Large Vascular Bundle Phloem Area 4 enhances grain yield and quality in rice via source-sink-flow.

In rice (Oryza sativa L.), vascular bundle phloem tissue in the panicle neck is vital for the transport of photosynthetic products from leaf to panicle and is positively associated with grain yield. However, genetic regulation of the single large vascular bundle phloem area (LVPA) in rice panicle neck tissue remains poorly understood. In this study, we carried out genome-wide association analysis of LVPA in the panicle neck using 386 rice accessions and isolated and characterized the gene LVPA4, which is allelic to NARROW LEAF1 (NAL1). Phenotypic analyses were carried out on the near-isogenic line (NIL) NIL-LVPA4LT in the high-yielding indica (xian) cultivar Teqing and on overexpression lines transformed with a vector carrying the Lemont alleles of LVPA4. Both NIL-LVPA4LT and LVPA4 overexpression lines exhibited significantly increased LVPA, enlarged flag leaf size, and improved panicle type. NIL-LVPA4LT had a 7.6%-9.6% yield increase, mainly due to the significantly higher filled grain number per panicle, larger vascular system for transporting photoassimilates to spikelets, and more sufficient source supply that could service the increased sink capacity. Moreover, NIL-LVPA4LT had improved grain quality compared with Teqing, which was mainly attributed to substantial improvement in grain filling, especially for inferior spikelets in NIL-LVPA4LT. The single-nucleotide variation in the third exon of LVPA4 was associated with LVPA, spikelet number, and leaf size throughout sequencing analysis in 386 panels. The results demonstrate that LVPA4 has synergistic effects on source capacity, sink size, and flow transport and plays crucial roles in rice productivity and grain quality, thus revealing the value of LVPA4 in rice breeding programs for improved varieties.

Bundle sheath suberisation is required for C4 photosynthesis in a Setaria viridis mutant.

C4 photosynthesis provides an effective solution for overcoming the catalytic inefficiency of Rubisco. The pathway is characterised by a biochemical CO2 concentrating mechanism that operates across mesophyll and bundle sheath (BS) cells and relies on a gas tight BS compartment. A screen of a mutant population of Setaria viridis, an NADP-malic enzyme type C4 monocot, generated using N-nitroso-N-methylurea identified a mutant with an amino acid change in the gene coding region of the ABCG transporter, a step in the suberin synthesis pathway. Here, Nile red staining, TEM, and GC/MS confirmed the alteration in suberin deposition in the BS cell wall of the mutant. We show that this has disrupted the suberin lamellae of BS cell wall and increased BS conductance to CO2 diffusion more than two-fold in the mutant. Consequently, BS CO2 partial pressure is reduced and CO2 assimilation was impaired in the mutant. Our findings provide experimental evidence that a functional suberin lamellae is an essential anatomical feature for efficient C4 photosynthesis in NADP-ME plants like S. viridis and have implications for engineering strategies to ensure future food security.

Genetic basis of vascular bundle variations in rice revealed by genome-wide association study.

The vascular bundles play important roles in transportation of photoassimilate, and the number, size, and capacity of vascular bundles influence the transportation efficiency. Dissecting the genetic basis may help to make better use of naturally occurring vascular bundle variations. Here, we conducted a genome-wide association study (GWAS) of the vascular bundle variations in a worldwide collection of 529 Oryza sativa accessions. A total of 42 and 93 significant association loci were identified in the neck panicle and flag leaf, respectively. The introgression lines showing extreme values of the target traits harbored at least one GWAS signal, indicating the reliability of the GWAS loci. Based on the data of near-isogenic lines and transgenic plants, Grain number, plant height, and heading date7 (Ghd7) was identified as a major locus for the natural variation of vascular bundles in the neck panicle at the heading stage. In addition, Narrow leaf1 (NAL1) was found to influence the vascular bundles in both the neck panicle and flag leaf, and the effects of the major haplotypes of NAL1 were characterized. The loci or candidate genes identified would help to improve vascular bundle system in rice breeding.

Conserved LBL1-ta-siRNA and miR165/166-RLD1/2 modules regulate root development in maize.

Root system architecture and anatomy of monocotyledonous maize is significantly different from dicotyledonous model Arabidopsis The molecular role of non-coding RNA (ncRNA) is poorly understood in maize root development. Here, we address the role of LEAFBLADELESS1 (LBL1), a component of maize trans-acting short-interfering RNA (ta-siRNA), in maize root development. We report that root growth, anatomical patterning, and the number of lateral roots (LRs), monocot-specific crown roots (CRs) and seminal roots (SRs) are significantly affected in lbl1-rgd1 mutant, which is defective in production of ta-siRNA, including tasiR-ARF that targets AUXIN RESPONSE FACTOR3 (ARF3) in maize. Altered accumulation and distribution of auxin, due to differential expression of auxin biosynthesis and transporter genes, created an imbalance in auxin signalling. Altered expression of microRNA165/166 (miR165/166) and its targets, ROLLED1 and ROLLED2 (RLD1/2), contributed to the changes in lbl1-rgd1 root growth and vascular patterning, as was evident by the altered root phenotype of Rld1-O semi-dominant mutant. Thus, LBL1/ta-siRNA module regulates root development, possibly by affecting auxin distribution and signalling, in crosstalk with miR165/166-RLD1/2 module. We further show that ZmLBL1 and its Arabidopsis homologue AtSGS3 proteins are functionally conserved.

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.

Specification and regulation of vascular tissue identity in the Arabidopsis embryo.

Development of plant vascular tissues involves tissue identity specification, growth, pattern formation and cell-type differentiation. Although later developmental steps are understood in some detail, it is still largely unknown how the tissue is initially specified. We used the early Arabidopsis embryo as a simple model to study this process. Using a large collection of marker genes, we found that vascular identity was specified in the 16-cell embryo. After a transient precursor state, however, there was no persistent uniform tissue identity. Auxin is intimately connected to vascular tissue development. We found that, although an AUXIN RESPONSE FACTOR5/MONOPTEROS (ARF5/MP)-dependent auxin response was required, it was not sufficient for tissue specification. We therefore used a large-scale enhanced yeast one-hybrid assay to identify potential regulators of vascular identity. Network and functional analysis of candidate regulators suggest that vascular identity is under robust, complex control. We found that one candidate regulator, the G-class bZIP transcription factor GBF2, can modulate vascular gene expression by tuning MP output through direct interaction. Our work uncovers components of a gene regulatory network that controls the initial specification of vascular tissue identity.

A coherent feed-forward loop drives vascular regeneration in damaged aerial organs of plants growing in a normal developmental context.

Aerial organs of plants, being highly prone to local injuries, require tissue restoration to ensure their survival. However, knowledge of the underlying mechanism is sparse. In this study, we mimicked natural injuries in growing leaves and stems to study the reunion between mechanically disconnected tissues. We show that PLETHORA (PLT) and AINTEGUMENTA (ANT) genes, which encode stem cell-promoting factors, are activated and contribute to vascular regeneration in response to these injuries. PLT proteins bind to and activate the CUC2 promoter. PLT proteins and CUC2 regulate the transcription of the local auxin biosynthesis gene YUC4 in a coherent feed-forward loop, and this process is necessary to drive vascular regeneration. In the absence of this PLT-mediated regeneration response, leaf ground tissue cells can neither acquire the early vascular identity marker ATHB8, nor properly polarise auxin transporters to specify new venation paths. The PLT-CUC2 module is required for vascular regeneration, but is dispensable for midvein formation in leaves. We reveal the mechanisms of vascular regeneration in plants and distinguish between the wound-repair ability of the tissue and its formation during normal development.

Expression Atlas of Selaginella moellendorffii Provides Insights into the Evolution of Vasculature, Secondary Metabolism, and Roots.

Selaginella moellendorffii is a representative of the lycophyte lineage that is studied to understand the evolution of land plant traits such as the vasculature, leaves, stems, roots, and secondary metabolism. However, only a few studies have investigated the expression and transcriptional coordination of Selaginella genes, precluding us from understanding the evolution of the transcriptional programs behind these traits. We present a gene expression atlas comprising all major organs, tissue types, and the diurnal gene expression profiles for S. moellendorffii We show that the transcriptional gene module responsible for the biosynthesis of lignocellulose evolved in the ancestor of vascular plants and pinpoint the duplication and subfunctionalization events that generated multiple gene modules involved in the biosynthesis of various cell wall types. We demonstrate how secondary metabolism is transcriptionally coordinated and integrated with other cellular pathways. Finally, we identify root-specific genes and show that the evolution of roots did not coincide with an increased appearance of gene families, suggesting that the development of new organs does not coincide with increased fixation of new gene functions. Our updated database at conekt.plant.tools represents a valuable resource for studying the evolution of genes, gene families, transcriptomes, and functional gene modules in the Archaeplastida kingdom.

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.

Histone Deacetylase HDT1 is Involved in Stem Vascular Development in Arabidopsis.

Histone acetylation and deacetylation play essential roles in eukaryotic gene regulation. HD2 (HD-tuins) proteins were previously identified as plant-specific histone deacetylases. In this study, we investigated the function of the HDT1 gene in the formation of stem vascular tissue in Arabidopsis thaliana. The height and thickness of the inflorescence stems in the hdt1 mutant was lower than that of wild-type plants. Paraffin sections showed that the cell number increased compared to the wild type, while transmission electron microscopy showed that the size of individual tracheary elements and fiber cells significantly decreased in the hdt1 mutant. In addition, the cell wall thickness of tracheary elements and fiber cells increased. We also found that the lignin content in the stem of the hdt1 mutants increased compared to that of the wild type. Transcriptomic data revealed that the expression levels of many biosynthetic genes related to secondary wall components, including cellulose, lignin biosynthesis, and hormone-related genes, were altered, which may lead to the altered phenotype in vascular tissue of the hdt1 mutant. These results suggested that HDT1 is involved in development of the vascular tissue of the stem by affecting cell proliferation and differentiation.

Turing-like mechanism in a stochastic reaction-diffusion model recreates three dimensional vascular patterning of plant stems.

Vascular tissue in plants provides a resource distribution network for water and nutrients that exhibits remarkable diversity in patterning among different species. In many succulent plants, the vascular network includes longitudinally-oriented supplemental vascular bundles (SVBs) in the central core of the succulent stems and roots in addition to the more typical zone of vascular tissue development (vascular cambium) in a cylinder at the periphery of the succulent organ. Plant SVBs evolved in over 38 plant families often in tandem with evolutionary increases in stem and root parenchyma storage tissue, so it is of interest to understand the evolutionary-developmental processes responsible for their recurrent evolution and patterning. Previous mathematical models have successfully recreated the two-dimensional vascular patterns in stem and root cross sections, but such models have yet to recreate three-dimensional vascular patterning. Here, a stochastic reaction-diffusion model of plant vascular bundle patterning is developed in an effort to highlight a potential mechanism of three dimensional patterning-Turing pattern formation coupled with longitudinal efflux of a regulatory molecule. A relatively simple model of four or five molecules recreated empirical SVB patterns and many other common vascular arrangements. SVBs failed to develop below a threshold width of parenchymatous tissues, suggesting a mechanism of evolutionary character loss due to changes in the spatial context in which development takes place. Altered diffusion rates of the modeled activator and substrate molecules affected the number and size of the simulated SVBs. This work provides a first mathematical model employing a stochastic Turing-type mechanism that recreates three dimensional vascular patterns seen in plant stems. The model offers predictions that can be tested using molecular-genetic approaches. Evolutionary-developmental ramifications concerning evolution of diffusion rates, organ size and geometry are discussed.

The Ca2+ Channel CNGC19 Regulates Arabidopsis Defense Against Spodoptera Herbivory.

Cellular calcium elevation is an important signal used by plants for recognition and signaling of environmental stress. Perception of the generalist insect, Spodoptera litura, by Arabidopsis (Arabidopsis thaliana) activates cytosolic Ca2+ elevation, which triggers downstream defense. However, not all the Ca2+ channels generating the signal have been identified, nor are their modes of action known. We report on a rapidly activated, leaf vasculature- and plasma membrane-localized, CYCLIC NUCLEOTIDE GATED CHANNEL19 (CNGC19), which activates herbivory-induced Ca2+ flux and plant defense. Loss of CNGC19 function results in decreased herbivory defense. The cngc19 mutant shows aberrant and attenuated intravascular Ca2+ fluxes. CNGC19 is a Ca2+-permeable channel, as hyperpolarization of CNGC19-expressing Xenopus oocytes in the presence of both cyclic adenosine monophosphate and Ca2+ results in Ca2+ influx. Breakdown of Ca2+-based defense in cngc19 mutants leads to a decrease in herbivory-induced jasmonoyl-l-isoleucine biosynthesis and expression of JA responsive genes. The cngc19 mutants are deficient in aliphatic glucosinolate accumulation and hyperaccumulate its precursor, methionine. CNGC19 modulates aliphatic glucosinolate biosynthesis in tandem with BRANCHED-CHAIN AMINO ACID TRANSAMINASE4, which is involved in the chain elongation pathway of Met-derived glucosinolates. Furthermore, CNGC19 interacts with herbivory-induced CALMODULIN2 in planta. Together, our work reveals a key mechanistic role for the Ca2+ channel CNGC19 in the recognition of herbivory and the activation of defense signaling.

Getting closer: vein density in C4 leaves.

Contents Summary 1260 I. Introduction 1260 II. Molecular and genetic mechanisms of C4 leaf venation 1262 III. Conclusions and future perspectives 1266 Acknowledgements 1266 References 1266 SUMMARY: C4 grasses are major contributors to the world's food supply. Their highly efficient method of carbon fixation is a unique adaptation that combines close vein spacing and distinct photosynthetic cell types. Despite its importance, the molecular genetic basis of C4 leaf development is still poorly understood. Here we summarize current knowledge of leaf venation and review recent progress in understanding molecular and genetic regulation of vascular patterning events in C4 plants. Evidence points to the interplay of auxin, brassinosteroids, SHORTROOT/SCARECROW and INDETERMINATE DOMAIN transcription factors. Identification and functional characterization of candidate regulators acting early in vascular development will be essential for further progress in understanding the precise regulation of these processes.

The CLE9/10 secretory peptide regulates stomatal and vascular development through distinct receptors.

The frequency and orientation of cell division are regulated by intercellular signalling molecules; however, tissue-specific regulatory systems for cell divisions are only partially understood. Here, we report that the peptide hormone CLAVATA3/ESR-RELATED 9/10 (CLE9/10) regulates two different developmental processes, stomatal lineage development and xylem development, through two distinct receptor systems in Arabidopsis thaliana. We show that the receptor kinase HAESA-LIKE 1 (HSL1) is a CLE9/10 receptor that regulates stomatal lineage cell division, and BARELY NO MERISTEM (BAM) class receptor kinases are CLE9/10 receptors that regulate periclinal cell division of xylem precursor cells. Both HSL1 and BAM1 bind to CLE9/10, but only HSL1 recruits SOMATIC EMBRYOGENESIS RECEPTOR KINASES as co-receptors in the presence of CLE9/10, suggesting different signalling modes for these receptor systems.

Histochemical observations and gene expression changes related to internal browning in tuberous roots of sweet potato (Ipomea batatas).

The mechanism underlying internal browning (IB), or brown discoloration, of the central region of tuberous roots of sweet potato (Ipomoea batatas) was examined. IB disorder begins in roots from approx. 90 days after transplanting, and the severity increases significantly with time. IB damage initially occurs in cells around the secondary vascular tissue, and the area per cell occupied by starch grains in this region was larger than in the unaffected region. High levels of reducing sugars, polyphenol oxidase (PPO) activities, chlorogenic acid, and hydrogen peroxide (H2O2) were detected in cells from the IB damaged regions. The content of sugar and polyphenols was higher in disks (transverse sections) with larger amounts of damaged tissues than in disks of sound root. The transcript levels of acid invertase (IbAIV) tended to be higher with greater IB severity, whereas fluctuation patterns of ADP-glucose pyrophosphorylase (IbAGPase), granule bound starch synthase (IbGBSS), and starch branching enzyme 1 (IbSBE1) were lower with higher IB severity. These observations suggest that the incidence of IB disorder in sweet potato is largely dependent on the excessive generation of reactive oxygen species (ROS) in cells around the secondary vascular tissues due to the abundant accumulation of sugar and/or starch grains during the root maturation period.

From reconstruction to C4 metabolic engineering: A case study for overproduction of polyhydroxybutyrate in bioenergy grasses.

The compartmentalization of C4 plants increases photosynthetic efficiency, while constraining how material and energy must flow in leaf tissues. To capture this metabolic phenomenon, a generic plant metabolic reconstruction was replicated into four connected spatiotemporal compartments, namely bundle sheath (B) and mesophyll (M) across the day and night cycle. The C4 leaf model was used to explore how amenable polyhydroxybutyrate (PHB) production is with these four compartments working cooperatively. A strategic pattern of metabolite conversion and exchange emerged from a systems-level network that has very few constraints imposed; mainly the sequential two-step carbon capture in mesophyll, then bundle sheath and photosynthesis during the day only. The building of starch reserves during the day and their mobilization during the night connects day and night metabolism. Flux simulations revealed that PHB production did not require rerouting of metabolic pathways beyond what is already utilised for growth. PHB yield was sensitive to photoassimilation capacity, availability of carbon reserves, ATP maintenance, relative photosynthetic activity of B and M, and type of metabolites exchanged in the plasmodesmata, but not sensitive towards compartmentalization. Hence, the compartmentalization issues currently encountered are likely to be kinetic or thermodynamic limitations rather than stoichiometric.

Gain-of-function mutation of AtDICE1, encoding a putative endoplasmic reticulum-localized membrane protein, causes defects in anisotropic cell elongation by disturbing cell wall integrity in Arabidopsis.

Background and Aims: Anisotropic cell elongation depends on cell wall relaxation and cellulose microfibril arrangement. The aim of this study was to characterize the molecular function of AtDICE1 encoding a novel transmembrane protein involved in anisotropic cell elongation in Arabidopsis. Methods: Phenotypic characterizations of transgenic Arabidopsis plants mis-regulating AtDICE1 expression with different pharmacological treatments were made, and biochemical, cell biological and transcriptome analyses were performed. Key Results: Upregulation of AtDICE1 in Arabidopsis (35S::AtDICE1) resulted in severe dwarfism, probably caused by defects in anisotropic cell elongation. Epidermal cell swelling was evident in all tissues, and abnormal secondary wall thickenings were observed in pith cells of stems. These phenotypes were reproduced not only by inducible expression of AtDICE1 but also by overexpression of its poplar homologue in Arabidopsis. RNA interference suppression lines of AtDICE1 resulted in no observable phenotypic changes. Interestingly, wild-type plants treated with isoxaben, a cellulose biosynthesis inhibitor, phenocopied the 35S::AtDICE1 plants, suggesting that cellulose biosynthesis was compromised in the 35S::AtDICE1 plants. Indeed, disturbed cortical microtubule arrangements in 35S::AtDICE1/GFP-TuA6 plants were observed, and the cellulose content was significantly reduced in 35S::AtDICE1 plants. A promoter::GUS analysis showed that AtDICE1 is mainly expressed in vascular tissue, and transient expression of GFP:AtDICE1 in tobacco suggests that AtDICE1 is probably localized in the endoplasmic reticulum (ER). In addition, the external N-terminal conserved domain of AtDICE1 was found to be necessary for AtDICE1 function. Whole transcriptome analyses of 35S::AtDICE1 revealed that many genes involved in cell wall modification and stress/defence responses were mis-regulated. Conclusions: AtDICE1, a novel ER-localized transmembrane protein, may contribute to anisotropic cell elongation in the formation of vascular tissue by affecting cellulose biosynthesis.

Transcriptome dynamics at Arabidopsis graft junctions reveal an intertissue recognition mechanism that activates vascular regeneration.

The ability for cut tissues to join and form a chimeric organism is a remarkable property of many plants; however, grafting is poorly characterized at the molecular level. To better understand this process, we monitored genome-wide gene expression changes in grafted Arabidopsis thaliana hypocotyls. We observed a sequential activation of genes associated with cambium, phloem, and xylem formation. Tissues above and below the graft rapidly developed an asymmetry such that many genes were more highly expressed on one side than on the other. This asymmetry correlated with sugar-responsive genes, and we observed an accumulation of starch above the graft junction. This accumulation decreased along with asymmetry once the sugar-transporting vascular tissues reconnected. Despite the initial starvation response below the graft, many genes associated with vascular formation were rapidly activated in grafted tissues but not in cut and separated tissues, indicating that a recognition mechanism was activated independently of functional vascular connections. Auxin, which is transported cell to cell, had a rapidly elevated response that was symmetric, suggesting that auxin was perceived by the root within hours of tissue attachment to activate the vascular regeneration process. A subset of genes was expressed only in grafted tissues, indicating that wound healing proceeded via different mechanisms depending on the presence or absence of adjoining tissues. Such a recognition process could have broader relevance for tissue regeneration, intertissue communication, and tissue fusion events.

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.

A profilin gene promoter from switchgrass (Panicum virgatum L.) directs strong and specific transgene expression to vascular bundles in rice.

KEY MESSAGE: A switchgrass vascular tissue-specific promoter (PvPfn2) and its 5'-end serial deletions drive high levels of vascular bundle transgene expression in transgenic rice. Constitutive promoters are widely used for crop genetic engineering, which can result in multiple off-target effects, including suboptimal growth and epigenetic gene silencing. These problems can be potentially avoided using tissue-specific promoters for targeted transgene expression. One particularly urgent need for targeted cell wall modification in bioenergy crops, such as switchgrass (Panicum virgatum L.), is the development of vasculature-active promoters to express cell wall-affective genes only in the specific tissues, i.e., xylem and phloem. From a switchgrass expression atlas we identified promoter sequence upstream of a vasculature-specific switchgrass profilin gene (PvPfn2), especially in roots, nodes and inflorescences. When the putative full-length (1715 bp) and 5'-end serial deletions of the PvPfn2 promoter (shortest was 413 bp) were used to drive the GUS reporter expression in stably transformed rice (Oryza sativa L.), strong vasculature-specificity was observed in various tissues including leaves, leaf sheaths, stems, and flowers. The promoters were active in both phloem and xylem. It is interesting to note that the promoter was active in many more tissues in the heterologous rice system than in switchgrass. Surprisingly, all four 5'-end promoter deletions, including the shortest fragment, had the same expression patterns as the full-length promoter and with no attenuation in GUS expression in rice. These results indicated that the PvPfn2 promoter variants are new tools to direct transgene expression specifically to vascular tissues in monocots. Of special interest is the very compact version of the promoter, which could be of use for vasculature-specific genetic engineering in monocots.