Role of light and jasmonic acid signaling in regulating foliar phloem cell wall ingrowth development

Foliar Phenotypic Plasticity Reflects Adaptation to Environmental Variability

Arabidopsis thaliana ecotypes adapted to native habitats with different daylengths, temperatures, and precipitation were grown experimentally under seven combinations of light intensity and leaf temperature to assess their acclimatory phenotypic plasticity in foliar structure and function. There were no differences among ecotypes when plants developed under moderate conditions of 400 µmol photons m-2 s-1 and 25 °C. However, in response to more extreme light or temperature regimes, ecotypes that evolved in habitats with pronounced differences in either the magnitude of changes in daylength or temperature or in precipitation level exhibited pronounced adjustments in photosynthesis and transpiration, as well as anatomical traits supporting these functions. Specifically, when grown under extremes of light intensity (100 versus 1000 µmol photons m-2 s-1) or temperature (8 °C versus 35 °C), ecotypes from sites with the greatest range of daylengths and temperature over the growing season exhibited the greatest differences in functional and structural features related to photosynthesis (light- and CO2-saturated capacity of oxygen evolution, leaf dry mass per area or thickness, phloem cells per minor vein, and water-use efficiency of CO2 uptake). On the other hand, the ecotype from the habitat with the lowest precipitation showed the greatest plasticity in features related to water transport and loss (vein density, ratio of water to sugar conduits in foliar minor veins, and transpiration rate). Despite these differences, common structure-function relationships existed across all ecotypes and growth conditions, with significant positive, linear correlations (i) between photosynthetic capacity (ranging from 10 to 110 µmol O2 m-2 s-1) and leaf dry mass per area (from 10 to 75 g m-2), leaf thickness (from 170 to 500 µm), and carbohydrate-export infrastructure (from 6 to 14 sieve elements per minor vein, from 2.5 to 8 µm2 cross-sectional area per sieve element, and from 16 to 82 µm2 cross-sectional area of sieve elements per minor vein); (ii) between transpiration rate (from 1 to 17 mmol H2O m-2 s-1) and water-transport infrastructure (from 3.5 to 8 tracheary elements per minor vein, from 13.5 to 28 µm2 cross-sectional area per tracheary element, and from 55 to 200 µm2 cross-sectional area of tracheary elements per minor vein); (iii) between the ratio of transpirational water loss to CO2 fixation (from 0.2 to 0.7 mol H2O to mmol-1 CO2) and the ratio of water to sugar conduits in minor veins (from 0.4 to 1.1 tracheary to sieve elements, from 4 to 6 µm2 cross-sectional area of tracheary to sieve elements, and from 2 to 6 µm2 cross-sectional area of tracheary elements to sieve elements per minor vein); (iv) between sugar conduits and sugar-loading cells; and (v) between water conducting and sugar conducting cells. Additionally, the proportion of water conduits to sugar conduits was greater for all ecotypes grown experimentally under warm-to-hot versus cold temperature. Thus, developmental acclimation to the growth environment included ecotype-dependent foliar structural and functional adjustments resulting in multiple common structural and functional relationships.

Asymmetric wall ingrowth deposition in Arabidopsis phloem parenchyma transfer cells is tightly associated with sieve elements

In Arabidopsis, polarized deposition of wall ingrowths in phloem parenchyma (PP) transfer cells (TCs) occurs adjacent to cells of the sieve element/companion cell (SE/CC) complex. However, the spatial relationships between these different cell types in minor veins, where phloem loading occurs, are poorly understood. PP TC development and wall ingrowth localization were compared with those of other phloem cells in leaves of Col-0 and the transgenic lines AtSUC2::AtSTP9-GFP (green fluorescent protein) and AtSWEET11::AtSWEET11-GFP that identify CCs and PP cells, respectively. The development of PP TCs in minor veins, indicated by deposition of wall ingrowths, proceeded basipetally in leaves. However, not all PP cells develop wall ingrowths, and higher levels of deposition occur in abaxial- compared with adaxial-positioned PP TCs. Furthermore, the deposition of wall ingrowths was exclusively initiated on and preferentially covered the PP TC/SE interface, rather than the PP TC/CC interface, and only occurred in PP cells that were adjacent to SEs. Collectively, these results demonstrate a tight association between SEs and wall ingrowth deposition in PP TCs and suggest the existence of two subtypes of PP cells in leaf minor veins. Compared with PP cells, PP TCs showed more abundant accumulation of AtSWEET11-GFP, indicating functional differences in phloem loading between PP and PP TCs.

Immunocytochemical Analysis of the Wall Ingrowths in the Digestive Gland Transfer Cells in Aldrovanda vesiculosa L. (Droseraceae)

Carnivorous plants are unique due to their ability to attract small animals or protozoa, retain them in specialized traps, digest them, and absorb nutrients from the dissolved prey material; however, to this end, these plants need a special secretion-digestive system (glands). A common trait of the digestive glands of carnivorous plants is the presence of transfer cells. Using the aquatic carnivorous species Aldrovanda vesiculosa, we showed carnivorous plants as a model for studies of wall ingrowths/transfer cells. We addressed the following questions: Is the cell wall ingrowth composition the same between carnivorous plant glands and other plant system models? Is there a difference in the cell wall ingrowth composition between various types of gland cells (glandular versus endodermoid cells)? Fluorescence microscopy and immunogold electron microscopy were employed to localize carbohydrate epitopes associated with major cell wall polysaccharides and glycoproteins. The cell wall ingrowths were enriched with arabinogalactan proteins (AGPs) localized with the JIM8, JIM13, and JIM14 epitopes. Both methylesterified and de-esterified homogalacturonans (HGs) were absent or weakly present in the wall ingrowths in transfer cells (stalk cells and head cells of the gland). Both the cell walls and the cell wall ingrowths in the transfer cells were rich in hemicelluloses: xyloglucan (LM15) and galactoxyloglucan (LM25). There were differences in the composition between the cell wall ingrowths and the primary cell walls in A. vesiculosa secretory gland cells in the case of the absence or inaccessibility of pectins (JIM5, LM19, JIM7, LM5, LM6 epitopes); thus, the wall ingrowths are specific cell wall microdomains. Even in the same organ (gland), transfer cells may differ in the composition of the cell wall ingrowths (glandular versus endodermoid cells). We found both similarities and differences in the composition of the cell wall ingrowths between the A. vesiculosa transfer cells and transfer cells of other plant species.

Foliar sieve elements: Nexus of the leaf

In this review, a central position of foliar sieve elements in linking leaf structure and function is explored. Results from studies involving plants grown under, and acclimated to, different growth regimes are used to identify significant, linear relationships between features of minor vein sieve elements and those of 1) leaf photosynthetic capacity that drives sugar synthesis, 2) overall leaf structure that serves as the platform for sugar production, 3) phloem components that facilitate the loading of sugars (companion & phloem parenchyma cells), and 4) the tracheary elements that import water to support photosynthesis (and stomatal opening) as well as mass flow of sugars out of the leaf. Despite comprising only a small fraction of physical space within the leaf, sieve elements represent a hub through which multiple functions of the leaf intersect. As the conduits for export of energy-rich carbohydrates, essential mineral nutrients, and information carriers, sieve elements play a central role in fueling and orchestrating development and function of the plant as well as, by extension, of natural and human communities that depend on plants as producers and partners in the global carbon cycle.

Photoprotection contributes to freezing tolerance as revealed by RNA-seq profiling of rhododendron leaves during cold acclimation and deacclimation over time

Cold acclimation (CA) and deacclimation (DA), which are often accompanied by changes in freezing tolerance (FT), carbohydrates and hormones, are crucial for winter survival, especially under global warming. Plants with weak CA and premature DA caused by warm winters and/or unseasonal warm spells can be easily injured by adverse reactions to cold. Thus, understanding the molecular mechanisms of FT is imperative. In this study, we used high-throughput RNA-seq to profile the CA and DA of leaves of overwintering Rhododendron "Miyo-no-Sakae" over time; these leaves do not undergo dormancy but do undergo photoprotection during CA, and they do not grow during DA. Using Mfuzz and weighted gene coexpression network analysis, we identified specific transcriptional characteristics in each phase of CA and DA and proposed networks involving coexpressed genes and physiological traits. In particular, we discovered that the circadian rhythm is critical for obtaining the strongest FT, and high expression of circadian rhythm-related genes might be linked to sugar accumulation during winter. Furthermore, evergreen leaves exhibited robust photoprotection during winter, as revealed by high values of nonphotochemical quenching, high expression of transcripts annotated as "early light-induced proteins", loss of granum stacks and destacking of thylakoids, all of which were alleviated during DA. The strong requirement of photoprotection could be the reason for decreased abscisic acid (ABA) and jasmonic acid (JA) contents during CA, and decreases in ABA and JA contents may contribute to decreases in lignin content. Our data suggest that the molecular mechanisms of FT in overwintering leaves are unique, which may be due to the high requirements for photoprotection during winter.

Photosynthesis and foliar vascular adjustments to growth light intensity in summer annual species with symplastic and apoplastic phloem loading

Concomitant adjustments in photosynthetic capacity and size, composition, and/or density of minor foliar veins in response to growth environment were previously described primarily for winter annuals that load sugars into foliar phloem apoplastically. Here, common trends, differences associated with phloem-loading mechanism, and species-dependent differences are identified for summer annuals (loading sugars either symplastically [cucumber, pumpkin, and basil] or apoplastically [tomato and sunflower]) that were grown in low and high light. Photosynthetic capacity per leaf area was significantly positively correlated with leaf-level volume of phloem-loading cells (LCs), sugar-export conduits (sieve elements), and water conduits (tracheary elements) irrespective of phloem-loading mechanism. The relative contribution to leaf-level volume of LC numbers versus individual LC size was greater in apoplastic loaders than in symplastic loaders. Species-dependent differences included different vein density within each loading group and either greater or lower numbers of cells per minor vein (especially of tracheary elements in the symplastic loaders basil versus cucumber, respectively), which may be due to genetic adaptation to different environmental conditions. These results indicate considerable plasticity in foliar vascular features in summer annuals as well as some loading-mechanism-dependent trends.

Review: More than sweet: New insights into the biology of phloem parenchyma transfer cells in Arabidopsis

Transfer cells (TCs) develop extensive wall ingrowths to facilitate enhanced rates of membrane transport. In Arabidopsis, TCs trans-differentiate from phloem parenchyma (PP) cells abutting the sieve element/companion cell complex in minor veins of foliar tissues and, based on anatomy and expression of SWEET sucrose uniporters, are assumed to play pivotal roles in phloem loading. While wall ingrowth deposition in PP TCs is a dynamic process responding to abiotic stresses such as high light and cold, the transcriptional control of PP TC development, including deposition of the wall ingrowths themselves, is not understood. PP TC development is a trait of vegetative phase change, potentially linking wall ingrowth deposition with floral induction. Transcript profiling by RNA-seq identified NAC056 and NAC018 (NARS1 and NARS2) as putative regulators of wall ingrowth deposition, while recent single cell RNA-seq analysis of leaf vasculature identified PP-specific expression of NAC056. Numerous membrane transporters, particularly of the UmamiT family of amino acid efflux carriers, were also identified. Collectively, these findings, and the recent discovery that wall ingrowth deposition is regulated by sucrose-dependent loading activity of these cells, provide new insights into the biology of PP TCs and their importance to phloem loading in Arabidopsis, establishing these cells as a key transport hub for phloem loading.

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.

Sucrose Utilization for Improved Crop Yields: A Review Article

Photosynthetic carbon converted to sucrose is vital for plant growth. Sucrose acts as a signaling molecule and a primary energy source that coordinates the source and sink development. Alteration in source-sink balance halts the physiological and developmental processes of plants, since plant growth is mostly triggered when the primary assimilates in the source leaf balance with the metabolic needs of the heterotrophic sinks. To measure up with the sink organ's metabolic needs, the improvement of photosynthetic carbon to synthesis sucrose, its remobilization, and utilization at the sink level becomes imperative. However, environmental cues that influence sucrose balance within these plant organs, limiting positive yield prospects, have also been a rising issue over the past few decades. Thus, this review discusses strategies to improve photosynthetic carbon assimilation, the pathways actively involved in the transport of sucrose from source to sink organs, and their utilization at the sink organ. We further emphasize the impact of various environmental cues on sucrose transport and utilization, and the strategic yield improvement approaches under such conditions.

Transfer cells: what regulates the development of their intricate wall labyrinths?

Transfer cells (TCs) support high nutrient rates into, or at symplasmic discontinuities within, the plant body. Their transport capacity is conferred by an amplified plasma membrane surface area, enriched in nutrient transporters, supported on an intricately invaginated wall labyrinth (WL). Thus, development of the WL is at the heart of TC function. Enquiry has shifted from describing WL architecture and formation to discovering mechanisms regulating WL assembly. Experimental systems used to examine these phenomena are critiqued. Considerable progress has been made in identifying master regulators that commit stem cells to a TC fate (e.g. the maize Myeloblastosis (MYB)-related R1-type transcription factor) and signals that induce differentiated cells to undergo trans-differentiation to a TC phenotype (e.g. sugar, auxin and ethylene). In addition, signals that provide positional information for assembly of the WL include apoplasmic hydrogen peroxide and cytosolic Ca²⁺ plumes. The former switches on, and specifies the intracellular site for WL construction, while the latter creates subdomains to direct assembly of WL invaginations. Less is known about macromolecule species and their spatial organization essential for WL assembly. Emerging evidence points to a dependency on methyl-esterified homogalacturonan accumulation, unique patterns of cellulose and callose deposition and spatial positioning of arabinogalactan proteins.

Sucrose regulates wall ingrowth deposition in phloem parenchyma transfer cells in Arabidopsis via affecting phloem loading activity

In Arabidopsis thaliana, phloem parenchyma transfer cells (PPTCs) occur in leaf minor veins and play a pivotal role in phloem loading. Wall ingrowth formation in PPTCs is induced by the phloem loading activity of these cells, which is regulated by sucrose (Suc). The effects of endogenous versus exogenous Suc on wall ingrowth deposition, however, differ. Elevating endogenous Suc levels by increased light enhanced wall ingrowth formation, whereas lowering endogenous Suc levels by dark treatment or genetically in ch-1 resulted in lower levels of deposition. In contrast, exogenously applied Suc, or Suc derived from other organs, repressed wall ingrowth deposition. Analysis of pAtSUC2::GFP plants, used as a marker for phloem loading status, suggested that wall ingrowth formation is correlated with phloem loading activity. Gene expression analysis revealed that exogenous Suc down-regulated expression of AtSWEET11 and 12, whereas endogenous Suc up-regulated AtSWEET11 expression. Analysis of a TREHALOSE 6-PHOSPHATE (T6P) SYNTHASE overexpression line and the hexokinase (HXK)-null mutant, gin2-1, suggested that Suc signalling of wall ingrowth formation is independent of T6P and HXK. Collectively, these results are consistent with the conclusion that Suc regulates wall ingrowth formation via affecting Suc exporting activity in PPTCs.

Heterogeneous Light Conditions Reduce the Assimilate Translocation Towards Maize Ears

The border row crop in strip intercropped maize is often exposed to heterogeneous light conditions, resulting in increased photosynthesis and yield decreased. Previous studies have focused on photosynthetic productivity, whereas carbon allocation could also be one of the major causes of decreased yield. However, carbon distribution remains unclear in partially shaded conditions. In the present study, we applied heterogeneous light conditions (T), and one side of plants was shaded (T-30%), keeping the other side fully exposed to light (T-100%), as compared to control plants that were exposed entirely to full-light (CK). Dry weight, carbon assimilation, ¹³C abundance, and transport tissue structure were analyzed to clarify the carbon distribution in partial shading of plants. T caused a marked decline in dry weight and harvest index (HI), whereas dry weight in unshaded and shaded leaves did not differ. Net photosynthesis rate (Pn), the activity of sucrose phosphate synthase enzymes (SPS), and sucrose concentration increased in unshaded leaves. Appropriately, 5.7% of the ¹³C from unshaded leaves was transferred to shaded leaves. Furthermore, plasmodesma density in the unshaded (T-100%) and shaded (T-30%) leaves in T was not significantly different but was lower than that of CK. Similarly, the vascular bundle total area of T was decreased. ¹³C transfer from unshaded leaves to ear in T was decreased by 18.0% compared with that in CK. Moreover, ¹³C and sucrose concentration of stem in T were higher than those in CK. Our results suggested that, under heterogeneous light, shaded leaves as a sink imported the carbohydrates from the unshaded leaves. Ear and shaded leaf competed for carbohydrates, and were not conducive to tissue structure of sucrose transport, resulting in a decrease in the carbon proportion in the ear, harvest index, and ear weight.

Methods of Phloem Visualization: A Clear Future in Sight?

There have been exciting new results in phloem research in recent years, at least in part made possible by the rapid advancement of microscopic techniques. Several methods for visualizing phloem cells are available. The suitability of each method depends on the organ and species being studied, and on the scientific question being addressed. This review will briefly explain the specific challenges associated with phloem cell visualization. It will then focus on common methods currently being used for studying phloem anatomy, development, and function. Emphasis will be placed on the most recent improvements in imaging techniques which had, or most certainly will have, an impact on phloem research.

Abscisic Acid and Jasmonate Metabolisms Are Jointly Regulated During Senescence in Roots and Leaves of Populus trichocarpa

Plant senescence is a highly regulated process that allows nutrients to be mobilized from dying tissues to other organs. Despite that senescence has been extensively studied in leaves, the senescence of ephemeral organs located underground is still poorly understood, especially in the context of phytohormone engagement. The present study focused on filling this knowledge gap by examining the roles of abscisic acid (ABA) and jasmonate in the regulation of senescence of fine, absorptive roots and leaves of Populus trichocarpa. Immunohistochemical (IHC), chromatographic, and molecular methods were utilized to achieve this objective. A transcriptomic analysis identified significant changes in gene expression that were associated with the metabolism and signal transduction of phytohormones, especially ABA and jasmonate. The increased level of these phytohormones during senescence was detected in both organs and was confirmed by IHC. Based on the obtained data, we suggest that phytohormonal regulation of senescence in roots and leaves is organ-specific. We have shown that the regulation of ABA and JA metabolism is tightly regulated during senescence processes in both leaves and roots. The results were discussed with respect to the role of ABA in cold tolerance and the role of JA in resistance to pathogens.

Less photoprotection can be good in some genetic and environmental contexts

Antioxidant systems modulate oxidant-based signaling networks and excessive removal of oxidants can prevent beneficial acclimation responses. Evidence from mutant, transgenic, and locally adapted natural plant systems is used to interpret differences in the capacity for antioxidation and formulate hypotheses for future inquiry. We focus on the first line of chloroplast antioxidant defense, pre-emptive thermal dissipation of excess absorbed light (monitored as nonphotochemical fluorescence quenching, NPQ) as well as on tocopherol-based antioxidation. Findings from NPQ-deficient and tocopherol-deficient mutants that exhibited enhanced biomass production and/or enhanced foliar water-transport capacity are reviewed and discussed in the context of the impact of lower levels of antioxidation on plant performance in hot/dry conditions, under cool temperature, and in the presence of biotic stress. The complexity of cellular redox-signaling networks is related to the complexity of environmental and endogenous inputs as well as to the need for intensified training and collaboration in the study of plant-environment interactions across biological sub-disciplines.

Quantification of Leaf Phloem Anatomical Features with Microscopy

Measurements of vein density and foliar minor vein phloem cell numbers, minor vein phloem cell sizes, and transfer cell wall ingrowths provide quantitative proxies for the leaf's capacities to load and export photosynthates. While overall infrastructural capacity for sugar loading and sugar export correlated positively and closely with photosynthetic capacity, the specific targets of the adjustment of minor vein organization varied with phloem-loading mechanism, plant life-cycle characteristics, and environmental growth conditions. Among apoplastic loaders, for which sugar loading into the phloem depends on cell membrane-spanning transport proteins, variation in minor vein density, phloem cell number, and level of cell wall ingrowth (when present) were consistently associated with photosynthetic capacity. Among active symplastic loaders, for which sugar loading into the phloem depends on cytosolic enzymes, variation in vein density and phloem cell size were consistently associated with photosynthetic capacity. All of these anatomical features were also subject to acclimatory adjustment depending on species and environmental conditions, with increased levels of these features supporting higher rates of photosynthesis. We present a procedure for the preparation of leaf tissue for minor vein analysis, using both light and transmission electron microscopy, that facilitates quantification of not only phloem features but also xylem features that provide proxies for foliar water import capacity.

Transcript Profiling Identifies NAC-Domain Genes Involved in Regulating Wall Ingrowth Deposition in Phloem Parenchyma Transfer Cells of Arabidopsis thaliana

Transfer cells (TCs) play important roles in facilitating enhanced rates of nutrient transport at key apoplasmic/symplasmic junctions along the nutrient acquisition and transport pathways in plants. TCs achieve this capacity by developing elaborate wall ingrowth networks which serve to increase plasma membrane surface area thus increasing the cell's surface area-to-volume ratio to achieve increased flux of nutrients across the plasma membrane. Phloem parenchyma (PP) cells of Arabidopsis leaf veins trans-differentiate to become PP TCs which likely function in a two-step phloem loading mechanism by facilitating unloading of photoassimilates into the apoplasm for subsequent energy-dependent uptake into the sieve element/companion cell (SE/CC) complex. We are using PP TCs in Arabidopsis as a genetic model to identify transcription factors involved in coordinating deposition of the wall ingrowth network. Confocal imaging of pseudo-Schiff propidium iodide-stained tissue revealed different profiles of temporal development of wall ingrowth deposition across maturing cotyledons and juvenile leaves, and a basipetal gradient of deposition across mature adult leaves. RNA-Seq analysis was undertaken to identify differentially expressed genes common to these three different profiles of wall ingrowth deposition. This analysis identified 68 transcription factors up-regulated two-fold or more in at least two of the three experimental comparisons, with six of these transcription factors belonging to Clade III of the NAC-domain family. Phenotypic analysis of these NAC genes using insertional mutants revealed significant reductions in levels of wall ingrowth deposition, particularly in a double mutant of NAC056 and NAC018, as well as compromised sucrose-dependent root growth, indicating impaired capacity for phloem loading. Collectively, these results support the proposition that Clade III members of the NAC-domain family in Arabidopsis play important roles in regulating wall ingrowth deposition in PP TCs.

Arabidopsis thaliana Ei-5: Minor Vein Architecture Adjustment Compensates for Low Vein Density in Support of Photosynthesis

An Arabidopsis thaliana accession with naturally low vein density, Eifel-5 (Ei-5), was compared to Columbia-0 (Col-0) with respect to rosette growth, foliar vein architecture, photosynthesis, and transpiration. In addition to having to a lower vein density, Ei-5 grew more slowly, with significantly lower rates of rosette expansion, but had similar capacities for photosynthetic oxygen evolution on a leaf area basis compared to Col-0. The individual foliar minor veins were larger in Ei-5, with a greater number of vascular cells per vein, compared to Col-0. This compensation for low vein density resulted in similar values for the product of vein density × phloem cell number per minor vein in Ei-5 and Col-0, which suggests a similar capacity for foliar sugar export to support similar photosynthetic capacities per unit leaf area. In contrast, the product of vein density × xylem cell number per minor vein was significantly greater in Ei-5 compared to Col-0, and was associated not only with a higher ratio of water-transporting tracheary elements versus sugar-transporting sieve elements but also significantly higher foliar transpiration rates per leaf area in Ei-5. In contrast, previous studies in other systems had reported higher ratios of tracheary to sieve elements and higher transpiration rate to be associated with higher - rather than lower - vein densities. The Ei-5 accession thus further underscores the plasticity of the foliar vasculature by illustrating an example where a higher ratio of tracheary to sieve elements is associated with a lower vein density. Establishment of the Ei-5 accession, with a low vein density but an apparent overcapacity for water flux through the foliar xylem network, may have been facilitated by a higher level of precipitation in its habitat of origin compared to that of the Col-0 accession.

Arabidopsis thaliana Ei-5: Minor Vein Architecture Adjustment Compensates for Low Vein Density in Support of Photosynthesis

An Arabidopsis thaliana accession with naturally low vein density, Eifel-5 (Ei-5), was compared to Columbia-0 (Col-0) with respect to rosette growth, foliar vein architecture, photosynthesis, and transpiration. In addition to having to a lower vein density, Ei-5 grew more slowly, with significantly lower rates of rosette expansion, but had similar capacities for photosynthetic oxygen evolution on a leaf area basis compared to Col-0. The individual foliar minor veins were larger in Ei-5, with a greater number of vascular cells per vein, compared to Col-0. This compensation for low vein density resulted in similar values for the product of vein density × phloem cell number per minor vein in Ei-5 and Col-0, which suggests a similar capacity for foliar sugar export to support similar photosynthetic capacities per unit leaf area. In contrast, the product of vein density × xylem cell number per minor vein was significantly greater in Ei-5 compared to Col-0, and was associated not only with a higher ratio of water-transporting tracheary elements versus sugar-transporting sieve elements but also significantly higher foliar transpiration rates per leaf area in Ei-5. In contrast, previous studies in other systems had reported higher ratios of tracheary to sieve elements and higher transpiration rate to be associated with higher - rather than lower - vein densities. The Ei-5 accession thus further underscores the plasticity of the foliar vasculature by illustrating an example where a higher ratio of tracheary to sieve elements is associated with a lower vein density. Establishment of the Ei-5 accession, with a low vein density but an apparent overcapacity for water flux through the foliar xylem network, may have been facilitated by a higher level of precipitation in its habitat of origin compared to that of the Col-0 accession.

Transcript Profiling Identifies Gene Cohorts Controlled by Each Signal Regulating Trans-Differentiation of Epidermal Cells of Vicia faba Cotyledons to a Transfer Cell Phenotype

Transfer cells (TCs) support high rates of membrane transport of nutrients conferred by a plasma membrane area amplified by lining a wall labyrinth comprised of an uniform wall layer (UWL) upon which intricate wall ingrowth (WI) papillae are deposited. A signal cascade of auxin, ethylene, extracellular hydrogen peroxide (H₂O₂) and cytosolic Ca²⁺ regulates wall labyrinth assembly. To identify gene cohorts regulated by each signal, a RNA- sequencing study was undertaken using Vicia faba cotyledons. When cotyledons are placed in culture, their adaxial epidermal cells spontaneously undergo trans-differentiation to epidermal TCs (ETCs). Expressed genes encoding proteins central to wall labyrinth formation (signaling, intracellular organization, cell wall) and TC function of nutrient transport were assembled. Transcriptional profiles identified 9,742 annotated ETC-specific differentially expressed genes (DEGs; Log₂fold change > 1; FDR p ≤ 0.05) of which 1,371 belonged to signaling (50%), intracellular organization (27%), cell wall (15%) and nutrient transporters (9%) functional categories. Expression levels of 941 ETC-specific DEGs were found to be sensitive to the known signals regulating ETC trans-differentiation. Significantly, signals acting alone, or in various combinations, impacted similar numbers of ETC-specific DEGs across the four functional gene categories. Amongst the signals acting alone, H₂O₂ exerted most influence affecting expression levels of 56% of the ETC-specific DEGs followed by Ca²⁺ (21%), auxin (18%) and ethylene (5%). The dominance by H₂O₂ was evident across all functional categories, but became more attenuated once trans-differentiation transitioned into WI papillae formation. Amongst the eleven signal combinations, H₂O₂/Ca²⁺ elicited the greatest impact across all functional categories accounting for 20% of the ETC-specific DEG cohort. The relative influence of the other signals acting alone, or in various combinations, varied across the four functional categories and two phases of wall labyrinth construction. These transcriptome data provide a powerful information platform from which to examine signal transduction pathways and how these regulate expression of genes encoding proteins engaged in intracellular organization, cell wall construction and nutrient transport.

Acclimation of Swedish and Italian ecotypes of Arabidopsis thaliana to light intensity

This study addressed whether ecotypes of Arabidopsis thaliana from Sweden and Italy exhibited differences in foliar acclimation to high versus low growth light intensity, and compared CO₂ uptake under growth conditions with light- and CO₂-saturated intrinsic photosynthetic capacity and leaf morphological and vascular features. Differential responses between ecotypes occurred mainly at the scale of leaf architecture, with thicker leaves with higher intrinsic photosynthetic capacities and chlorophyll contents per leaf area, but no difference in photosynthetic capacity on a chlorophyll basis, in high light-grown leaves of the Swedish versus the Italian ecotype. Greater intrinsic photosynthetic capacity per leaf area in the Swedish ecotype was accompanied by a greater capacity of vascular infrastructure for sugar and water transport, but this was not associated with greater CO₂ uptake rates under growth conditions. The Swedish ecotype with its thick leaves is thus constructed for high intrinsic photosynthetic and vascular flux capacity even under growth chamber conditions that may not permit full utilization of this potential. Conversely, the Swedish ecotype was less tolerant of low growth light intensity than the Italian ecotype, with smaller rosette areas and lesser aboveground biomass accumulation in low light-grown plants. Foliar vein density and stomatal density were both enhanced by high growth light intensity with no significant difference between ecotypes, and the ratio of water to sugar conduits was also similar between the two ecotypes during light acclimation. These findings add to the understanding of the foliar vasculature's role in plant photosynthetic acclimation and adaptation.

Wall ingrowth deposition in phloem parenchyma transfer cells in Arabidopsis: Heteroblastic variations and a potential role in pathogen defence

Transfer cell (TCs) develop unique wall ingrowth networks which amplify plasma membrane surface area and thus maximize nutrient transporter density at key anatomic sites for nutrient exchange within plants and their external environment. These sites fall into 4 main groups corresponding to 4 categories of trans-membrane flux: absorption/secretion of solutes from or to the external environment, and absorption/secretion of solutes from or to internal, extra-cytoplasmic compartments. Research on TC biology over recent decades has demonstrated correlations between wall ingrowth deposition in TCs and enhanced transport capacity in many major agricultural species such as pea, fava bean, cotton and maize. Consequently, there is general consensus that the existence of wall ingrowth morphology implies an augmentation in membrane transport capacity. However, this may not be entirely applicable for phloem parenchyma (PP) TCs in Arabidopsis. Our recent survey of PP TC abundance and distribution in Arabidopsis veins indicated that PP TC development reflects heteroblastic status. A consequence of this observation is the suggestion that PP TCs, or at least wall ingrowth deposition in these cells, potentially act as a physical barrier to defend access of invading pathogens to sugar-rich sieve elements rather than solely in facilitating the export of photoassimilate from collection phloem in leaves.

Light, temperature and tocopherol status influence foliar vascular anatomy and leaf function in Arabidopsis thaliana

This study addressed whether the winter annual Arabidopsis thaliana can adjust foliar phloem and xylem anatomy both differentially and in parallel. In plants acclimated to hot vs cool temperature, foliar minor vein xylem-to-phloem ratio was greater, whereas xylem and phloem responded concomitantly to growth light intensity. Across all growth conditions, xylem anatomy correlated with transpiration rate, while phloem anatomy correlated with photosynthetic capacity for two plant lines (wild-type Col-0 and tocopherol-deficient vte1 mutant) irrespective of tocopherol status. A high foliar vein density (VD) was associated with greater numbers and cross-sectional areas of both xylem and phloem cells per vein as well as higher rates of both photosynthesis and transpiration under high vs low light intensities. Under hot vs cool temperature, high foliar VD was associated with a higher xylem-to-phloem ratio and greater relative rates of transpiration to photosynthesis. Tocopherol status affected development of foliar vasculature as dependent on growth environment. The most notable impact of tocopherol deficiency was seen under hot growth temperature, where the vte1 mutant exhibited greater numbers of tracheary elements (TEs) per vein, a greater ratio of TEs to sieve elements, with smaller individual sizes of TEs, and resulting similar total areas of TEs per vein and transpiration rates compared with Col-0 wild-type. These findings illustrate the plasticity of foliar vascular anatomy acclimation to growth environment resulting from independent adjustments of the vasculature's components.

Habitat Temperature and Precipitation of Arabidopsis thaliana Ecotypes Determine the Response of Foliar Vasculature, Photosynthesis, and Transpiration to Growth Temperature

Acclimatory adjustments of foliar vascular architecture, photosynthetic capacity, and transpiration rate in Arabidopsis thaliana ecotypes (Italian, Polish [Col-0], Swedish) were characterized in the context of habitat of origin. Temperatures of the habitat of origin decreased linearly with increasing habitat latitude, but habitat precipitation was greatest in Italy, lowest in Poland, and intermediate in Sweden. Plants of the three ecotypes raised under three different growth temperature regimes (low, moderate, and high) exhibited highest photosynthetic capacities, greatest leaf thickness, highest chlorophyll a/b ratio and levels of β-carotene, and greatest levels of wall ingrowths in phloem transfer cells, and, in the Col-0 and Swedish ecotypes, of phloem per minor vein in plants grown at the low temperature. In contrast, vein density and minor vein tracheary to sieve element ratio increased with increasing growth temperature - most strongly in Col-0 and least strongly in the Italian ecotype - and transpirational water loss correlated with vein density and number of tracheary elements per minor vein. Plotting of these vascular features as functions of climatic conditions in the habitat of origin suggested that temperatures during the evolutionary history of the ecotypes determined acclimatory responses of the foliar phloem and photosynthesis to temperature in this winter annual that upregulates photosynthesis in response to lower temperature, whereas the precipitation experienced during the evolutionary history of the ecotypes determined adjustment of foliar vein density, xylem, and transpiration to temperature. In particular, whereas photosynthetic capacity, leaf thickness, and foliar minor vein phloem features increased linearly with increasing latitude and decreasing temperature of the habitats of origin in response to experimental growth at low temperature, transpiration rate, foliar vein density, and minor vein tracheary element numbers and cross-sectional areas increased linearly with decreasing precipitation level in the habitats of origin in response to experimental growth at high temperature. This represents a situation where temperature acclimation of the apparent capacity for water flux through the xylem and transpiration rate in a winter annual responded differently from that of photosynthetic capacity, in contrast to previous reports of strong relationships between hydraulic conductance and photosynthesis in other studies.

Venus flytrap carnivorous lifestyle builds on herbivore defense strategies

Although the concept of botanical carnivory has been known since Darwin's time, the molecular mechanisms that allow animal feeding remain unknown, primarily due to a complete lack of genomic information. Here, we show that the transcriptomic landscape of the Dionaea trap is dramatically shifted toward signal transduction and nutrient transport upon insect feeding, with touch hormone signaling and protein secretion prevailing. At the same time, a massive induction of general defense responses is accompanied by the repression of cell death-related genes/processes. We hypothesize that the carnivory syndrome of Dionaea evolved by exaptation of ancient defense pathways, replacing cell death with nutrient acquisition.

Oil Secretory System in Vegetative Organs of Three Arnica Taxa: Essential Oil Synthesis, Distribution and Accumulation

Arnica, a genus including the medicinal species A. montana, in its Arbo variety, and A. chamissonis, is among the plants richest in essential oils used as pharmaceutical materials. Despite its extensive use, the role of anatomy and histochemistry in the internal secretory system producing the essential oil is poorly understood. Anatomical sections allowed differentiation between two forms of secretory structures which differ according to their distribution in plants. The first axial type is connected to the vascular system of all vegetative organs and forms canals lined with epithelial cells. The second cortical type is represented by elongated intercellular spaces filled with oil formed only between the cortex cells of roots and rhizomes at maturity, with canals lacking an epithelial layer.Only in A. montana rhizomes do secretory structures form huge characteristic reservoirs. Computed tomography illustrates their spatial distribution and fusiform shape. The axial type of root secretory canals is formed at the interface between the endodermis and cortex parenchyma, while, in the stem, they are located in direct contact with veinal parenchyma. The peripheral phloem parenchyma cells are arranged in strands around sieve tube elements which possess a unique ability to accumulate large amounts of oil bodies. The cells of phloem parenchyma give rise to the aforementioned secretory structures while the lipid components (triacylglycerols) stored there support the biosynthesis of essential oils by later becoming a medium in which these oils are dissolved. The results indicate the integrity of axial secretory structures forming a continuous system in vegetative plant organs.

Manipulation of the Xanthophyll Cycle Increases Plant Susceptibility to Sclerotinia sclerotiorum

The xanthophyll cycle is involved in dissipating excess light energy to protect the photosynthetic apparatus in a process commonly assessed from non-photochemical quenching (NPQ) of chlorophyll fluorescence. Here, it is shown that the xanthophyll cycle is modulated by the necrotrophic pathogen Sclerotinia sclerotiorum at the early stage of infection. Incubation of Sclerotinia led to a localized increase in NPQ even at low light intensity. Further studies showed that this abnormal change in NPQ was closely correlated with a decreased pH caused by Sclerotinia-secreted oxalate, which might decrease the ATP synthase activity and lead to a deepening of thylakoid lumen acidification under continuous illumination. Furthermore, suppression (with dithiothreitol) or a defect (in the npq1-2 mutant) of violaxanthin de-epoxidase (VDE) abolished the Sclerotinia-induced NPQ increase. HPLC analysis showed that the Sclerotinia-inoculated tissue accumulated substantial quantities of zeaxanthin at the expense of violaxanthin, with a corresponding decrease in neoxanthin content. Immunoassays revealed that the decrease in these xanthophyll precursors reduced de novo abscisic acid (ABA) biosynthesis and apparently weakened tissue defense responses, including ROS induction and callose deposition, resulting in enhanced plant susceptibility to Sclerotinia. We thus propose that Sclerotinia antagonizes ABA biosynthesis to suppress host defense by manipulating the xanthophyll cycle in early pathogenesis. These findings provide a model of how photoprotective metabolites integrate into the defense responses, and expand the current knowledge of early plant-Sclerotinia interactions at infection sites.

High-resolution confocal imaging of wall ingrowth deposition in plant transfer cells: Semi-quantitative analysis of phloem parenchyma transfer cell development in leaf minor veins of Arabidopsis

BACKGROUND

Transfer cells (TCs) are trans-differentiated versions of existing cell types designed to facilitate enhanced membrane transport of nutrients at symplasmic/apoplasmic interfaces. This transport capacity is conferred by intricate wall ingrowths deposited secondarily on the inner face of the primary cell wall, hence promoting the potential trans-membrane flux of solutes and consequently assigning TCs as having key roles in plant growth and productivity. However, TCs are typically positioned deep within tissues and have been studied mostly by electron microscopy. Recent advances in fluorophore labelling of plant cell walls using a modified pseudo-Schiff-propidium iodide (mPS-PI) staining procedure in combination with high-resolution confocal microscopy have allowed visualization of cellular details of individual tissue layers in whole mounts, hence enabling study of tissue and cellular architecture without the need for tissue sectioning. Here we apply a simplified version of the mPS-PI procedure for confocal imaging of cellulose-enriched wall ingrowths in vascular TCs at the whole tissue level.

RESULTS

The simplified mPS-PI staining procedure produced high-resolution three-dimensional images of individual cell types in vascular bundles and, importantly, wall ingrowths in phloem parenchyma (PP) TCs in minor veins of Arabidopsis leaves and companion cell TCs in pea. More efficient staining of tissues was obtained by replacing complex clearing procedures with a simple post-fixation bleaching step. We used this modified procedure to survey the presence of PP TCs in other tissues of Arabidopsis including cotyledons, cauline leaves and sepals. This high-resolution imaging enabled us to classify different stages of wall ingrowth development in Arabidopsis leaves, hence enabling semi-quantitative assessment of the extent of wall ingrowth deposition in PP TCs at the whole leaf level. Finally, we conducted a defoliation experiment as an example of using this approach to statistically analyze responses of PP TC development to leaf ablation.

CONCLUSIONS

Use of a modified mPS-PI staining technique resulted in high-resolution confocal imaging of polarized wall ingrowth deposition in TCs. This technique can be used in place of conventional electron microscopy and opens new possibilities to study mechanisms determining polarized deposition of wall ingrowths and use reverse genetics to identify regulatory genes controlling TC trans-differentiation.

De novo assembly of a genome-wide transcriptome map of Vicia faba (L.) for transfer cell research

Vicia faba (L.) is an important cool-season grain legume species used widely in agriculture but also in plant physiology research, particularly as an experimental model to study transfer cell (TC) development. TCs are specialized nutrient transport cells in plants, characterized by invaginated wall ingrowths with amplified plasma membrane surface area enriched with transporter proteins that facilitate nutrient transfer. Many TCs are formed by trans-differentiation from differentiated cells at apoplasmic/symplasmic boundaries in nutrient transport. Adaxial epidermal cells of isolated cotyledons can be induced to form functional TCs, thus providing a valuable experimental system to investigate genetic regulation of TC trans-differentiation. The genome of V. faba is exceedingly large (ca. 13 Gb), however, and limited genomic information is available for this species. To provide a resource for future transcript profiling of epidermal TC differentiation, we have undertaken de novo assembly of a genome-wide transcriptome map for V. faba. Illumina paired-end sequencing of total RNA pooled from different tissues and different stages, including isolated cotyledons induced to form epidermal TCs, generated 69.5 M reads, of which 65.8 M were used for assembly following trimming and quality control. Assembly using a De-Bruijn graph-based approach generated 21,297 contigs, of which 80.6% were successfully annotated against GO terms. The assembly was validated against known V. faba cDNAs held in GenBank, including transcripts previously identified as being specifically expressed in epidermal cells across TC trans-differentiation. This genome-wide transcriptome map therefore provides a valuable tool for future transcript profiling of epidermal TC trans-differentiation, and also enriches the genetic resources available for this important legume crop species.

Diffusion or bulk flow: how plasmodesmata facilitate pre-phloem transport of assimilates

Assimilates synthesized in the mesophyll of mature leaves move along the pre-phloem transport pathway to the bundle sheath of the minor veins from which they are loaded into the phloem. The present review discusses the most probable driving force(s) for the pre-phloem pathway, diffusion down the concentration gradient or bulk flow along a pressure gradient. The driving force seems to depend on the mode of phloem loading. In a majority of plant species phloem loading is a thermodynamically active process, involving the activity of membrane transporters in the sieve-element companion cell complex. Since assimilate movement includes an apoplasmic step, this mode is called apoplasmic loading. Well established is also the polymer-trap loading mode, where the phloem-transport sugars are raffinose-family oligomers in herbaceous plants. Also this mode depends on the investment of energy, here for sugar oligomerization, and leads to a high sugar accumulation in the phloem, even though the phloem is not symplasmically isolated, but well coupled by plasmodesmata (PD). Hence the mode polymer-trap mode is also designated active symplasmic loading. For woody angiosperms and gymnosperms an alternate loading mode is currently matter of discussion, called passive symplasmic loading. Based on the limited material available, this review compares the different loading modes and suggests that diffusion is the driving force in apoplasmic loaders, while bulk flow plays an increasing role in plants having a continuous symplasmic pathway from mesophyll to sieve elements. Crucial for the driving force is the question where water enters the pre-phloem pathway. Surprisingly, the role of PD in water movement has not been addressed so far appropriately. Modeling of assimilate and water fluxes indicates that in symplasmic loaders a considerable part of water flux happens through the PD between bundle sheath and phloem.

Lipoxygenase Activation in Peanut Seed Cultivars Resistant and Susceptible to Aspergillus parasiticus Colonization

Accumulative evidence indicates that the lipoxygenase (LOX) pathway plays a significant role in the Aspergillus-seed interaction, such as interfering with activities of endogenous fungal oxylipins or producing antimicrobial compounds and signaling molecules. In this study, we characterized the LOX pathway in peanut seed during Aspergillus parasiticus colonization in a model of two cultivars distinguished as resistant ('PI337394') and susceptible ('Florman INTA') to Aspergillus spp. infection and aflatoxin contamination. The LOX activity together with the content of LOX substrate and LOX products demonstrated the presence of a differential response mechanism to A. parasiticus infection between cultivars. Our findings suggest that this mechanism is under transcriptional control of previously identified (LOX 2 and LOX 3) and novel (LOX 4 and LOX 5) LOX genes. The results of this study support the role of these enzymes in defense during fungus infection in peanut seed.

Association between photosynthesis and contrasting features of minor veins in leaves of summer annuals loading phloem via symplastic versus apoplastic routes

Foliar vascular anatomy and photosynthesis were evaluated for a number of summer annual species that either load sugars into the phloem via a symplastic route (Cucumis sativus L. cv. Straight Eight; Cucurbita pepo L. cv. Italian Zucchini Romanesco; Citrullus lanatus L. cv. Faerie Hybrid; Cucurbita pepo L. cv. Autumn Gold) or an apoplastic route (Nicotiana tabacum L.; Solanum lycopersicum L. cv. Brandywine; Gossypium hirsutum L.; Helianthus annuus L. cv. Soraya), as well as winter annual apoplastic loaders (Spinacia oleracea L. cv. Giant Nobel; Arabidopsis thaliana (L.) Heynhold Col-0, Swedish and Italian ecotypes). For all summer annuals, minor vein cross-sectional xylem area and tracheid number as well as the ratio of phloem loading cells to phloem sieve elements, each when normalized for foliar vein density (VD), was correlated with photosynthesis. These links presumably reflect (1) the xylem's role in providing water to meet foliar transpirational demand supporting photosynthesis and (2) the importance of the driving force of phloem loading as well as the cross-sectional area for phloem sap flux to match foliar photosynthate production. While photosynthesis correlated with the product of VD and cross-sectional phloem cell area among symplastic loaders, photosynthesis correlated with the product of VD and phloem cell number per vein among summer annual apoplastic loaders. Phloem cell size has thus apparently been a target of selection among symplastic loaders (where loading depends on enzyme concentration within loading cells) versus phloem cell number among apoplastic loaders (where loading depends on membrane transporter numbers).

Leaf anatomical and photosynthetic acclimation to cool temperature and high light in two winter versus two summer annuals

Acclimation of foliar features to cool temperature and high light was characterized in winter (Spinacia oleracea L. cv. Giant Nobel; Arabidopsis thaliana (L.) Heynhold Col-0 and ecotypes from Sweden and Italy) versus summer (Helianthus annuus L. cv. Soraya; Cucurbita pepo L. cv. Italian Zucchini Romanesco) annuals. Significant relationships existed among leaf dry mass per area, photosynthesis, leaf thickness and palisade mesophyll thickness. While the acclimatory response of the summer annuals to cool temperature and/or high light levels was limited, the winter annuals increased the number of palisade cell layers, ranging from two layers under moderate light and warm temperature to between four and five layers under cool temperature and high light. A significant relationship was also found between palisade tissue thickness and either cross-sectional area or number of phloem cells (each normalized by vein density) in minor veins among all four species and growth regimes. The two winter annuals, but not the summer annuals, thus exhibited acclimatory adjustments of minor vein phloem to cool temperature and/or high light, with more numerous and larger phloem cells and a higher maximal photosynthesis rate. The upregulation of photosynthesis in winter annuals in response to low growth temperature may thus depend on not only (1) a greater volume of photosynthesizing palisade tissue but also (2) leaf veins containing additional phloem cells and presumably capable of exporting a greater volume of sugars from the leaves to the rest of the plant.

Multiple feedbacks between chloroplast and whole plant in the context of plant adaptation and acclimation to the environment

This review focuses on feedback pathways that serve to match plant energy acquisition with plant energy utilization, and thereby aid in the optimization of chloroplast and whole-plant function in a given environment. First, the role of source-sink signalling in adjusting photosynthetic capacity (light harvesting, photochemistry and carbon fixation) to meet whole-plant carbohydrate demand is briefly reviewed. Contrasting overall outcomes, i.e. increased plant growth versus plant growth arrest, are described and related to respective contrasting environments that either do or do not present opportunities for plant growth. Next, new insights into chloroplast-generated oxidative signals, and their modulation by specific components of the chloroplast's photoprotective network, are reviewed with respect to their ability to block foliar phloem-loading complexes, and, thereby, affect both plant growth and plant biotic defences. Lastly, carbon export capacity is described as a newly identified tuning point that has been subjected to the evolution of differential responses in plant varieties (ecotypes) and species from different geographical origins with contrasting environmental challenges.

Associations between the acclimation of phloem-cell wall ingrowths in minor veins and maximal photosynthesis rate

The companion cells (CCs) and/or phloem parenchyma cells (PCs) in foliar minor veins of some species exhibit invaginations that are amplified when plants develop in high light (HL) compared to low light (LL). Leaves of plants that develop under HL also exhibit greater maximal rates of photosynthesis compared to those that develop under LL, suggesting that the increased membrane area of CCs and PCs of HL-acclimated leaves may provide for greater levels of transport proteins facilitating enhanced sugar export. Furthermore, the degree of wall invagination in PCs (Arabidopsis thaliana) or CCs (pea) of fully expanded LL-acclimated leaves increased to the same level as that present in HL-acclimated leaves 7 days following transfer to HL, and maximal photosynthesis rates of transferred leaves of both species likewise increased to the same level as in HL-acclimated leaves. In contrast, transfer of Senecio vulgaris from LL to HL resulted in increased wall invagination in CCs, but not PCs, and such leaves furthermore exhibited only partial upregulation of photosynthetic capacity following LL to HL transfer. Moreover, a significant linear relationship existed between the level of cell wall ingrowths and maximal photosynthesis rates across all three species and growth light regimes. A positive linear relationship between these two parameters was also present for two ecotypes (Sweden, Italy) of the winter annual A. thaliana in response to growth at different temperatures, with significantly greater levels of PC wall ingrowths and higher rates of photosynthesis in leaves that developed at cooler versus warmer temperatures. Treatment of LL-acclimated plants with the stress hormone methyl jasmonate also resulted in increased levels of wall ingrowths in PCs of A. thaliana and S. vulgaris but not in CCs of pea and S. vulgaris. The possible role of PC wall ingrowths in sugar export versus as physical barriers to the movement of pathogens warrants further attention.

Not to be suppressed? Rethinking the host response at a root-parasite interface

Root-knot nematodes are highly efficient plant parasites that establish permanent feeding sites within host roots. The initiation of this feeding site is critical for parasitic success and requires an interaction with multiple signaling pathways involved in plant development and environmental response. Resistance against root-knot nematodes is relatively rare amongst their broad host range and they remain a major threat to agriculture. The development of effective and sustainable control strategies depends on understanding how host signaling pathways are manipulated during invasion of susceptible hosts. It is generally understood that root-knot nematodes either suppress host defense signaling during infestation or are able to avoid detection altogether, explaining their profound success as parasites. However, when compared to the depth of knowledge from other well-studied pathogen interactions, the published data on host responses to root-knot nematode infestation do not yet provide convincing support for this hypothesis and alternative explanations also exist. It is equally possible that defense-like signaling responses are actually induced and required during the early stages of root-knot nematode infestation. We describe how defense-signaling is highly context-dependent and that caution is necessary when interpreting transcriptional responses in the absence of appropriate control data or stringent validation of gene annotation. Further hypothesis-driven studies on host defense-like responses are required to account for these limitations and advance our understanding of root-knot nematode parasitism of plants.

Source-to-sink transport of sugar and regulation by environmental factors

Source-to-sink transport of sugar is one of the major determinants of plant growth and relies on the efficient and controlled distribution of sucrose (and some other sugars such as raffinose and polyols) across plant organs through the phloem. However, sugar transport through the phloem can be affected by many environmental factors that alter source/sink relationships. In this paper, we summarize current knowledge about the phloem transport mechanisms and review the effects of several abiotic (water and salt stress, mineral deficiency, CO2, light, temperature, air, and soil pollutants) and biotic (mutualistic and pathogenic microbes, viruses, aphids, and parasitic plants) factors. Concerning abiotic constraints, alteration of the distribution of sugar among sinks is often reported, with some sinks as roots favored in case of mineral deficiency. Many of these constraints impair the transport function of the phloem but the exact mechanisms are far from being completely known. Phloem integrity can be disrupted (e.g., by callose deposition) and under certain conditions, phloem transport is affected, earlier than photosynthesis. Photosynthesis inhibition could result from the increase in sugar concentration due to phloem transport decrease. Biotic interactions (aphids, fungi, viruses…) also affect crop plant productivity. Recent breakthroughs have identified some of the sugar transporters involved in these interactions on the host and pathogen sides. The different data are discussed in relation to the phloem transport pathways. When possible, the link with current knowledge on the pathways at the molecular level will be highlighted.

Association between minor loading vein architecture and light- and CO2-saturated rates of photosynthetic oxygen evolution among Arabidopsis thaliana ecotypes from different latitudes

Through microscopic analysis of veins and assessment of light- and CO2-saturated rates of photosynthetic oxygen evolution, we investigated the relationship between minor loading vein anatomy and photosynthesis of mature leaves in three ecotypes of Arabidopsis thaliana grown under four different combinations of temperature and photon flux density (PFD). All three ecotypes exhibited greater numbers and cross-sectional area of phloem cells as well as higher photosynthesis rates in response to higher PFD and especially lower temperature. The Swedish ecotype exhibited the strongest response to these conditions, the Italian ecotype the weakest response, and the Col-0 ecotype exhibited an intermediate response. Among all three ecotypes, strong linear relationships were found between light- and CO2-saturated rates of photosynthetic oxygen evolution and the number and area of either sieve elements or of companion and phloem parenchyma cells in foliar minor loading veins, with the Swedish ecotype showing the highest number of cells in minor loading veins (and largest minor veins) coupled with unprecedented high rates of photosynthesis. Linear, albeit less significant, relationships were also observed between number and cross-sectional area of tracheids per minor loading vein versus light- and CO2-saturated rates of photosynthetic oxygen evolution. We suggest that sugar distribution infrastructure in the phloem is co-regulated with other features that set the upper limit for photosynthesis. The apparent genetic differences among Arabidopsis ecotypes should allow for future identification of the gene(s) involved in augmenting sugar-loading and -transporting phloem cells and maximal rates of photosynthesis.

Intersection of transfer cells with phloem biology-broad evolutionary trends, function, and induction

Transfer cells (TCs) are ubiquitous throughout the plant kingdom. Their unique ingrowth wall labyrinths, supporting a plasma membrane enriched in transporter proteins, provides these cells with an enhanced membrane transport capacity for resources. In certain plant species, TCs have been shown to function to facilitate phloem loading and/or unloading at cellular sites of intense resource exchange between symplasmic/apoplasmic compartments. Within the phloem, the key cellular locations of TCs are leaf minor veins of collection phloem and stem nodes of transport phloem. In these locations, companion and phloem parenchyma cells trans-differentiate to a TC morphology consistent with facilitating loading and re-distribution of resources, respectively. At a species level, occurrence of TCs is significantly higher in transport than in collection phloem. TCs are absent from release phloem, but occur within post-sieve element unloading pathways and particularly at interfaces between generations of developing Angiosperm seeds. Experimental accessibility of seed TCs has provided opportunities to investigate their inductive signaling, regulation of ingrowth wall formation and membrane transport function. This review uses this information base to explore current knowledge of phloem transport function and inductive signaling for phloem-associated TCs. The functional role of collection phloem and seed TCs is supported by definitive evidence, but no such information is available for stem node TCs that present an almost intractable experimental challenge. There is an emerging understanding of inductive signals and signaling pathways responsible for initiating trans-differentiation to a TC morphology in developing seeds. However, scant information is available to comment on a potential role for inductive signals (auxin, ethylene and reactive oxygen species) that induce seed TCs, in regulating induction of phloem-associated TCs. Biotic phloem invaders have been used as a model to speculate on involvement of these signals.

Foliar phloem infrastructure in support of photosynthesis

Acclimatory adjustments of foliar minor loading veins in response to growth at different temperatures and light intensities are evaluated. These adjustments are related to their role in providing infrastructure for the export of photosynthetic products as a prerequisite for full acclimation of photosynthesis to the respective environmental conditions. Among winter-active apoplastic loaders, higher photosynthesis rates were associated with greater numbers of sieve elements per minor vein as well as an increased apparent total membrane area of cells involved in phloem loading (greater numbers of cells and/or greater cell wall invaginations). Among summer-active apoplastic loaders, higher photosynthesis rates were associated with increased vein density and, possibly, a greater number of sieve elements and companion cells per minor vein. Among symplastic loaders, minor loading vein architecture (number per vein and arrangement of cells) was apparently constrained, but higher photosynthesis rates were associated with higher foliar vein densities and larger intermediary cells (presumably providing a greater volume for enzymes involved in active raffinose sugar synthesis). Winter-active apoplastic loaders thus apparently place emphasis on adjustments of cell membrane area (presumably available for transport proteins active in loading of minor veins), while symplastic loaders apparently place emphasis on increasing the volume of cells in which their active loading step takes place. Presumably to accommodate a greater flux of photosynthate through the foliar veins, winter-active apoplastic loaders also have a higher number of sieve elements per minor loading vein, whereas symplastic loaders and summer-active apoplastic loaders have a higher total number of veins per leaf area. These latter adjustments in the vasculature (during leaf development) may also apply to the xylem (via greater numbers of tracheids per vein and/or greater vein density per leaf area) serving to increase water flux to mesophyll tissues in support of high rates of transpiration typically associated with high rates of photosynthesis.

Emerging trade-offs - impact of photoprotectants (PsbS, xanthophylls, and vitamin E) on oxylipins as regulators of development and defense

This review summarizes evidence for a mechanistic link between plant photoprotection and the synthesis of oxylipin hormones as regulators of development and defense. Knockout mutants of Arabidopsis, deficient in various key components of the chloroplast photoprotection system, consistently produced greater concentrations of the hormone jasmonic acid or its precursor 12- oxo-phytodienoic acid (OPDA), both members of the oxylipin messenger family. Characterized plants include several mutants deficient in PsbS (an intrinsic chlorophyll-binding protein of photosystem II) or pigments (zeaxanthin and/or lutein) required for photoprotective thermal dissipation of excess excitation energy in the chloroplast and a mutant deficient in reactive oxygen detoxification via the antioxidant vitamin E (tocopherol). Evidence is also presented that certain plant defenses against herbivores or pathogens are elevated for these mutants. This evidence furthermore indicates that wild-type Arabidopsis plants possess less than maximal defenses against herbivores or pathogens, and suggest that plant lines with superior defenses against abiotic stress may have lower biotic defenses. The implications of this apparent trade-off between abiotic and biotic plant defenses for plant ecology as well as for plant breeding/engineering are explored, and the need for research further addressing this important issue is highlighted.

Non-photochemical quenching capacity in Arabidopsis thaliana affects herbivore behaviour

Under natural conditions, plants have to cope with numerous stresses, including light-stress and herbivory. This raises intriguing questions regarding possible trade-offs between stress defences and growth. As part of a program designed to address these questions we have compared herbivory defences and damage in wild type Arabidopsis thaliana and two “photoprotection genotypes”, npq4 and oePsbS, which respectively lack and overexpress PsbS (a protein that plays a key role in qE-type non-photochemical quenching). In dual-choice feeding experiments both a specialist (Plutella xylostella) and a generalist (Spodoptera littoralis) insect herbivore preferred plants that expressed PsbS most strongly. In contrast, although both herbivores survived equally well on each of the genotypes, for oviposition female P. xylostella adults preferred plants that expressed PsbS least strongly. However, there were no significant differences between the genotypes in levels of the 10 most prominent glucosinolates; key substances in the Arabidopsis anti-herbivore chemical defence arsenal. After transfer from a growth chamber to the field we detected significant differences in the genotypes’ metabolomic profiles at all tested time points, using GC-MS, but no consistent “metabolic signature” for the lack of PsbS. These findings suggest that the observed differences in herbivore preferences were due to differences in the primary metabolism of the plants rather than their contents of typical “defence compounds”. A potentially significant factor is that superoxide accumulated most rapidly and to the highest levels under high light conditions in npq4 mutants. This could trigger changes in planta that are sensed by herbivores either directly or indirectly, following its dismutation to H₂O₂.

Minor loading vein acclimation for three Arabidopsis thaliana ecotypes in response to growth under different temperature and light regimes

In light of the important role of foliar phloem as the nexus between energy acquisition through photosynthesis and distribution of the products of photosynthesis to the rest of the plant, as well as communication between the whole plant and its leaves, we examined whether foliar minor loading veins in three Arabidopsis thaliana ecotypes undergo acclimation to the growth environment. As a winter annual exhibiting higher rates of photosynthesis in response to cooler vs. warmer temperatures, this species might be expected to adjust the structure of its phloem to accommodate greater fluxes of sugars in response to growth at low temperature. Minor (fourth- and third-order) veins had 14 or fewer sieve elements and phloem tissue comprised 50% or more of the cross-sectional area. The number of phloem cells per minor loading vein was greater in leaves grown under cool temperature and high light vs. warm temperature and moderate light. This effect was greatest in an ecotype from Sweden, in which growth under cool temperature and high light resulted in minor veins with an even greater emphasis on phloem (50% more phloem cells with more than 100% greater cross-sectional area of phloem) compared to growth under warm temperature and moderate light. Likewise, the number of sieve elements per minor vein increased linearly with growth temperature under moderate light, almost doubling over a 27°C temperature range (21°C leaf temperature range) in the Swedish ecotype. Increased emphasis on cells involved in sugar loading and transport may be critical for maintaining sugar export from leaves of an overwintering annual such as A. thaliana, and particularly for the ecotype from the northern-most population experiencing the lowest temperatures.

Extracellular hydrogen peroxide, produced through a respiratory burst oxidase/superoxide dismutase pathway, directs ingrowth wall formation in epidermal transfer cells of Vicia faba cotyledons

The intricate, and often polarized, ingrowth walls of transfer cells (TCs) amplify their plasma membrane surface areas to confer a transport function of supporting high rates of nutrient exchange across apo-/symplasmic interfaces. The TC ingrowth wall comprises a uniform wall layer on which wall ingrowths are deposited. Signals and signal cascades inducing trans-differentiation events leading to formation of TC ingrowth walls are poorly understood. Vicia faba cotyledons offer a robust experimental model to examine TC induction as, when placed into culture, their adaxial epidermal cells rapidly (h) and synchronously form polarized ingrowth walls accessible for experimental observations. Using this model, we recently reported findings consistent with extracellular hydrogen peroxide, produced through a respiratory burst oxidase homolog/superoxide dismutase pathway, initiating cell wall biosynthetic activity and providing directional information guiding deposition of the polarized uniform wall. Our conclusions rested on observations derived from pharmacological manipulations of hydrogen peroxide production and correlative gene expression data sets. A series of additional studies were undertaken, the results of which verify that extracellular hydrogen peroxide contributes to regulating ingrowth wall formation and is generated by a respiratory burst oxidase homolog/superoxide dismutase pathway.

Reactive oxygen species form part of a regulatory pathway initiating trans-differentiation of epidermal transfer cells in Vicia faba cotyledons

Various cell types can trans-differentiate to a transfer cell (TC) morphology characterized by deposition of polarized ingrowth walls comprised of a uniform layer on which wall ingrowths (WIs) develop. WIs form scaffolds supporting amplified plasma membrane areas enriched in transporters conferring a cellular capacity for high rates of nutrient exchange across apo- and symplasmic interfaces. The hypothesis that reactive oxygen species (ROS) are a component of the regulatory pathway inducing ingrowth wall formation was tested using Vicia faba cotyledons. Vicia faba cotyledons offer a robust experimental model to examine TC induction as, on being placed into culture, their adaxial epidermal cells rapidly (hours) form ingrowth walls on their outer periclinal walls. These are readily visualized by electron microscopy, and epidermal peels of their trans-differentiating cells allow measures of cell-specific gene expression. Ingrowth wall formation responded inversely to pharmacological manipulation of ROS levels, indicating that a flavin-containing enzyme (NADPH oxidase) and superoxide dismutase cooperatively generate a regulatory H(2)O(2) signature. Extracellular H(2)O(2) fluxes peaked prior to the appearance of WIs and were followed by a slower rise in H(2)O(2) flux that occurred concomitantly, and co-localized, with ingrowth wall formation. De-localizing the H(2)O(2) signature caused a corresponding de-localization of cell wall deposition. Temporal and epidermal cell-specific expression profiles of VfrbohA and VfrbohC coincided with those of extracellular H(2)O(2) production and were regulated by cross-talk with ethylene. It is concluded that H(2)O(2) functions, downstream of ethylene, to activate cell wall biosynthesis and direct polarized deposition of a uniform wall on which WIs form.

Glucose and ethylene signalling pathways converge to regulate trans-differentiation of epidermal transfer cells in Vicia narbonensis cotyledons

Transfer cells are specialized transport cells containing invaginated wall ingrowths that provide an amplified plasma membrane surface area with high densities of transporter proteins. They trans-differentiate from differentiated cells at sites where enhanced rates of nutrient transport occur across apo/symplasmic boundaries. Despite their physiological importance, the signal(s) and signalling cascades responsible for initiating their trans-differentiation are poorly understood. In culture, adaxial epidermal cells of Vicia narbonensis cotyledons were induced to trans-differentiate to a transfer cell morphology. Manipulating their intracellular glucose concentrations by transgenic knock-down of ADP-glucose pyrophosphorylase expression and/or culture on a high-glucose medium demonstrated that glucose functioned as a negative regulator of wall ingrowth induction. In contrast, glucose had no detectable effect on wall ingrowth morphology. The effect on wall ingrowth induction of culture on media containing glucose analogues suggested that glucose acts through a hexokinase-dependent signalling pathway. Elevation of an epidermal cell-specific ethylene signal alone, or in combination with glucose analogues, countered the negative effect of glucose on wall ingrowth induction. Glucose modulated the amplitude of ethylene-stimulated wall ingrowth induction by down-regulating the expression of ethylene biosynthetic genes and an ethylene insensitive 3 (EIN3)-like gene (EIL) encoding a key transcription factor in the ethylene signalling cascade. A model is presented describing the interaction between glucose and ethylene signalling pathways regulating the induction of wall ingrowth formation in adaxial epidermal cells.

Geminiviruses subvert ubiquitination by altering CSN-mediated derubylation of SCF E3 ligase complexes and inhibit jasmonate signaling in Arabidopsis thaliana

Viruses must create a suitable cell environment and elude defense mechanisms, which likely involves interactions with host proteins and subsequent interference with or usurpation of cellular machinery. Here, we describe a novel strategy used by plant DNA viruses (Geminiviruses) to redirect ubiquitination by interfering with the activity of the CSN (COP9 signalosome) complex. We show that geminiviral C2 protein interacts with CSN5, and its expression in transgenic plants compromises CSN activity on CUL1. Several responses regulated by the CUL1-based SCF ubiquitin E3 ligases (including responses to jasmonates, auxins, gibberellins, ethylene, and abscisic acid) are altered in these plants. Impairment of SCF function is confirmed by stabilization of yellow fluorescent protein-GAI, a substrate of the SCF(SLY1). Transcriptomic analysis of these transgenic plants highlights the response to jasmonates as the main SCF-dependent process affected by C2. Exogenous jasmonate treatment of Arabidopsis thaliana plants disrupts geminivirus infection, suggesting that the suppression of the jasmonate response might be crucial for infection. Our findings suggest that C2 affects the activity of SCFs, most likely through interference with the CSN. As SCFs are key regulators of many cellular processes, the capability of viruses to selectively interfere with or hijack the activity of these complexes might define a novel and powerful strategy in viral infections.

Phloem loading, plant growth form, and climate

Plasmodesmatal frequencies in the phloem of leaf minor veins vary considerably, suggesting that photoassimilate is loaded into the phloem by different strategies. The ecophysiological basis for multiple loading types is unknown. We updated the analysis of van Bel and Gamalei (Plant Cell Environ 15: 265-270, 1992) with more current phylogenetic data and by treating separately two symplastic loading types, those that load actively by polymer trapping (synthesis of raffinose family oligosaccharides--RFOs), and those that load passively, by diffusion. The results indicate a stronger association between passive, symplastic loading and the tree growth form than previously recognized. Apoplastic loading is highly correlated with the herbaceous habit. There is no correlation between RFO families and growth form. At the family level, there are no correlations between minor vein types and climate that cannot be explained by the dearth of woody plants in the arctic for reasons unassociated with phloem loading. However, at the species level, a floristic analysis uncovered a correlation between the RFO trait and species frequency in tropical and subtropical regions of the world. The correlations between loading types and both growth form and climate are subtle, probably indirect, and poorly understood.

GIGANTEA is a component of a regulatory pathway determining wall ingrowth deposition in phloem parenchyma transfer cells of Arabidopsis thaliana

Transfer cells are specialised transport cells containing invaginated wall ingrowths that generate an amplified plasma membrane surface area with high densities of transporter proteins. They trans-differentiate from differentiated cells at sites at which enhanced rates of nutrient transport occur across apo/symplasmic boundaries. Despite their physiological importance, little is known of the molecular mechanisms regulating construction of their intricate wall ingrowths. We investigated the genetic control of wall ingrowth formation in phloem parenchyma transfer cells of leaf minor veins in Arabidopsis thaliana. Wall ingrowth development in these cells is substantially enhanced upon exposing plants to high-light or cold treatments. A hierarchical bioinformatic analysis of public microarray datasets derived from the leaves of plants subjected to these treatments identified GIGANTEA (GI) as one of 46 genes that are commonly up-regulated twofold or more under both high-light and cold conditions. Histological analysis of the GI mutants gi-2 and gi-3 showed that the amount of phloem parenchyma containing wall ingrowths was reduced 15-fold compared with wild-type. Discrete papillate wall ingrowths were formed in gi-2 plants but failed to develop into branched networks. Wall ingrowth development in gi-2 was not rescued by exposing these plants to high-light or cold conditions. In contrast, over-expression of GI in the gi-2 background restored wall ingrowth deposition to wild-type levels. These results indicate that GI regulates the ongoing development of wall ingrowth networks at a point downstream of inputs from environmental signals.