Evidence for symplastic phloem unloading in sink leaves of barley

Coordination of carbon assimilation, allocation, and utilization for systemic improvement of cereal yield

The growth of yield outputs is dwindling after the first green revolution, which cannot meet the demand for the projected population increase by the mid-century, especially with the constant threat from extreme climates. Cereal yield requires carbon (C) assimilation in the source for subsequent allocation and utilization in the sink. However, whether the source or sink limits yield improvement, a crucial question for strategic orientation in future breeding and cultivation, is still under debate. To narrow the knowledge gap and capture the progress, we focus on maize, rice, and wheat by briefly reviewing recent advances in yield improvement by modulation of i) leaf photosynthesis; ii) primary C allocation, phloem loading, and unloading; iii) C utilization and grain storage; and iv) systemic sugar signals (e.g., trehalose 6-phosphate). We highlight strategies for optimizing C allocation and utilization to coordinate the source-sink relationships and promote yields. Finally, based on the understanding of these physiological mechanisms, we envisage a future scenery of "smart crop" consisting of flexible coordination of plant C economy, with the goal of yield improvement and resilience in the field population of cereals crops.

Foxtail mosaic virus: A tool for gene function analysis in maize and other monocots

Many plant viruses have been engineered into vectors for use in functional genomics studies, expression of heterologous proteins, and, most recently, gene editing applications. The use of viral vectors overcomes bottlenecks associated with mutagenesis and transgenesis approaches often implemented for analysis of gene function. There are several engineered viruses that are demonstrated or suggested to be useful in maize through proof-of-concept studies. However, foxtail mosaic virus (FoMV), which has a relatively broad host range, is emerging as a particularly useful virus for gene function studies in maize and other monocot crop or weed species. A few clones of FoMV have been independently engineered, and they have different features and capabilities for virus-induced gene silencing (VIGS) and virus-mediated overexpression (VOX) of proteins. In addition, FoMV can be used to deliver functional guide RNAs in maize and other plants expressing the Cas9 protein, demonstrating its potential utility in virus-induced gene editing applications. There is a growing number of studies in which FoMV vectors are being applied for VIGS or VOX in maize and the vast majority of these are related to maize-microbe interactions. In this review, we highlight the biology and engineering of FoMV as well as its applications in maize-microbe interactions and more broadly in the context of the monocot functional genomics toolbox.

Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink

Sucrose is transported from sources (mature leaves) to sinks (importing tissues such as roots, stems, fruits, and seeds) through the phloem tissues in veins. In many herbaceous crop species, sucrose must first be effluxed to the cell wall by a sugar transporter of the SWEET family prior to being taken up into phloem companion cells or sieve elements by a different sugar transporter, called SUT or SUC. The import of sucrose into these cells is termed apoplasmic phloem loading. In sinks, sucrose can similarly exit the phloem apoplasmically or, alternatively, symplasmically through plasmodesmata into connecting parenchyma storage cells. Recent advances describing the regulation and manipulation of sugar transporter expression and activities provide stimulating new insights into sucrose phloem loading in sources and unloading processes in sink tissues. Additionally, new breakthroughs have revealed distinct subpopulations of cells in leaves with different functions pertaining to phloem loading. These and other discoveries in sucrose transport are discussed.

The Role of Sugar Transporter CsSWEET7a in Apoplasmic Phloem Unloading in Receptacle and Nectary During Cucumber Anthesis

During anthesis, there is an increased demand for carbohydrates due to pollen maturation and nectary secretion that warrants a systematic phloem unloading strategy for sugar partitioning. Sugar transporters are key components of the apoplasmic phloem unloading strategy and control the sugar flux needed for plant development. Currently, the phloem unloading strategy during anthesis has not been explored in cucumber, and the question of which sugar transporters are active during flower anthesis is poorly understood. In this study, a study utilizing the phloem-mobile symplasmic tracer carboxyfluorescein (CF) suggested that the phloem unloading was symplasmically isolated in the receptacle and nectary of cucumber flowers at anthesis. We also identified a hexose transporter that is highly expressed in cucumber flower, Sugar Will Eventually be Exported Transporter 7a (SWEET7a). CsSWEET7a was mainly expressed in receptacle and nectary tissues in both male and female flowers, where its expression level increased rapidly right before anthesis. At anthesis, the CsSWEET7a protein was specifically localized to the phloem region of the receptacle and nectary, indicating that CsSWEET7a may function in the apoplasmic phloem unloading during flower anthesis. Although cucumber mainly transports raffinose family oligosaccharides (RFOs) in the phloem, sucrose, glucose, and fructose are the major sugars in the flower receptacle and the nectary as well as in nectar at anthesis. In addition, the transcript levels of genes encoding soluble sugar hydrolases (α-galactosidase, sucrose synthase, cytoplasmic invertase, and cell wall invertase) were correlated with that of CsSWEET7a. These results indicated that CsSWEET7a may be involved in sugar partitioning as an exporter in the phloem of the receptacle and nectary to supply carbohydrates for flower anthesis and nectar secretion in cucumber.

A review of starch biosynthesis in cereal crops and its potential breeding applications in rice (Oryza Sativa L.)

Starch provides primary storage of carbohydrates, accounting for approximately 85% of the dry weight of cereal endosperm. Cereal seeds contribute to maximum annual starch production and provide the primary food for humans and livestock worldwide. However, the growing demand for starch in food and industry and the increasing loss of arable land with urbanization emphasizes the urgency to understand starch biosynthesis and its regulation. Here, we first summarized the regulatory signaling pathways about leaf starch biosynthesis. Subsequently, we paid more attention to how transcriptional factors (TFs) systematically respond to various stimulants via the regulation of the enzymes during starch biosynthesis. Finally, some strategies to improve cereal yield and quality were put forward based on the previous reports. This review would collectively help to design future studies on starch biosynthesis in cereal crops.

Switch from symplasmic to aspoplasmic phloem unloading in Xanthoceras sorbifolia fruit and sucrose influx XsSWEET10 as a key candidate for Sugar transport

The process of phloem unloading and post-unloading transport of photoassimilate is critical to crop output. Xanthoceras sorbifolia is a woody oil species with great biomass energy prospects in China; however, underproduction of seeds seriously restricts its development. Here, our cytological studies by ultrastructural observation revealed that the sieve element-companion cell complex in carpellary bundle was symplasmically interconnected with surrounding parenchyma cells at the early and late fruit developmental stages, whereas it was symplasmically isolated at middle stage. Consistently, real-time imaging showed that fluorescent tracer 6(5)carboxyfluorescein was confined to phloem strands at middle stage but released into surrounding parenchymal cells at early and late stages. Enzymatic assay showed that sucrose synthase act as the key enzyme catalyzing the progress of Suc degradation post-unloading pathway whether in pericarp or in seed, while vacuolar acid invertase and neutral invertase play compensation roles in sucrose decomposition. Sugar transporter XsSWEET10 had a high expression profile in fruit, especially at middle stage. XsSWEET10 is a plasma membrane-localized protein and heterologous expression in SUC2-deficient yeast strain SUSY7/ura3 confirmed its ability to uptake sucrose. These findings approved the transition from symplasmic to apoplasmic phloem unloading in Xanthoceras sorbifolia fruit and XsSWEET10 as a key candidate in sugar transport.

Association among starch storage, metabolism, related genes and growth of Moso bamboo (Phyllostachys heterocycla) shoots

BACKGROUND

Both underground rhizomes/buds and above-ground Moso bamboo (Phyllostachys heterocycla) shoots/culms/branches are connected together into a close inter-connecting system in which nutrients are transported and shared among each organ. However, the starch storage and utilization mechanisms during bamboo shoot growth remain unclear. This study aimed to reveal in which organs starch was stored, how carbohydrates were transformed among each organ, and how the expression of key genes was regulated during bamboo shoot growth and developmental stages which should lay a foundation for developing new theoretical techniques for bamboo cultivation.

RESULTS

Based on changes of the NSC content, starch metabolism-related enzyme activity and gene expression from S0 to S3, we observed that starch grains were mainly elliptical in shape and proliferated through budding and constriction. Content of both soluble sugar and starch in bamboo shoot peaked at S0, in which the former decreased gradually, and the latter initially decreased and then increased as shoots grew. Starch synthesis-related enzymes (AGPase, GBSS and SBE) and starch hydrolase (α-amylase and β-amylase) activities exhibited the same dynamic change patterns as those of the starch content. From S0 to S3, the activity of starch synthesis-related enzyme and starch amylase in bamboo rhizome was significantly higher than that in bamboo shoot, while the NSC content in rhizomes was obviously lower than that in bamboo shoots. It was revealed by the comparative transcriptome analysis that the expression of starch synthesis-related enzyme-encoding genes were increased at S0, but reduced thereafter, with almost the same dynamic change tendency as the starch content and metabolism-related enzymes, especially during S0 and S1. It was revealed by the gene interaction analysis that AGPase and SBE were core genes for the starch and sucrose metabolism pathway.

CONCLUSIONS

Bamboo shoots were the main organ in which starch was stored, while bamboo rhizome should be mainly functioned as a carbohydrate transportation channel and the second carbohydrate sink. Starch metabolism-related genes were expressed at the transcriptional level during underground growth, but at the post-transcriptional level during above-ground growth. It may be possible to enhance edible bamboo shoot quality for an alternative starch source through genetic engineering.

The plant axis as the command centre for (re)distribution of sucrose and amino acids

Along with the increase in size required for optimal colonization of terrestrial niches, channels for bidirectional bulk transport of materials in land plants evolved during a period of about 100 million years. These transport systems are essentially still in operation - though perfected over the following 400 million years - and make use of hydrostatic differentials. Substances are accumulated or released at the loading and unloading ends, respectively, of the transport channels. The intermediate stretch between the channel termini is bifunctional and executes orchestrated release and retrieval of solutes. Analyses of anatomical and physiological data demonstrate that the release/retrieval zone extends deeper into sources and sinks than is commonly thought and covers usually much more than 99% of the translocation stretch. This review sketches the significance of events in the intermediate stretch for distribution of organic materials over the plant body. Net leakage from the channels does not only serve maintenance and growth of tissues along the pathway, but also diurnal, short-term or seasonal storage of reserve materials, and balanced distribution of organic C- and N-compounds over axial and terminal sinks. Release and retrieval are controlled by plasma-membrane transporters at the vessel/parenchyma interface in the contact pits along xylem vessels and by plasma-membrane transporters at the interface between companion cells and phloem parenchyma along sieve tubes. The xylem-to-phloem pathway vice versa is a bifacial, radially oriented system comprising a symplasmic pathway, of which entrance and exit are controlled at specific membrane checkpoints, and a parallel apoplasmic pathway. A broad range of specific sucrose and amino-acid transporters are deployed at the checkpoint plasma membranes. SUCs, SUTs, STPs, SWEETs, and AAPs, LTHs, CATs are localized to the plasma membranes in question, both in monocots and eudicots. Presence of Umamits in monocots is uncertain. There is some evidence for endo- and exocytosis at the vessel/parenchyma interface supplementary to the transporter-mediated uptake and release. Actions of transporters at the checkpoints are equally decisive for storage and distribution of amino acids and sucrose in monocots and eudicots, but storage and distribution patterns may differ between both taxa. While the majority of reserves is sequestered in vascular parenchyma cells in dicots, lack of space in monocot vasculature urges "outsourcing" of storage in ground parenchyma around the translocation path. In perennial dicots, specialized radial pathways (rays) include the sites for seasonal alternation of storage and mobilization. In dicots, apoplasmic phloem loading and a correlated low rate of release along the path would favour supply with photoassimilates of terminal sinks, while symplasmic phloem loading and a correlated higher rate of release along the path favours supply of axial sinks and transfer to the xylem. The balance between the resource acquisition by terminal and axial sinks is an important determinant of relative growth rate and, hence, for the fitness of plants in various habitats. Body enlargement as the evolutionary drive for emergence of vascular systems and mass transport propelled by hydrostatic differentials.

Identification of HvLRX, a new dehydration and light responsive gene in Tibetan hulless barley (Hordeum vulgare var. nudum)

BACKGROUND

Tibetan hulless barley (Hordeum vulgare var. nudum), adjusting to the harsh environment on Qinghai-Tibet Plateau, is a good subject for analyzing drought tolerance mechanism. Several unannotated differentially expressed genes (DEGs) were identified through our previous RNA-Seq study using two hulless barley accessions with contrasting drought tolerance. One of these DEGs, HVU010048.2, showed up-regulated pattern under dehydration stress in both drought tolerant (DT) and drought susceptible (DS) accessions, while its function in drought resistance remains unknown. This new gene was named as HvLRX (light responsive X), because its expression was induced under high light intensity while suppressed under dark.

OBJECTIVE

To provide preliminary bioinformatics prediction, expression pattern, and drought resistance function of this new gene.

METHODS

Bioinformatics analysis of HvLRX were conducted by MEGA, PlantCARE, ProtParam, CELLO et al. The expression pattern of HvLRX under different light intensity, dehydration shock, gradual drought stress, NaCl stress, polyethylene glycol (PEG) 6000 stress and abscisic acid (ABA) treatment was investigated by quantitative reverse transcription-polymerase chain reaction (RT-qPCR). The function of HvLRX was analyzed by virus induced gene silencing (VIGS) in hulless barley and by transgenic method in tobacco.

RESULTS

Full cDNAs of HvLRX were cloned and compared in three hulless barley accessions. Homologues of HvLRX protein in other plants were excavated and their phylogenetic relationship was analyzed. Several light responsive elements (ATC-motif, Box 4, G-box, Sp1, and chs-CMA1a) were identified in its promoter region. Its expression can be promoted under high light intensity, dehydration shock, gradual drought stress, PEG 6000, and NaCl stress, but was almost unchanged in ABA treatment. HvLRX-silenced plants had a higher leaf water loss rate (WLR) and a lower survival rate (SR) compared with controls under dehydration stress. The infected leaves of HvLRX-silenced plants lost their water content quickly and became withered at 10 dpi. The SR of HvLRX overexpressed transgenic tobacco plants was significantly higher than that of wild-type plants. These results indicated HvLRX play a role in drought resistance. Besides, retarded vegetative growth was detected in HvLRX-silenced hulless barley plants, which suggested that this gene is important for plant development.

CONCLUSIONS

This study provided data of bioinformatics, expression pattern, and function of HvLRX. To our knowledge, this is the first report of this new dehydration and light responsive gene.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

Change in Sucrose Cleavage Pattern and Rapid Starch Accumulation Govern Lily Shoot-to-Bulblet Transition in vitro

In bulb crops, bulbing is a key progress in micropropagation and is the feature that most distinguishes bulbous crops from other plants. Generally, bulbing involves a shoot-to-bulblet transition; however, the underlying mechanism remains elusive. We explored this process by tracking the shoot-to-bulblet transition under different culture conditions. Rapid starch accumulation occurred at 15 days after transplanting (DAT) in the bulblet-inducing treatments as confirmed via histological observations and the significant elevation of starch synthesis related-gene transcription, including LohAGPS, LohAGPL, LohGBSS, LohSS, and LohSBE. However, for shoots that did not transition to bulblets and maintained the shoot status, much higher soluble sugars were detected. Interestingly, we observed a clear shift from invertase-catalyzed to sucrose synthase-catalyzed sucrose cleavage pattern based on the differential expression of LohCWIN and LohSuSy during the key transition stage (prior to and after bulbing at 0-15 DAT). Shoots that transitioned into bulblets showed significantly higher LohSuSy expression, especially LohSuSy4 expression, than shoots that did not transition. A symplastic phloem unloading pathway at the bulblet emergence stage (15 DAT) was verified via the 6(5)-carboxyfluorescein diacetate fluorescent tracer. We propose that starch is the fundamental compound in the shoot-to-bulblet transition and that starch synthesis is likely triggered by the switch from apoplastic to symplastic sucrose unloading, which may be related to sucrose depletion. Furthermore, this study is the first to provide a complete inventory of the genes involved in starch metabolism based on our transcriptome data. Two of these genes, LohAGPS1.2b and LohSSIIId, were verified by rapid amplification of cDNA ends cloning, and these data will provide additional support for Lilium research since whole genome is currently lacking.

Proteinaceous effector discovery and characterization in filamentous plant pathogens

The complicated interplay of plant-pathogen interactions occurs on multiple levels as pathogens evolve to constantly evade the immune responses of their hosts. Many economically important crops fall victim to filamentous pathogens that produce small proteins called effectors to manipulate the host and aid infection/colonization. Understanding the effector repertoires of pathogens is facilitating an increased understanding of the molecular mechanisms underlying virulence as well as guiding the development of disease control strategies. The purpose of this review is to give a chronological perspective on the evolution of the methodologies used in effector discovery from physical isolation and in silico predictions, to functional characterization of the effectors of filamentous plant pathogens and identification of their host targets.

Genome-Wide Identification and Expression Profiling of Monosaccharide Transporter Genes Associated with High Harvest Index Values in Rapeseed (Brassica napus L.)

Sugars are important throughout a plant’s lifecycle. Monosaccharide transporters (MST) are essential sugar transporters that have been identified in many plants, but little is known about the evolution or functions of MST genes in rapeseed (Brassica napus). In this study, we identified 175 MST genes in B. napus, 87 in Brassica oleracea, and 83 in Brassica rapa. These genes were separated into the sugar transport protein (STP), polyol transporter (PLT), vacuolar glucose transporter (VGT), tonoplast monosaccharide transporter (TMT), inositol transporter (INT), plastidic glucose transporter (pGlcT), and ERD6-like subfamilies, respectively. Phylogenetic and syntenic analysis indicated that gene redundancy and gene elimination have commonly occurred in Brassica species during polyploidization. Changes in exon-intron structures during evolution likely resulted in the differences in coding regions, expression patterns, and functions seen among BnMST genes. In total, 31 differentially expressed genes (DEGs) were identified through RNA-seq among materials with high and low harvest index (HI) values, which were divided into two categories based on the qRT-PCR results, expressed more highly in source or sink organs. We finally identified four genes, including BnSTP5, BnSTP13, BnPLT5, and BnERD6-like14, which might be involved in monosaccharide uptake or unloading and further affect the HI of rapeseed. These findings provide fundamental information about MST genes in Brassica and reveal the importance of BnMST genes to high HI in B. napus.

Recent developments and applications of genetic transformation and genome editing technologies in wheat

Wheat (Triticum aestivum) is a staple crop across the world and plays a remarkable role in food supplying security. Over the past few decades, basic and applied research on wheat has lagged behind other cereal crops due to the complex and polyploid genome and difficulties in genetic transformation. A breakthrough called as PureWheat was made in the genetic transformation of wheat in 2014 in Asia, leading to a noticeable progress of wheat genome editing. Due to this great achievement, it is predicated that wheat biotechnology revolution is arriving. Genome editing technologies using zinc finger nucleases, transcription activator-like effector nuclease, and clustered regularly interspaced short palindromic repeats-associated endonucleases (CRISR/Cas) are becoming powerful tools for crop modification which can help biologists and biotechnologists better understand the processes of mutagenesis and genomic alteration. Among the three genome editing systems, CRISR/Cas has high specificity and activity, and therefore it is widely used in genetic engineering. Generally, the genome editing technologies depend on an efficient genetic transformation system. In this paper, we summarize recent progresses and applications on genetic transformation and genome editing in wheat. We also examine the future aspects of genetic transformation and genome editing. We believe that the technologies for wheat efficient genetic engineering and functional studies will become routine with the emergence of high-quality genomic sequences.

Symplasmic phloem unloading and post-phloem transport during bamboo internode elongation

In traditional opinions, no radial transportation was considered to occur in the bamboo internodes but was usually considered to occur in the nodes. Few studies have involved the phloem unloading and post-phloem transport pathways in the rapid elongating bamboo shoots. Our observations indicated a symplastic pathway in phloem unloading and post-unloading pathways in the culms of Fargesiayunnanensis Hsueh et Yi, based on a 5,6-carboxyfluorescein diacetate tracing experiment. Significant lignification and suberinization in fiber and parenchyma cell walls in maturing internodes blocked the apoplastic transport. Assimilates were transported out of the vascular bundles in four directions in the inner zones but in two directions in the outer zones via the continuum of parenchyma cells. In transverse sections, assimilates were outward transported from the inner zones to the outer zones. Assimilates transport velocities varied with time, with the highest values at 0):00 h, which were affected by water transport. The assimilate transport from the adult culms to the young shoots also varied with the developmental degree of bamboo shoots, with the highest transport velocities in the rapidly elongating internodes. The localization of sucrose, glucose, starch grains and the related enzymes reconfirmed that the parenchyma cells in and around the vascular bundles constituted a symplastic pathway for the radial transport of sugars and were the main sites for sugar metabolism. The parenchyma cells functioned as the 'rays' for the radial transport in and between vascular bundles in bamboo internodes. These results systematically revealed the transport mechanism of assimilate and water in the elongating bamboo shoots.

sRNA Profiling Combined With Gene Function Analysis Reveals a Lack of Evidence for Cross-Kingdom RNAi in the Wheat - Zymoseptoria tritici Pathosystem

Cross-kingdom small RNA (sRNA) silencing has recently emerged as a mechanism facilitating fungal colonization and disease development. Here we characterized RNAi pathways in Zymoseptoria tritici, a major fungal pathogen of wheat, and assessed their contribution to pathogenesis. Computational analysis of fungal sRNA and host mRNA sequencing datasets was used to define the global sRNA populations in Z. tritici and predict their mRNA targets in wheat. 389 in planta-induced sRNA loci were identified. sRNAs generated from some of these loci were predicted to target wheat mRNAs including those potentially involved in pathogen defense. However, molecular approaches failed to validate targeting of selected wheat mRNAs by fungal sRNAs. Mutant strains of Z. tritici carrying deletions of genes encoding key components of RNAi such as Dicer-like (DCL) and Argonaute (AGO) proteins were generated, and virulence bioassays suggested that these are dispensable for full infection of wheat. Nonetheless, our results did suggest the existence of non-canonical DCL-independent pathway(s) for sRNA biogenesis in Z. tritici. dsRNA targeting essential fungal genes applied in vitro or generated from an RNA virus vector in planta in a procedure known as HIGS (Host-Induced Gene Silencing) was ineffective in preventing Z. tritici growth or disease. We also demonstrated that Z. tritici is incapable of dsRNA uptake. Collectively, our data suggest that RNAi approaches for gene function analyses in this fungal species and potentially also as a control measure may not be as effective as has been demonstrated for some other plant pathogenic fungi.

Transcriptomic and functional analysis of cucumber (Cucumis sativus L.) fruit phloem during early development

The phloem of the Cucurbitaceae has long been a subject of interest due to its complex nature and the economic importance of the family. As in a limited number of other families, cucurbit phloem is bicollateral, i.e. with sieve tubes on both sides of the xylem. To date little is known about the specialized functions of the internal phloem (IP) and external phloem (EP). Here, a combination of microscopy, fluorescent dye transport analysis, micro-computed tomography, laser capture microdissection and RNA-sequencing (RNA-Seq) were used to study the functions of IP and EP in the vascular bundles (VBs) of cucumber fruit. There is one type of VB in the peduncle, but four in the fruit: peripheral (PeVB), main (MVB), carpel (CVB) and placental (PlVB). The VBs are bicollateral, except for the CVB and PlVB. Phloem mobile tracers and ¹⁴ C applied to leaves are transported primarily in the EP, and to a lesser extent in the IP. RNA-Seq data indicate preferential gene transcription in the IP related to differentiation/development, hormone transport, RNA or protein modification/processing/transport, and nitrogen compound metabolism and transport. The EP preferentially expresses genes for stimulus/stress, defense, ion transport and secondary metabolite biosynthesis. The MVB phloem is preferentially involved in photoassimilate transport, unloading and long-distance signaling, while the PeVB plays a more substantial role in morphogenesis and/or development and defense response. CVB and PlVB transcripts are biased toward development of reproductive organs. These findings provide an integrated view of the differentiated structure and function of the vascular tissue in cucumber fruit.

Photoinducible DRONPA-s: a new tool for investigating cell-cell connectivity

The development of multicellular plants relies on the ability of their cells to exchange solutes, proteins and signalling compounds through plasmodesmata, symplasmic pores in the plant cell wall. The aperture of plasmodesmata is regulated in response to developmental cues or external factors such as pathogen attack. This regulation enables tight control of symplasmic cell-to-cell transport. Here we report on an elegant non-invasive method to quantify the passive movement of protein between selected cells even in deeper tissue layers. The system is based on the fluorescent protein DRONPA-s, which can be switched on and off repeatedly by illumination with different light qualities. Using transgenic 35S::DRONPA-s Arabidopsis thaliana and a confocal microscope it was possible to activate DRONPA-s fluorescence in selected cells of the root meristem. This enabled us to compare movement of DRONPA-s from the activated cells into the respective neighbouring cells. Our analyses showed that pericycle cells display the highest efflux capacity with a good lateral connectivity. In contrast, root cap cells showed the lowest efflux of DRONPA-s. Plasmodesmata of quiescent centre cells mediated a stronger efflux into columella cells than into stele initials. To simplify measurements of fluorescence intensity in a complex tissue we developed software that allows simultaneous analyses of fluorescence intensities of several neighbouring cells. Our DRONPA-s system generates reproducible data and is a valuable tool for studying symplasmic connectivity.

A New Barley Stripe Mosaic Virus Allows Large Protein Overexpression for Rapid Function Analysis

Understanding the genetic and molecular bases of gene function is of increasing importance to harness their potential to produce plants with novel traits. One important objective is the improvement of plant productivity to meet future demands in food crop production. Gene function is mostly characterized through overexpression or silencing in transgenic plants. This approach is a lengthy procedure, especially in cereals. Plant viral expression systems can be used for rapid expression of proteins. However, current systems have a small cargo capacity and have mostly been used for gene silencing. Here, a four-component barley stripe mosaic virus-based system with high cargo capacity was constructed for the rapid and stable expression of recombinant proteins in different plant species, allowing function analyses at different stages of development. Fluorescent marker proteins are expressed at high levels within 1 week, and a proof of efficient function analysis is shown using the aluminum malate transporter1 gene. In addition to the ability of gene cotransformation, this work demonstrates that the four-component barley stripe mosaic virus-based system allows the overexpression of cDNAs of up to 2,100 nucleotides (encoding a protein of ∼78 kD), thereby providing an invaluable tool to accelerate functional genomics and proteomic research in monocot and dicot species.

High-efficiency gene targeting in hexaploid wheat using DNA replicons and CRISPR/Cas9

The ability to edit plant genomes through gene targeting (GT) requires efficient methods to deliver both sequence-specific nucleases (SSNs) and repair templates to plant cells. This is typically achieved using Agrobacterium T-DNA, biolistics or by stably integrating nuclease-encoding cassettes and repair templates into the plant genome. In dicotyledonous plants, such as Nicotinana tabacum (tobacco) and Solanum lycopersicum (tomato), greater than 10-fold enhancements in GT frequencies have been achieved using DNA virus-based replicons. These replicons transiently amplify to high copy numbers in plant cells to deliver abundant SSNs and repair templates to achieve targeted gene modification. In the present work, we developed a replicon-based system for genome engineering of cereal crops using a deconstructed version of the wheat dwarf virus (WDV). In wheat cells, the replicons achieve a 110-fold increase in expression of a reporter gene relative to non-replicating controls. Furthermore, replicons carrying CRISPR/Cas9 nucleases and repair templates achieved GT at an endogenous ubiquitin locus at frequencies 12-fold greater than non-viral delivery methods. The use of a strong promoter to express Cas9 was critical to attain these high GT frequencies. We also demonstrate gene-targeted integration by homologous recombination (HR) in all three of the homoeoalleles (A, B and D) of the hexaploid wheat genome, and we show that with the WDV replicons, multiplexed GT within the same wheat cell can be achieved at frequencies of ~1%. In conclusion, high frequencies of GT using WDV-based DNA replicons will make it possible to edit complex cereal genomes without the need to integrate GT reagents into the genome.

Sucrose Transporter ZmSut1 Expression and Localization Uncover New Insights into Sucrose Phloem Loading

Sucrose transporters (SUTs) translocate sucrose (Suc) across cellular membranes, and in eudicots, multiple SUTs are known to function in Suc phloem loading in leaves. In maize (Zea mays), the Sucrose Transporter1 (ZmSut1) gene has been implicated in Suc phloem loading based upon RNA expression in leaves, electrophysiological experiments, and phenotypic analysis of zmsut1 mutant plants. However, no previous studies have examined the cellular expression of ZmSut1 RNA or the subcellular localization of the ZmSUT1 protein to assess the gene's hypothesized function in Suc phloem loading or to evaluate its potential roles, such as phloem unloading, in nonphotosynthetic tissues. To this end, we performed RNA in situ hybridization experiments, promoter-reporter gene analyses, and ZmSUT1 localization studies to elucidate the cellular expression pattern of the ZmSut1 transcript and protein. These data showed that ZmSut1 was expressed in multiple cell types throughout the plant and indicated that it functions in phloem companion cells to load Suc and also in other cell types to retrieve Suc from the apoplasm to prevent its accumulation and loss to the transpiration stream. Additionally, by comparing a phloem-mobile tracer with ZmSut1 expression, we determined that developing maize leaves dynamically switch from symplasmic to apoplasmic phloem unloading, reconciling previously conflicting reports, and suggest that ZmSut1 does not have an apparent function in either unloading process. A model for the dual roles for ZmSut1 function (phloem loading and apoplasmic recycling), Sut1 evolution, and its possible use to enhance Suc export from leaves in engineering C3 grasses for C4 photosynthesis is discussed.

Functional characterization and expression analysis of cucumber (Cucumis sativus L.) hexose transporters, involving carbohydrate partitioning and phloem unloading in sink tissues

Many hexose transporters (HTs) have been reported to play roles in sucrose-transporting plants. However, little information about roles of HTs in RFOs (raffinose family oligosaccharides)-transporting plants has been reported. Here, three hexose transporters (CsHT2, CsHT3, and CsHT4) were cloned from Cucumis sativus L. Heterologous expression in yeast demonstrated that CsHT3 transported glucose, galactose and mannose, with a K(m) of 131.9 μM for glucose, and CsHT4 only transported galactose, while CsHT2 was non-functional. Both CsHT3 and CsHT4 were targeted to the plasma membrane of cucumber protoplasts. Spatio-temporal expression indicated that transcript level of CsHT3 was much higher than that of CsHT2 and CsHT4 in most tissues, especially in peduncles and fruit tissues containing vascular bundles. GUS staining of CsHT3-promoter-β-glucuronidase (GUS) transgenic Arabidopsis plants revealed CsHT3 expression in tissues with high metabolic turnover, suggesting that CsHT3 is involved in sugar competition among different sink organs during plant development. The transcript levels of CsHT3 and cell wall invertase genes increased in peduncles and fruit tissues along with cucumber fruit enlargement, and CsHT3 localized to phloem tissues by immunohistochemical localization; These results suggest that CsHT3 probably plays an important role in apoplastic phloem unloading of cucumber fruit.

A tale of two neglected systems-structure and function of the thin- and thick-walled sieve tubes in monocotyledonous leaves

There is a large body of information relating to the ontogeny, development and the vasculature of eudicotyledonous leaves. However, there is less information available concerning the vascular anatomy of monocotyledonous leaves. This is surprising, given that there are two uniquely different phloem systems present in large groups such as grasses and sedges. Monocotyledonous leaves contain marginal, large, intermediate, and small longitudinal veins that are interconnected by numerous transverse veins. The longitudinal veins contain two metaphloem sieve tube types, which, based upon their ontogeny and position within the phloem, are termed early (thin-walled) and late (thick-walled) sieve tubes. Early metaphloem comprises sieve tubes, companion cells and vascular parenchyma (VP) cells, whilst the late metaphloem, contains thick-walled sieve tubes (TSTs) that lack companion cells. TSTs are generally adjacent to, or no more than one cell removed from the metaxylem. Unlike thin-walled sieve tube (ST) -companion cell complexes, TSTs are connected to parenchyma by pore-plasmodesma units and are generally symplasmically isolated from the STs. This paper addresses key structural and functional differences between thin- and thick-walled sieve tubes and explores the unique advantages of alternate transport strategies that this 5-7 million years old dual system may offer. It would seem that these two systems may enhance, add to, or play a significant role in increasing the efficiency of solute retrieval as well as of assimilate transfer.

Citrus leaf blotch virus invades meristematic regions in Nicotiana benthamiana and citrus

To invade systemically host plants, viruses need to replicate in the infected cells, spread to neighbouring cells through plasmodesmata and move to distal parts of the plant via sieve tubes to start new infection foci. To monitor the infection of Nicotiana benthamiana plants by Citrus leaf blotch virus (CLBV), leaves were agroinoculated with an infectious cDNA clone of the CLBV genomic RNA expressing green fluorescent protein (GFP) under the transcriptional control of a duplicate promoter of the coat protein subgenomic RNA. Fluorescent spots first appeared in agroinfiltrated leaves 11-12 days after infiltration, indicating CLBV replication. Then, after entering the phloem vascular system, CLBV was unloaded in the upper parts of the plant and invaded all tissues, including flower organs and meristems. GFP fluorescence was not visible in citrus plants infected with CLBV-GFP. Therefore, to detect CLBV in meristematic regions, Mexican lime (Citrus aurantifolia) plants were graft inoculated with CLBV, with Citrus tristeza virus (CTV), a virus readily eliminated by shoot-tip grafting in vitro, or with both simultaneously. Although CLBV was detected by hybridization and real-time reverse transcription-polymerase chain reaction (RT-PCR) in 0.2-mm shoot tips in all CLBV-inoculated plants, CTV was not detected. These results explain the difficulty in eliminating CLBV by shoot-tip grafting in vitro.

Rationale for developing new virus vectors to analyze gene function in grasses through virus-induced gene silencing

The exploding availability of genome and EST-based sequences from grasses requires a technology that allows rapid functional analysis of the multitude of genes that these resources provide. There are several techniques available to determine a gene's function. For gene knockdown studies, silencing through RNAi is a powerful tool. Gene silencing can be accomplished through stable transformation or transient expression of a fragment of a target gene sequence. Stable transformation in rice, maize, and a few other species, although routine, remains a relatively low-throughput process. Transformation in other grass species is difficult and labor-intensive. Therefore, transient gene silencing methods including Agrobacterium-mediated and virus-induced gene silencing (VIGS) have great potential for researchers studying gene function in grasses. VIGS in grasses already has been used to determine the function of genes during pathogen challenge and plant development. It also can be used in moderate-throughput reverse genetics screens to determine gene function. However, the number of viruses modified to serve as silencing vectors in grasses is limited, and the silencing phenotype induced by these vectors is not optimal: the phenotype being transient and with moderate penetration throughout the tissue. Here, we review the most recent information available for VIGS in grasses and summarize the strengths and weaknesses in current virus-grass host systems. We describe ways to improve current virus vectors and the potential of other grass-infecting viruses for VIGS studies. This work is necessary because VIGS for the foreseeable future remains a higher throughput and more rapid system to evaluate gene function than stable transformation.

Does Don Fisher's high-pressure manifold model account for phloem transport and resource partitioning?

The pressure flow model of phloem transport envisaged by Münch (1930) has gained wide acceptance. Recently, however, the model has been questioned on structural and physiological grounds. For instance, sub-structures of sieve elements may reduce their hydraulic conductances to levels that impede flow rates of phloem sap and observed magnitudes of pressure gradients to drive flow along sieve tubes could be inadequate in tall trees. A variant of the Münch pressure flow model, the high-pressure manifold model of phloem transport introduced by Donald Fisher may serve to reconcile at least some of these questions. To this end, key predicted features of the high-pressure manifold model of phloem transport are evaluated against current knowledge of the physiology of phloem transport. These features include: (1) An absence of significant gradients in axial hydrostatic pressure in sieve elements from collection to release phloem accompanied by transport properties of sieve elements that underpin this outcome; (2) Symplasmic pathways of phloem unloading into sink organs impose a major constraint over bulk flow rates of resources translocated through the source-path-sink system; (3) Hydraulic conductances of plasmodesmata, linking sieve elements with surrounding phloem parenchyma cells, are sufficient to support and also regulate bulk flow rates exiting from sieve elements of release phloem. The review identifies strong circumstantial evidence that resource transport through the source-path-sink system is consistent with the high-pressure manifold model of phloem transport. The analysis then moves to exploring mechanisms that may link demand for resources, by cells of meristematic and expansion/storage sinks, with plasmodesmal conductances of release phloem. The review concludes with a brief discussion of how these mechanisms may offer novel opportunities to enhance crop biomass yields.

Symplasmic, long-distance transport in xylem and cambial regions in branches of Acer pseudoplatanus (Aceraceae) and Populus tremula x P. tremuloides (Salicaceae)

PREMISE OF THE STUDY

The picture of how long-distance transport proceeds in trees is still far from being complete. Beside the apoplasmic pathway, transport undoubtedly also takes place within the system of living cells in the secondary xylem and cambial region. Because detailed, thorough studies of the symplasmic routes in woody branches, using direct localization with fluorescent tracers, had not been done, here we focused on the main routes of long-distance symplasmic transport in xylem and cambial tissues and analyzed in detail tracer distribution in the rays on the extended cambial surface in branches of Acer pseudoplatanus and Populus tremula ×P. tremuloides.

METHODS

Fluorescent tracers were loaded into branches through the vascular system, then their distribution in xylem and cambial regions was analyzed.

KEY RESULTS

Tracer signal was present in the symplast of axial and radial xylem parenchyma cells and in both types of cambial cells. The living cells of xylem parenchyma and of the cambium were symplasmically interconnected via xylem rays. Tracer distribution in rays was uneven on the extended cambial surface; cambial regions with intensively or sparsely dyed rays alternated along the vertical axis of analyzed branches.

CONCLUSIONS

Symplasmic, long-distance transport is present between the living cells of xylem and the cambial region in woody branches. The uneven distribution of fluorescent tracers in cambial rays along the stems is surprising and suggests the presence of an intrinsic pattern caused by an unknown mechanism.

Phloem unloading follows an extensive apoplasmic pathway in cucumber (Cucumis sativus L.) fruit from anthesis to marketable maturing stage

The phloem unloading pathway remains unclear in fruits of Cucurbitaceae, a classical stachyose-transporting species with bicollateral phloem. Using a combination of electron microscopy, transport of phloem-mobile symplasmic tracer carboxyfluorescein, assays of acid invertase and sucrose transporter, and [(14)C]sugar uptake, the phloem unloading pathway was studied in cucumber (Cucumis sativus) fruit from anthesis to the marketable maturing stage. Structural investigations showed that the sieve element-companion cell (SE-CC) complex of the vascular bundles feeding fruit flesh is apparently symplasmically restricted. Imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the vascular bundles in the whole fruit throughout the stages examined. A 37 kDa acid invertase was located predominantly in the cell walls of SE-CC complexes and parenchyma cells. Studies of [(14)C]sugar uptake suggested that energy-driven transporters may be functional in sugar trans-membrane transport within symplasmically restricted SE-CC complex, which was further confirmed by the existence of a functional plasma membrane sucrose transporter (CsSUT4) in cucumber fruit. These data provide a clear evidence for an apoplasmic phloem unloading pathway in cucumber fruit. A presumption that putative raffinose or stachyose transporters may be involved in soluble sugars unloading was discussed.

The Barley stripe mosaic virus system used for virus-induced gene silencing in cereals differentially affects susceptibility to fungal pathogens in wheat

Barley stripe mosaic virus (BSMV) has emerged as a vector for virus-induced gene silencing (VIGS) in cereals, having been used to study a number of genes involved in resistance in both wheat and barley. However, the effects of the BSMV vector on plant physiology and disease resistance in plants remains unexplored. The BSMV inoculation control vector, BSMV:GFP was shown to cause severe viral symptoms in wheat, displaying chlorosis, leaf curling and growth inhibition typical of the symptoms seen in BSMV-infected barley. These viral symptoms were accompanied by induction of genes implicated in defense against pathogens, namely PR1, PR4, PR5, PR10 and PAL. Subsequent inoculation of BSMV:GFP-infected wheat with a wheat pathotype of Magnaporthe oryzae, the blast pathogen, resulted in decreased susceptibility. Penetration of epidermal cells and subsequent multiple cell colonization by M. oryzae was significantly reduced. This increased restriction of pathogen growth observed for BSMV:GFP infections with and without the viral coat protein gene. However, prior infection with BSMV:GFP had no effect on the development of a compatible isolate of Blumeria graminis f. sp. tritici, the causal agent of powdery mildew.

Efficient and stable expression of GFP through Wheat streak mosaic virus-based vectors in cereal hosts using a range of cleavage sites: formation of dense fluorescent aggregates for sensitive virus tracking

A series of Wheat streak mosaic virus (WSMV)-based expression vectors were developed by engineering a cycle 3 GFP (GFP) cistron between P1 and HC-Pro cistrons with several catalytic/cleavage peptides at the C-terminus of GFP. WSMV-GFP vectors with the Foot-and-mouth disease virus 1D/2A or 2A catalytic peptides cleaved GFP from HC-Pro but expressed GFP inefficiently. WSMV-GFP vectors with homologous NIa-Pro heptapeptide cleavage sites did not release GFP from HC-Pro, but efficiently expressed GFP as dense fluorescent aggregates. However, insertion of one or two spacer amino acids on either side of NIb/CP heptapeptide cleavage site or deletion in HC-Pro cistron improved processing by NIa-Pro. WSMV-GFP vectors were remarkably stable in wheat for seven serial passages and for 120 days postinoculation. Mite transmission efficiencies of WSMV-GFP vectors correlated with the amount of free GFP produced. WSMV-GFP vectors infected the same range of cereal hosts as wild-type virus, and GFP fluorescence was detected in most wheat tissues.

Intracellular expression of a host-selective toxin, ToxA, in diverse plants phenocopies silencing of a ToxA-interacting protein, ToxABP1

*ToxA, a host-selective toxin of wheat, can be detected within ToxA-sensitive mesophyll cells, where it localizes to chloroplasts and induces necrosis. Interaction of ToxA with the chloroplast-localized protein ToxABP1 has been implicated in this process. Therefore, we hypothesized that silencing of ToxABP1 in wheat would lead to a necrotic phenotype. Also, because ToxABP1 is highly conserved in plants, internal expression of ToxA in plants that do not normally internalize ToxA should result in cell death. *Reduction of ToxABP1 expression was achieved using Barley stripe mosaic virus (BSMV)-mediated, viral-induced gene silencing. The BSMV system was modified for use as an internal expression vector for ToxA in monocots. Agrobacterium-mediated expression of ToxA in a dicot (tobacco-Nicotiana benthamiana) was also performed. *Viral-induced gene silencing of ToxABP1 partially recapitulates the phenotype of ToxA treatment and wheat plants with reduced ToxABP1 also have reduced sensitivity to ToxA. When ToxA is expressed in ToxA-insensitive wheat, barley (Hordeum vulgare) and tobacco, cell death ensues. *ToxA accumulation in any chloroplast-containing cell is likely to result in cell death. Our data indicate that the ToxA-ToxABP1 interaction alters ToxABP1 function. This interaction is a critical, although not exclusive, component of the ToxA-induced cell death cascade.

BAX INHIBITOR-1 is required for full susceptibility of barley to powdery mildew

BAX INHIBITOR-1 (BI-1) is one of the few proteins known to have cross-kingdom conserved functions in negative control of programmed cell death. Additionally, barley BI-1 (HvBI-1) suppresses defense responses and basal resistance to the powdery mildew fungus Blumeria graminis f. sp. hordei and enhances resistance to cell death-provoking fungi when overexpressed in barley. Downregulation of HvBI-1 by transient-induced gene silencing or virus-induced gene silencing limited susceptibility to B. graminis f. sp. hordei, suggesting that HvBI-1 is a susceptibility factor toward powdery mildew. Transient silencing of BI-1 did not limit supersusceptibility induced by overexpression of MLO. Transgenic barley plants harboring an HvBI-1 RNA interference (RNAi) construct displayed lower levels of HvBI-1 transcripts and were less susceptible to powdery mildew than wild-type plants. At the cellular level, HvBI-1 RNAi plants had enhanced resistance to penetration by B. graminis f. sp. hordei. These data support a function of BI-1 in modulating cell-wall-associated defense and in establishing full compatibility of B. graminis f. sp. hordei with barley.

The predominance of the apoplasmic phloem-unloading pathway is interrupted by a symplasmic pathway during Chinese jujube fruit development

Phloem unloading plays a pivotal part in photoassimilate transport and partitioning into sink organs. However, it remains unclear whether the unloading pathway alters to adapt to developmental transitions in sinks, especially in fleshy fruits accumulating a high level of soluble sugars. Using a combination of electron microscopy, transport of the phloem-mobile symplasmic tracer carboxyfluorescein and assays of acid invertase, the pathway of unloading was investigated in different varieties of Chinese jujube fruit (Zizyphus jujuba Mill. cv Dongzao and Lizao). Structural investigation showed that the sieve element-companion-cell complex of bundles feeding the fruit flesh is symplasmically connected with surrounding parenchyma cells at the middle stage, and isolated during the early and late stages. Numerous plasmodesmata are present between phloem parenchyma cells and flesh cells throughout fruit development. Confocal laser scanning images of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands during the early and late stages of fruit development, whereas it was released from the functional phloem at the middle stage. The levels of both the expression and activities of cell wall acid invertase and soluble acid invertase varied in an inverse pattern relative to fruit development. These results provided clear evidence for the predominance of an apoplasmic phloem unloading pathway interrupted with a symplasmic pathway and simultaneous symplasmic and apoplasmic unloading pathways in post-phloem transport during fruit development. Similar unloading pathways were obtained in different varieties of jujube fruit. The mechanisms and significance of the adaptive switch in the phloem-unloading pathway during fruit development were discussed.

Activation of sucrose transport in defoliated Lolium perenne L.: an example of apoplastic phloem loading plasticity

The pathway of carbon phloem loading was examined in leaf tissues of the forage grass Lolium perenne. The effect of defoliation (leaf blade removal) on sucrose transport capacity was assessed in leaf sheaths as the major carbon source for regrowth. The pathway of carbon transport was assessed via a combination of electron microscopy, plasmolysis experiments and plasma membrane vesicles (PMVs) purified by aqueous two-phase partitioning from the microsomal fraction. Results support an apoplastic phloem loading mechanism. Imposition of an artificial proton-motive force to PMVs from leaf sheaths energized an active, transient and saturable uptake of sucrose (Suc). The affinity of Suc carriers for Suc was 580 microM in leaf sheaths of undefoliated plants. Defoliation induced a decrease of K(m) followed by an increase of V(max). A transporter was isolated from stubble (including leaf sheaths) cDNA libraries and functionally expressed in yeast. The level of L.perenne SUcrose Transporter 1 (LpSUT1) expression increased in leaf sheaths in response to defoliation. Taken together, the results indicate that Suc transport capacity increased in leaf sheaths of L. perenne in response to leaf blade removal. This increase might imply de novo synthesis of Suc transporters, including LpSUT1, and may represent one of the mechanisms contributing to rapid refoliation.

Tie-dyed1 encodes a novel, phloem-expressed transmembrane protein that functions in carbohydrate partitioning

Carbon is partitioned between export from the leaf and retention within the leaf, and this process is essential for all aspects of plant growth and development. In most plants, sucrose is loaded into the phloem of carbon-exporting leaves (sources), transported through the veins, and unloaded into carbon-importing tissues (sinks). We have taken a genetic approach to identify genes regulating carbon partitioning in maize (Zea mays). We identified a collection of mutants, called the tie-dyed (tdy) loci, that hyperaccumulate carbohydrates in regions of their leaves. To understand the molecular function of Tdy1, we cloned the gene. Tdy1 encodes a novel transmembrane protein present only in grasses, although two protein domains are conserved across angiosperms. We found that Tdy1 is expressed exclusively in phloem cells of both source and sink tissues, suggesting that Tdy1 may play a role in phloem loading and unloading processes. In addition, Tdy1 RNA accumulates in protophloem cells upon differentiation, suggesting that Tdy1 may function as soon as phloem cells become competent to transport assimilates. Monitoring the movement of a fluorescent, soluble dye showed that tdy1 leaves have retarded phloem loading. However, once the dye entered into the phloem, solute transport appeared equal in wild-type and tdy1 mutant plants, suggesting that tdy1 plants are not defective in phloem unloading. Therefore, even though Tdy1 RNA accumulates in source and sink tissues, we propose that TDY1 functions in carbon partitioning by promoting phloem loading. Possible roles for TDY1 are discussed.

Identification and characterization of a novel efficient resistance response to the furoviruses SBWMV and SBCMV in barley

The interaction between the furoviruses Soilborne cereal mosaic virus (SBCMV) and Soilborne wheat mosaic virus (SBWMV) and their main host wheat is well documented; however, to date, only a few reports have addressed the response of other cereal species to these viruses. Here, we show that, in contrast to wheat, barley germplasm is a rich source of resistance to furoviruses. Moreover, we demonstrate that barley genotypes respond differentially to SBCMV and SBWMV, thereby providing an additional biological basis for classification of these viruses as two separate species. Following natural (soil) inoculation, some barley genotypes permitted foliar infection by SBWMV, whereas all 22 genotypes tested were resistant to SBCMV. Resistance is unlikely to be directed toward the virus vector, because Polymyxa graminis DNA was detected in the roots of all tested genotypes. Resistance to SBCMV in some barley genotypes was overcome by artificial virus inoculation onto the leaves, suggesting a block on virus translocation from roots to shoots as in resistant wheat genotypes. However, other genotypes were fully resistant following both inoculation techniques. One barley genotype, 'Dayton,' exhibited extreme resistance to both furoviruses. Further molecular analyses suggested that this novel and highly efficient resistance to furoviruses in barley operates by limiting virus spread from the primary inoculated cells.

The silver lining of a viral agent: increasing seed yield and harvest index in Arabidopsis by ectopic expression of the potato leaf roll virus movement protein

Ectopic expression of viral movement proteins (MPs) has previously been shown to alter plasmodesmata (PD) function and carbon partitioning in transgenic plants, giving rise to the view of PD being dynamic and highly regulated structures that allow resource allocation to be adapted to environmental and developmental needs. However, most work has been restricted to solanaceous species and the potential use of MP expression to improve biomass and yield parameters has not been addressed in detail. Here we demonstrate that MP-mediated modification of PD function can substantially alter assimilate allocation, biomass production, and reproductive growth in Arabidopsis (Arabidopsis thaliana). These effects were achieved by constitutive expression of the potato leaf roll virus 17-kD MP (MP17) fused to green fluorescent protein (GFP) in different Arabidopsis ecotypes. The resulting transgenic plants were analyzed for PD localization of the MP17:GFP fusion protein and different lines with low to high expression levels were selected for further analysis. Low-level accumulation of MP17 resulted in enhanced sucrose efflux from source leaves and a considerably increased vegetative biomass production. In contrast, high MP17 levels impaired sucrose export, resulting in source leaf-specific carbohydrate accumulation and a strongly reduced vegetative growth. Surprisingly, later during development the MP17-mediated inhibition of resource allocation was reversed, and final seed yield increased in average up to 30% in different transgenic lines as compared to wild-type plants. This resulted in a strongly improved harvest index. The release of the assimilate export block was paralleled by a reduced PD binding of MP17 in senescing leaves, indicating major structural changes of PD during leaf senescence.

A shift of Phloem unloading from symplasmic to apoplasmic pathway is involved in developmental onset of ripening in grape berry

It remains unclear whether the phloem unloading pathway alters to adapt to developmental transition in fleshy fruits that accumulate high level of soluble sugars. Using a combination of electron microscopy, transport of the phloem-mobile symplasmic tracer carboxyfluorescein, movement of the companion cell-expressed and the green fluorescent protein-tagged viral movement protein, and assays of the sucrose cleavage enzymes, the pathway of phloem unloading was studied in the berries of a hybrid grape (Vitis vinifera x Vitis labrusca). Structural investigations showed that the sieve element-companion cell complex is apparently symplasmically connected through plasmodesmata with surrounding parenchyma cells throughout fruit development, though a small portion of plasmodesmata are apparently blocked in the ripening stage. Both carboxyfluorescein and the green fluorescent protein-tagged viral movement protein were released from the functional phloem strands during the early and middle stages of fruit development, whereas the two symplasmic tracers were confined to the phloem strands during the late stage. This reveals a shift of phloem unloading from symplasmic to apoplasmic pathway during fruit development. The turning point of the phloem unloading pathways was further shown to be at or just before onset of ripening, an important developmental checkpoint of grape berry. In addition, the levels of both the expression and activities of cell wall acid invertase increased around the onset of ripening and reached a high level in the late stage, providing further evidence for an operation of the apoplasmic unloading pathway after onset of ripening. These data demonstrate clearly the occurrence of an adaptive shift of phloem unloading pathway to developmental transition from growing phase to ripening in grape berry.

Development of Bean pod mottle virus-based vectors for stable protein expression and sequence-specific virus-induced gene silencing in soybean

Plant virus-based vectors provide valuable tools for expression of foreign proteins in plants and for gene function studies. None of the presently available virus vectors is suitable for use in soybean. In the present study, we produced Bean pod mottle virus (BPMV)-based vectors that are appropriate for gene expression and virus-induced gene silencing (VIGS) in soybean. The genes of interest were inserted into the RNA2-encoded polyprotein open reading frame between the movement protein (MP) and the large coat protein (L-CP) coding regions. Additional proteinase cleavage sites were created to flank the foreign protein by duplicating the MP/L-CP cleavage site. To minimize the chances of homologous recombination and thus insert instability, we took advantage of the genetic code degeneracy and altered the nucleotide sequence of the duplicated regions without affecting amino acid sequences. The recombinant BPMV constructs were stable following several serial passages in soybean and relatively high levels of protein expression were attained. Successful expression of several proteins with different biological activities was demonstrated from the BPMV vector. These included the reporter proteins GFP and DsRed, phosphinothricin acetyltransferase (encoded by the herbicide resistance bar gene), and the RNA silencing suppressors encoded by Tomato bushy stunt virus, Turnip crinkle virus, Tobacco etch virus, and Soybean mosaic virus. The possible use of BPMV as a VIGS vector to study gene function in soybean was also demonstrated with the phytoene desaturase gene. Our results suggest that the BPMV-based vectors are suitable for expression of foreign proteins in soybean and for functional genomics applications.

Virus-induced gene silencing-based functional characterization of genes associated with powdery mildew resistance in barley

We successfully implemented virus-induced gene silencing (VIGS) in barley (Hordeum vulgare) for the functional characterization of genes required for Mla13-mediated resistance toward the biotrophic barley pathogen Blumeria graminis f. sp. hordei. Initially, barley cultivars were screened for their ability to host the barley stripe mosaic virus (BSMV)-VIGS vector by allowing its replication and systemic movement without causing excessive symptoms. Phytoene desaturase silencing leading to photobleaching was used as a phenotypic marker alongside reverse transcription-PCR data to characterize the silencing response at the molecular level. Barley cultivar Clansman, harboring the Mla13 resistance gene, was chosen as the most suitable host for BSMV-VIGS-based functional characterization of Rar1, Sgt1, and Hsp90 in the Mla-mediated resistance toward powdery mildew. BSMV-induced gene silencing of these candidate genes, which are associated in many but not all race-specific pathways, proved to be robust and could be detected at both mRNA and protein levels for up to 21 d postinoculation. Systemic silencing was observed not only in the newly developed leaves from the main stem but also in axillary shoots. By examining fungal development from an incompatible mildew strain carrying the cognate Avr13 gene on plants BSMV silenced for Rar1, Sgt1, and Hsp90, a resistance-breaking phenotype was observed, while plants infected with BSMV control constructs remained resistant. We demonstrate that Hsp90 is a required component for Mla13-mediated race-specific resistance and that BSMV-induced VIGS is a powerful tool to characterize genes involved in pathogen resistance in barley.

Phloem unloading in developing walnut fruit is symplasmic in the seed pericarp and apoplasmic in the fleshy pericarp

The sieve element-companion cell (SE-CC) complex of the sepal bundles feeding the fleshy pericarp of developing walnut (Juglans regia L.) fruit is structurally symplasmically isolated, but the SE-CC complex of the minor ventral carpellary bundles located in the seed pericarp and feeding the seed is structurally symplasmically connected to its adjacent parenchyma cells. 14C-autoradiography indicated that the phloem of both the sepal and carpellary bundles was functional for unloading. Confocal laser scanning microscopy imaging of carboxyfluorescein unloading showed that the dye is confined to the phloem strands of the sepal bundles in the fleshy pericarp, but released from the phloem strands of the minor ventral carpellary bundles into the surrounding parenchyma cells in the seed pericarp. A 60-kDa acid invertase was immunolocalized to the cell wall of SE-CC complex and parenchyma cells in both the fleshy and seed pericarp. These data provide clear evidence for an apoplasmic phloem unloading pathway in the fleshy pericarp and a predominant symplasmic phloem unloading pathway parallel with a possible apoplasmic path as suggested by the presence of the extracellular invertase in the seed pericarp. A model of complex phloem unloading pathways in developing walnut fruit has been proposed.

Expression and localisation analysis of the wheat sucrose transporter TaSUT1 in vegetative tissues

Previously we reported the isolation of three sucrose transporter genes, TaSUT1A, 1B and 1D, all expressed at high levels in the developing grains of hexaploid wheat ( Triticum aestivum L.), but also in a variety of other tissues. In order to further characterise the expression of the TaSUT1 genes in wheat plants, we have analysed TaSUT1 expression in their vegetative tissues using semi-quantitative reverse transcription-polymerase chain reaction, in situ hybridisation and immunolocalisation. The three TaSUT1 genes, which encode 98% identical SUT proteins, all appeared to be expressed at the same level in leaf blades, leaf sheaths and internodes, as well as developing grains, of hexaploid wheat. In mature leaf blades, TaSUT1 protein localised to the plasma membrane of phloem sieve elements in all classes of veins. In contrast, TaSUT1 mRNA was found to be localised to phloem companion cells. A similar localisation pattern for TaSUT1 protein was observed in veins of leaf sheaths and internodes. These results suggest that the wheat SUT1 has a transport function in enucleate sieve elements, in both veins responsible for loading photoassimilates, and in veins for axial transport. Furthermore, transport of the fluorescent dye carboxyfluorescein was used to investigate symplasmic connectivity between sieve element-companion cell complexes and non-phloem cells. Observations in source leaves indicated that sieve element-companion cell complexes of minor veins were symplasmically restricted, suggesting a role of TaSUT1 in apoplasmic phloem loading. In contrast, the dye was able to move symplasmically out of the phloem in internodes. In these circumstances TaSUT1 may also have a role in retrieving sucrose leaked to the phloem apoplasm.

Evidence for apoplasmic phloem unloading in developing apple fruit

The phloem unloading pathway remains unclear in fleshy fruits accumulating a high level of soluble sugars. A structural investigation in apple fruit (Malus domestica Borkh. cv Golden Delicious) showed that the sieve element-companion cell complex of the sepal bundles feeding the fruit flesh is symplasmically isolated over fruit development. 14C-autoradiography indicated that the phloem of the sepal bundles was functional for unloading. Confocal laser scanning microscopy imaging of carboxyfluorescein unloading showed that the dye remained confined to the phloem strands of the sepal bundles from the basal to the apical region of the fruit. A 52-kD putative monosaccharide transporter was immunolocalized predominantly in the plasma membrane of both the sieve elements and parenchyma cells and its amount increased during fruit development. A 90-kD plasma membrane H(+)-ATPase was also localized in the plasma membrane of the sieve element-companion cell complex. Studies of [14C]sorbitol unloading suggested that an energy-driven monosaccharide transporter may be functional in phloem unloading. These data provide clear evidence for an apoplasmic phloem unloading pathway in apple fruit and give information on the structural and molecular features involved in this process.

Analysis of the systemic colonization of cucumber plants by Cucumber green mottle mosaic virus

Systemic movement of Cucumber green mottle mosaic virus (CGMMV) in cucumber plants was shown to be from photoassimilate source to sink, thus indicating phloem transport. Nevertheless, CGMMV was not detected by immunocytochemical procedures in the intermediary cell-sieve element complex in inoculated cotyledons, where photoassimilate loading occurs. In stem internodes, CGMMV was first localized in the companion cells of the external phloem and subsequently in all tissues except the medulla, therefore suggesting leakage of the virus from, and reloading into, the transport phloem during systemic movement. In systemically infected sink leaves, CGMMV was simultaneously detected in the xylem and phloem. Interestingly, CGMMV accumulated to high levels in the differentiating tracheids of young leaves implying that the xylem could be involved in the systemic movement of CGMMV. This possibility was tested using plants in which cell death was induced in a portion of the stem by steam treatment. At 24 degrees C, steam treatment effectively prevented the systemic movement of CGMMV, even though viral RNA was detected in washes of the xylem above the steamed internode suggesting that xylem circulation occurred. At 29 degrees C, CGMMV systemically infected steam-treated cucumber plants, indicating that CGMMV can move systemically via the xylem. Xylem transport of CGMMV was, however, less efficient than phloem transport in terms of the time required for systemic infection and the percentage of plants infected.

Enhancement of virus-induced gene silencing through viral-based production of inverted-repeats

Plant virus-based vectors carrying sequences homologous to endogenous genes trigger silencing through a homology-dependent RNA degradation mechanism. This phenomenon, called virus-induced gene silencing (VIGS), has potential as a powerful reverse-genetics tool in functional genomic programmes through transient, loss-of-function screens. Here, we describe a method to enhance the robustness of the VIGS phenotype by increasing the level of dsRNA molecule production, a critical step in the VIGS response. Incorporation of 40-60 base direct inverted-repeats into a plant viral vector generates RNA molecules that form dsRNA hairpins. A tobacco mosaic virus (TMV)-based vector carrying such inverted-repeats, homologous to a green fluorescent protein (gfp) transgene or an endogenous phytoene desaturase (pds) gene, generated a stronger and more pervasive VIGS phenotype than constructs carrying corresponding cDNA fragments in sense or antisense orientation. Real-time RT-PCR indicated that there was up to a threefold reduction in target mRNA accumulation in the tissues where VIGS was triggered by constructs carrying inverted-repeats compared to those where it was triggered by sense or antisense constructs. Moreover, an enhanced VIGS pds phenotype was observed using a different vector, based on barley stripe mosaic virus, in the monocotyledonous host barley. This demonstrates that VIGS can be significantly improved through the inclusion of small inverted-repeats in plant virus-based vectors, generating a more robust loss-of-function phenotype. This suggests that dsRNA formation can be a limiting factor in the VIGS phenomenon.

The sucrose transporter gene family in rice

In this paper we report the identification, cloning and expression analysis of four putative sucrose transporter (SUT) genes from rice, designated OsSUT2, 3, 4 and 5. Three of the four genes were identified through extensive searches of the recently published draft sequence of the rice genome. Along with the previously reported OsSUT1 we propose that these five genes comprise the rice SUT gene family. Complementary DNA clones were isolated for the four newly identified genes. The deduced proteins of all five SUT genes were predicted to contain 12 membrane-spanning helices and a domain highly conserved throughout all known plant SUTs, suggesting the four additional OsSUT genes encode functional SUTs. Reverse transcription-PCR analysis was performed in order to investigate the expression pattern of each member of the SUT family in rice. A differing but overlapping expression pattern was observed for each member of the SUT family at different stages through plant development. These results, together with the structural variations apparent from the deduced protein sequences, suggest that the five SUTs possess diverse roles in both sink and source tissues. We also discuss the classification and evolution of the rice SUT gene family, using a comparison of the gene structures and deduced amino acid sequences with other known plant SUT genes.