Plant vascular development: mechanisms and environmental regulation

Uncovering differences in cadmium accumulation capacity of different Ipomoea aquatica cultivars at the level of root cell types

Water spinach (Ipomoea aquatica) can accumulate cadmium (Cd) even in mildly contaminated soils, but the roles of its root tip cell types in Cd fixation and transport remain unclear. Single-cell RNA sequencing revealed nine cell types in root tips in both the QLQ cultivar (low Cd accumulation) and the T308 cultivar (high Cd accumulation). High expression of LAC2 and PER72 in the QLQ epidermis was associated with enhanced lignin deposition, which may facilitate fixation of Cd and reduce its translocation to the shoot. In T308, PER72 and hormone-related genes (PIN1, ARF8, IAA17, and EIN3) were upregulated, which was hypothesized to promote xylem and trichoblast development, potentially facilitating Cd uptake and transport. Fluorescence assays suggested that the higher pectin demethylation and lignin content in QLQ may limit Cd movement, whereas the more developed tissues in T308 may contribute to increased Cd accumulation in the shoots. These findings clarify the mechanisms by which Cd accumulates in water spinach and offer insights into mitigating Cd uptake in crops.

Divergent evolution of a thermospermine-dependent regulatory pathway in land plants

Plants adapted to life on land by developing diverse anatomical features across lineages. The molecular basis of these innovations often involves the emergence of new genes or establishing new connections between conserved elements, though evidence for evolutionary genetic circuit rewiring remains scarce. Here, we show that the thermospermine-dependent pathway regulating vascular cell proliferation in Arabidopsis thaliana operates as two distinct modules with different functions in the bryophyte Marchantia polymorpha. One module controls dichotomous branching at meristems, while the other one modulates gemmae and rhizoid production in the thallus. Heterologous assays and comparative expression analyses reveal that the molecular links between these modules, forming a unified circuit in vascular plants, emerged early in tracheophyte evolution. Our results illustrate how the thermospermine-dependent circuit elements followed two divergent evolutionary trajectories in bryophytes and tracheophytes, eventually influencing distinct developmental processes.

Function of WAKs in Regulating Cell Wall Development and Responses to Abiotic Stress

Receptor-like kinases (RLKs) are instrumental in regulating plant cell surface sensing and vascular tissue differentiation. Wall-associated kinases (WAKs) are a unique group of RLKs that have been identified as key cell wall integrity (CWI) sensors. WAK signaling is suggested to be activated during growth in response to cell expansion or when the cell wall is damaged, for example, during pathogen attack. WAKs are proposed to interact with pectins or pectin fragments that are enriched in primary walls. Secondary walls have low levels of pectins, yet recent studies have shown important functions of WAKs during secondary wall development. Several wak mutants show defects in secondary wall thickening of the xylem vessels and fibers or the development of vascular bundles. This review will discuss the recent advances in our understanding of WAK functions during plant development and responses to abiotic stresses and the regulation of vascular tissue secondary wall development.

Transcriptome sequencing-based analysis of primary vein development in Betula pendula 'Dalecarlica'

Keymessage The study revealed the major biological processes occurred at three developmental stages and identified candidate genes involved in primary vein development of birch plants. Vascular tissues usually mirror the surrounding leaf shape and its development plays a fundamental role in plant performance. However, the information of vascular development in birch trees, especially primary vein development, remains unclear. Therefore, we conducted the anatomical observation on primary veins from leaves at different development stages in Betula pendula 'Dalecarlica'. With the development of primary vein, dynamic changes in mechanical tissue thickness and primary vein diameter were consistent with each other, and the sum of phloem, xylem and cambium thickness was significantly varied. Transcriptome analysis indicated that primary vein development could be divided into three stages, namely Stage I, II and III, which were in aggreement with anatomical observation. Expression of marker genes associated with vascular tissues revealed that pro-vasculature development occurred at Stage I and II, and phloem development occurred at Stage III. GO enrichment analysis of differentially expressed genes (DEGs) showed that shared DEGs at Stage II were mainly engaged in cell division and cell cycle, and shared DEGs at Stage III were mainly engaged in phosphorylation. Decreased cell division and cell cycle as well as activation of lignin biosynthesis might contribute to primary vein development. Combining phenotypic traits, we performed weighted gene co-expression network analysis and identified a cytochrome P450 84A (CYP84A) family gene (BpF5H1). Based on analyses of gene families, expression patterns and yeast-two hybrid assay results, we proposed a potential electron transfer pathway involving BpF5H1 and three cytochrome b5 proteins during primary vein development in B. pendula 'Dalecarlica'. These results could shed some light on which biological processes occurred during primary vein formation and provide some valuable clues for vascular morphogenesis in woody plants.

Identification of a potential homeodomain-like gene governing leaf size and venation architecture in birch

Leaf vein, an essential part of leaf architecture, plays significant roles in shaping the proper leaf size. To date, the molecular mechanisms governing leaf development including leaf venation patterning remains poorly understood in birch. Here, we performed the genome-wide identification of homeodomain-like (HD-like) superfamily genes using phylogenetic analysis and revealed the functional role of a potential HD-like gene in leaf growth and development using transgenic technology and transcriptomic sequencing. A total of 267 HD-like genes were identified based on Arabidopsis HD-containing transcription factors, which were members of KNOTTED1-like homeobox (KNOX) family, BELL1-like homeobox (BLH) family, Zinc finger-HD (ZHD) family, HD-leucine zipper (HD-Zip) family, Golden2, ARR-B, Psr1 (GARP) family, WUSCHEL-related homeobox (WOX) family, and Myeloblastosis (MYB) and MYB-like family. Further, 41 HD-like genes showing co-expression with marker genes related to leaf vascular tissues exhibited differential expression during primary vein development. Among them, a potential HD-like gene (BpPHD4) of GARP family served as a negative factor in governing leaf size and venation patterning. Compared to non-transgenic plants, BpPHD4 repression transgenic plants showed increased leaf length, leaf width, leaf area, leaf thickness, spongy tissue thickness, stomata number, epidermal cell size, primary vein length, the distance between the secondary veins, and primary vein diameter, which was opposite to those of BpPHD4 overexpression transgenic plants. Meanwhile, reduced expression levels of BpPHD4 could remarkably promote phloem tissue development. Transcriptome analysis of BpPHD4 overexpression transgenic plants showed two candidate genes (Bpev01.c0518.g0018 and Bpev01.c2797.g0002) probably regulated by BpPHD4. To conclude, our findings contribute to a better understanding of HD-like superfamily genes and unravel the role of a potential HD-like gene in genetically controlling leaf size and venation patterning in birch, which provides clues to genetic improvement of woody plants with diverse geometric and topological properties of leaf vascular network.

Role of ROS signaling in the plant defense against vascular pathogens

Reactive oxygen species (ROS) is a collective term for highly reactive oxygen derivatives, including singlet oxygen, hydroxyl radicals, superoxide anions, and hydrogen peroxide. In plants, ROS are produced in apoplasts, chloroplasts, mitochondria, and peroxisomes. Although ROS are toxic when their levels exceed a certain threshold, low-concentration ROS can serve as essential signaling molecules for plant growth and development, as well as plant responses to abiotic and biotic stresses. Various aspects of the role of ROS in plants have been discussed in previous reviews. In this review, we first summarize recent progress in the regulatory mechanisms of apoplastic ROS signaling and then propose its potential roles in plant defense against vascular pathogens to provide new ideas for the prevention and control of vascular diseases.

Evaluation of novel promoters for vascular tissue-specific gene expression in Populus

Due to the extended generation cycle of trees, the breeding process for forest trees tends to be time-consuming. Genetic engineering has emerged as a viable approach to expedite the genetic breeding of forest trees. However, current genetic engineering techniques employed in forest trees often utilize continuous expression promoters such as CaMV 35S, which may result in unintended consequences by introducing genes into non-target tissues. Therefore, it is imperative to develop specific promoters for forest trees to facilitate targeted and precise design and breeding. In this study, we utilized single-cell RNA-Seq data and co-expression network analysis during wood formation to identify three vascular tissue-specific genes in poplar, PP2-A10, PXY, and VNS07, which are expressed in the phloem, cambium/expanding xylem, and mature xylem, respectively. Subsequently, we cloned the promoters of these three genes from '84K' poplar and constructed them into a vector containing the eyGFPuv visual selection marker, along with the 35S mini enhancer to drive GUS gene expression. Transgenic poplars expressing the ProPagPP2-A10::GUS, ProPagPXY::GUS, and ProPagVNS07::GUS constructs were obtained. To further elucidate the tissue specificity of these promoters, we employed qPCR, histochemical staining, and GUS enzyme activity. Our findings not only establish a solid foundation for the future utilization of these promoters to precisely express of specific functional genes in stems but also provide a novel perspective for the modular breeding of forest trees.

WALL-ASSOCIATED KINASE Like 14 regulates vascular tissue development in Arabidopsis and tomato

Initiation of plant vascular tissue is regulated by transcriptional networks during development and in response to environmental stimuli. The WALL-ASSOCIATED KINASES (WAKs) and WAK-likes (WAKLs) are cell surface receptors involved in cell expansion and defence in cells with primary walls, yet their roles in regulation of vascular tissue development that contain secondary walls remains unclear. In this study, we showed tomato (Solanum lycopersicum) SlWAKL2 and the orthologous gene in Arabidopsis thaliana, AtWAKL14, were specifically expressed in vascular tissues. SlWAKL2-RNAi tomato plants displayed smaller fruit size with fewer seeds and vascular bundles compared to wild-type (WT) and over-expression (OE) lines. RNA-seq data showed that SlWAKL2-RNAi fruits down-regulated transcript levels of genes related to vascular tissue development compared to WT. Histological analysis showed T-DNA insertion mutant wakl14-1 had reduced plant stem length with fewer number of xylem vessels and interfascicular fibres compared to WT, with no significant differences in cellulose and lignin content. Mutant wakl14-1 also showed reduced number of vascular bundles in fruit. A proWAKL14::mCherry-WAKL14 fusion protein was able to complement wakl14-1 phenotypes and showed mCherry-WAKL14 associated with the plasma membrane. In vitro binding assays showed both SlWAKL2 and AtWAKL14 can interact with pectin and oligogalacturonides. Our results reveal novel roles of WAKLs in regulating vascular tissue development.

Identification of MeC3HDZ1/MeCNA as a potential regulator of cassava storage root development

The storage root (SR) of cassava is the main staple food in sub-Saharan Africa, where it feeds over 500 million people. However, little is known about the genetic and molecular regulation underlying its development. Unraveling such regulation would pave the way for biotechnology approaches aimed at enhancing cassava productivity. Anatomical studies indicate that SR development relies on the massive accumulation of xylem parenchyma, a cell-type derived from the vascular cambium. The C3HDZ family of transcription factors regulate cambial cells proliferation and xylem differentiation in Arabidopsis and other species. We thus aimed at identifying C3HDZ proteins in cassava and determining whether any of them shows preferential activity in the SR cambium and/or xylem. Using phylogeny and synteny studies, we identified eight C3HDZ proteins in cassava, namely MeCH3DZ1-8. We observed that MeC3HDZ1 is the MeC3HDZ gene displaying the highest expression in SR and that, within that organ, the gene also shows high expression in cambium and xylem. In-silico analyses revealed the existence of a number of potential C3HDZ targets displaying significant preferential expression in the SR. Subsequent Y1H analyses proved that MeC3HDZ1 can bind canonical C3HDZ binding sites, present in the promoters of these targets. Transactivation assays demonstrated that MeC3HDZ1 can regulate the expression of genes downstream of promoters harboring such binding sites, thereby demonstrating that MeC3HDZ1 has C3HDZ transcription factor activity. We conclude that MeC3HDZ1 may be a key factor for the regulation of storage root development in cassava, holding thus great promise for future biotechnology applications.

Monitoring Xylem Transport in Arabidopsis thaliana Seedlings Using Fluorescent Dyes

Fluorescent dyes are often used to observe transport mechanisms in plant vascular tissues. However, it has been technically challenging to apply fluorescent dyes on roots to monitor xylem transport in vivo. Here, we present a fast, noninvasive, and high-throughput protocol to monitor xylem transport in seedlings. Using the fluorescent dyes 5(6)-carboxyfluorescein diacetate (CFDA) and Rhodamine WT, we were able to observe xylem transport on a cellular level in Arabidopsis thaliana roots. We describe how to apply these dyes on primary roots of young seedlings, how to monitor root-to-shoot xylem transport, and how to measure xylem transport velocity in roots. Moreover, we show that our protocol can also be applied to lateral roots and grafted seedlings to assess xylem (re)connection. Altogether, these techniques are useful for investigating xylem functionality in diverse experimental setups.

Game of thrones among AUXIN RESPONSE FACTORs-over 30 years of MONOPTEROS research

For many years, research has been carried out with the aim of understanding the mechanism of auxin action, its biosynthesis, catabolism, perception, and transport. One central interest is the auxin-dependent gene expression regulation mechanism involving AUXIN RESPONSE FACTOR (ARF) transcription factors and their repressors, the AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA) proteins. Numerous studies have been focused on MONOPTEROS (MP)/ARF5, an activator of auxin-dependent gene expression with a crucial impact on plant development. This review summarizes over 30 years of research on MP/ARF5. We indicate the available analytical tools to study MP/ARF5 and point out the known mechanism of MP/ARF5-dependent regulation of gene expression during various developmental processes, namely embryogenesis, leaf formation, vascularization, and shoot and root meristem formation. However, many questions remain about the auxin dose-dependent regulation of gene transcription by MP/ARF5 and its isoforms in plant cells, the composition of the MP/ARF5 protein complex, and, finally, all the genes under its direct control. In addition, information on post-translational modifications of MP/ARF5 protein is marginal, and knowledge about their consequences on MP/ARF5 function is limited. Moreover, the epigenetic factors and other regulators that act upstream of MP/ARF5 are poorly understood. Their identification will be a challenge in the coming years.

Connecting emerging with existing vasculature above and below ground

The vascular system was essential for plants to colonize land by facilitating the transport of water, nutrients, and minerals throughout the body. Our current knowledge on the molecular-genetic control of vascular tissue specification and differentiation is mostly based on studies in the Arabidopsis primary root. To what degree these regulatory mechanisms in the root meristem can be extrapolated to vascular tissue development in other organs is a question of great interest. In this review, we discuss the most recent progress on cotyledon vein formation, with a focus on polar auxin transport-dependent and -independent mechanisms. We also provide an overview of vasculature formation in postembryonic organs, namely lateral roots, which is more complex than anticipated as several tissues of the parent root must act in a spatio-temporally coordinated manner.

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.

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

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

Comprehensive Time-Course Transcriptome Reveals the Crucial Biological Pathways Involved in the Seasonal Branch Growth in Siberian Elm (Ulmus pumila)

Timber, the most prevalent organic material on this planet, is the result of a secondary xylem emerging from vascular cambium. Yet, the intricate processes governing its seasonal generation are largely a mystery. To better understand the cyclic growth of vascular tissues in elm, we undertook an extensive study examining the anatomy, physiology, and genetic expressions in Ulmus pumila. We chose three robust 15-year-old elm trees for our study. The cultivars used in this study were collected from the Inner Mongolia Autonomous Region in China and nurtured in the tree farm of Shandong Normal University. Monthly samples of 2-year-old elm branches were taken from the tree from February to September. Marked seasonal shifts in elm branch vascular tissues were observed by phenotypic observation: In February, the cambium of the branch emerged from dormancy, spurring growth. By May, elms began generating secondary xylem, or latewood, recognized by its tiny pores and dense cell structure. From June to August, there was a marked increase in the thickness of the secondary xylem. Transcriptome sequencing provides a potential molecular mechanism for the thickening of elm branches and their response to stress. In February, the tree enhanced its genetic responses to cold and drought stress. The amplified expression of CDKB, CYCB, WOX4, and ARF5 in the months of February and March reinforced their essential role in the development of the vascular cambium in elm. Starting in May, the elm deployed carbohydrates as a carbon resource to synthesize the abundant cellulose and lignin necessary for the formation of the secondary wall. Major genes participating in cellulose (SUC and CESA homologs), xylan (UGD, UXS, IRX9, IRX10, and IRX14), and lignin (PAL, C4H, 4CL, HCT, C3H, COMT, and CAD) biosynthetic pathways for secondary wall formation were up-regulated by May or/and June. In conclusion, our findings provided a foundation for an in-depth exploration of the molecular processes dictating the seasonal growth of elm timber.

The genome of Acorus deciphers insights into early monocot evolution

Acorales is the sister lineage to all the other extant monocot plants. Genomic resource enhancement of this genus can help to reveal early monocot genomic architecture and evolution. Here, we assemble the genome of Acorus gramineus and reveal that it has ~45% fewer genes than the majority of monocots, although they have similar genome size. Phylogenetic analyses based on both chloroplast and nuclear genes consistently support that A. gramineus is the sister to the remaining monocots. In addition, we assemble a 2.2 Mb mitochondrial genome and observe many genes exhibit higher mutation rates than that of most angiosperms, which could be the reason leading to the controversies of nuclear genes- and mitochondrial genes-based phylogenetic trees existing in the literature. Further, Acorales did not experience tau (τ) whole-genome duplication, unlike majority of monocot clades, and no large-scale gene expansion is observed. Moreover, we identify gene contractions and expansions likely linking to plant architecture, stress resistance, light harvesting, and essential oil metabolism. These findings shed light on the evolution of early monocots and genomic footprints of wetland plant adaptations.

Comparative Microscopic, Transcriptome and IAA Content Analyses Reveal the Stem Growth Variations in Two Cultivars Ilex verticillata

Ilex verticillata is not only an excellent ornamental tree species for courtyards, but it is also a popular bonsai tree. 'Oosterwijk' and 'Red sprite' are two varieties of Ilex verticillata. The former has a long stem with few branches, while the latter has a short stem. In order to explain the stem growth differences between the two cultivars 'Oosterwijk' and 'Red sprite', determination of the microstructure, transcriptome sequence and IAA content was carried out. The results showed that the xylem thickness, vessel area and vessel number of 'Oosterwijk' were larger than in 'Red sprite'. In addition, our analysis revealed that the differentially expressed genes which were enriched in phenylpropanoid biosynthesis; phenylalanine metabolism and phenylalanine, tyrosine and tryptophan biosynthesis in the black and tan modules of the two varieties. We found that AST, HCT and bHLH 94 may be key genes in the formation of shoot difference. Moreover, we found that the IAA content and auxin-related DEGs GH3.6, GH3, ATRP5, IAA27, SAUR36-like, GH3.6-like and AIP 10A5-like may play important roles in the formation of shoot differences. In summary, these results indicated that stem growth variations of 'Oosterwijk' and 'Red sprite' were associated with DEGs related to phenylpropanoid biosynthesis, phenylalanine metabolism and phenylalanine, tyrosine and tryptophan biosynthesis, as well as auxin content and DEGs related to the auxin signaling pathway.

Grafting in plants: recent discoveries and new applications

Grafting is a traditional horticultural technique that makes use of plant wound healing mechanisms to join two different genotypes together to form one plant. In many agricultural systems, grafting with rootstocks controls the vigour of the scion and/or provides tolerance to deleterious soil conditions such as the presence of soil pests or pathogens or limited or excessive water or mineral nutrient supply. Much of our knowledge about the limits to grafting different genotypes together comes from empirical knowledge of horticulturalists. Until recently, researchers believed that grafting monocotyledonous plants was impossible, because they lack a vascular cambium, and that graft compatibility between different scion/rootstock combinations was restricted to closely related genotypes. Recent studies have overturned these ideas and open up the possibility of new research directions and applications for grafting in agriculture. The objective of this review is to describe and assess these recent advances in the field of grafting and, in particular, the molecular mechanisms underlining graft union formation and graft compatibility between different genotypes. The challenges of characterizing the different stages of graft union formation and phenotyping graft compatibility are examined.

Xylem development and phloem conductivity in relation to the stem mechanical strength of Paeonia lactiflora

The quality of cut Paeonia lactiflora flowers is limited by their low stem mechanical strength, but the underlying mechanism of this low strength is poorly understood. In this study, two P. lactiflora cultivars with distinct stem mechanical strengths (Chui Touhong with low stem mechanical strength and Da Fugui with high stem mechanical strength) were used as test materials. The xylem development was examined at the cellular level, and the phloem conductivity was analyzed by evaluating phloem geometry. The results showed that the secondary cell wall formation of the xylem of Chui Touhong was affected primarily in fiber cells but was affected little in vessel cells. The formation of the secondary cell walls in the xylem fiber cells of Chui Touhong was delayed, resulting in longer and thinner fiber cells with a lack of cellulose and S-lignin in the secondary cell walls. Moreover, the phloem conductivity of Chui Touhong was lower than that of Da Fugui, and more callose was accumulated in the lateral walls of the phloem sieve elements of Chui Touhong. Consequently, the delayed deposition of the secondary cell walls of the xylem fiber cells was the main factor leading to the low stem mechanical strength of Chui Touhong, and the low stem mechanical strength was closely related to the low conductivity of sieve tubes and extensive callose accumulation in the phloem. These findings provide a new perspective on enhancing P. lactiflora stem mechanical strength by targeting single cell level, and lay the foundation for future works on the correlation between phloem long-distance transport and stem mechanical strength.

Environmental Regulation in Evolution and Governance Strategies

Environmental regulation faces theoretical and practical research challenges on global scale, due to differences such as language and policy environment. Research reflects the beneficial exploration of scholars, policymakers, and enterprises in the cognitive and behavioural norms of economic development, environmental protection, and social governance. This study demonstrated that the relevant research is motivated by the enaction of environmental regulations and discussed the influence of relevant research on the evolution of environmental regulations. Under the assumption that environmental regulations are consistent with related research, this study used 9185 papers in the field of environmental regulation from 2000 to 2019 to construct a research network panorama and explored the development and revelation of environmental regulation. The results revealed that environmental regulation research is motivated by the introduction of policies, and regulation is influenced by research evolution with the path of 'competitiveness-technological change-innovation.' In addition, after the twenty-first session of the Conference of the Parties (COP21), the number of studies increased significantly, with the USA in the leading position in the research field. Furthermore, governance strategies were inspired by real events, including the increasing concern with climate change, regional research preferences, and the promotion of information disclosure. These results suggest that environmental governors should focus on climate change, localisation, and mechanisms of information disclosure.

PtrVCS2 Regulates Drought Resistance by Changing Vessel Morphology and Stomatal Closure in Populus trichocarpa

Drought has severe effects on plant growth, forest productivity, and survival throughout the world. Understanding the molecular regulation of drought resistance in forest trees can enable effective strategic engineering of novel drought-resistant genotypes of tree species. In this study, we identified a gene, PtrVCS2, encoding a zinc finger (ZF) protein of the ZF-homeodomain transcription factor in Populus trichocarpa (Black Cottonwood) Torr. & A. Gray. ex Hook. Overexpression of PtrVCS2 (OE-PtrVCS2) in P. trichocarpa resulted in reduced growth, a higher proportion of smaller stem vessels, and strong drought-resistance phenotypes. Stomatal movement experiments revealed that the OE-PtrVCS2 transgenics showed lower stomata apertures than wild-type plants under drought conditions. RNA-seq analysis of the OE-PtrVCS2 transgenics showed that PtrVCS2 regulates the expression of multiple genes involved in regulation of stomatal opening and closing, particularly the PtrSULTR3;1-1 gene, and several genes related to cell wall biosynthesis, such as PtrFLA11-12 and PtrPR3-3. Moreover, we found that the water use efficiency of the OE-PtrVCS2 transgenic plants was consistently higher than that of wild type plants when subjected to chronic drought stress. Taken together, our results suggest that PtrVCS2 plays a positive role in improving drought adaptability and resistance in P. trichocarpa.

Molecular mechanisms of vascular tissue patterning in Arabidopsis thaliana L. roots

A vascular system in plants is a product of aromorphosis that enabled them to colonize land because it delivers water, mineral and organic compounds to plant organs and provides effective communications between organs and mechanical support. Vascular system development is a common object of fundamental research in plant development biology. In the model plant Arabidopsis thaliana, early stages of vascular tissue formation in the root are a bright example of the self-organization of a bisymmetric (having two planes of symmetry) pattern of hormone distribution, which determines vascular cell fates. In the root, vascular tissue development comprises four stages: (1) specification of progenitor cells for the provascular meristem in early embryonic stages, (2) the growth and patterning of the embryo provascular meristem, (3) postembryonic maintenance of the cell identity in the vascular tissue initials within the root apical meristem, and (4) differentiation of their descendants. Although the anatomical details of A. thaliana root vasculature development have long been known and described in detail, our knowledge of the underlying molecular and genetic mechanisms remains limited. In recent years, several important advances have been made, shedding light on the regulation of the earliest events in provascular cells specification. In this review, we summarize the latest data on the molecular and genetic mechanisms of vascular tissue patterning in A. thaliana root. The first part of the review describes the root vasculature ontogeny, and the second reconstructs the sequence of regulatory events that underlie this histogenesis and determine the development of the progenitors of the vascular initials in the embryo and organization of vascular initials in the seedling root.

Transcription factor NTL9 negatively regulates Arabidopsis vascular cambium development during stem secondary growth

In plant stems, secondary vascular development is established through the differentiation of cylindrical vascular cambium, producing secondary xylem (wood) and phloem (bast), which have economic importance. However, there is a dearth of knowledge on the genetic mechanism underlying this process. NAC with Transmembrane Motif 1-like transcription factor 9 (NTL9) plays a central role in abiotic and immune signaling responses. Here, we investigated the role of NTL9 in vascular cambium development in Arabidopsis (Arabidopsis thaliana) inflorescence stems by identifying and characterizing an Arabidopsis phloem circular-timing (pct) mutant. The pct mutant exhibited enhanced vascular cambium formation following secondary phloem production. In the pct mutant, although normal organization in vascular bundles was maintained, vascular cambium differentiation occurred at an early stage of stem development, which was associated with increased expression of cambium-/phloem-related genes and enhanced cambium activity. The pct mutant stem phenotype was caused by a recessive frameshift mutation that disrupts the transmembrane (TM) domain of NTL9. Our results indicate that NTL9 functions as a negative regulator of cambial activity and has a suppressive role in developmental transition to the secondary growth phase in stem vasculature, which is necessary for its precise TM domain-mediated regulation.

The Regulation of Xylem Development by Transcription Factors and Their Upstream MicroRNAs

Xylem, as a unique organizational structure of vascular plants, bears water transport and supports functions necessary for plant survival. Notably, secondary xylem in the stem (i.e., wood) also has important economic and ecological value. In view of this, the regulation of xylem development has been widely concerned. In recent years, studies on model plants Arabidopsis and poplar have shown that transcription factors play important regulatory roles in various processes of xylem development, including the directional differentiation of procambium and cambium into xylem, xylem arrangement patterns, secondary cell wall formation and programmed cell death. This review focuses on the regulatory roles of widely and thoroughly studied HD-ZIP, MYB and NAC transcription factor gene families in xylem development, and it also pays attention to the regulation of their upstream microRNAs. In addition, the existing questions in the research and future research directions are prospected.

Genetic and molecular factors in determining grain number per panicle of rice

It was suggested that the most effective way to improve rice grain yield is to increase the grain number per panicle (GN) through the breeding practice in recent decades. GN is a representative quantitative trait affected by multiple genetic and environmental factors. Understanding the mechanisms controlling GN has become an important research field in rice biotechnology and breeding. The regulation of rice GN is coordinately controlled by panicle architecture and branch differentiation, and many GN-associated genes showed pleiotropic effect in regulating tillering, grain size, flowering time, and other domestication-related traits. It is also revealed that GN determination is closely related to vascular development and the metabolism of some phytohormones. In this review, we summarize the recent findings in rice GN determination and discuss the genetic and molecular mechanisms of GN regulators.

A genetic framework for proximal secondary vein branching in the Arabidopsis thaliana embryo

Over time, plants have evolved flexible self-organizing patterning mechanisms to adapt tissue functionality for continuous organ growth. An example of this process is the multicellular organization of cells into a vascular network in foliar organs. An important, yet poorly understood component of this process is secondary vein branching, a mechanism employed to extend vascular tissues throughout the cotyledon surface. Here, we uncover two distinct branching mechanisms during embryogenesis by analyzing the discontinuous vein network of the double mutant cotyledon vascular pattern 2 (cvp2) cvp2-like 1 (cvl1). Similar to wild-type embryos, distal veins in cvp2 cvl1 embryos arise from the bifurcation of cell files contained in the midvein, whereas proximal branching is absent in this mutant. Restoration of this process can be achieved by increasing OCTOPUS dosage as well as by silencing RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2) expression. Although RPK2-dependent rescue of cvp2 cvl1 is auxin- and CLE peptide-independent, distal branching involves polar auxin transport and follows a distinct regulatory mechanism. Our work defines a genetic network that confers plasticity to Arabidopsis embryos to spatially adapt vascular tissues to organ growth.

The warfare for plant highway: vascular plant-microbe interaction pinpoints lignin

Plant vascular pathogens are one kind of destructive pathogens in agricultural production. However, mechanisms behind the vascular pathogen-recognition and the subsequent defense responses of plants are not well known. A recent pioneering study on plant vascular immunity discovered a conserved MKP1-MPK-MYB signaling cascade that activates lignin biosynthesis in vascular tissues to confer vascular resistance in both monocot rice and the dicot Arabidopsis. The breakthrough provides a novel view on plant immunity to vascular pathogens and offers a potential strategy for the future breeding of disease-resistant crops.

Storage roots

Vanderschuren and Agusti introduce plant storage roots.

Comparative Transcriptome Analysis Reveals Candidate Genes and Pathways for Potential Branch Growth in Elm (Ulmus pumila) Cultivars

Wood plays a vital role in human life. It is important to study the thickening mechanism of tree branches and explore the mechanism of wood formation. Elm (Ulmus pumila) is a strong essential wood, and it is widely used in cabinets, sculptures, and ship making. In the present study, phenotypic and comparative transcriptomic analyses were performed in U. pumila fast- (UGu17 and UZuantian) and slow-growing cultivars (U81-07 and U82-39). Phenotypic observation showed that the thickness of secondary xylem of 2-year-old fast-growing branches was greater compared with slow-growing cultivars. A total of 9367 (up = 4363, down = 5004), 7159 (3413/3746), 7436 (3566/3870), and 5707 (2719/2988) differentially expressed genes (DEGs) were identified between fast- and slow-growing cultivars. Moreover, GO and KEGG enrichment analyses predicted that many pathways were involved in vascular development and transcriptional regulation in elm, such as "plant-type secondary cell wall biogenesis", "cell wall thickening", and "phenylpropanoid biosynthesis". NAC domain transcriptional factors (TFs) and their master regulators (VND1/MYB26), cellulose synthase catalytic subunits (CESAs) (such as IRX5/IRX3/IRX1), xylan synthesis, and secondary wall thickness (such as IRX9/IRX10/IRX8) were supposed to function in the thickening mechanism of elm branches. Our results indicated that the general phenylpropanoid pathway (such as PAL/C4H/4CL) and lignin metabolism (such as HCL/CSE/CCoAOMT/CCR/F5H) had vital functions in the growth of elm branches. Our transcriptome data were consistent with molecular results for branch thickening in elm cultivars.

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.

Wood Formation under Severe Drought Invokes Adjustment of the Hormonal and Transcriptional Landscape in Poplar

Drought is a severe environmental stress that exerts negative effects on plant growth. In trees, drought leads to reduced secondary growth and altered wood anatomy. The mechanisms underlying wood stress adaptation are not well understood. Here, we investigated the physiological, anatomical, hormonal, and transcriptional responses of poplar to strong drought. Drought-stressed xylem was characterized by higher vessel frequencies, smaller vessel lumina, and thicker secondary fiber cell walls. These changes were accompanied by strong increases in abscisic acid (ABA) and antagonistic changes in salicylic acid in wood. Transcriptional evidence supported ABA biosynthesis and signaling in wood. Since ABA signaling activates the fiber-thickening factor NST1, we expected upregulation of the secondary cell wall (SCW) cascade under stress. By contrast, transcription factors and biosynthesis genes for SCW formation were down-regulated, whereas a small set of cellulose synthase-like genes and a huge array of genes involved in cell wall modification were up-regulated in drought-stressed wood. Therefore, we suggest that ABA signaling monitors normal SCW biosynthesis and that drought causes a switch from normal to "stress wood" formation recruiting a dedicated set of genes for cell wall biosynthesis and remodeling. This proposition implies that drought-induced changes in cell wall properties underlie regulatory mechanisms distinct from those of normal wood.

How roots and shoots communicate through stressful times

When plants face an environmental stress such as water deficit, soil salinity, high temperature, or shade, good communication between above- and belowground organs is necessary to coordinate growth and development. Various signals including hormones, peptides, proteins, hydraulic signals, and metabolites are transported mostly through the vasculature to distant tissues. How shoots and roots synchronize their response to stress using mobile signals is an emerging field of research. We summarize recent advances on mobile signals regulating shoot stomatal movement and root development in response to highly localized environmental cues. In addition, we highlight how the vascular system is not only a conduit but is also flexible in its development in response to abiotic stress.

Towards understanding the origin of animal development

Almost all animals undergo embryonic development, going from a single-celled zygote to a complex multicellular adult. We know that the patterning and morphogenetic processes involved in development are deeply conserved within the animal kingdom. However, the origins of these developmental processes are just beginning to be unveiled. Here, we focus on how the protist lineages sister to animals are reshaping our view of animal development. Most intriguingly, many of these protistan lineages display transient multicellular structures, which are governed by similar morphogenetic and gene regulatory processes as animal development. We discuss here two potential alternative scenarios to explain the origin of animal embryonic development: either it originated concomitantly at the onset of animals or it evolved from morphogenetic processes already present in their unicellular ancestors. We propose that an integrative study of several unicellular taxa closely related to animals will allow a more refined picture of how the last common ancestor of animals underwent embryonic development.