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.

Anatomical insights into the vascular layout of the barley rachis: implications for transport and spikelet connection.

BACKGROUND AND AIMS: Vascular patterning is intimately related to plant form and function. Here, using barley (Hordeum vulgare) as a model, we studied the vascular anatomy of the spike-type inflorescence. The main aim of the present work was to clarify the relationship between rachis (spike axis) vasculature and spike size, to define vascular dynamics and to discuss the implications for transport capacity and its interaction with the spikelets. METHODS: We used serial transverse internode sections to determine the internode area, vascular area and number of veins along the rachis of several barley lines. KEY RESULTS: Internode area and total vascular area show a clear positive correlation with spike size, whereas the number of veins is only weakly correlated. The lateral periphery of the rachis contains large mature veins of constant size, whereas the central part is occupied by small immature veins. Spikelet-derived veins entering the rachis often merge with the immature rachis veins but never merge with the mature veins. An increase in floret fertility through the conversion of a two-rowed barley into an isogenic six-rowed line, in addition to a decrease in floret fertility owing to enhanced pre-anthesis tip degeneration caused by the mutation tip sterile 2.b (tst2.b), significantly affected vein size but had limited to no effects on the number of veins or internode area. CONCLUSIONS: The rachis vasculature is the result of a two-step process involving an initial layout followed by size adjustment according to floret fertility/spike size. The restriction of large mature vessels to the periphery and that of small immature vessels to the centre of the rachis suggests that long-distance transport and local supply to spikelets are spatially separated processes. The identification of spikelet-derived veins entering the rachis without fusing with its vasculature indicates that a vascular continuity between rachis and spikelets might be non-essential.

CsIVP functions in vasculature development and downy mildew resistance in cucumber.

Domesticated crops with high yield and quality are frequently susceptible to pathogen attack, whereas enhancement of disease resistance generally compromises crop yield. The underlying mechanisms of how plant development and disease resistance are coordinately programed remain elusive. Here, we showed that the basic Helix-Loop-Helix (bHLH) transcription factor Cucumis sativus Irregular Vasculature Patterning (CsIVP) was highly expressed in cucumber vascular tissues. Knockdown of CsIVP caused severe vasculature disorganization and abnormal organ morphogenesis. CsIVP directly binds to vascular-related regulators YABBY5 (CsYAB5), BREVIPEDICELLUS (CsBP), and AUXIN/INDOLEACETIC ACIDS4 (CsAUX4) and promotes their expression. Knockdown of CsYAB5 resulted in similar phenotypes as CsIVP-RNA interference (RNAi) plants, including disturbed vascular configuration and abnormal organ morphology. Meanwhile, CsIVP-RNAi plants were more resistant to downy mildew and accumulated more salicylic acid (SA). CsIVP physically interacts with NIM1-INTERACTING1 (CsNIMIN1), a negative regulator in the SA signaling pathway. Thus, CsIVP is a novel vasculature regulator functioning in CsYAB5-mediated organ morphogenesis and SA-mediated downy mildew resistance in cucumber.

OsARF11 Promotes Growth, Meristem, Seed, and Vein Formation during Rice Plant Development.

The plant hormone auxin acts as a mediator providing positional instructions in a range of developmental processes. Studies in Arabidopsis thaliana L. show that auxin acts in large part via activation of Auxin Response Factors (ARFs) that in turn regulate the expression of downstream genes. The rice (Oryza sativa L.) gene OsARF11 is of interest because of its expression in developing rice organs and its high sequence similarity with MONOPTEROS/ARF5, a gene with prominent roles in A. thaliana development. We have assessed the phenotype of homozygous insertion mutants in the OsARF11 gene and found that in relation to wildtype, osarf11 seedlings produced fewer and shorter roots as well as shorter and less wide leaves. Leaves developed fewer veins and larger areoles. Mature osarf11 plants had a reduced root system, fewer branches per panicle, fewer grains per panicle and fewer filled seeds. Mutants had a reduced sensitivity to auxin-mediated callus formation and inhibition of root elongation, and phenylboronic acid (PBA)-mediated inhibition of vein formation. Taken together, our results implicate OsARF11 in auxin-mediated growth of multiple organs and leaf veins. OsARF11 also appears to play a central role in the formation of lateral root, panicle branch, and grain meristems.

Molecular Changes Concomitant with Vascular System Development in Mature Galls Induced by Root-Knot Nematodes in the Model Tree Host Populus tremula × P. alba.

One of the most striking features occurring in the root-knot nematode Meloidogyne incognita induced galls is the reorganization of the vascular tissues. During the interaction of the model tree species Populus and M. incognita, a pronounced xylem proliferation was previously described in mature galls. To better characterise changes in expression of genes possibly involved in the induction and the formation of the de novo developed vascular tissues occurring in poplar galls, a comparative transcript profiling of 21-day-old galls versus uninfected root of poplar was performed. Genes coding for transcription factors associated with procambium maintenance and vascular differentiation were shown to be differentially regulated, together with genes partaking in phytohormones biosynthesis and signalling. Specific signatures of transcripts associated to primary cell wall biosynthesis and remodelling, as well as secondary cell wall formation (cellulose, xylan and lignin) were revealed in the galls. Ultimately, we show that molecules derived from the monolignol and salicylic acid pathways and related to secondary cell wall deposition accumulate in mature galls.

The α-Aurora Kinases Function in Vascular Development in Arabidopsis.

The Aurora kinases are serine/threonine kinases with conserved functions in mitotic cell division in eukaryotes. In Arabidopsis, Aurora kinases play important roles in primary meristem maintenance, but their functions in vascular development are still elusive. We report a dominant xdi-d mutant showing the xylem development inhibition (XDI) phenotype. Gene identification and transgenic overexpression experiments indicated that the activation of the Arabidopsis Aurora 2 (AtAUR2) gene is responsible for the XDI phenotype. In contrast, the aur1-2 aur2-2 double mutant plants showed enhanced differentiation of phloem and xylem cells, indicating that the Aurora kinases negatively affect xylem differentiation. The transcript levels of key regulatory genes in vascular cell differentiation, i.e. ALTERED PHLOEM DEVELOPMENT (APL), VASCULAR-RELATED NAC-DOMAIN 6 (VND6) and VND7, were higher in the aur1-2 aur2-2 double mutant and lower in xdi-d mutants compared with the wild-type plants, further supporting the functions of α-Aurora kinases in vascular development. Gene mutagenesis and transgenic studies showed that protein phosphorylation and substrate binding, but not protein dimerization and ubiquitination, are critical for the biological function of AtAUR2. These results indicate that α-Aurora kinases play key roles in vascular cell differentiation in Arabidopsis.

Mosaic modularity: an updated perspective and research agenda for the evolution of vascular cambial growth.

Secondary growth from a vascular cambium, present today only in seed plants and isoetalean lycophytes, has a 400-million-yr evolutionary history that involves considerably broader taxonomic diversity, most of it hidden in the fossil record. Approaching vascular cambial growth as a complex developmental process, we review data from living plants and fossils that reveal diverse modes of secondary growth. These are consistent with a modular nature of secondary growth, when considered as a tracheophyte-wide structural feature. This modular perspective identifies putative constituent developmental modules of cambial growth, for which we review developmental anatomy and regulation. Based on these data, we propose a hypothesis that explains the sources of diversity of secondary growth, considered across the entire tracheophyte clade, and opens up new avenues for exploring the origin of secondary growth. In this hypothesis, various modes of secondary growth reflect a mosaic pattern of expression of different developmental-regulatory modules among different lineages. We outline an approach that queries three information systems (living seed plants, living seed-free plants, and fossils) and integrates data on developmental regulation, anatomy, gene evolution and phylogeny to test the mosaic modularity hypothesis and its implications, and to inform efforts aimed at understanding the evolution of secondary growth.

On the mechanisms of development in monocot and eudicot leaves.

Contents Summary 706 I. Introduction 707 II. Leaf zones in monocot and eudicot leaves 707 III. Monocot and eudicot leaf initiation: differences in degree and timing, but not kind 710 IV. Reticulate and parallel venation: extending the model? 711 V. Flat laminar growth: patterning and coordination of adaxial-abaxial and mediolateral axes 713 VI. Stipules and ligules: ontogeny of primordial elaborations 715 VII. Leaf architecture 716 VIII. Stomatal development: shared and diverged mechanisms for making epidermal pores 717 IX. Conclusion 719 Acknowledgements 720 References 720 SUMMARY: Comparisons of concepts in monocot and eudicot leaf development are presented, with attention to the morphologies and mechanisms separating these angiosperm lineages. Monocot and eudicot leaves are distinguished by the differential elaborations of upper and lower leaf zones, the formation of sheathing/nonsheathing leaf bases and vasculature patterning. We propose that monocot and eudicot leaves undergo expansion of mediolateral domains at different times in ontogeny, directly impacting features such as venation and leaf bases. Furthermore, lineage-specific mechanisms in compound leaf development are discussed. Although models for the homologies of enigmatic tissues, such as ligules and stipules, are proposed, tests of these hypotheses are rare. Likewise, comparisons of stomatal development are limited to Arabidopsis and a few grasses. Future studies may investigate correlations in the ontogenies of parallel venation and linear stomatal files in monocots, and the reticulate patterning of veins and dispersed stoma in eudicots. Although many fundamental mechanisms of leaf development are shared in eudicots and monocots, variations in the timing, degree and duration of these ontogenetic events may contribute to key differences in morphology. We anticipate that the incorporation of an ever-expanding number of sequenced genomes will enrich our understanding of the developmental mechanisms generating eudicot and monocot leaves.

Uncovering C4-like photosynthesis in C3 vascular cells.

In C4 plants, the vascularization of the leaf is extended to include a ring of photosynthetic bundle sheath cells, which have essential and specific functions. In contrast to the substantial knowledge of photosynthesis in C4 plants, relatively little is known about photosynthesis in C3 plant veins, which differs substantially from that in C3 mesophyll cells. In this review we highlight the specific photosynthetic machinery present in C3 vascular cells, which likely evolved prior to the divergence between C3 and C4 plants. The associated primary processes of carbon recapture, nitrogen transport, and antioxidant metabolism are discussed. This review of the basal C4 photosynthesis in C3 plants is significant in the context of promoting the potential for biotechnological development of C4-transgenic rice crops.

Extended differentiation of veins and stomata is essential for the expansion of large leaves in Rheum rhabarbarum.

PREMISE OF THE STUDY: The densities of veins and stomata govern leaf water supply and gas exchange. They are coordinated to avoid overproduction of either veins or stomata. In many species, where leaf area is greater at low light, this coordination is primarily achieved through differential cell expansion, resulting in lower stomatal and vein density in larger leaves. This mechanism would, however, create highly inefficient leaves in species in which leaf area is greater at high light. Here we investigate the role of cell expansion and differentiation as regulators of vein and stomatal density in Rheum rhabarbarum, which produces large leaves under high light. METHODS: Rheum rhabarbarum plants were grown under full sunlight and 7% of full sunlight. Leaf area, stomatal density, and vein density were measured from leaves harvested at different intervals. KEY RESULTS: Leaves of R. rhabarbarum expanded at high light were six times larger than leaves expanded at low light, yet vein and stomatal densities were similar. In high light-expanded leaves, minor veins were continuously initiated as the leaves expanded, while an extended period of stomatal initiation, compared to leaves expanded at low light, occurred early in leaf development. CONCLUSIONS: We demonstrate that R. rhabarbarum adjusts the initiation of stomata and minor veins at high light, allowing for the production of larger leaves uncoupled from lower vein and stomatal densities. We also present evidence for an independent control of vein and stomatal initiation, suggesting that this adjustment must involve some unknown developmental mechanism.

Distinct transgenic effects of poplar TDIF genes on vascular development in Arabidopsis.

KEY MESSAGE: Poplar CLE genes encoding TDIF motifs differentially regulate vascular cambial cell division and woody tissue organization in transgenic Arabidopsis. In Arabidopsis, CLE41 and CLE44 genes encode the tracheary element differentiation inhibitory factor (TDIF) peptide, which functions as a non-cell autonomous signal to regulate vascular development, and overexpression of AtCLE41/CLE44 generate similar phenotypic defects. In poplar, there are six CLE genes (PtTDIF1-4 and PtTDIF-like1-2) encoding two TDIF peptides (TDIF and TDIF-like peptide), which exhibit nearly same activities when exogenously applied to Arabidopsis seedlings. In this work, for each TDIF peptide, we chose two poplar CLE genes (PtTDIF2 and 3 for TDIF, and PtTDIF-like1-2 for TDIF-like peptide) to compare their in vivo effects in transgenic Arabidopsis. Our results showed that transgenic Arabidopsis lines overexpressing each individual PtTDIF gene exhibited dramatically distinct phenotypes associated with vascular development, demonstrating that TDIF motif is not the only functional determinant after genetic transformation. Moreover, we revealed that overexpressed poplar TDIFs enhanced the proliferation of (pro)cambial cells only in hypocotyls, but not in inflorescence stems by differentially regulating the transcriptional levels of WOX4 and WOX14 in these two tissues.

Dynamic Roles for Small RNAs and DNA Methylation during Ovule and Fiber Development in Allotetraploid Cotton.

DNA methylation is essential for plant and animal development. In plants, methylation occurs at CG, CHG, and CHH (H = A, C or T) sites via distinct pathways. Cotton is an allotetraploid consisting of two progenitor genomes. Each cotton fiber is a rapidly-elongating cell derived from the ovule epidermis, but the molecular basis for this developmental transition is unknown. Here we analyzed methylome, transcriptome, and small RNAome and revealed distinct changes in CHH methylation during ovule and fiber development. In ovules, CHH hypermethylation in promoters correlated positively with siRNAs, inducing RNA-dependent DNA methylation (RdDM), and up-regulation of ovule-preferred genes. In fibers, the ovule-derived cells generated additional heterochromatic CHH hypermethylation independent of RdDM, which repressed transposable elements (TEs) and nearby genes including fiber-related genes. Furthermore, CHG and CHH methylation in genic regions contributed to homoeolog expression bias in ovules and fibers. Inhibiting DNA methylation using 5-aza-2'-deoxycytidine in cultured ovules has reduced fiber cell number and length, suggesting a potential role for DNA methylation in fiber development. Thus, RdDM-dependent methylation in promoters and RdDM-independent methylation in TEs and nearby genes could act as a double-lock feedback mechanism to mediate gene and TE expression, potentiating the transition from epidermal to fiber cells during ovule and seed development.

Arabidopsis UNHINGED encodes a VPS51 homolog and reveals a role for the GARP complex in leaf shape and vein patterning.

Asymmetric localization of PIN proteins controls directionality of auxin transport and many aspects of plant development. Directionality of PIN1 within the marginal epidermis and the presumptive veins of developing leaf primordia is crucial for establishing leaf vein pattern. One mechanism that controls PIN protein distribution within the cell membranes is endocytosis and subsequent transport to the vacuole for degradation. The Arabidopsis mutant unhinged-1 (unh-1) has simpler leaf venation with distal non-meeting of the secondary veins and fewer higher order veins, a narrower leaf with prominent serrations, and reduced root and shoot growth. We identify UNH as the Arabidopsis vacuolar protein sorting 51 (VPS51) homolog, a member of the Arabidopsis Golgi-associated retrograde protein (GARP) complex, and show that UNH interacts with VPS52, another member of the complex and colocalizes with trans Golgi network and pre-vacuolar complex markers. The GARP complex in yeast and metazoans retrieves vacuolar sorting receptors to the trans-Golgi network and is important in sorting proteins for lysosomal degradation. We show that vacuolar targeting is reduced in unh-1. In the epidermal cells of unh-1 leaf margins, PIN1 expression is expanded. The unh-1 leaf phenotype is partially suppressed by pin1 and cuc2-3 mutations, supporting the idea that the phenotype results from expanded PIN1 expression in the marginal epidermis. Our results suggest that UNH is important for reducing expression of PIN1 within margin cells, possibly by targeting PIN1 to the lytic vacuole.

Physical rupture of the xylem in developing sweet cherry fruit causes progressive decline in xylem sap inflow rate.

MAIN CONCLUSION: Xylem flow is progressively shut down during maturation beginning with minor veins at the stylar end and progressing to major veins and finally to bundles at the stem end. This study investigates the functionality of the xylem vascular system in developing sweet cherry fruit (Prunus avium L.). The tracers acid fuchsin and gadoteric acid were fed to the pedicel of detached fruit. The tracer distribution was studied using light microscopy and magnetic resonance imaging. The vasculature of the sweet cherry comprises five major bundles. Three of these supply the flesh; two enter the pit to supply the ovules. All vascular bundles branch into major and minor veins that interconnect via numerous anastomoses. The flow in the xylem as indexed by the tracer distribution decreases continuously during development. The decrease is first evident at the stylar (distal) end of the fruit during pit hardening and progresses basipetally towards the pedicel (proximal) end of the fruit at maturity. That growth strains are the cause of the decreased conductance is indicated by: elastic strain relaxation after tissue excision, the presence of ruptured vessels in vivo, the presence of intrafascicular cavities, and the absence of tyloses.

OCTOPUS-LIKE 2, a novel player in Arabidopsis root and vascular development, reveals a key role for OCTOPUS family genes in root metaphloem sieve tube differentiation.

Protophloem and metaphloem sieve tubes are essential for transporting carbohydrates and signalling molecules towards sink tissues. OCTOPUS (OPS) was previously identified as an important regulator of protophloem differentiation in Arabidopsis roots. Here, we investigated the role of OCTOPUS-LIKE 2 (OPL2), a gene homologous to OPS. OPL2 expression patterns were analysed, and functional equivalence of OPS and OPL2 was tested. Mutant and double mutant phenotypes were investigated. OPS and OPL2 displayed overlapping expression patterns and a high degree of functional overlap. A mutation in OPL2 revealed redundant functions of OPS and OPL2 in developmental processes in which OPS was known to play a role, notably cotyledon vascular patterning and protophloem development. Moreover, we also uncovered redundant roles for OPS and OPL2 in leaf vascular patterning and, most interestingly, metaphloem sieve tube differentiation. Our results reveal a novel OPS-like protein that, together with OPS, is an important regulator of vascular patterning, root growth and phloem development. OPS and OPL2 are the first genes identified that play a role in metaphloem sieve tube differentiation.

Combined Chlorophyll Fluorescence and Transcriptomic Analysis Identifies the P3/P4 Transition as a Key Stage in Rice Leaf Photosynthetic Development.

Leaves are derived from heterotrophic meristem tissue that, at some point, must make the transition to autotrophy via the initiation of photosynthesis. However, the timing and spatial coordination of the molecular and cellular processes underpinning this switch are poorly characterized. Here, we report on the identification of a specific stage in rice (Oryza sativa) leaf development (P3/P4 transition) when photosynthetic competence is first established. Using a combined physiological and molecular approach, we show that elements of stomatal and vascular differentiation are coordinated with the onset of measurable light absorption for photosynthesis. Moreover, by exploring the response of the system to environmental perturbation, we show that the earliest stages of rice leaf development have significant plasticity with respect to elements of cellular differentiation of relevance for mature leaf photosynthetic performance. Finally, by performing an RNA sequencing analysis targeted at the early stages of rice leaf development, we uncover a palette of genes whose expression likely underpins the acquisition of photosynthetic capability. Our results identify the P3/P4 transition as a highly dynamic stage in rice leaf development when several processes for the initiation of photosynthetic competence are coordinated. As well as identifying gene targets for future manipulation of rice leaf structure/function, our data highlight a developmental window during which such manipulations are likely to be most effective.

Myrosin cell development is regulated by endocytosis machinery and PIN1 polarity in leaf primordia of Arabidopsis thaliana.

Myrosin cells, which accumulate myrosinase to produce toxic compounds when they are ruptured by herbivores, form specifically along leaf veins in Arabidopsis thaliana. However, the mechanism underlying this pattern formation is unknown. Here, we show that myrosin cell development requires the endocytosis-mediated polar localization of the auxin-efflux carrier PIN1 in leaf primordia. Defects in the endocytic/vacuolar SNAREs (syp22 and syp22 vti11) enhanced myrosin cell development. The syp22 phenotype was rescued by expressing SYP22 under the control of the PIN1 promoter. Additionally, myrosin cell development was enhanced either by lacking the activator of endocytic/vacuolar RAB5 GTPase (VPS9A) or by PIN1 promoter-driven expression of a dominant-negative form of RAB5 GTPase (ARA7). By contrast, myrosin cell development was not affected by deficiencies of vacuolar trafficking factors, including the vacuolar sorting receptor VSR1 and the retromer components VPS29 and VPS35, suggesting that endocytic pathway rather than vacuolar trafficking pathway is important for myrosin cell development. The phosphomimic PIN1 variant (PIN1-Asp), which is unable to be polarized, caused myrosin cells to form not only along leaf vein but also in the intervein leaf area. We propose that Brassicales plants might arrange myrosin cells near vascular cells in order to protect the flux of nutrients and water via polar PIN1 localization.

Mechanical Stress Induces Remodeling of Vascular Networks in Growing Leaves.

Differentiation into well-defined patterns and tissue growth are recognized as key processes in organismal development. However, it is unclear whether patterns are passively, homogeneously dilated by growth or whether they remodel during tissue expansion. Leaf vascular networks are well-fitted to investigate this issue, since leaves are approximately two-dimensional and grow manyfold in size. Here we study experimentally and computationally how vein patterns affect growth. We first model the growing vasculature as a network of viscoelastic rods and consider its response to external mechanical stress. We use the so-called texture tensor to quantify the local network geometry and reveal that growth is heterogeneous, resembling non-affine deformations in composite materials. We then apply mechanical forces to growing leaves after veins have differentiated, which respond by anisotropic growth and reorientation of the network in the direction of external stress. External mechanical stress appears to make growth more homogeneous, in contrast with the model with viscoelastic rods. However, we reconcile the model with experimental data by incorporating randomness in rod thickness and a threshold in the rod growth law, making the rods viscoelastoplastic. Altogether, we show that the higher stiffness of veins leads to their reorientation along external forces, along with a reduction in growth heterogeneity. This process may lead to the reinforcement of leaves against mechanical stress. More generally, our work contributes to a framework whereby growth and patterns are coordinated through the differences in mechanical properties between cell types.

Integration of hormonal signaling networks and mobile microRNAs is required for vascular patterning in Arabidopsis roots.

As multicellular organisms grow, positional information is continually needed to regulate the pattern in which cells are arranged. In the Arabidopsis root, most cell types are organized in a radially symmetric pattern; however, a symmetry-breaking event generates bisymmetric auxin and cytokinin signaling domains in the stele. Bidirectional cross-talk between the stele and the surrounding tissues involving a mobile transcription factor, SHORT ROOT (SHR), and mobile microRNA species also determines vascular pattern, but it is currently unclear how these signals integrate. We use a multicellular model to determine a minimal set of components necessary for maintaining a stable vascular pattern. Simulations perturbing the signaling network show that, in addition to the mutually inhibitory interaction between auxin and cytokinin, signaling through SHR, microRNA165/6, and PHABULOSA is required to maintain a stable bisymmetric pattern. We have verified this prediction by observing loss of bisymmetry in shr mutants. The model reveals the importance of several features of the network, namely the mutual degradation of microRNA165/6 and PHABULOSA and the existence of an additional negative regulator of cytokinin signaling. These components form a plausible mechanism capable of patterning vascular tissues in the absence of positional inputs provided by the transport of hormones from the shoot.

MicroRNA857 Is Involved in the Regulation of Secondary Growth of Vascular Tissues in Arabidopsis.

MicroRNAs (miRNAs) are endogenous small RNAs that repress target gene expression posttranscriptionally, and are critically involved in various developmental processes and responses to environmental stresses in eukaryotes. MiRNA857 is not widely distributed in plants and is encoded by a single gene, AtMIR857, in Arabidopsis (Arabidopsis thaliana). The functions of miR857 and its mechanisms in regulating plant growth and development are still unclear. Here, by means of genetic analysis coupled with cytological studies, we investigated the expression pattern and regulation mechanism of miR857 and its biological functions in Arabidopsis development. We found that miR857 regulates its target gene, Arabidopsis LACCASE7, at the transcriptional level, thereby reducing laccase activity. Using stimulated Raman scattering and x-ray microtomography three-dimensional analyses, we showed that miR857 was involved in the regulation of lignin content and consequently morphogenesis of the secondary xylem. In addition, miR857 was activated by SQUAMOSA PROMOTER BINDING PROTEIN-LIKE7 in response to low copper conditions. Collectively, these findings demonstrate the role of miR857 in the regulation of secondary growth of vascular tissues in Arabidopsis and reveal a unique control mechanism for secondary growth based on the miR857 expression in response to copper deficiency.