression of either MYB75^T131E or MYB75^T131A phosphovariants, from the endogenous MYB75 promoter, in Arabidopsis myb75^- mutants (Nossen background), resulted in severely impaired germination rates, and developmental arrest at early seedling stages. Arabidopsis plants overexpressing MYB75^T131E from a strong constitutive Cauliflower mosaic virus (CaMV35S) promoter displayed slower development, with delayed bolting, flowering and onset of senescence. Conversely, MYB75^T131A -overexpressing lines flowered and set seed earlier than either Col-0 WT controls or other MYB75-overexpressors (MYB75^WT and MYB75^T131E ). Histochemical analysis of mature stems also revealed ectopic vessel development in plants overexpressing MYB75; this phenotype was particularly prominent in the MYB75^T131E phosphovariant. These data suggest that MYB75 plays a significant role in plant development, and that this aspect of MYB75 function is influenced by its phosphorylation status.

Dynamic cytokinin signalling landscapes during lateral root formation in Arabidopsis

By forming lateral roots, plants expand their root systems to improve anchorage and absorb more water and nutrients from the soil. Each phase of this developmental process in Arabidopsis is tightly regulated by dynamic and continuous signalling of the phytohormones cytokinin and auxin. While the roles of auxin in lateral root organogenesis and spatial accommodation by overlying cell layers have been well studied, insights on the importance of cytokinin is still somewhat limited. Cytokinin is a negative regulator of lateral root formation with versatile modes of action being activated at different root developmental zones. Here, we review the latest progress made towards our understanding of these spatially separated mechanisms of cytokinin-mediated signalling that shape lateral root initiation, outgrowth and emergence and highlight some of the enticing open questions.

Arabidopsis primary root growth: let it grow, can't hold it back anymore!

In multicellular organisms, growth is defined by those processes that allow an organ to increase in mass, namely cell proliferation - that increases the number of cells - and cell expansion - that increases their volume. For an organ to achieve a functional shape and a characteristic final size both these processes need to be tightly coordinated. In roots, these processes stand behind root primary growth, which results in lengthening of the root along its longitudinal axis, and secondary growth, which results in an increase of the root thickness. In this review, we will analyze latest advances in the study of the molecular mechanisms involved in root primary growth, focusing on the model species Arabidopsis thaliana, where some molecular factors and networks responsible for regulating its self-organized primary growth have been identified.

Pluripotent Pericycle Cells Trigger Different Growth Outputs by Integrating Developmental Cues into Distinct Regulatory Modules

During post-embryonic development, the pericycle specifies the stem cells that give rise to both lateral roots (LRs) and the periderm, a suberized barrier that protects the plant against biotic and abiotic stresses. Comparable auxin-mediated signaling hubs regulate meristem establishment in many developmental contexts; however, it is unknown how specific outputs are achieved. Using the Arabidopsis root as a model, we show that while LR formation is the main auxin-induced program after de-etiolation, plants with age become competent to form a periderm in response to auxin. The establishment of the vascular cambium acts as the developmental switch required to trigger auxin-mediated periderm initiation. Moreover, distinct auxin signaling components and targets control LR versus periderm formation. Among the periderm-specific-promoting transcription factors, WUSCHEL-RELATED HOMEOBOX 4 (WOX4) and KNAT1/BREVIPEDICELLUS (BP) stand out as their specific overexpression in the periderm results in an increased number of periderm layers, a trait of agronomical importance in breeding programs targeting stress tolerance. These findings reveal that specificity in pericycle stem cell fate is achieved by the integration of developmental cues into distinct regulatory modules.

Establishment of Pluripotent Cell Cultures to Explore Allelopathic Activity of Coffee Cells by Protoplast Co-Culture Bioassay Method

We focused on the demonstration of a new pluripotent coffee cell culture system to control the growth and metabolic functions. Somatic cells in the epidermal layer of in vitro somatic embryos (SEs) of Coffea canephora expressed higher pluripotency to produce secondary SEs than primary or secondary meristematic tissue. SEs were ideal explants to selectively induce functionally-differentiated cell lines, both non-embryogenic callus (nEC) and embryogenic callus (EC). The protoplast co-culture bioassay method was used to explore allelopathic activity of these cultured coffee cells. Cell wall formation of lettuce protoplasts varied after five days of co-culture. A strong stimulative reaction was observed at lower nEC protoplast densities, whereas growth was inhibited at higher densities. The reaction of lettuce protoplasts after 12 days of co-culture was recognized as an inhibitory reaction of colony formation.

Root secondary growth: an unexplored component of soil resource acquisition

BACKGROUND AND AIMS

Despite recent progress in elucidating the molecular basis of secondary growth (cambial growth), the functional implications of this developmental process remain poorly understood. Targeted studies exploring how abiotic and biotic factors affect this process, as well as the relevance of secondary growth to fitness of annual dicotyledonous crop species under stress, are almost entirely absent from the literature. Specifically, the physiological role of secondary growth in roots has been completely neglected yet entails a unique array of implications for plant performance that are distinct from secondary growth in shoot tissue.

SCOPE

Since roots are directly responsible for soil resource capture, understanding of the fitness landscape of root phenotypes is important in both basic and applied plant biology. Interactions between root secondary growth, edaphic conditions and soil resource acquisition may have significant effects on plant fitness. Our intention here is not to provide a comprehensive review of a sparse and disparate literature, but rather to highlight knowledge gaps, propose hypotheses and identify opportunities for novel and agriculturally relevant research pertaining to secondary growth of roots. This viewpoint: (1) summarizes evidence from our own studies and other published work; (2) proposes hypotheses regarding the fitness landscape of secondary growth of roots in annual dicotyledonous species for abiotic and biotic stress; and (3) highlights the importance of directing research efforts to this topic within an agricultural context.

CONCLUSIONS

Secondary growth of the roots of annual dicots has functional significance with regards to soil resource acquisition and transport, interactions with soil organisms and carbon sequestration. Research on these topics would contribute significantly toward understanding the agronomic value of secondary growth of roots for crop improvement.

Early-stage sugar beet taproot development is characterized by three distinct physiological phases

Despite the agronomic importance of sugar beet (Beta vulgaris L.), the early-stage development of its taproot has only been poorly investigated. Thus, the mechanisms that determine growth and sugar accumulation in sugar beet are largely unknown. In the presented study, a physiological characterization of early-stage sugar beet taproot development was conducted. Activities were analyzed for fourteen key enzymes of carbohydrate metabolism in developing taproots over the first 80 days after sowing. In addition, we performed in situ localizations of selected carbohydrate-metabolic enzyme activities, anatomical investigations, and quantifications of soluble carbohydrates, hexose phosphates, and phytohormones. Based on the accumulation dynamics of biomass and sucrose, as well as on anatomical parameters, the early phase of taproot development could be subdivided into three stages-prestorage, transition, secondary growth and sucrose accumulation stage-each of which was characterized by distinct metabolic and phytohormonal signatures. The enzyme activity signatures corresponding to these stages were also shown to be robustly reproducible in experiments conducted in two additional locations. The results from this physiological phenotyping approach contribute to the identification of the key regulators of sugar beet taproot development and open up new perspectives for sugar beet crop improvement concerning both physiological marker-based breeding and biotechnological approaches.

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.

The barrier function of plant roots: biological bases for selective uptake and avoidance of soil compounds

The root is the main organ through which water and mineral nutrients enter the plant organism. In addition, root fulfils several other functions. Here, we propose that the root also performs the barrier function, which is essential not only for plant survival but for plant acclimation and adaptation to a constantly changing and heterogeneous soil environment. This function is related to selective uptake and avoidance of some soil compounds at the whole plant level. We review the toolkit of morpho-anatomical, structural, and other components that support this view. The components of the root structure involved in selectivity, permeability or barrier at a cellular, tissue, and organ level and their properties are discussed. In consideration of the arguments supporting barrier function of plant roots, evolutionary aspects of this function are also reviewed. Additionally, natural variation in selective root permeability is discussed which suggests that the barrier function is constantly evolving and is subject of natural selection.

The Control of Developmental Phase Transitions by microRNAs and Their Targets in Seed Plants

Seed plants usually undergo various developmental phase transitions throughout their lifespan, mainly including juvenile-to-adult and vegetative-to-reproductive transitions, as well as developmental transitions within organ/tissue formation. MicroRNAs (miRNAs), as a class of small endogenous non-coding RNAs, are involved in the developmental phase transitions in plants by negatively regulating the expression of their target genes at the post-transcriptional level. In recent years, cumulative evidence has revealed that five miRNAs, miR156, miR159, miR166, miR172, and miR396, are key regulators of developmental phase transitions in plants. In this review, the advanced progress of the five miRNAs and their targets in regulating plant developmental transitions, especially in storage organ formation, are summarized and discussed, combining our own findings with the literature. In general, the functions of the five miRNAs and their targets are relatively conserved, but their functional divergences also emerge to some extent. In addition, potential research directions of miRNAs in regulating plant developmental phase transitions are prospected.

Parallel evolution of arborescent carrots (Daucus) in Macaronesia

PREMISE

Despite intensive research, the pathways and driving forces behind the evolution of derived woodiness on oceanic islands remain obscure. The genus Daucus comprises mostly herbs (therophytes, hemicryptophytes) with few rosette treelets (chamaephytes) endemic to various Macaronesian archipelagos, suggesting their independent evolution. To elucidate the evolutionary pathways to derived woodiness, we examined phylogenetic relationships and the habit and secondary xylem evolution in Daucus and related taxa.

METHODS

Sixty taxa were surveyed for molecular markers, life history, and habit traits. Twenty-one species were considered for wood anatomical characters. A dated phylogeny was estimated using Bayesian methods. The evolution of selected traits was reconstructed using parsimony and maximum likelihood.

RESULTS

Daucus dispersed independently to the Canary Islands (and subsequently to Madeira), Cape Verde, and the Azores in the late Miocene and Pleistocene. Life span, reproductive strategy, and life form were highly homoplastic; the ancestor of Daucus was probably a monocarpic, biennial hemicryptophyte. Rosette treelets evolved independently in the Canarian-Madeiran lineage and in Cape Verde, the latter within the last 0.13 Myr. Treelets and hemicryptophytes did not differ in wood anatomy. Pervasive axial parenchyma in wood occurred more often in polycarpic rather than monocarpic species.

CONCLUSIONS

Life span and life form in Daucus are evolutionarily labile and may change independently of wood anatomy, which is related to plant reproductive strategy rather than to life form. Insular woodiness may evolve rapidly (as demonstrated in D. bischoffii), and in Daucus, it does not seem to be an adaptation to lower the risk of xylem embolism.

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.

Wood Transcriptome Profiling Identifies Critical Pathway Genes of Secondary Wall Biosynthesis and Novel Regulators for Vascular Cambium Development in Populus

Wood, the most abundant biomass on Earth, is composed of secondary xylem differentiated from vascular cambium. However, the underlying molecular mechanisms of wood formation remain largely unclear. To gain insight into wood formation, we performed a series of wood-forming tissue-specific transcriptome analyses from a hybrid poplar (Populus alba × P. glandulosa, clone BH) using RNA-seq. Together with shoot apex and leaf tissue, cambium and xylem tissues were isolated from vertical stem segments representing a gradient of secondary growth developmental stages (i.e., immature, intermediate, and mature stem). In a comparative transcriptome analysis of the 'developing xylem' and 'leaf' tissue, we could identify critical players catalyzing each biosynthetic step of secondary wall components (e.g., cellulose, xylan, and lignin). Several candidate genes involved in the initiation of vascular cambium formation were found via a co-expression network analysis using abundantly expressed genes in the 'intermediate stem-derived cambium' tissue. We found that transgenic Arabidopsis plants overexpressing the PtrHAM4-1, a GRAS family transcription factor, resulted in a significant increase of vascular cambium development. This phenotype was successfully reproduced in the transgenic poplars overexpressing the PtrHAM4-1. Taken together, our results may serve as a springboard for further research to unravel the molecular mechanism of wood formation, one of the most important biological processes on this planet.

Plant Vascular Tissues-Connecting Tissue Comes in All Shapes

For centuries, humans have grown and used structures based on vascular tissues in plants. One could imagine that life would have developed differently without wood as a resource for building material, paper, heating energy, or fuel and without edible tubers as a food source. In this review, we will summarise the status of research on Arabidopsis thaliana vascular development and subsequently focus on how this knowledge has been applied and expanded in research on the wood of trees and storage organs of crop plants. We will conclude with an outlook on interesting open questions and exciting new research opportunities in this growing and important field.

Jasmonic acid to boost secondary growth in hemp hypocotyl

The application of jasmonic acid results in an increased secondary growth, as well as additional secondary phloem fibres and higher lignin content in the hypocotyl of textile hemp (Cannabis sativa L.). Secondary growth provides most of the wood in lignocellulosic biomass. Textile hemp (Cannabis sativa L.) is cultivated for its phloem fibres, whose secondary cell wall is rich in crystalline cellulose with a limited amount of lignin. Mature hemp stems and older hypocotyls are characterised by large blocks of secondary phloem fibres which originate from the cambium. This study aims at investigating the role of exogenously applied jasmonic acid on the differentiation of secondary phloem fibres. We show indeed that the exogenous application of this plant growth regulator on young hemp plantlets promotes secondary growth, differentiation of secondary phloem fibres, expression of lignin-related genes, and lignification of the hypocotyl. This work paves the way to future investigations focusing on the molecular network underlying phloem fibre development.