Sieve element occlusion (SEO) genes encode structural phloem proteins involved in wound sealing of the phloem

Filamentous sieve element proteins are able to limit phloem mass flow, but not phytoplasma spread

In Fabaceae, dispersion of forisomes-highly ordered aggregates of sieve element proteins-in response to phytoplasma infection was proposed to limit phloem mass flow and, hence, prevent pathogen spread. In this study, the involvement of filamentous sieve element proteins in the containment of phytoplasmas was investigated in non-Fabaceae plants. Healthy and infected Arabidopsis plants lacking one or two genes related to sieve element filament formation-AtSEOR1 (At3g01680), AtSEOR2 (At3g01670), and AtPP2-A1 (At4g19840)-were analysed. TEM images revealed that phytoplasma infection induces phloem protein filament formation in both the wild-type and mutant lines. This result suggests that, in contrast to previous hypotheses, sieve element filaments can be produced independently of AtSEOR1 and AtSEOR2 genes. Filament presence was accompanied by a compensatory overexpression of sieve element protein genes in infected mutant lines in comparison with wild-type lines. No correlation was found between phloem mass flow limitation and phytoplasma titre, which suggests that sieve element proteins are involved in defence mechanisms other than mechanical limitation of the pathogen.

The Nucleolar Fibrillarin Protein Is Required for Helper Virus-Independent Long-Distance Trafficking of a Subviral Satellite RNA in Plants

RNA trafficking plays pivotal roles in regulating plant development, gene silencing, and adaptation to environmental stress. Satellite RNAs (satRNAs), parasites of viruses, depend on their helper viruses (HVs) for replication, encapsidation, and efficient spread. However, it remains largely unknown how satRNAs interact with viruses and the cellular machinery to undergo trafficking. Here, we show that the P20 protein of Bamboo mosaic potexvirus satRNA (satBaMV) can functionally complement in trans the systemic trafficking of P20-defective satBaMV in infected Nicotiana benthamiana The transgene-derived satBaMV, uncoupled from HV replication, was able to move autonomously across a graft union identified by RT-qPCR, RNA gel blot, and in situ RT-PCR analyses. Coimmunoprecipitation experiments revealed that the major nucleolar protein fibrillarin is coprecipitated in the P20 protein complex. Notably, silencing fibrillarin suppressed satBaMV-, but not HV-, phloem-based movement following grafting or coinoculation with HV Confocal microscopy revealed that the P20 protein colocalized with fibrillarin in the nucleoli and formed punctate structures associated with plasmodesmata. The mobile satBaMV RNA appears to exist as ribonucleoprotein (RNP) complex composed of P20 and fibrillarin, whereas BaMV movement proteins, capsid protein, and BaMV RNA are recruited with HV coinfection. Taken together, our findings provide insight into movement of satBaMV via the fibrillarin-satBaMV-P20 RNP complex in phloem-mediated systemic trafficking.

Proteomics and metabolomics analyses reveal the cucurbit sieve tube system as a complex metabolic space

The plant vascular system, and specifically the phloem, plays a pivotal role in allocation of fixed carbon to developing sink organs. Although the processes involved in loading and unloading of sugars and amino acids are well characterized, little information is available regarding the nature of other metabolites in the sieve tube system (STS) at specific sites along the pathway. Here, we elucidate spatial features of metabolite composition mapped with phloem enzymes along the cucurbit STS. Phloem sap (PS) was collected from the loading (source), unloading (apical sink region) and shoot-root junction regions of cucumber, watermelon and pumpkin. Our PS analyses revealed significant differences in the metabolic and proteomic profiles both along the source-sink pathway and between the STSs of these three cucurbits. In addition, metabolite profiles established for PS and vascular tissue indicated the presence of distinct compositions, consistent with the operation of the STS as a unique symplasmic domain. In this regard, at various locations along the STS we could map metabolites and their related enzymes to specific metabolic pathways. These findings are discussed with regard to the function of the STS as a unique and highly complex metabolic space within the plant vascular system.

Looking inside phytoplasma-infected sieve elements: A combined microscopy approach using Arabidopsis thaliana as a model plant

Phytoplasmas are phloem-inhabiting plant pathogens that affect over one thousand plant species, representing a severe threat to agriculture. The absence of an effective curative strategy and the economic importance of many affected crops make a priority of studying how plants respond to phytoplasma infection. Nevertheless, the study of phytoplasmas has been hindered by the extreme difficulty of culturing them in vitro and by impediments to natural host plant surveys such as low phytoplasma titre, long plant life cycle and poor knowledge of natural host-plant biology. Stating correspondence between macroscopic symptoms of phytoplasma infected Arabidopsis thaliana and those observed in natural host plants, over the last decade some authors have started to use this plant as a model for studying phytoplasma-plant interactions. Nevertheless, the morphological and ultrastructural modifications occurring in A. thaliana tissues following phytoplasma infection have never been described in detail. In this work, we adopted a combined-microscopy approach to verify if A. thaliana can be considered a reliable model for the study of phytoplasma-plant interactions at the microscopical level. The consistent presence of phytoplasma in infected phloem allowed detailed study of the infection process and the relationship established by phytoplasmas with different components of the sieve elements. In infected A. thaliana, phytoplasmas induced strong disturbances of host plant development that were mainly due to phloem disorganization and impairment. Light microscopy showed collapse, necrosis and hyperplasia of phloem cells. TEM observations of sieve elements identified two common plant-responses to phytoplasma infection: phloem protein agglutination and callose deposition.

Combined microscopy and molecular analyses show phloem occlusions and cell wall modifications in tomato leaves in response to 'Candidatus Phytoplasma solani'

Callose deposition, phloem-protein conformational changes and cell wall thickening are calcium-mediated occlusions occurring in the plant sieve elements in response to different biotic and abiotic stresses. However, the significance of these structures in plant-phytoplasma interactions requires in-depth investigations. We adopted a novel integrated approach, based on the combined use of microscopic and molecular analyses, to investigate the structural modifications induced in tomato leaf tissues in presence of phytoplasmas, focusing on vascular bundles and on the occlusion structures. Phloem hyperplasia and string-like arrangement of xylem vessels were found in infected vascular tissue. The diverse occlusion structures were differentially modulated in the phloem in response to phytoplasma infection. Callose amount was higher in midribs from infected plants than in healthy ones. Callose was observed at sieve plates but not at pore-plasmodesma units. A putative callose synthase gene encoding a protein with high similarity to Arabidopsis CalS7, responsible for callose deposition at sieve plates, was upregulated in symptomatic leaves, indicating a modulation in the response to stolbur infection. P-proteins showed configuration changes in infected sieve elements, exhibiting condensation of the filaments. The transcripts for a putative P-protein 2 and a sieve element occlusion-related protein were localized in the phloem but only the first one was modulated in the infected tissues.

Ectopic expression of phloem motor protein pea forisome PsSEO-F1 enhances salinity stress tolerance in tobacco

KEY MESSAGE

PsSEOF-1 binds to calcium and its expression is upregulated by salinity treatment. PsSEOF - 1 -overexpressing transgenic tobacco showed enhanced salinity stress tolerance by maintaining cellular ion homeostasis and modulating ROS-scavenging pathway. Calcium (Ca(2+)) plays important role in growth, development and stress tolerance in plants. Cellular Ca(2+) homeostasis is achieved by the collective action of channels, pumps, antiporters and by Ca(2+) chelators present in the cell like calcium-binding proteins. Forisomes are ATP-independent mechanically active motor proteins known to function in wound sealing of injured sieve elements of phloem tissue. The Ca(2+)-binding activity of forisome and its role in abiotic stress signaling were largely unknown. Here we report the Ca(2+)-binding activity of pea forisome (PsSEO-F1) and its novel function in promoting salinity tolerance in transgenic tobacco. Native PsSEO-F1 promoter positively responded in salinity stress as confirmed using GUS reporter. Overexpression of PsSEO-F1 tobacco plants confers salinity tolerance by alleviating ionic toxicity and increased ROS scavenging activity which probably results in reduced membrane damage and improved yield under salinity stress. Evaluation of several physiological indices shows an increase in relative water content, electrolyte leakage, proline accumulation and chlorophyll content in transgenic lines as compared with null-segregant control. Expression of several genes involved in cellular homeostasis is perturbed by PsSEO-F1 overexpression. These findings suggest that PsSEO-F1 provides salinity tolerance through cellular Ca(2+) homeostasis which in turn modulates ROS machinery providing indirect link between Ca(2+) and ROS signaling under salinity-induced perturbation. PsSEO-F1 most likely functions in salinity stress tolerance by improving antioxidant machinery and mitigating ion toxicity in transgenic lines. This finding should make an important contribution in our better understanding of the significance of calcium signaling in phloem tissue leading to salinity stress tolerance.

Functional Evaluation of Proteins in Watery and Gel Saliva of Aphids

Gel and watery saliva are regarded as key players in aphid-pIant interactions. The salivary composition seems to be influenced by the variable environment encountered by the stylet tip. Milieu sensing has been postulated to provide information needed for proper stylet navigation and for the required switches between gel and watery saliva secretion during stylet progress. Both the chemical and physical factors involved in sensing of the stylet's environment are discussed. To investigate the salivary proteome, proteins were collected from dissected gland extracts or artificial diets in a range of studies. We discuss the advantages and disadvantages of either collection method. Several proteins were identified by functional assays or by use of proteomic tools, while most of their functions still remain unknown. These studies disclosed the presence of at least two proteins carrying numerous sulfhydryl groups that may act as the structural backbone of the salivary sheath. Furthermore, cell-wall degrading proteins such a pectinases, pectin methylesterases, polygalacturonases, and cellulases as well as diverse Ca²⁺-binding proteins (e.g., regucalcin, ARMET proteins) were detected. Suppression of the plant defense may be a common goal of salivary proteins. Salivary proteases are likely involved in the breakdown of sieve-element proteins to invalidate plant defense or to increase the availability of organic N compounds. Salivary polyphenoloxidases, peroxidases and oxidoreductases were suggested to detoxify, e.g., plant phenols. During the last years, an increasing number of salivary proteins have been categorized under the term 'effector'. Effectors may act in the suppression (C002 or MIF cytokine) or the induction (e.g., Mp10 or Mp 42) of plant defense, respectively. A remarkable component of watery saliva seems the protein GroEL that originates from Buchnera aphidicola, the obligate symbiont of aphids and probably reflects an excretory product that induces plant defense responses. Furthermore, chitin fragments in the saliva may trigger defense reactions (e.g., callose deposition). The functions of identified proteins and protein classes are discussed with regard to physical and chemical characteristics of apoplasmic and symplasmic plant compartments.

Similar Intracellular Location and Stimulus Reactivity, but Differential Mobility of Tailless (Vicia faba) and Tailed Forisomes (Phaseolus vulgaris) in Intact Sieve Tubes

Sieve elements of legumes contain forisomes—fusiform protein bodies that are responsible for sieve-tube occlusion in response to damage or wound signals. Earlier work described the existence of tailless and tailed forisomes. This study intended to quantify and compare location and position of tailless (in Vicia faba) and tailed (in Phaseolus vulgaris) forisomes inside sieve elements and to assess their reactivity and potential mobility in response to a remote stimulus. Location (distribution within sieve elements) and position (forisome tip contacts) of more than altogether 2000 forisomes were screened in 500 intact plants by laser scanning confocal microscopy in the transmission mode. Furthermore, we studied the dispersion of forisomes at different locations in different positions and their positional behaviour in response to distant heat shocks. Forisome distribution turned out to be species-specific, whereas forisome positions at various locations were largely similar in bushbean (Phaseolus) and broadbean (Vicia). In general, the tailless forisomes had higher dispersion rates in response to heat shocks than the tailed forisomes and forisomes at the downstream (basal) end dispersed more frequently than those at the upstream end (apical). In contrast to the tailless forisomes that only oscillate in response to heat shocks, downstream-located tailed forisomes can cover considerable distances within sieve elements. This displacement was prevented by gentle rubbing of the leaf (priming) before the heat shock. Movement of these forisomes was also prohibited by Latrunculin A, an inhibitor of actin polymerization. The apparently active mobility of tailed forisomes gives credence to the idea that at least the latter forisomes are not free-floating, but connected to other sieve-element structures.

Aphid salivary proteases are capable of degrading sieve-tube proteins

Sieve tubes serve as transport conduits for photo-assimilates and other resources in angiosperms and are profitable targets for piercing-sucking insects such as aphids. Sieve-tube sap also contains significant amounts of proteins with diverse functions, for example in signalling, metabolism, and defence. The identification of salivary proteases in Acyrthosiphon pisum led to the hypothesis that aphids might be able to digest these proteins and by doing so suppress plant defence and access additional nitrogen sources. Here, the scarce knowledge of proteases in aphid saliva is briefly reviewed. In order to provide a better platform for discussion, we conducted a few tests on in vitro protease activity and degradation of sieve-tube sap proteins of Cucurbita maxima by watery saliva. Inhibition of protein degradation by EDTA indicates the presence of different types of proteases (e.g. metalloproteses) in saliva of A. pisum. Proteases in the watery saliva from Macrosiphum euphorbiae and A. pisum were able to degrade the most abundant phloem protein, which is phloem protein 1. Our results provide support for the breakdown of sieve-element proteins by aphid saliva in order to suppress/neutralize the defence responses of the plant and to make proteins of sieve-tube sap accessible as a nitrogen source, as is discussed in detail. Finally, we discuss whether glycosylation of sieve-element proteins and the presence of protease inhibitors may confer partial protection against the proteolytic activity of aphid saliva.

Identification of mRNAs that move over long distances using an RNA-Seq analysis of Arabidopsis/Nicotiana benthamiana heterografts

Phloem is a conductive tissue that allocates nutrients from mature source leaves to sinks such as young developing tissues. Phloem also delivers proteins and RNA species, such as small RNAs and mRNAs. Intensive studies on plant systemic signaling revealed the essential roles of proteins and RNA species. However, many of their functions are still largely unknown, with the roles of transported mRNAs being particularly poorly understood. A major difficulty is the absence of an accurate and comprehensive list of mobile transcripts. In this study, we used a hetero-graft system with Nicotiana benthamiana as the recipient scion and Arabidopsis as the donor stock, to identify transcripts that moved long distances across the graft union. We identified 138 Arabidopsis transcripts as mobile mRNAs, which we collectively termed the mRNA mobilome. Reverse transcription-PCR, quantitative real-time PCR and droplet digital PCR analyses confirmed the mobility. The transcripts included potential signaling factors and, unexpectedly, more general factors. In our investigations, we found no preferred transcript length, no previously known sequence motifs in promoter or transcript sequences and no similarities between the level of the transcripts and that in the source leaves. Grafting experiments regarding the function of ERECTA, an identified transcript, showed that no function of the transcript mobilized. To our knowledge, this is the first report identifying transcripts that move over long distances using a hetero-graft system between different plant taxa.

Identification of phloem-mobile mRNA

Signaling between cells, tissues and organs is essential for multicellular organisms to coordinate and adapt their development and growth to internal and environmental changes. Plants have evolved a plant-specific symplasmic pathway, called plasmodesmata, for efficient intercellular communication, in addition to the receptor-ligand-based apoplasmic pathway. Long-distance signaling between distant organs is enabled via the phloem tube system, where plasmodesmata contribute to phloem loading and unloading for photosynthate allocation. In addition to signaling by small molecules such as metabolites and phytohormones, the transport of proteins, small RNAs and mRNAs is also considered an important mechanism to achieve long-distance signaling in plants. Recent studies on phloem-mobile proteins and small RNAs have revealed their role in crucial physiological processes including flowering, systemic silencing and nutrient allocation. However, the biological role of mRNAs found in the phloem tube is not yet clear, though their mobility over long-distances has been well evidenced. To gain this knowledge, it is important to collect further information on mRNA profiles in the phloem translocation stream. In this review, I summarize the current approaches to identifying the mRNA population in the phloem translocation system, and discuss the possible role of short- and long-distance mRNA transport.

Dynamics of long-distance signaling via plant vascular tissues

Plant vascular systems are constructed by specific cell wall modifications through which cells are highly specialized to make conduits for water and nutrients. Xylem vessels are formed by thickened cell walls that remain after programmed cell death, and serve as water conduits from the root to the shoot. In contrast, phloem tissues consist of a complex of living cells, including sieve tube elements and their neighboring companion cells, and translocate photosynthetic assimilates from mature leaves to developing young tissues. Intensive studies on the content of vascular flow fluids have unveiled that plant vascular tissues transport various types of gene product, and the transport of some provides the molecular basis for the long-distance communications. Analysis of xylem sap has demonstrated the presence of proteins in the xylem transpiration stream. Recent studies have revealed that CLE and CEP peptides secreted in the roots are transported to above ground via the xylem in response to plant-microbe interaction and soil nitrogen starvation, respectively. Their leucine-rich repeat transmembrane receptors localized in the shoot phloem are required for relaying the signal from the shoot to the root. These findings well-fit to the current scenario of root-to-shoot-to-root feedback signaling, where peptide transport achieves the root-to-shoot signaling, the first half of the signaling process. Meanwhile, it is now well-evidenced that proteins and a range of RNAs are transported via the phloem translocation system, and some of those can exert their physiological functions at their destinations, including roots. Thus, plant vascular systems may serve not only as conduits for the translocation of essential substances but also as long-distance communication pathways that allow plants to adapt to changes in internal and external environments at the whole plant level.

cDNA-AFLP analysis reveals the adaptive responses of citrus to long-term boron-toxicity

BACKGROUND

Boron (B)-toxicity is an important disorder in agricultural regions across the world. Seedlings of 'Sour pummelo' (Citrus grandis) and 'Xuegan' (Citrus sinensis) were fertigated every other day until drip with 10 μM (control) or 400 μM (B-toxic) H3BO3 in a complete nutrient solution for 15 weeks. The aims of this study were to elucidate the adaptive mechanisms of citrus plants to B-toxicity and to identify B-tolerant genes.

RESULTS

B-toxicity-induced changes in seedlings growth, leaf CO2 assimilation, pigments, total soluble protein, malondialdehyde (MDA) and phosphorus were less pronounced in C. sinensis than in C. grandis. B concentration was higher in B-toxic C. sinensis leaves than in B-toxic C. grandis ones. Here we successfully used cDNA-AFLP to isolate 67 up-regulated and 65 down-regulated transcript-derived fragments (TDFs) from B-toxic C. grandis leaves, whilst only 31 up-regulated and 37 down-regulated TDFs from B-toxic C. sinensis ones, demonstrating that gene expression is less affected in B-toxic C. sinensis leaves than in B-toxic C. grandis ones. These differentially expressed TDFs were related to signal transduction, carbohydrate and energy metabolism, nucleic acid metabolism, protein and amino acid metabolism, lipid metabolism, cell wall and cytoskeleton modification, stress responses and cell transport. The higher B-tolerance of C. sinensis might be related to the findings that B-toxic C. sinensis leaves had higher expression levels of genes involved in photosynthesis, which might contribute to the higher photosyntheis and light utilization and less excess light energy, and in reactive oxygen species (ROS) scavenging compared to B-toxic C. grandis leaves, thus preventing them from photo-oxidative damage. In addition, B-toxicity-induced alteration in the expression levels of genes encoding inorganic pyrophosphatase 1, AT4G01850 and methionine synthase differed between the two species, which might play a role in the B-tolerance of C. sinensis.

CONCLUSIONS

C. sinensis leaves could tolerate higher level of B than C. grandis ones, thus improving the B-tolerance of C. sinensis plants. Our findings reveal some novel mechanisms on the tolerance of plants to B-toxicity at the gene expression level.

Characterization of five subgroups of the sieve element occlusion gene family in Glycine max reveals genes encoding non-forisome P-proteins, forisomes and forisome tails

P-proteins are structural phloem proteins discussed to be involved in the rapid sealing of injured sieve elements. P-proteins are found in all dicotyledonous and some monocotyledonous plants, but additional crystalloid P-proteins, known as forisomes, have evolved solely in the Fabaceae. Both types are encoded by members of the sieve element occlusion (SEO) gene family, which comprises seven phylogenetic subgroups. The Fabaceae-specific subgroup 1 contains genes encoding forisome subunits in e.g. Medicago truncatula, Vicia faba, Dipteryx panamensis and Canavalia gladiata whereas basal subgroup 5 encodes P-proteins in Nicotiana tabacum (tobacco) and Arabidopsis thaliana. The function of remaining subgroups is still unknown. We chose Glycine max (soybean) as a model to investigate SEO proteins representing different subgroups in one species. We isolated native P-proteins to determine the SEO protein composition and analyzed the expression pattern, localization and structure of the G. max SEO proteins representing five of the subgroups. We found that subgroup 1 GmSEO genes encode forisome subunits, a member of subgroup 5 encodes a non-forisome P-protein and subgroup 2 GmSEO genes encode the components of forisome tails, which are present in a restricted selection of Fabaceaen species. We therefore present the first molecular characterization of a Fabaceae non-forisome P-protein and the first evidence that forisome tails are encoded by a phylogenetically-distinct branch of the SEO gene family.

Molecular and ultrastructural analysis of forisome subunits reveals the principles of forisome assembly

BACKGROUND AND AIMS

Forisomes are specialized structural phloem proteins that mediate sieve element occlusion after wounding exclusively in papilionoid legumes, but most studies of forisome structure and function have focused on the Old World clade rather than the early lineages. A comprehensive phylogenetic, molecular, structural and functional analysis of forisomes from species covering a broad spectrum of the papilionoid legumes was therefore carried out, including the first analysis of Dipteryx panamensis forisomes, representing the earliest branch of the Papilionoideae lineage. The aim was to study the molecular, structural and functional conservation among forisomes from different tribes and to establish the roles of individual forisome subunits.

METHODS

Sequence analysis and bioinformatics were combined with structural and functional analysis of native forisomes and artificial forisome-like protein bodies, the latter produced by expressing forisome genes from different legumes in a heterologous background. The structure of these bodies was analysed using a combination of confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the function of individual subunits was examined by combinatorial expression, micromanipulation and light microscopy.

KEY RESULTS

Dipteryx panamensis native forisomes and homomeric protein bodies assembled from the single sieve element occlusion by forisome (SEO-F) subunit identified in this species were structurally and functionally similar to forisomes from the Old World clade. In contrast, homomeric protein bodies assembled from individual SEO-F subunits from Old World species yielded artificial forisomes differing in proportion to their native counterparts, suggesting that multiple SEO-F proteins are required for forisome assembly in these plants. Structural differences between Medicago truncatula native forisomes, homomeric protein bodies and heteromeric bodies containing all possible subunit combinations suggested that combinations of SEO-F proteins may fine-tune the geometric proportions and reactivity of forisomes.

CONCLUSIONS

It is concluded that forisome structure and function have been strongly conserved during evolution and that species-dependent subsets of SEO-F proteins may have evolved to fine-tune the structure of native forisomes.

SEORious business: structural proteins in sieve tubes and their involvement in sieve element occlusion

The phloem provides a network of sieve tubes for long-distance translocation of photosynthates. For over a century, structural proteins in sieve tubes have presented a conundrum since they presumably increase the hydraulic resistance of the tubes while no potential function other than sieve tube or wound sealing in the case of injury has been suggested. Here we summarize and critically evaluate current speculations regarding the roles of these proteins. Our understanding suffers from the suggestive power of images; what looks like a sieve tube plug on micrographs may not actually impede translocation very much. Recent reports of an involvement of SEOR (sieve element occlusion-related) proteins, a class of P-proteins, in the sealing of injured sieve tubes are inconclusive; various lines of evidence suggest that, in neither intact nor injured plants, are SEORs determinative of translocation stoppage. Similarly, the popular notion that P-proteins serve in the defence against phloem sap-feeding insects is unsupported by empirical facts; it is conceivable that in functional sieve tubes, aphids actually could benefit from inducing a plug. The idea that rising cytosolic Ca(2+) generally triggers sieve tube blockage by P-proteins appears widely accepted, despite lacking experimental support. Even in forisomes, P-protein assemblages restricted to one single plant family and the only Ca(2+)-responsive P-proteins known, the available evidence does not unequivocally suggest that plug formation is the cause rather than a consequence of translocation stoppage. We conclude that the physiological roles of structural P-proteins remain elusive, and that in vivo studies of their dynamics in continuous sieve tube networks combined with flow velocity measurements will be required to (hopefully) resolve this scientific roadblock.

Spread the news: systemic dissemination and local impact of Ca²⁺ signals along the phloem pathway

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca(2+) influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca(2+)-permeable channels, some of which have been localized, is required for Ca(2+)-modulated events in sieve elements. There, Ca(2+)-permeable channels are clustered and create so-called Ca(2+) hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca(2+)-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca(2+) ions and Ca(2+)-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca(2+) influx and handling patterns (Ca(2+) signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.

Plant Vascular Biology 2013: vascular trafficking

About 200 researchers from around the world attended the Third International Conference on Plant Vascular Biology (PVB 2013) held in July 2013 at the Rantapuisto Conference Center, in Helsinki, Finland (http://www.pvb2013.org). The plant vascular system, which connects every organ in the mature plant, continues to attract the interest of researchers representing a wide range of disciplines, including development, physiology, systems biology, and computational biology. At the meeting, participants discussed the latest research advances in vascular development, long- and short-distance vascular transport and long-distance signalling in plant defence, in addition to providing a context for how these studies intersect with each other. The meeting provided an opportunity for researchers working across a broad range of fields to share ideas and to discuss future directions in the expanding field of vascular biology. In this report, the latest advances in understanding the mechanism of vascular trafficking presented at the meeting have been summarized.

Pectin methylesterase-generated methanol may be involved in tobacco leaf growth

Plant leaves undergo a sink-source modification of intercellular macromolecular transport during the transition from carbon import to carbon export. After assessing the role of metabolite signaling in gene regulation in Nicotiana tabacum sink and source leaves, we observed increased pectin methylesterase (PME)-mediated methanol generation in immature leaves. Using suppression subtractive hybridization (SSH), we identified a number of genes whose activity changes from sink to source leaves. The most abundant SSH-identified genes appeared to be sensitive to methanol. We hypothesize that tobacco leaf maturation and the sink-source transition are accompanied by a change in mRNA levels of genes that function in methanol-dependent cell signaling.

Cloning and functional characterization of the promoter of PsSEOF1 gene from Pisum sativum under different stress conditions using Agrobacterium-mediated transient assay

PsSEOF1, a SEO (sieve element occlusion) gene family protein (forisome) is calcium powered motor protein and is located close to plasma membrane of sieve element. In sieve element (SE) it senses the calcium ion levels and undergoes ATP-independent conformational shifts. Forisome, meaning gate-bodies (Latin foris: wing of a gate; Greek soma: body). Recent reports show that SEO gene family protein can prevent the loss of nutrient rich photoassimilate upon wound injury. The regulation of SEO protein forisome under abiotic/ biotic stress is still unknown. The analysis of cis-regulatory element present in the upstream region is not well understood. Tissue specific promoters guarantee correct expression when it perceives particular stimuli. Here we report isolation of tissue specific promoter of PsSEOF1 was isolated by gene walking PCR from P. sativum (pea) genomic DNA library constructed by BD genome walker kit. In silico analysis revealed several putative cis element within this promoter sequence like wound response, cold, dehydration. Putative elements which might be required for its vascular tissue specificity has also been identified. The GUS activities of PsSEOF1 promoter-GUS chimeric construct in the agroinfiltrated leaves under different environmental stress abiotic and biotic like wound, cold, salt and phytohormones has shown high level of GUS activity. To identify the activity of PsSEOF1 promoter under different stress condition an Agrobacterium-mediated transient expression of tobacco plants were subjected to histochemical GUS staining. Stress-inducible nature of PsSEOF1 promoter opens possibility for the study of the PsSEOF1 gene regulation under stress condition. The isolated promoter sequence could serve as an important candidate for tissue specific promoter in genetic engineering of plant under stress conditions.

Uncertain role of MtSEO-F3 in assembly of Medicago truncatula forisomes

Forisomes are specialized multimeric protein complexes found only in the papilionoid legumes. They undergo a reversible conformational change in response to phloem injury to enable the occlusion of sieve tubes, thus preventing the loss of photoassimilates. The individual subunits are designated by the letters SEO-F (sieve element occlusion by forisomes) and are part of the larger SEO protein family, which also includes the typical P-proteins found in most dicots and some monocots. When specific SEO-F subunits from different species are expressed in a heterologous background, they self-assemble into fully-functional artificial forisomes. However, with the exception of basal species such as Dipteryx panamensis, the geometry of these artificial forisomes differs from that of their native counterparts. Studies involving SEO-F proteins from the model legume Medicago truncatula have shown that a combination of 3 of the 4 subunits can fine-tune the geometry of artificial forisomes. However, MtSEO-F3 was excluded from these studies because it was not incorporated into either the native or artificial forisomes in our original experiments. In this addendum, we present further data concerning the interactive properties of the SEO-F proteins and confirm that all 4 MtSEO-F proteins interact in all possible pairwise combinations. These data indicate that the exclusion of MtSEO-F3 from the compact forisome may reflect the steric hindrance of binding sites rather than an inability to interact with other forisome subunits.

Calcium powered phloem protein of SEO gene family "Forisome" functions in wound sealing and act as biomimetic smart materials

Forisomes protein belongs to SEO gene family and is unique to Fabaceae family. These proteins are located in sieve tubes of phloem and function to prevent loss of nutrient-rich photoassimilates, upon mechanical injury/wounding. Forisome protein is also known as ATP independent, mechanically active proteins. Despite the wealth of information role of forisome in plants are not yet fully understood. Recent reports suggest that forisomes protein can act as ideal model to study self assembly mechanism for development of nanotechnological devices like microfluidic system application in space exploration mission. Improvement in micro instrument is highly demanding and has been a key technology by NASA in future space exploration missions. Based on its physical parameters, forisome are found to be ideal biomimetic materials for micro fluidic system because the conformational shifts can be replicated in vitro and are fully reversible over large number of cycles. By the use of protein engineering forisome recombinant protein can be tailored. Due to its unique ability to convert chemical energy into mechanical energy forisome has received much attention. For nanotechnological application and handling biomolecules such as DNA, RNA, protein and cell as a whole microfluidic system will be the most powerful technology. The discovery of new biomimetic smart materials has been a key factor in development of space science and its requirements in such a challenging environment. The field of microfludic, particularly in terms of development of its components along with identification of new biomimetic smart materials, deserves more attention. More biophysical investigation is required to characterize it to make it more suitable under parameters of performance.

Phloem transport: a review of mechanisms and controls

It is generally believed that an osmotically generated pressure gradient drives the phloem mass flow. So far, this widely accepted Münch theory has required remarkably few adaptations, but the debate on alternative and additional hypotheses is still ongoing. Recently, a possible shortcoming of the Münch theory has been pointed out, suggesting that the Münch pressure flow is more suitable for herbs than for trees. Estimation of the phloem resistance indicates that a point might be reached in long sieve tubes where the pressure required to drive the Münch flow cannot be generated. Therefore, the relay hypothesis regained belief as it implies that the sieve tubes are shorter then the plant's axial axis. In the source phloem, three different loading strategies exist which probably result from evolutionary advantages. Passive diffusion seems to be the most primitive one, whereas active loading strategies substantially increase the growth potential. Along the transport phloem, a leakage-retrieval mechanism is observed. Appreciable amounts of carbohydrates are lost from the sieve tubes to feed the lateral sinks, while a part of these lost carbohydrates is subsequently reloaded into the sieve tubes. This mechanism is probably involved to buffer short-term irregularities in phloem turgor and gradient. In the long term, the mechanism controls the replenishment and remobilization of lateral stem storage tissues. As phloem of higher plants has multiple functions in plant development, reproduction, signalling, and growth, the fundamental understanding of the mechanisms behind phloem transport should be elucidated to increase our ability to influence plant growth and development.

TIME FOR COFFEE is an essential component in the maintenance of metabolic homeostasis in Arabidopsis thaliana

Plants often respond to environmental changes by reprogramming metabolic and stress-associated pathways. Homeostatic integration of signaling is a central requirement for ensuring metabolic stability in living organisms. Under diurnal conditions, properly timed rhythmic metabolism provides fitness benefits to plants. TIME FOR COFFEE (TIC) is a circadian regulator known to be involved in clock resetting at dawn. Here we explored the mechanism of influence of TIC in plant growth and development, as initiated by a microarray analysis. This global profiling showed that a loss of TIC function causes a major reprogramming of gene expression that predicts numerous developmental, metabolic, and stress-related phenotypes. This led us to demonstrate that this mutant exhibits late flowering, a plastochron defect, and diverse anatomical phenotypes. We further observed a starch-excess phenotype and altered soluble carbohydrate levels. tic exhibited hypersensitivity to oxidative stress and abscisic acid, and this was associated with a striking resistance to drought. These phenotypes were connected to an increase in total glutathione levels that correlated with a readjustment of amino acids and polyamine pools. By comparatively analyzing our transcriptomic and metabolomic data, we concluded that TIC is a central element in plant homeostasis that integrates and coordinates developmental, metabolic, and environmental signals.

Calcium-energized motor protein forisome controls damage in phloem: potential applications as biomimetic "smart" material

Forisomes are ATP independent, mechanically active proteins from the Fabaceae family (also called Leguminosae). These proteins are located in sieve tubes of phloem and function to prevent loss of nutrient-rich photoassimilates, upon mechanical injury/wounding. Forisomes are SEO (sieve element occlusion) gene family proteins that have recently been shown to be involved in wound sealing mechanism. Recent findings suggest that forisomes could act as an ideal model to study self assembly mechanism for the development of nanotechnological devices like microinstruments, the microfluidic system frequently used in space exploration missions. Technology enabling improvement in micro instruments has been identified as a key technology by NASA in future space exploration missions. Forisomes are designated as biomimetic smart materials which are calcium-energized motor proteins. Since forisomes are biomolecules from plant systems it can be doctored through genetic engineering. In contrast, "smart" materials which are not derived from plants are difficult to modify in their properties. Current levels of understanding about forisomes conformational shifts with respect to calcium ions and pH changes requires supplement of future advances with relation to its 3D structure to understand self assembly processes. In plant systems it forms blood clots in the form of occlusions to prevent nutrient fluid leakage and thus proves to be a unique damage control system of phloem tissue.

How phloem-feeding insects face the challenge of phloem-located defenses

Due to the high content of nutrient, sieve tubes are a primary target for pests, e.g., most phytophagous hemipteran. To protect the integrity of the sieve tubes as well as their content, plants possess diverse chemical and physical defense mechanisms. The latter mechanisms are important because they can potentially interfere with the food source accession of phloem-feeding insects. Physical defense mechanisms are based on callose as well as on proteins and often plug the sieve tube. Insects that feed from sieve tubes are potentially able to overwhelm these defense mechanisms using their saliva. Gel saliva forms a sheath in the apoplast around the stylet and is suggested to seal the stylet penetration site in the cell plasma membrane. In addition, watery saliva is secreted into penetrated cells including sieve elements; the presence of specific enzymes/effectors in this saliva is thought to interfere with plant defense responses. Here we detail several aspects of plant defense and discuss the interaction of plants and phloem-feeding insects. Recent agro-biotechnological phloem-located aphid control strategies are presented.

P-proteins in Arabidopsis are heteromeric structures involved in rapid sieve tube sealing

Structural phloem proteins (P-proteins) are characteristic components of the sieve elements in all dicotyledonous and many monocotyledonous angiosperms. Tobacco P-proteins were recently confirmed to be encoded by the widespread sieve element occlusion (SEO) gene family, and tobacco SEO proteins were shown to be directly involved in sieve tube sealing thus preventing the loss of photosynthate. Analysis of the two Arabidopsis SEO proteins (AtSEOa and AtSEOb) indicated that the corresponding P-protein subunits do not act in a redundant manner. However, there are still pending questions regarding the interaction properties and specific functions of AtSEOa and AtSEOb as well as the general function of structural P-proteins in Arabidopsis. In this study, we characterized the Arabidopsis P-proteins in more detail. We used in planta bimolecular fluorescence complementation assays to confirm the predicted heteromeric interactions between AtSEOa and AtSEOb. Arabidopsis mutants depleted for one or both AtSEO proteins lacked the typical P-protein structures normally found in sieve elements, underlining the identity of AtSEO proteins as P-proteins and furthermore providing the means to determine the role of Arabidopsis P-proteins in sieve tube sealing. We therefore developed an assay based on phloem exudation. Mutants with reduced AtSEO expression levels lost twice as much photosynthate following injury as comparable wild-type plants, confirming that Arabidopsis P-proteins are indeed involved in sieve tube sealing.

Structural and functional heterogeneity in phloem loading and transport

The phloem is often regarded as a relatively straightforward transport system composed of loading (collection), long-distance (transport), and unloading (release) zones. While this simple view is necessary and useful in many contexts, it belies the reality, which is that the phloem is inherently complex. At least three types of sieve element-companion cell complexes are found in minor veins of leaves. Individual species may have more than one type, indicating that they employ multiple loading strategies, even in the same vein. Gene expression data in particular point to heterogeneity in sieve element-companion cell complexes of minor veins, perhaps in all flowering plants. Phloem heterogeneity in the transport phloem is also evident in many species based on anatomical, biochemical and gene expression data. In this regard, members of the Cucurbitaceae are especially complex and interesting. We conclude that a hidden world of specialized phloem function awaits discovery.

Interactions among tobacco sieve element occlusion (SEO) proteins

Angiosperms transport their photoassimilates through sieve tubes, which comprise longitudinally-connected sieve elements. In dicots and also some monocots, the sieve elements contain parietal structural proteins known as phloem proteins or P-proteins. Following injury, P proteins disperse and accumulate as viscous plugs at the sieve plates to prevent the loss of valuable transport sugars. Tobacco (Nicotiana tabacum) P-proteins are multimeric complexes comprising subunits encoded by members of the SEO (sieve element occlusion) gene family. The existence of multiple subunits suggests that P-protein assembly involves interactions between SEO proteins, but this process is largely uncharacterized and it is unclear whether the different subunits perform unique roles or are redundant. We therefore extended our analysis of the tobacco P-proteins NtSEO1 and NtSEO2 to investigate potential interactions between them, and found that both proteins can form homomeric and heteromeric complexes in planta.