Parallel auxin transport via PINs and plasmodesmata during the Arabidopsis leaf hyponasty response

KEY MESSAGE

The leaf hyponasty response depends on tip-to-petiole auxin transport. This transport can happen through two parallel pathways: active trans-membrane transport mediated by PIN proteins and passive diffusion through plasmodesmata. A plant's ability to counteract potential shading by neighboring plants depends on transport of the hormone auxin. Neighbor sensing at the leaf tip triggers auxin production. Once this auxin reaches the abaxial petiole epidermis, it causes cell elongation, which leads to leaf hyponasty. Two pathways are known to contribute to this intercellular tip-to-petiole auxin movement: (i) transport facilitated by plasma membrane-localized PIN auxin transporters and (ii) diffusion enabled by plasmodesmata. We tested if these two modes of transport are arranged sequentially or in parallel. Moreover, we investigated if they are functionally linked. Mutants in which one of the two pathways is disrupted indicated that both pathways are necessary for a full hyponasty response. Visualization of PIN3-GFP and PIN7-GFP localization indicated PIN-mediated transport in parallel to plasmodesmata-mediated transport along abaxial midrib epidermis cells. We found plasmodesmata-mediated cell coupling in the pin3pin4pin7 mutant to match wild-type levels, indicating no redundancy between pathways. Similarly, PIN3, PIN4 and PIN7 mRNA levels were unaffected in a mutant with disrupted plasmodesmata pathway. Our results provide mechanistic insight on leaf hyponasty, which might facilitate the manipulation of the shade avoidance response in crops.

Diameters of phloem sieve elements can predict stem growth rates of woody plants

Understanding forest dynamics is crucial to addressing climate change and reforestation challenges. Plant anatomy can help predict growth rates of woody plants, contributing key information on forest dynamics. Although features of the water-transport system (xylem) have long been used to predict plant growth, the potential contribution of carbon-transporting tissue (phloem) remains virtually unexplored. Here, we use data from 347 woody plant species to investigate whether species-specific stem diameter growth rates can be predicted by the diameter of both the xylem and phloem conducting cells when corrected for phylogenetic relatedness. We found positive correlations between growth rate, phloem sieve element diameter and xylem vessel diameter in liana species sampled in the field. Moreover, we obtained similar results for data extracted from the Xylem Database, an online repository of functional, anatomical and image data for woody plant species. Information from this database confirmed the correlation of sieve element diameter and growth rate across woody plants of various growth forms. Furthermore, we used data subsets to explore potential influences of biomes, growth forms and botanical family association. Subsequently, we combined anatomical and geoclimatic data to train an artificial neural network to predict growth rates. Our results demonstrate that sugar transport architecture is associated with growth rate to a similar degree as water-transport architecture. Furthermore, our results illustrate the potential value of artificial neural networks for modeling plant growth under future climatic scenarios.

Arabidopsis plants engineered for high root sugar secretion enhance the diversity of soil microorganisms

Plants secrete sugars from their roots into the soil, presumably to support beneficial plant-microbe interactions. Accordingly, manipulation of sugar secretion might be a viable strategy to enhance plant health and productivity. To evaluate the effect of increased root sugar secretion on plant performance and the soil microbiome, we overexpressed glucose and sucrose-specific membrane transporters in root epidermal cells of the model plant Arabidopsis thaliana. These plants showed strongly increased rates of sugar secretion in a hydroponic culture system. When grown on soil, the transporter-overexpressor plants displayed a higher photosynthesis rate, but reduced shoot growth compared to the wild-type control. Amplicon sequencing and qPCR analysis of rhizosphere soil samples indicated a limited effect on the total abundance of bacteria and fungi, but a strong effect on community structure in soil samples associated with the overexpressors. Notable changes included the increased abundance of bacteria belonging to the genus Rhodanobacter and the fungi belonging to the genus Cutaneotrichosporon, while Candida species abundance was reduced. The potential influences of the altered soil microbiome on plant health and productivity are discussed. The results indicate that the engineering of sugar secretion can be a viable pathway to enhancing the carbon sequestration rate and optimizing the soil microbiome. This article is protected by copyright. All rights reserved.

Evidence for conifer sucrose transporters' functioning in the light-dependent adjustment of sugar allocation

Sucrose is the central unit of carbon and energy in plants. Active intercellular transport of sucrose is mediated by sucrose transporters (SUTs), genes for which have been found in the genomes of all land plants. However, they have only been assigned functions in angiosperm species. Here, we cloned two types of SUTs from two gymnosperms, the conifers Cedrus deodara (Roxb. G. Don) and Pinus massoniana Lambert, and analyzed their sucrose transport activities. Uptake of the fluorescent sucrose-analog esculin into tobacco epidermis cells expressing the conifer SUT confirmed their transport ability. To determine their function in planta, we investigated their mRNA abundance in relation to photosynthesis and sugar levels in leaves, inner bark, wood and roots. Combined with measurements of protein abundance and immunolocalization of C. deodara SUTs, our results suggest a role for CdSUT1G and CdSUT2 in supporting phloem transport under varying environmental conditions. The implications of these findings regarding conifer physiology and SUT evolution are discussed.

Mutation of CESA1 phosphorylation site influences pectin synthesis and methylesterification with a role in seed development

Cell wall biogenesis is required for the production of seeds of higher plants. However, little is known about regulatory mechanisms underlying cell wall biogenesis during seed formation. Here we show a role for the phosphorylation of Arabidopsis cellulose synthase 1 (AtCESA1) in modulating pectin synthesis and methylesterification in seed coat mucilage. A phosphor-null mutant of AtCESA1 on T166 (AtCESA1^T166A) was constructed and introduced into a null mutant of AtCESA1 (Atcesa1-1). The resulting transgenic lines showed a slight but significant decrease in cellulose contents in mature seeds. Defects in cellulosic ray architecture along with reduced levels of non-adherent and adherent mucilage were observed on the seeds of the AtCESA1^T166A mutant. Reduced mucilage pectin synthesis was also reflected by a decrease in the level of uronic acid. Meanwhile, an increase in the degree of pectin methylesterification was also observed in the seed coat mucilage of AtCESA1^T166A mutant. Change in seed development was further reflected by a delayed germination and about 50% increase in the accumulation of proanthocyanidins, which is known to bind pectin and inhibit seed germination as revealed by previous studies. Taken together, the results suggest a role of AtCESA1 phosphorylation on T166 in modulating mucilage pectin synthesis and methylesterification as well as cellulose synthesis with a role in seed development.

Comparative Analysis of Herbaceous and Woody Cell Wall Digestibility by Pathogenic Fungi

Fungal pathogens have evolved combinations of plant cell-wall-degrading enzymes (PCWDEs) to deconstruct host plant cell walls (PCWs). An understanding of this process is hoped to create a basis for improving plant biomass conversion efficiency into sustainable biofuels and bioproducts. Here, an approach integrating enzyme activity assay, biomass pretreatment, field emission scanning electron microscopy (FESEM), and genomic analysis of PCWDEs were applied to examine digestibility or degradability of selected woody and herbaceous biomass by pathogenic fungi. Preferred hydrolysis of apple tree branch, rapeseed straw, or wheat straw were observed by the apple-tree-specific pathogen Valsa mali, the rapeseed pathogen Sclerotinia sclerotiorum, and the wheat pathogen Rhizoctonia cerealis, respectively. Delignification by peracetic acid (PAA) pretreatment increased PCW digestibility, and the increase was generally more profound with non-host than host PCW substrates. Hemicellulase pretreatment slightly reduced or had no effect on hemicellulose content in the PCW substrates tested; however, the pretreatment significantly changed hydrolytic preferences of the selected pathogens, indicating a role of hemicellulose branching in PCW digestibility. Cellulose organization appears to also impact digestibility of host PCWs, as reflected by differences in cellulose microfibril organization in woody and herbaceous PCWs and variation in cellulose-binding domain organization in cellulases of pathogenic fungi, which is known to influence enzyme access to cellulose. Taken together, this study highlighted the importance of chemical structure of both hemicelluloses and cellulose in host PCW digestibility by fungal pathogens.

Persulfidation-induced structural change in SnRK2.6 establishes intramolecular interaction between phosphorylation and persulfidation

Post-translational modifications (PTMs), including phosphorylation and persulfidation, regulate the activity of SNF1-RELATED PROTEIN KINASE2.6 (SnRK2.6). Here, we report how persulfidations and phosphorylations of SnRK2.6 influence each other. The persulfidation of cysteine C131/C137 alters SnRK2.6 structure and brings the serine S175 residue closer to the aspartic acid D140 that acts as ATP-γ-phosphate proton acceptor, thereby improving the transfer efficiency of phosphate groups to S175 to enhance the phosphorylation level of S175. Interestingly, we predicted that S267 and C137 were predicted to lie in close proximity on the protein surface and found that the phosphorylation status of S267 positively regulates the persulfidation level at C137. Analyses of the responses of dephosphorylated and depersulfidated mutants to abscisic acid and the H₂S-donor NaHS during stomatal closure, water loss, gas exchange, Ca²⁺ influx, and drought stress revealed that S175/S267-associated phosphorylation and C131/137-associated persulfidation are essential for SnRK2.6 function in vivo. In light of these findings, we propose a mechanistic model in which certain phosphorylations facilitate persulfidation, thereby changing the structure of SnRK2.6 and increasing its activity.

Sugar export from Arabidopsis leaves: actors and regulatory strategies

Plant acclimation and stress responses depend on the dynamic optimization of carbon balance between source and sink organs. This optimization also applies to the leaf export rate of photosynthetically produced sugars. So far, investigations into the molecular mechanisms of how the rate is controlled have focused on sugar transporters responsible for loading sucrose into the phloem sieve element-companion cell complex of leaf veins. Here, we take a broader view of the various proteins with potential direct influence on the leaf sugar export rate in the model plant Arabidopsis thaliana, helped by the cell type-specific transcriptome data that have recently become available. Furthermore, we integrate current information on the regulation of these potential target proteins. Our analysis identifies putative control points and units of transcriptionally and post-transcriptionally co-regulated genes. Most notable is the potential regulatory unit of sucrose transporters (SUC2, SWEET11, SWEET12, and SUC4) and proton pumps (AHA3 and AVP1). Our analysis can guide future research aimed at understanding the regulatory network controlling leaf sugar export by providing starting points for characterizing regulatory strategies and identifying regulatory factors that link sugar export rate to the major signaling pathways.

The mechanism of sugar export from long conifer needles

The green leaves of plants are optimised for carbon fixation and the production of sugars, which are used as central units of carbon and energy throughout the plant. However, there are physical limits to this optimisation that remain insufficiently understood. Here, quantitative anatomical analysis combined with mathematical modelling and sugar transport rate measurements were used to determine how effectively sugars are exported from the needle-shaped leaves of conifers in relation to leaf length. Mathematical modelling indicated that phloem anatomy constrains sugar export in long needles. However, we identified two mechanisms by which this constraint is overcome, even in needles longer than 20 cm: (1) the grouping of transport conduits, and (2) a shift in the diurnal rhythm of sugar metabolism and export in needle tips. The efficiency of sugar transport in the phloem can have a significant influence on leaf function. The constraints on sugar export described here for conifer needles are likely to also be relevant in other groups of plants, such as grasses and angiosperm trees.

Reduced pectin content of cell walls prevents stress-induced root cell elongation in Arabidopsis

The primary cell walls of plants provide mechanical strength while maintaining the flexibility needed for cell extension growth. Cell extension involves loosening the bonds between cellulose microfibrils, hemicelluloses and pectins. Pectins have been implicated in this process, but it remains unclear if this depends on the abundance of certain pectins, their modifications, and/or structure. Here, cell wall-related mutants of the model plant Arabidopsis were characterized by biochemical and immunohistochemical methods and Fourier-transform infrared microspectroscopy. Mutants with reduced pectin or hemicellulose content showed no root cell elongation in response to simulated drought stress, in contrast to wild-type plants or mutants with reduced cellulose content. While no association was found between the degrees of pectin methylesterification and cell elongation, cell wall composition analysis suggested an important role of the pectin rhamnogalacturonan II (RGII), which was corroborated in experiments with the RGII-modifying chemical 2β-deoxy-Kdo. The results were complemented by expression analysis of cell wall synthesis genes and microscopic analysis of cell wall porosity. It is concluded that a certain amount of pectin is necessary for stress-induced root cell elongation, and hypotheses regarding the mechanistic basis of this result are formulated.

Pseudohyphal growth in Saccharomyces cerevisiae involves protein kinase-regulated lipid flippases

Lipid flippases of the P4 ATPase family establish phospholipid asymmetry in eukaryotic cell membranes and are involved in many essential cellular processes. The yeast Saccharomyces cerevisiae contains five P4 ATPases, among which Dnf3p is poorly characterized. Here, we demonstrate that Dnf3p is a flippase that catalyzes translocation of major glycerophospholipids, including phosphatidylserine, towards the cytosolic membrane leaflet. Deletion of the genes encoding Dnf3p and the distantly related P4 ATPases Dnf1p and Dnf2p results in yeast mutants with aberrant formation of pseudohyphae, suggesting that the Dnf1p-Dnf3p proteins have partly redundant functions in the control of this specialized form of polarized growth. Furthermore, as previously demonstrated for Dnf1 and Dnf2p, the phospholipid flipping activity of Dnf3p is positively regulated by flippase kinase 1 (Fpk1p) and Fpk2p. Phylogenetic analyses demonstrate that Dnf3p belongs to a subfamily of P4 ATPases specific for fungi and are likely to represent a hallmark of fungal evolution.

Non-invasive Quantification of Cell Wall Porosity by Fluorescence Quenching Microscopy

All bacteria, fungi and plant cells are surrounded by a cell wall. This complex network of polysaccharides and glycoproteins provides mechanical support, defines cell shape, controls cell growth and influences the exchange of substances between the cell and its surroundings. Despite its name, the cell wall is a flexible, dynamic structure. However, due to the lack of non-invasive methods to probe the structure, relatively little is known about the synthesis and dynamic remodeling of cell walls. Here, we describe a non-invasive method that quantifies a key physiological parameter of cell walls, the porosity, i.e., the size of spaces between cell wall components. This method measures the porosity-dependent decrease of the plasma membrane-localized fluorescent dye FM4-64 in the presence of the extracellular quencher Trypan blue. This method is applied to bacteria, fungi and plant cell walls to detect dynamic changes of porosity in response to environmental cues.

Direct Comparison of Leaf Plasmodesma Structure and Function in Relation to Phloem-Loading Type

The export of photosynthetically produced sugars from leaves depends on plasmodesmatal transport of sugar molecules from mesophyll to phloem. Traditionally, the density of plasmodesmata (PD) along this phloem-loading pathway has been used as a defining feature of different phloem-loading types, with species proposed to have either many or few PD between the phloem and surrounding cells of the leaf. However, quantitative determination of PD density has rarely been performed. Moreover, the structure of PD has not been considered, even though it could impact permeability, and functional data are only available for very few species. Here, a comparison of PD density, structure, and function using data from transmission electron microscopy and live-cell microscopy was conducted for all relevant cell-cell interfaces in leaves of nine species. These species represent the three principal phloem-loading types currently discussed in literature. Results show that relative PD density among the different cell-cell interfaces in one species, but not absolute PD density, is indicative of phloem-loading type. PD density data of single interfaces, even combined with PD diameter and length data, did not correlate with the intercellular diffusion capacity measured by the fluorescence loss in photobleaching method. This means that PD substructure not visible on standard transmission electron micrographs may have a strong influence on permeability. Furthermore, the results support a proposed passive symplasmic loading mechanism in the tree species horse chestnut (Aesculus hippocastanum), white birch (Betula pubescens), orchard apple (Malus domestica), and gray poplar (Populus x canescens) as functional cell coupling and PD structure differed from active symplasmic and apoplasmic phloem-loading species.

Novel tool to quantify cell wall porosity relates wall structure to cell growth and drug uptake

Even though cell walls have essential functions for bacteria, fungi, and plants, tools to investigate their dynamic structure in living cells have been missing. Here, it is shown that changes in the intensity of the plasma membrane dye FM4-64 in response to extracellular quenchers depend on the nano-scale porosity of cell walls. The correlation of quenching efficiency and cell wall porosity is supported by tests on various cell types, application of differently sized quenchers, and comparison of results with confocal, electron, and atomic force microscopy images. The quenching assay was used to investigate how changes in cell wall porosity affect the capability for extension growth in the model plant Arabidopsis thaliana Results suggest that increased porosity is not a precondition but a result of cell extension, thereby providing new insight on the mechanism plant organ growth. Furthermore, it was shown that higher cell wall porosity can facilitate the action of antifungal drugs in Saccharomyces cerevisiae, presumably by facilitating uptake.

Environmental conditions, not sugar export efficiency, limit the length of conifer leaves

Most conifer species have needle-shaped leaves that are only a few centimeters long. In general, variation in leaf size has been associated with environmental factors, such as cold or drought stress. However, it has recently been proposed that sugar export efficiency is the limiting factor for conifer needle length, based on the results obtained using a mathematical model of phloem transport. Here, phloem transport rates in long conifer needles were experimentally determined to test if the mathematical model accurately represents phloem transport. The validity of the model's assumptions was tested by anatomical analyses and sugar quantification. Furthermore, various environmental and physiological factors were tested for their correlation with needle length. The results indicate that needle length is not limited by sugar transport efficiency, but, instead, by winter temperatures and light availability. The identification of factors that influence needle size is instrumental for using this trait as a variable in breeding programs.

Noninvasive Determination of Phloem Transport Speed with Carbon-14 (14C)

Studying the phloem, through which organic substances are distributed between plant organs, is challenging because of its position deep inside the plant body and its sensitivity to manipulation. The speed of phloem transport can be studied by tracers. Here a protocol for the use of ¹⁴C-labeled photoassimilate to measure phloem transport speed is provided. A major advantage of this method is its noninvasiveness, as the isotope is supplied as ¹⁴CO₂, which is converted in source leaves to ¹⁴C-sugars, whose movement is then followed by photomultiplier-based X-ray detectors positioned close to the stem. The same method can be used to determine partitioning among sinks over time and rates of export from sources. The relatively simple handling enables medium throughput experiments under controlled conditions.

Phloem transport in gymnosperms: a question of pressure and resistance

Even in the highest trees, carbon is efficiently distributed from leaves to heterotrophic tissues like fruit, flowers and roots. This long-distance transport happens in the highly specialized sieve elements of the phloem. In gymnosperms, sieve element anatomy appears to be less suited for mass flow of phloem sap than that of angiosperms. This review covers available data on gymnosperm phloem to evaluate if it functions differently from that of angiosperms. Although current evidence suggests that, despite a higher pathway resistance, a single source-to-sink turgor pressure gradient can drive mass flow, several questions remain unanswered. These include how endoplasmic reticulum-complexes in sieve elements influence flow, as well as what the effect of symplasmic coupling along the whole phloem pathway could be.

Ethylene-Induced Hydrogen Sulfide Negatively Regulates Ethylene Biosynthesis by Persulfidation of ACO in Tomato Under Osmotic Stress

A number of recent studies identified hydrogen sulfide (H₂S) as an important signal in plant development and adaptation to environmental stress. H₂S has been proven to participate in ethylene-induced stomatal closure, but how the signaling pathways of H₂S and ethylene interact is still unclear. Here, we reveal how H₂S controls the feedback-regulation of ethylene biosynthesis in tomato (Solanum lycopersicum) under osmotic stress. We found that ethylene induced the production of H₂S in guard cells. The supply of hypotaurine (HT; a H₂S scavenger) or DL-pro-pargylglycine (PAG; a synthetic inhibitor of H₂S) removed the effect of ethylene or osmotic stress on stomatal closure. This suggests that ethylene-induced H₂S is a downstream component of osmotic stress signaling, which is required for ethylene-induced stomatal closure under osmotic stress. We further found that H₂S inhibited ethylene synthesis through inhibiting the activity of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidases (ACOs) by persulfidation. A modified biotin-switch method (MBST) showed that H₂S can induce persulfidation of LeACO1 and LeACO2 in a dose-dependent manner, and that persulfidation inhibits the activity of LeACO1 and LeACO2. We also found that LeACO1 is persulfidated at cysteine 60. These data suggested that ethylene-induced H₂S negatively regulates ethylene biosynthesis by persulfidation of LeACOs. In addition, H₂S was also found to inhibit the expression of LeACO genes. The results provide insight on the general mode of action of H₂S and contribute to a better understanding of a plant's response to osmotic stress.

Regulation of Sucrose Transporters and Phloem Loading in Response to Environmental Cues

Suc transporters (SUTs) play a key role in the allocation and partitioning of photosynthetically fixed carbon in plants. While a function could be assigned to many members of the SUT family, almost no information is available on their regulation. Here, the transcriptional regulation of SUTs in response to various environmental stimuli in the leaves of five dicots (Arabidopsis [Arabidopsis thaliana], soybean [Glycine max], potato [Solanum tuberosum], tomato [Solanumlycopersicum], and poplar [Populus spp.]) and four monocots (maize [Zeamays], rice [Oryza sativa], wheat [Triticum aestivum], and barley [Hordeum vulgare]) was investigated. Extensive data on expression of SUTs in relation to changes of environmental conditions were obtained through a global analysis of 168 transcriptomics data sets. Results were validated by quantitative PCR measurements and extended by the measurement of photosynthesis rate and phloem sugar content to draw insight on the correlation of SUT expression and sugar export from leaves. For the apoplasmic phloem loaders, a clear difference in transcriptional regulation in response to different environmental stimuli was observed. The consistent patterns of SUT expression under abiotic stress indicates which types of SUTs are involved in the regulation of leaf sugar status and in stress signaling. Furthermore, it is shown that down-regulation of phloem loading is likely to be caused by transcriptional regulation of SUTs, while up-regulation depends on post-transcriptional regulation. In poplar, expression of PtaSUT4 was found to consistently respond to environmental stimuli, suggesting a significant role in the regulation of sugar export from leaves in this passive symplasmic phloem loader.

Ethylene promotes pollen tube growth by affecting actin filament organization via the cGMP-dependent pathway in Arabidopsis thaliana

Ethylene and cGMP are key regulators of plant developmental processes. In this study, we demonstrate that ethylene or cGMP promote pollen tube growth in a dose-dependent manner. The etr1-1 mutant was found to be insensitive to ethylene with regard to pollen tube growth, while the growth-promoting effect of ethylene in etr2-2, ein4-4, or ein4-7 did not change, suggesting that ethylene signaling was mainly perceived by ETR1. However, the function of cGMP was not inhibited in etr1-1 and pollen tubes became insensitive to ethylene when the endogenous cGMP level was artificially decreased. This shows that cGMP is necessary for the control of pollen tube growth and that it might be a downstream component of ETR1 in the ethylene signaling pathway. Our study also found that ethylene or cGMP increase the actin bundles and elevated the percentage of relative amount of F-actin, while removal of cGMP decreased actin bundles abundance and altered the ratio of F-actin in the tip and base regions of pollen tubes. In conclusion, our data suggests that ethylene functions as the upstream signal of cGMP, and that both signals promote pollen germination and tube growth by regulating F-actin, which is essential for vesicular transport and cytoplasmic streaming.

An update on phloem transport: a simple bulk flow under complex regulation

The phloem plays a central role in transporting resources and signalling molecules from fully expanded leaves to provide precursors for, and to direct development of, heterotrophic organs located throughout the plant body. We review recent advances in understanding mechanisms regulating loading and unloading of resources into, and from, the phloem network; highlight unresolved questions regarding the physiological significance of the vast array of proteins and RNAs found in phloem saps; and evaluate proposed structure/function relationships considered to account for bulk flow of sap, sustained at high rates and over long distances, through the transport phloem.

Sucrose transporters and plasmodesmal regulation in passive phloem loading

An essential step for the distribution of carbon throughout the whole plant is the loading of sugars into the phloem in source organs. In many plants, accumulation of sugars in the sieve element-companion cell (SE-CC) complex is mediated and regulated by active processes. However, for poplar and many other tree species, a passive symplasmic mechanism of phloem loading has been proposed, characterized by symplasmic continuity along the pre-phloem pathway and the absence of active sugar accumulation in the SE-CC complex. A high overall leaf sugar concentration is thought to enable diffusion of sucrose into the phloem. In this review, we critically evaluate current evidence regarding the mechanism of passive symplasmic phloem loading, with a focus on the potential influence of active sugar transport and plasmodesmal regulation. The limited experimental data, combined with theoretical considerations, suggest that a concomitant operation of passive symplasmic and active phloem loading in the same minor vein is unlikely. However, active sugar transport could well play an important role in how passively loading plants might modulate the rate of sugar export from leaves. Insights into the operation of this mechanism has direct implications for our understanding of how these plants utilize assimilated carbon.

Height-related scaling of phloem anatomy and the evolution of sieve element end wall types in woody plants

In the sieve elements (SEs) of the phloem, carbohydrates are transported throughout the whole plant from their site of production to sites of consumption or storage. SE structure, especially of the pore-rich end walls, has a direct effect on translocation efficiency. Differences in pore size and other features were interpreted as an evolutionary trend towards reduced hydraulic resistance. However, this has never been confirmed. Anatomical data of 447 species of woody angiosperms and gymnosperms were used for a phylogenetic analysis of end wall types, calculation of hydraulic resistance and correlation analysis with morphological and physiological variables. end wall types were defined according to pore arrangement: either grouped into a single area (simple) or into multiple areas along the end wall (compound). Convergent evolution of end wall types was demonstrated in woody angiosperms. In addition, an optimization of end wall resistance with plant height was discovered, but found to be independent of end wall type. While physiological factors also showed no correlation with end wall types, the number of sieve areas per end wall was found to scale with SE length. The results exclude the minimization of hydraulic resistance as evolutionary driver of different end wall types, contradicting this long-standing assumption. Instead, end wall type might depend on SE length.

Mother-plant-mediated pumping of zinc into the developing seed

Insufficient intake of zinc and iron from a cereal-based diet is one of the causes of 'hidden hunger' (micronutrient deficiency), which affects some two billion people(1,2). Identifying a limiting factor in the molecular mechanism of zinc loading into seeds is an important step towards determining the genetic basis for variation of grain micronutrient content and developing breeding strategies to improve this trait(3). Nutrients are translocated to developing seeds at a rate that is regulated by transport processes in source leaves, in the phloem vascular pathway, and at seed sinks. Nutrients are released from a symplasmic maternal seed domain into the seed apoplasm surrounding the endosperm and embryo by poorly understood membrane transport processes(4-6). Plants are unique among eukaryotes in having specific P1B-ATPase pumps for the cellular export of zinc(7). In Arabidopsis, we show that two zinc transporting P1B-ATPases actively export zinc from the mother plant to the filial tissues. Mutant plants that lack both zinc pumps accumulate zinc in the seed coat and consequently have vastly reduced amounts of zinc inside the seed. Blockage of zinc transport was observed at both high and low external zinc supplies. The phenotype was determined by the mother plant and is thus due to a lack of zinc pump activity in the seed coat and not in the filial tissues. The finding that P1B-ATPases are one of the limiting factors controlling the amount of zinc inside a seed is an important step towards combating nutritional zinc deficiency worldwide.

Slower phloem transport in gymnosperm trees can be attributed to higher sieve element resistance

In trees, carbohydrates produced in photosynthesizing leaves are transported to roots and other sink organs over distances of up to 100 m inside a specialized transport tissue, the phloem. Angiosperm and gymnosperm trees have a fundamentally different phloem anatomy with respect to cell size, shape and connectivity. Whether these differences have an effect on the physiology of carbohydrate transport, however, is not clear. A meta-analysis of the experimental data on phloem transport speed in trees yielded average speeds of 56 cm h(-1) for angiosperm trees and 22 cm h(-1) for gymnosperm trees. Similar values resulted from theoretical modeling using a simple transport resistance model. Analysis of the model parameters clearly identified sieve element (SE) anatomy as the main factor for the significantly slower carbohydrate transport speed inside the phloem in gymnosperm compared with angiosperm trees. In order to investigate the influence of SE anatomy on the hydraulic resistance, anatomical data on SEs and sieve pores were collected by transmission electron microscopy analysis and from the literature for 18 tree species. Calculations showed that the hydraulic resistance is significantly higher in the gymnosperm than in angiosperm trees. The higher resistance is only partially offset by the considerably longer SEs of gymnosperms.

Scaling of phloem structure and optimality of photoassimilate transport in conifer needles

The phloem vascular system facilitates transport of energy-rich sugar and signalling molecules in plants, thus permitting long-range communication within the organism and growth of non-photosynthesizing organs such as roots and fruits. The flow is driven by osmotic pressure, generated by differences in sugar concentration between distal parts of the plant. The phloem is an intricate distribution system, and many questions about its regulation and structural diversity remain unanswered. Here, we investigate the phloem structure in the simplest possible geometry: a linear leaf, found, for example, in the needles of conifer trees. We measure the phloem structure in four tree species representing a diverse set of habitats and needle sizes, from 1 (Picea omorika) to 35 cm (Pinus palustris). We show that the phloem shares common traits across these four species and find that the size of its conductive elements obeys a power law. We present a minimal model that accounts for these common traits and takes into account the transport strategy and natural constraints. This minimal model predicts a power law phloem distribution consistent with transport energy minimization, suggesting that energetics are more important than translocation speed at the leaf level.

Cell wall staining with Trypan blue enables quantitative analysis of morphological changes in yeast cells

Yeast cells are protected by a cell wall that plays an important role in the exchange of substances with the environment. The cell wall structure is dynamic and can adapt to different physiological states or environmental conditions. For the investigation of morphological changes, selective staining with fluorescent dyes is a valuable tool. Furthermore, cell wall staining is used to facilitate sub-cellular localization experiments with fluorescently-labeled proteins and the detection of yeast cells in non-fungal host tissues. Here, we report staining of Saccharomyces cerevisiae cell wall with Trypan Blue, which emits strong red fluorescence upon binding to chitin and yeast glucan; thereby, it facilitates cell wall analysis by confocal and super-resolution microscopy. The staining pattern of Trypan Blue was similar to that of the widely used UV-excitable, blue fluorescent cell wall stain Calcofluor White. Trypan Blue staining facilitated quantification of cell size and cell wall volume when utilizing the optical sectioning capacity of a confocal microscope. This enabled the quantification of morphological changes during growth under anaerobic conditions and in the presence of chemicals, demonstrating the potential of this approach for morphological investigations or screening assays.

Quantification of plant cell coupling with live-cell microscopy

Movement of nutrients and signaling compounds from cell to cell is an essential process for plant growth and development. To understand processes such as carbon allocation, cell communication, and reaction to pathogen attack it is important to know a specific molecule's capacity to pass a specific cell wall interface. Transport through plasmodesmata, the cell wall channels that directly connect plant cells, is regulated not only by a fixed size exclusion limit, but also by physiological and pathological adaptation. The noninvasive approach described here offers the possibility of precisely determining the plasmodesmata-mediated cell wall permeability for small molecules in living cells.The method is based on photoactivation of the fluorescent tracer caged fluorescein. Non-fluorescent caged fluorescein is applied to a target tissue, where it is taken up passively into all cells. Imaged by confocal microscopy, loaded tracer is activated by UV illumination in a target cell and its spread to neighboring cells monitored. When combined with high-speed acquisition by resonant scanning or spinning disc confocal microscopy, the high signal-to-noise ratio of photoactivation allows collection of three-dimensional (3D) time series. These contain all necessary functional and anatomical data to measure cell coupling in complex tissues noninvasively.

Diffusion and bulk flow in phloem loading: a theoretical analysis of the polymer trap mechanism for sugar transport in plants

Plants create sugar in the mesophyll cells of their leaves by photosynthesis. This sugar, mostly sucrose, has to be loaded via the bundle sheath into the phloem vascular system (the sieve elements), where it is distributed to growing parts of the plant. We analyze the feasibility of a particular loading mechanism, active symplasmic loading, also called the polymer trap mechanism, where sucrose is transformed into heavier sugars, such as raffinose and stachyose, in the intermediary-type companion cells bordering the sieve elements in the minor veins of the phloem. Keeping the heavier sugars from diffusing back requires that the plasmodesmata connecting the bundle sheath with the intermediary cell act as extremely precise filters, which are able to distinguish between molecules that differ by less than 20% in size. In our modeling, we take into account the coupled water and sugar movement across the relevant interfaces, without explicitly considering the chemical reactions transforming the sucrose into the heavier sugars. Based on the available data for plasmodesmata geometry, sugar concentrations, and flux rates, we conclude that this mechanism can in principle function, but that it requires pores of molecular sizes. Comparing with the somewhat uncertain experimental values for sugar export rates, we expect the pores to be only 5%-10% larger than the hydraulic radius of the sucrose molecules. We find that the water flow through the plasmodesmata, which has not been quantified before, contributes only 10%-20% to the sucrose flux into the intermediary cells, while the main part is transported by diffusion. On the other hand, the subsequent sugar translocation into the sieve elements would very likely be carried predominantly by bulk water flow through the plasmodesmata. Thus, in contrast to apoplasmic loaders, all the necessary water for phloem translocation would be supplied in this way with no need for additional water uptake across the plasma membranes of the phloem.

Super-resolution imaging with Pontamine Fast Scarlet 4BS enables direct visualization of cellulose orientation and cell connection architecture in onion epidermis cells

BACKGROUND

In plants, a complex cell wall protects cells and defines their shape. Cellulose fibrils form a multilayered network inside the cell-wall matrix that plays a direct role in controlling cell expansion. Resolving the structure of this network will allow us to comprehend the relationship of cellulose fibril orientation and growth.The fluorescent dye Pontamine Fast Scarlet 4BS (PFS) was shown to stain cellulose with high specificity and could be used to visualize cellulose bundles in cell walls of Arabidopsis root epidermal cells with confocal microscopy. The resolution limit of confocal microscopy of some 200 nm in xy and 550 nm in z for green light, restricts the direct visualization of cellulose to relatively large bundles, whereas the structure of cellulose microfibrils with their diameter below 10 nm remains unresolved. Over the last decade, several so-called super-resolution microscopy approaches have been developed; in this paper we explore the potential of such approaches for the direct visualization of cellulose.

RESULTS

To ensure optimal imaging we determined the spectral properties of PFS-stained tissue. PFS was found not to affect cell viability in the onion bulb scale epidermis. We present the first super-resolution images of cellulose bundles in the plant cell wall produced by direct stochastic optical reconstruction microscopy (dSTORM) in combination with total internal reflection fluorescence (TIRF) microscopy. Since TIRF limits observation to the cell surface, we tested as alternatives 3D-structured illumination microscopy (3D-SIM) and confocal microscopy, combined with image deconvolution. Both methods offer lower resolution than STORM, but enable 3D imaging. While 3D-SIM produced strong artifacts, deconvolution gave good results. The resolution was improved over conventional confocal microscopy and the approach could be used to demonstrate differences in fibril orientation in different layers of the cell wall as well as particular cellulose fortifications around plasmodesmata.

CONCLUSIONS

Super-resolution light microscopy of PFS-stained cellulose fibrils is possible and the increased resolution over conventional approaches makes it a valuable tool for the investigation of the cell-wall structure. This is one step in method developments that will close the gap to more invasive techniques, such as atomic force and electron microscopy.

Modeling the parameters for plasmodesmal sugar filtering in active symplasmic phloem loaders

Plasmodesmata (PD) play a key role in loading of sugars into the phloem. In plant species that employ the so-called active symplasmic loading strategy, sucrose that diffuses into their unique intermediary cells (ICs) is converted into sugar oligomers. According to the prevalent hypothesis, the oligomers are too large to pass back through PD on the bundle sheath side, but can pass on into the sieve element to be transported in the phloem. Here, we investigate if the PD at the bundle sheath-IC interface can indeed fulfill the function of blocking transport of sugar oligomers while still enabling efficient diffusion of sucrose. Hindrance factors are derived via theoretical modeling for different PD substructure configurations: sub-nano channels, slit, and hydrogel. The results suggest that a strong discrimination could only be realized when the PD opening is almost as small as the sugar oligomers. In order to find model parameters that match the in vivo situation, we measured the effective diffusion coefficient across the interface in question in Cucurbita pepo with 3D-photoactivation microscopy. Calculations indicate that a PD substructure of several sub-nano channels with a radius around 7 Å, a 10.4 Å-wide slit or a hydrogel with 49% polymer fraction would be compatible with the effective diffusion coefficient. If these configurations can accommodate sufficient flux of sucrose into the IC, while blocking raffinose and stachyose movement was assessed using literature data. While the slit-configuration would efficiently prevent the sugar oligomers from "leaking" from the IC, none of the configurations could enable a diffusion-driven sucrose flux that matches the reported rates at a physiologically relevant concentration potential. The presented data provides a first insight on how the substructure of PD could enable selective transport, but indicates that additional factors are involved in efficient phloem loading in active symplasmic loading species.

Photoperiodic regulation of the sucrose transporter StSUT4 affects the expression of circadian-regulated genes and ethylene production

Several recent publications reported different subcellular localization of the sucrose transporters belonging to the SUT4 subfamily. The physiological function of the SUT4 sucrose transporters requires clarification, because down-regulation of the members of the SUT4 clade had different effects in rice, poplar, and potato. Here, we provide new data for the localization and function of the Solanaceous StSUT4 protein, further elucidating involvement in the onset of flowering, tuberization and in the shade avoidance syndrome of potato plants. Induction of an early flowering and a tuberization in the SUT4-inhibited potato plants correlates with increased sucrose export from leaves and increased sucrose and starch accumulation in terminal sink organs, such as developing tubers. SUT4 affects expression of the enzymes involved in gibberellin and ethylene biosynthesis, as well as the rate of ethylene biosynthesis in potato. In the SUT4-inhibited plants, the ethylene production no longer follows a diurnal rhythm. Thus it was concluded that StSUT4 controls circadian gene expression, potentially by regulating sucrose export from leaves. Furthermore, SUT4 expression affects clock-regulated genes such as StFT, StSOC1, and StCO, which might be also involved in a photoperiod-dependent tuberization. A model is proposed in which StSUT4 controls a phloem-mobile signaling molecule generated in leaves, which together with enhanced sucrose export affects developmental switches in apical meristems. SUT4 seems to link photoreceptor-perceived information about the light quality and day length with phytohormone biosynthesis and the expression of circadian-regulated genes.

Unraveling the structure of viral replication complexes at super-resolution

During infection, many RNA viruses produce characteristic inclusion bodies that contain both viral and host components. These structures were first described over a century ago and originally termed "X-bodies," as their function was not immediately appreciated. Whilst some inclusion bodies may represent cytopathic by-products of viral protein over-accumulation, X-bodies have emerged as virus "factories," quasi-organelles that coordinate diverse viral infection processes such as replication, protein expression, evasion of host defenses, virion assembly, and intercellular transport. Accordingly, they are now generally referred to as viral replication complexes (VRCs). We previously used confocal fluorescence microscopy to unravel the complex structure of X-bodies produced by Potato virus X (PVX). Here we used 3D-structured illumination (3D-SIM) super-resolution microscopy to map the PVX X-body at a finer scale. We identify a previously unrecognized membrane structure induced by the PVX "triple gene block" (TGB) proteins, providing new insights into the complex interplay between virus and host within the X-body.

Universality of phloem transport in seed plants

Since Münch in the 1920s proposed that sugar transport in the phloem vascular system is driven by osmotic pressure gradients, his hypothesis has been strongly supported by evidence from herbaceous angiosperms. Experimental constraints made it difficult to test this proposal in large trees, where the distance between source and sink might prove incompatible with the hypothesis. Recently, the theoretical optimization of the Münch mechanism was shown to lead to surprisingly simple predictions for the dimensions of the phloem sieve elements in relation to that of fast growing angiosperms. These results can be obtained in a very transparent way using a simple coupled resistor model. To test the universality of the Münch mechanism, we compiled anatomical data for 32 angiosperm and 38 gymnosperm trees with heights spanning 0.1-50 m. The species studied showed a remarkable correlation with the scaling predictions. The compiled data allowed calculating stem sieve element conductivity and predicting phloem sap flow velocity. The central finding of this work is that all vascular plants seem to have evolved efficient osmotic pumping units, despite their huge disparity in size and morphology. This contribution extends the physical understanding of phloem transport, and will facilitate detailed comparison between theory and field experiments.

In vivo quantification of cell coupling in plants with different phloem-loading strategies

Uptake of photoassimilates into the leaf phloem is the key step in carbon partitioning and phloem transport. Symplasmic and apoplasmic loading strategies have been defined in different plant taxa based on the abundance of plasmodesmata between mesophyll and phloem. For apoplasmic loading to occur, an absence of plasmodesmata is a sufficient but not a necessary criterion, as passage of molecules through plasmodesmata might well be blocked or restricted. Here, we present a noninvasive, whole-plant approach to test symplasmic coupling and quantify the intercellular flux of small molecules using photoactivation microscopy. Quantification of coupling between all cells along the prephloem pathways of the apoplasmic loader Vicia faba and Nicotiana tabacum showed, to our knowledge for the first time in vivo, that small solutes like sucrose can diffuse through plasmodesmata up to the phloem sieve element companion cell complex (SECCC). As expected, the SECCC was found to be symplasmically isolated for small solutes. In contrast, the prephloem pathway of the symplasmic loader Cucurbita maxima was found to be well coupled with the SECCC. Phloem loading in gymnosperms is not well understood, due to a profoundly different leaf anatomy and a scarcity of molecular data compared with angiosperms. A cell-coupling analysis for Pinus sylvestris showed high symplasmic coupling along the entire prephloem pathway, comprising at least seven cell border interfaces between mesophyll and sieve elements. Cell coupling together with measurements of leaf sap osmolality indicate a passive symplasmic loading type. Similarities and differences of this loading type with that of angiosperm trees are discussed.

Quantification of plant cell coupling with three-dimensional photoactivation microscopy

Plant cells are directly connected by plasmodesmata that form channels through the cell wall and enable the intercellular movement of cytosolic solutes, membrane lipids and signalling molecules. Transport through plasmodesmata is regulated not only by a fixed size-exclusion limit, but also by physiological and pathological adaptation. To understand plant cell communication, carbon allocation and pathogen attack, the capacities for a specific molecule to pass a specific cell-wall interface is an essential parameter. So far, the degree of cell coupling was derived from frequency and diameter of plasmodesmata in relevant tissues as assessed by electron microscopy of fixed material. However, plasmodesmata functionality and capacity can only be determined in live material, not from electron microscopy, which is static and prone to fixation artefacts. Plasmodesmata functionality was a few times assessed using fluorescent tracers with diffusion properties similar to cytosolic solutes. Here, we used three-dimensional photoactivation microscopy to quantify plasmodesmata-mediated cell-wall permeability between living Cucurbita maxima leaf mesophyll cells with caged fluorescein as tracer. For the first time, all necessary functional and anatomical data were gathered for each individual cell from three-dimensional time series. This approach utilized a confocal microscope equipped with resonant scanner, which provides the high acquisition speed necessary to record optical sections of whole cells and offers time resolution high enough to follow the kinetics of photoactivation. The results were compared to two-dimensional measurements, which are shown to give a good estimate of cell coupling adequate for homogenous tissues. The two-dimensional approach is limited whenever tissues interfaces are studied that couple different cell types with diverse cell geometries.

Sucrose transporter regulation at the transcriptional, post-transcriptional and post-translational level

Sucrose transporters are crucial to carbon partitioning in higher plants. With their role in loading of sucrose into the phloem they control sucrose distribution throughout the whole plant and drive the osmotic flow system in the phloem. Recently, first insight was obtained on the coordination of sucrose transporter action with plant growth and development. The analysis of transgenic plants with reduced or enhanced expression of sucrose transporters helped to elucidate their physiological function and regulation in detail and connections to light and hormone signalling pathways were discovered. Whereas members of the SUT1 subfamily of sucrose transporters seem to be tightly controlled at the transcriptional and post-translational level in solanaceous plants, other family members show primarily post-transcriptional control of their mRNA stability. Post-translational regulation of sucrose transporters might be affected by direct protein-protein interactions or by recycling of sucrose transporters at the plasma membrane. A model is proposed showing cell-to-cell movement of both the SUT1 mRNA as well as the SUT1 protein via the desmotubule connecting companion cells where transcription of sucrose transporters occurs, and the neighbouring sieve elements. We provide an overview over sucrose transporter regulation in Solanum species at the transcriptional, post-transcriptional and post-translational level with emphasis on the many old and new questions surrounding the topic and how they could be answered.

Symplasmic transport and phloem loading in gymnosperm leaves

Despite more than 130 years of research, phloem loading is far from being understood in gymnosperms. In part this is due to the special architecture of their leaves. They differ from angiosperm leaves among others by having a transfusion tissue between bundle sheath and the axial vascular elements. This article reviews the somewhat inaccessible and/or neglected literature and identifies the key points for pre-phloem transport and loading of photoassimilates. The pre-phloem pathway of assimilates is structurally characterized by a high number of plasmodesmata between all cell types starting in the mesophyll and continuing via bundle sheath, transfusion parenchyma, Strasburger cells up to the sieve elements. Occurrence of median cavities and branching indicates that primary plasmodesmata get secondarily modified and multiplied during expansion growth. Only functional tests can elucidate whether this symplasmic pathway is indeed continuous for assimilates, and if phloem loading in gymnosperms is comparable with the symplasmic loading mode in many angiosperm trees. In contrast to angiosperms, the bundle sheath has properties of an endodermis and is equipped with Casparian strips or other wall modifications that form a domain border for any apoplasmic transport. It constitutes a key point of control for nutrient transport, where the opposing flow of mineral nutrients and photoassimilates has to be accommodated in each single cell, bringing to mind the principle of a revolving door. The review lists a number of experiments needed to elucidate the mode of phloem loading in gymnosperms.

Recycling of Solanum sucrose transporters expressed in yeast, tobacco, and in mature phloem sieve elements

The plant sucrose transporter SUT1 (from Solanum tuberosum, S. lycopersicum, or Zea mays) exhibits redox-dependent dimerization and targeting if heterologously expressed in S. cerevisiae (Krügel et al., 2008). It was also shown that SUT1 is present in motile vesicles when expressed in tobacco cells and that its targeting to the plasma membrane is reversible. StSUT1 is internalized in the presence of brefeldin A (BFA) in yeast, plant cells, and in mature sieve elements as confirmed by immunolocalization. These results were confirmed here and the dynamics of intracellular SUT1 localization were further elucidated. Inhibitor studies revealed that vesicle movement of SUT1 is actin-dependent. BFA-mediated effects might indicate that anterograde vesicle movement is possible even in mature sieve elements, and could involve components of the cytoskeleton that were previously thought to be absent in SEs. Our results are in contradiction to this old dogma of plant physiology and the potential of mature sieve elements should therefore be re-evaluated. In addition, SUT1 internalization was found to be dependent on the plasma membrane lipid composition. SUT1 belongs to the detergent-resistant membrane (DRM) fraction in planta and is targeted to membrane raft-like microdomains when expressed in yeast (Krügel et al., 2008). Here, SUT1-GFP expression in different yeast mutants, which were unable to perform endocytosis and/or raft formation, revealed a strong link between SUT1 raft localization, the sterol composition and membrane potential of the yeast plasma membrane, and the capacity of the SUT1 protein to be internalized by endocytosis. The results provide new insight into the regulation of sucrose transport and the mechanism of endocytosis in plant cells.

Dimerization and endocytosis of the sucrose transporter StSUT1 in mature sieve elements

The sucrose transporter StSUT1 from Solanum tuberosum was shown to be regulated post-translationally by redox reagents. Its activity is increased at least 10-fold in the presence of oxidizing agents if expressed in yeast. Oxidation has also an effect on plasma membrane targeting and dimerization of the protein. In response to oxidizing agents, StSUT1 is targeted to lipid raft-like microdomains and SUT1 protein is detectable in the detergent resistant membrane fraction of plant plasma membranes. Interestingly, StSUT1 treated with brefeldin A seems to aggregate in endocytic compartments in mature sieve elements.1 Further analysis of SUT1 targeting will certainly provide more information about the putative involvement of lipid raft-like microdomains in endocytic events. We provide here additional information on the dimerization and endocytosis of the SUT1 protein. The oligomerization of overexpressed SoSUT1 from Spinacia oleracea in transgenic potato plants was analyzed by two-dimensional gel electrophoresis and endocytosis of the StSUT1 protein was confirmed by immunogold labeling.

Transport and sorting of the solanum tuberosum sucrose transporter SUT1 is affected by posttranslational modification

The plant sucrose transporter SUT1 from Solanum tuberosum revealed a dramatic redox-dependent increase in sucrose transport activity when heterologously expressed in Saccharomyces cerevisiae. Plant plasma membrane vesicles do not show any change in proton flux across the plasma membrane in the presence of redox reagents, indicating a SUT1-specific effect of redox reagents. Redox-dependent sucrose transport activity was confirmed electrophysiologically in Xenopus laevis oocytes with SUT1 from maize (Zea mays). Localization studies of green fluorescent protein fusion constructs showed that an oxidative environment increased the targeting of SUT1 to the plasma membrane where the protein concentrates in 200- to 300-nm raft-like microdomains. Using plant plasma membranes, St SUT1 can be detected in the detergent-resistant membrane fraction. Importantly, in yeast and in plants, oxidative reagents induced a shift in the monomer to dimer equilibrium of the St SUT1 protein and increased the fraction of dimer. Biochemical methods confirmed the capacity of SUT1 to form a dimer in plants and yeast cells in a redox-dependent manner. Blue native PAGE, chemical cross-linking, and immunoprecipitation, as well as the analysis of transgenic plants with reduced expression of St SUT1, confirmed the dimerization of St SUT1 and Sl SUT1 (from Solanum lycopersicum) in planta. The ability to form homodimers in plant cells was analyzed by the split yellow fluorescent protein technique in transiently transformed tobacco (Nicotiana tabacum) leaves and protoplasts. Oligomerization seems to be cell type specific since under native-like conditions, a phloem-specific reduction of the dimeric form of the St SUT1 protein was detectable in SUT1 antisense plants, whereas constitutively inhibited antisense plants showed reduction only of the monomeric form. The role of redox control of sucrose transport in plants is discussed.

Sucrose transporter StSUT4 from potato affects flowering, tuberization, and shade avoidance response

Sucrose (Suc) transporters belong to a large gene family. The physiological role of SUT1 proteins has been intensively investigated in higher plants, whereas that of SUT4 proteins is so far unknown. All three known Suc transporters from potato (Solanum tuberosum), SUT1, SUT2, and SUT4, are colocalized and their RNA levels not only follow a diurnal rhythm, but also oscillate in constant light. Here, we examined the physiological effects of transgenic potato plants on RNA interference (RNAi)-inactivated StSUT4 expression. The phenotype of StSUT4-RNAi plants includes early flowering, higher tuber production, and reduced sensitivity toward light enriched in far-red wavelength (i.e. in canopy shade). Inhibition of StSUT4 led to tuber production of the strict photoperiodic potato subsp. andigena even under noninductive long-day conditions. Accumulation of soluble sugars and Suc efflux from leaves of transgenic plants are modified in StSUT4-RNAi plants, leading to modified Suc levels in sink organs. StSUT4 expression of wild-type plants is induced by gibberellins and ethephon, and external supply of gibberellic acid leads to even more pronounced differences between wild-type and StSUT4-RNAi plants regarding tuber yield and internode elongation, indicating a reciprocal regulation of StSUT4 and gibberellins.