Introns control expression of sucrose transporter LeSUT1 in trichomes, companion cells and in guard cells

PhyB-dependent phosphorylation of mitogen-activated protein kinase cascade MKK2-MPK2 positively regulates red light-induced stomatal opening

Red light induces stomatal opening by affecting photosynthesis, metabolism and triggering signal transductions in guard cells. Phytochrome B (phyB) plays a positive role in mediating red light-induced stomatal opening. However, phyB-mediated red light guard cell signalling is poorly understood. Here, we found that phyB-mediated sequential phosphorylation of mitogen-activated protein kinase kinase 2 (MAPKK2, MKK2) and MPK2 in guard cells is essential for red light-induced stomatal opening. Mutations in MKK2 and MPK2 led to reduced stomatal opening in response to white light, and these phenotypes could be observed under red light, not blue light. MKK2 interacted with MPK2 in vitro and in plants. MPK2 was directly phosphorylated by MKK2 in vitro. Red light triggered the phosphorylation of MKK2 in guard cells, and MKK2 phosphorylation was greatly reduced in phyB mutant. Simultaneously, red light-stimulated MPK2 phosphorylation in guard cells was inhibited in mkk2 mutant. Furthermore, mkk2 and mpk2 mutants exhibit significantly smaller stomatal apertures than that of wild type during the stomatal opening stage in the diurnal stomatal movements. Our results indicate that red light-promoted phyB-dependent phosphorylation of MKK2-MPK2 cascade in guard cells is essential for stomatal opening, which contributes to the fine-tuning of stomatal opening apertures under light.

The intragenic cis-elements mediate temperature response of RrKSN

Gene regulation via intragenic sequences is becoming more recognized in many eukaryotes. However, the intragenic sequences mediated gene expressions in response to environmental stimuli have been largely uncharacterized. Here, we showed that the first intron of RrKSN from the Rosa rugosa cultivar 'Purple branch' had a positive effect on RrKSN expression, and the effect depends on its position and orientation. Further analyses revealed that the four adjacent cis-elements (T)CGATT/AATCG(A) within the first intron were critical for the positive regulation, and the RrKSN promotion was significantly suppressed with mutations of these elements. These cis-elements were further evidenced as binding sites for RrARR1, the homologous of Arabidopsis type-B ARABIDOPSIS RESPONSE REGULATOR 1 (ARR1) transcription factor. The first intron-mediated RrKSN expression was enhanced with over-expressing of RrARR1, but abolished with RrARR1 silencing in rose seedlings. Moreover, the expression difference of RrKSN between 16°C and 28°C was eliminated along with RrARR1-silencing. Taken together, these results suggested both RrARR1 and its binding elements are required for the first intron-mediated RrKSN expression in response to varying temperatures. Therefore, our results reveal a unique intragenic regulation mechanism of gene expression by which plants perceive the signal of ambient temperature in rose.

How pollen tubes fight for food: the impact of sucrose carriers and invertases of Arabidopsis thaliana on pollen development and pollen tube growth

Pollen tubes of higher plants grow very rapidly until they reach the ovules to fertilize the female gametes. This growth process is energy demanding, however, the nutrition strategies of pollen are largely unexplored. Here, we studied the function of sucrose transporters and invertases during pollen germination and pollen tube growth. RT-PCR analyses, reporter lines and knockout mutants were used to study gene expression and protein function in pollen. The genome of Arabidopsis thaliana contains eight genes that encode functional sucrose/H⁺ symporters. Apart from AtSUC2, which is companion cell specific, all other AtSUC genes are expressed in pollen tubes. AtSUC1 is present in developing pollen and seems to be the most important sucrose transporter during the fertilization process. Pollen of an Atsuc1 knockout plant contain less sucrose and have defects in pollen germination and pollen tube growth. The loss of other sucrose carriers affects neither pollen germination nor pollen tube growth. A multiple knockout line Atsuc1Atsuc3Atsuc8Atsuc9 shows a phenotype that is comparable to the Atsuc1 mutant line. Loss of AtSUC1 can`t be complemented by AtSUC9, suggesting a special function of AtSUC1. Besides sucrose carriers, pollen tubes also synthesize monosaccharide carriers of the AtSTP family as well as invertases. We could show that AtcwINV2 and AtcwINV4 are expressed in pollen, AtcwINV1 in the transmitting tissue and AtcwINV5 in the funiculi of the ovary. The vacuolar invertase AtVI2 is also expressed in pollen, and a knockout of AtVI2 leads to a severe reduction in pollen germination. Our data indicate that AtSUC1 mediated sucrose accumulation during late stages of pollen development and cleavage of vacuolar sucrose into monosaccharides is important for the process of pollen germination.

Introns in the Naa50 gene act as strong enhancers of tissue-specific expression in Arabidopsis

Naa50 is the catalytic subunit of N-terminal acetyltransferase complex E, which plays an important role in regulating plant development, endoplasmic reticulum stress and immune responses in Arabidopsis. In this study, the complete genomic sequence (but not the coding sequence) of Naa50 rescued the phenotype of Naa50 deletion mutants. Naa50 expression was noted in whole roots except for central root cap cells. The deletion of intron 1 resulted in a loss of Naa50 expression in the root meristem zone and in the epidermis, cortex and endodermis of the elongation zone and mature zone, while the deletion of intron 2 decreased Naa50 expression in the epidermis, cortex and endodermis of the root elongation zone and mature zone. The native Naa50 promoter together with introns 1 and 2 promotes the expression of Naa50 in sepal vascular bundles, filaments, pollen and stigmas; however, neither intron has positive effect on Naa50 expression in mature rosette leaves. The results of this study show that introns 1 and 2 in the Naa50 gene function as enhancers to promote the tissue-specific expression of Naa50.

Sugar transporters in grasses: Function and modulation in source and storage tissues

Carbohydrate partitioning, the process of transporting carbohydrates from photosynthetic (source) tissues, such as leaves, to non-photosynthetic (sink) tissues, such as stems, roots, and reproductive structures, is vital not only for the growth and development of plants but also for withstanding biotic and abiotic stress. In many plants, sucrose is the primary form of carbohydrate loaded into the phloem for long-distance transport and unloaded into the sink tissues for utilization or storage. We highlight recent findings about 1) phloem loading in grasses, 2) the principal families of sugar transporters involved in sucrose transport, and 3) novel mechanisms by which the activities of sugar transporters are modulated. We discuss exciting discoveries from eudicot species that provide valuable insights regarding the regulation of these sugar transporters, which may be translatable to monocot species. As we better understand the intricate pathways that control the activities of various sugar transporters, we can utilize this knowledge for developing improved crop varieties.

Genomic editing of intronic enhancers unveils their role in fine-tuning tissue-specific gene expression in Arabidopsis thaliana

Enhancers located in introns are abundant and play a major role in the regulation of gene expression in mammalian species. By contrast, the functions of intronic enhancers in plants have largely been unexplored and only a handful of plant intronic enhancers have been reported. We performed a genome-wide prediction of intronic enhancers in Arabidopsis thaliana using open chromatin signatures based on DNase I sequencing. We identified 941 candidate intronic enhancers associated with 806 genes in seedling tissue and 1,271 intronic enhancers associated with 1,069 genes in floral tissue. We validated the function of 15 of 21 (71%) of the predicted intronic enhancers in transgenic assays using a reporter gene. We also created deletion lines of three intronic enhancers associated with two different genes using CRISPR/Cas. Deletion of these enhancers, which span key transcription factor binding sites, did not abolish gene expression but caused varying levels of transcriptional repression of their cognate genes. Remarkably, the transcriptional repression of the deletion lines occurred at specific developmental stages and resulted in distinct phenotypic effects on plant morphology and development. Clearly, these three intronic enhancers are important in fine-tuning tissue- and development-specific expression of their cognate genes.

What determines the composition of the phloem sap? Is there any selectivity filter for macromolecules entering the phloem sieve elements?

In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young sieve elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.

Arabidopsis Sucrose Transporter AtSuc1 introns act as strong enhancers of expression

The expression of AtSUC1 is controlled by the promoter and intragenic sequences. AtSUC1 is expressed in roots, pollen and trichomes. However, AtSUC1 promoter-GUS transgenics only show expression in trichomes and pollen. Here, we show that the root expression of AtSUC1 is controlled by an interaction between the AtSUC1 promoter and two short introns. The deletion of either intron from whole-gene-GUS constructs results in no root expression, showing that both introns are required. The two introns in tandem, fused to GUS, produce high constitutive expression throughout the vegetative parts of the plant. When combined with the promoter, the expression driven by the introns is reduced and localized to the roots. In Arabidopsis seedlings, exogenously applied sucrose induces the expression of AtSUC1 in roots and causes anthocyanin accumulation. atsuc1 loss-of-function mutants are defective in sucrose-induced anthocyanin accumulation. We show that an AtSUC1 whole-gene-GUS construct expressing a nonfunctional AtSUC1 (D152N) mutant, that is transport inactive, is defective in sucrose-induced AtSUC1 expression when expressed in an atsuc1-null background. We also show that the transport-defective allele does not complement the loss of sucrose-induced anthocyanin accumulation in null atsuc1 mutants. The results indicate that sucrose uptake via AtSUC1 is required for sucrose-induced AtSUC1 expression and sucrose-induced anthocyanin accumulation and that the site for sucrose detection is intracellular.

Expression of Sucrose Transporters from Vitis vinifera Confer High Yield and Enhances Drought Resistance in Arabidopsis

Sucrose is the predominant form of sugar transported from photosynthetic (source) to non-photosynthetic (sink) organs in higher plants relying on the transporting function of sucrose transporters (SUTs or SUCs). Many SUTs have been identified and characterized in both monocots and dicots. However, the function of sucrose transporters (SUTs or SUCs) from Vitis is not clear. As the world’s most planted grape species, Vitis vinifera owns three sucrose transport activity verified SUTs. In this study, we constructed three kinds of VvSUC (Vitis vinifera SUC)-overexpressing transgenic Arabidopsis. VvSUC-overexpressing transgenic Arabidopsis was cultured on sucrose-supplemented medium. VvSUC11- and VvSUC12-overexpressing lines had similar thrived growth phenotypes, whereas the size and number of leaves and roots from VvSUC27-overexpressing lines were reduced compared with that of WT. When plants were cultured in soil, all SUT transgenic seedlings produced more number of leaves and siliques, resulting in higher yield (38.6% for VvSUC12-transformants) than that of WT. Besides, VvSUC27-transformants and VvSUC11-transformants enhanced drought resistance in Arabidopsis, providing a promising target for crop improvement

Role of blue and red light in stomatal dynamic behaviour

Plants experience changes in light intensity and quality due to variations in solar angle and shading from clouds and overlapping leaves. Stomatal opening to increasing irradiance is often an order of magnitude slower than photosynthetic responses, which can result in CO2 diffusional limitations on leaf photosynthesis, as well as unnecessary water loss when stomata continue to open after photosynthesis has reached saturation. Stomatal opening to light is driven by two distinct pathways; the 'red' or photosynthetic response that occurs at high fluence rates and saturates with photosynthesis, and is thought to be the main mechanism that coordinates stomatal behaviour with photosynthesis; and the guard cell-specific 'blue' light response that saturates at low fluence rates, and is often considered independent of photosynthesis, and important for early morning stomatal opening. Here we review the literature on these complicated signal transduction pathways and osmoregulatory processes in guard cells that are influenced by the light environment. We discuss the possibility of tuning the sensitivity and magnitude of stomatal response to blue light which potentially represents a novel target to develop ideotypes with the 'ideal' balance between carbon gain, evaporative cooling, and maintenance of hydraulic status that is crucial for maximizing crop performance and productivity.

Molecular Evolution and Interaction of Membrane Transport and Photoreception in Plants

Light is a vital regulator that controls physiological and cellular responses to regulate plant growth, development, yield, and quality. Light is the driving force for electron and ion transport in the thylakoid membrane and other membranes of plant cells. In different plant species and cell types, light activates photoreceptors, thereby modulating plasma membrane transport. Plants maximize their growth and photosynthesis by facilitating the coordinated regulation of ion channels, pumps, and co-transporters across membranes to fine-tune nutrient uptake. The signal-transducing functions associated with membrane transporters, pumps, and channels impart a complex array of mechanisms to regulate plant responses to light. The identification of light responsive membrane transport components and understanding of their potential interaction with photoreceptors will elucidate how light-activated signaling pathways optimize plant growth, production, and nutrition to the prevailing environmental changes. This review summarizes the mechanisms underlying the physiological and molecular regulations of light-induced membrane transport and their potential interaction with photoreceptors in a plant evolutionary and nutrition context. It will shed new light on plant ecological conservation as well as agricultural production and crop quality, bringing potential nutrition and health benefits to humans and animals.

Evolution of Guard-Cell Theories: The Story of Sugars

Stomata are dynamic pores in the impermeable cuticle that coats the aerial parts of vascular plants, allowing the entry of CO₂ for photosynthesis and controlling water loss. They are composed of two guard cells that can swell or shrink due to an increase or decrease in their osmotic pressure, respectively. Swelling opens the stomata and shrinking closes the stomata. For more than a century, scientists have been working to uncover the nature of the osmolytes that modulate osmotic pressure in guard cells. Recent discoveries have undermined long-standing theories in this area, reversing the understood roles of sugars and demonstrating the evolution of scientific theories. Here, we describe the evolution of guard-cell osmoregulation theories with an emphasis on the role of sugars.

Confirmation of mesophyll signals controlling stomatal responses by a newly devised transplanting method

There are opposing views on whether the responses of stomata to environmental stimuli are all autonomous reactions of stomatal guard cells or whether mesophyll is involved in these responses. Transplanting isolated epidermis onto mesophyll is a potent methodology for examining the roles of mesophyll-derived signals in stomatal responses. Here we report on development of a new transplanting method. Leaf segments of Commelina communis L. were pretreated in the light or dark at 10, 39 or 70Pa ambient CO2 for 1h. Then the abaxial epidermises were removed and the epidermal strips prepared from the other leaves kept in the dark at 39Pa CO2, were transplanted onto the mesophyll. After illumination of the transplants for 1h at 39Pa CO2, stomatal apertures were measured. We also examined the molecular sizes of the mesophyll signals by inserting the dialysis membrane permeable to molecules smaller than 100-500Da or 500-1000Da between the epidermis and mesophyll. Mesophyll pretreatments in the light at low CO2 partial pressures accelerated stomatal opening in the transplanted epidermal strips, whereas pretreatments at 70Pa CO2 suppressed stomatal opening. Insertion of these dialysis membranes did not suppress stomatal opening significantly at 10Pa CO2 in the light, whereas insertion of the 100-500Da membrane decelerated stomatal closure at high CO2. It is probable that the mesophyll signals inducing stomatal opening at low CO2 in the light would permeate both membranes, and that those inducing stomatal closure at high CO2 would not permeate the 100-500Da membrane. Possible signal compounds are discussed.

Genome-wide association study revealed that the TaGW8 gene was associated with kernel size in Chinese bread wheat

Using Wheat 90 K SNP assay, kernel-related traits of Chinese bread wheat were used to perform association mapping in 14 environments by GWAS. Results indicated that 996 and 953 of 4417 and 3172 significant SNPs for kernel length and thousand-kernel weight were located on the chromosome 7B. Haplotype analysis of these SNPs on 7B generated the block containing the predicted TaGW8-B1 gene. TaGW8-B1 gene was further cloned by sequencing in bread wheat and a 276-bp InDel was found in the first intron. TaGW8-B1 without and with the 276-bp InDel were designated as TaGW8-B1a and TaGW8-B1b, respectively. Analysis of agronomic traits indicated that cultivars with TaGW8-B1a possessed significantly wider kernel width, significantly more kernel number per spike, longer kernel length, higher thousand-kernel weight and more spikelet number per spike than cultivars with TaGW8-B1b. Furthermore, cultivars with TaGW8-B1a possessed significantly higher yield than cultivars with TaGW8-B1b. Therefore, TaGW8-B1a was considered as a potentially superior allele. Meanwhile, TaGW8-B1a possessed a significantly higher expression level than TaGW8-B1b in mature seeds by qRT-PCR. It possibly suggested that the high expression of TaGW8-B1 was positively associated with kernel size in bread wheat. Distribution of TaGW8-B1 allele indicated that TaGW8-B1a has been positively selected in Chinese wheat.

Immunohistochemical Localization of Proteins in the Phloem: Problems and Solutions

Immunolocalization of proteins in differentiated phloem cells is a challenging task given their special anatomy, organellar infrastructure, and the phloem tissue's heterogeneity. Incorporation of specific wall components in the thickened cell walls of phloem cells is often the source of unspecific labeling, leading to erroneous localization. Therefore, special care is required regarding generation and purification of specific antibodies. In addition, tissue preservation of phloem cells, which contain a high osmotic pressure in their functional state, is a very challenging task prone to various pitfalls. This chapter provides practical advice for cautious tissue preparation and antibody purification. Furthermore, methods that can be used to verify immunohistochemical localization data, such as promoter-reporter studies or activity tests, are discussed. Such confirmation experiments are essential for unambiguous determination of protein location in cells of the phloem.

Sucrose-induced stomatal closure is conserved across evolution

As plants evolved to function on land, they developed stomata for effective gas exchange, for photosynthesis and for controlling water loss. We have recently shown that sugars, as the end product of photosynthesis, close the stomata of various angiosperm species, to coordinate sugar production with water loss. In the current study, we examined the sugar responses of the stomata of phylogenetically different plant species and species that employ different photosynthetic mechanisms (i.e., C3, C4 and CAM). To examine the effect of sucrose on stomata, we treated leaves with sucrose and then measured their stomatal apertures. Sucrose reduced stomatal aperture, as compared to an osmotic control, suggesting that regulation of stomata by sugars is a trait that evolved early in evolutionary history and has been conserved across different groups of plants.

Tissue-specific transcriptomic profiling of Plantago major provides insights for the involvement of vasculature in phosphate deficiency responses

The vasculature of higher plants is important with transport of both nutrient and information molecules. To understand the correspondence of this tissue in molecular responses under phosphate (Pi) deficiency, Plantago major, a model plant for vasculature biology study, was chosen in our analysis. After RNA-Seq and de novo transcriptome assembly of 24 libraries prepared from the vasculature of P. major, 37,309 unigenes with a mean length of 1571 base pairs were obtained. Upon 24 h of Pi deficiency, 237 genes were shown to be differentially expressed in the vasculature of P. major. Among these genes, only 27 have been previously identified to be specifically expressed in the vasculature tissues in other plant species. Temporal expression of several marker genes associated with Pi deficiency showed that the time period of first 24 h is at the beginning stage of more dynamic expression patterns. In this study, we found several physiological processes, e.g., "phosphate metabolism and remobilization", "sucrose metabolism, loading and synthesis", "plant hormone metabolism and signal transduction", "transcription factors", and "metabolism of other minerals", were mainly involved in early responses to Pi deficiency in the vasculature. A number of vasculature genes with promising roles in Pi deficiency adaptation have been identified and deserve further functional characterization. This study clearly demonstrated that plant vasculature is actively involved in Pi deficiency responses and understanding of this process may help to create plants proficient to offset Pi deficiency.

Protoplast-Esculin Assay as a New Method to Assay Plant Sucrose Transporters: Characterization of AtSUC6 and AtSUC7 Sucrose Uptake Activity in Arabidopsis Col-0 Ecotype

The best characterized function of sucrose transporters of the SUC family in plants is the uptake of sucrose into the phloem for long-distance transport of photoassimilates. This important step is usually performed by one specific SUC in every species. However, plants possess small families of several different SUCs which are less well understood. Here, we report on the characterization of AtSUC6 and AtSUC7, two members of the SUC family in Arabidopsis thaliana. Heterologous expression in yeast (Saccharomyces cerevisiae) revealed that AtSUC6_Col-0 is a high-affinity H⁺-symporter that mediates the uptake of sucrose and maltose across the plasma membrane at exceptionally low pH values. Reporter gene analyses revealed a strong expression of AtSUC6_Col-0 in reproductive tissues, where the protein product might contribute to sugar uptake into pollen tubes and synergid cells. A knockout of AtSUC6 did not interfere with vegetative development or reproduction, which points toward physiological redundancy of AtSUC6_Col-0 with other sugar transporters. Reporter gene analyses showed that AtSUC7_Col-0 is expressed in roots and pollen tubes and that this sink specific expression of AtSUC7_Col-0 is regulated by intragenic regions. Transport activity of AtSUC7_Col-0 could not be analyzed in baker's yeast or Xenopus oocytes because the protein was not correctly targeted to the plasma membrane in both heterologous expression systems. Therefore, a novel approach to analyze sucrose transporters in planta was developed. Plasma membrane localized SUCs including AtSUC6_Col-0 and also sucrose specific SWEETs were able to mediate transport of the fluorescent sucrose analog esculin in transformed mesophyll protoplasts. In contrast, AtSUC7_Col-0 is not able to mediate esculin transport across the plasma membrane which implicates that AtSUC7_Col-0 might be a non-functional pseudogene. The novel protoplast assay provides a useful tool for the quick and quantitative analysis of sucrose transporters in an in planta expression system.

Suppression of sucrose synthase affects auxin signaling and leaf morphology in tomato

Metabolic enzymes have been found to play roles in plant development. Sucrose synthase (SUS) is one of the two enzyme families involved in sucrose cleavage in plants. In tomato, six SUS genes have been found. We generated transgenic tomato plants with RNAi suppression of SlSUS1, SlSUS3 and SlSUS4 genes. Independent transgenic lines with RNAi suppression of more than one SUS gene exhibited morphological effects on their cotyledons and leaf structure, but there were no significant effects on their carbohydrate levels, demonstrating that SUS has a developmental function, in addition to its metabolic function. Shoot apices of the transgenic lines showed elevated expression of JAGGED (JAG) and the auxin transporter PIN1. In a PIN1-GFP fusion reporter/SUS-RNAi hybrid, PIN1-GFP patterns were altered in developing leaves (as compared to control plants), indicating that SlSUS suppression alters auxin signaling. These results suggest possible roles for SUS in the regulation of plant growth and leaf morphology, in association with the auxin-signaling pathway.

Intronic Sequence Regulates Sugar-Dependent Expression of Arabidopsis thaliana Production of Anthocyanin Pigment-1/MYB75

Sucrose-specific regulation of gene expression is recognized as an important signaling response, distinct from glucose, which serves to modulate plant growth, metabolism, and physiology. The Arabidopsis MYB transcription factor Production of Anthocyanin Pigment-1 (PAP1) plays a key role in anthocyanin biosynthesis and expression of PAP1 is known to be regulated by sucrose. Sucrose treatment of Arabidopsis seedlings led to a 20-fold induction of PAP1 transcript, which represented a 6-fold increase over levels in glucose-treated seedlings. The PAP1 promoter was not sufficient for conferring a sucrose response to a reporter gene and did not correctly report expression of PAP1 in plants. Although we identified 3 putative sucrose response elements in the PAP1 gene, none were found to be necessary for this response. Using deletion analysis, we identified a 90 bp sequence within intron 1 of PAP1 that is necessary for the sucrose response. This sequence was sufficient for conferring a sucrose response to a minimal promoter: luciferase reporter when present in multiple copies upstream of the promoter. This work lays the foundation for dissecting the sucrose signaling pathway of PAP1 and contributes to understanding the interplay between sucrose signaling, anthocyanin biosynthesis, and stress responses.

The guard cell metabolome: functions in stomatal movement and global food security

Guard cells represent a unique single cell-type system for the study of cellular responses to abiotic and biotic perturbations that affect stomatal movement. Decades of effort through both classical physiological and functional genomics approaches have generated an enormous amount of information on the roles of individual metabolites in stomatal guard cell function and physiology. Recent application of metabolomics methods has produced a substantial amount of new information on metabolome control of stomatal movement. In conjunction with other "omics" approaches, the knowledge-base is growing to reach a systems-level description of this single cell-type. Here we summarize current knowledge of the guard cell metabolome and highlight critical metabolites that bear significant impact on future engineering and breeding efforts to generate plants/crops that are resistant to environmental challenges and produce high yield and quality products for food and energy security.

Lacking chloroplasts in guard cells of crumpled leaf attenuates stomatal opening: both guard cell chloroplasts and mesophyll contribute to guard cell ATP levels

Controversies regarding the function of guard cell chloroplasts and the contribution of mesophyll in stomatal movements have persisted for several decades. Here, by comparing the stomatal opening of guard cells with (crl-ch) or without chloroplasts (crl-no ch) in one epidermis of crl (crumpled leaf) mutant in Arabidopsis, we showed that stomatal apertures of crl-no ch were approximately 65-70% those of crl-ch and approximately 50-60% those of wild type. The weakened stomatal opening in crl-no ch could be partially restored by imposing lower extracellular pH. Correspondingly, the external pH changes and K(+) accumulations following fusicoccin (FC) treatment were greatly reduced in the guard cells of crl-no ch compared with crl-ch and wild type. Determination of the relative ATP levels in individual cells showed that crl-no ch guard cells contained considerably lower levels of ATP than did crl-ch and wild type after 2 h of white light illumination. In addition, guard cell ATP levels were lower in the epidermis than in leaves, which is consistent with the observed weaker stomatal opening response to white light in the epidermis than in leaves. These results provide evidence that both guard cell chloroplasts and mesophyll contribute to the ATP source for H(+) extrusion by guard cells.

Mesophyll photosynthesis and guard cell metabolism impacts on stomatal behaviour

Stomata control gaseous fluxes between the internal leaf air spaces and the external atmosphere. Guard cells determine stomatal aperture and must operate to ensure an appropriate balance between CO2 uptake for photosynthesis (A) and water loss, and ultimately plant water use efficiency (WUE). A strong correlation between A and stomatal conductance (gs ) is well documented and often observed, but the underlying mechanisms, possible signals and metabolites that promote this relationship are currently unknown. In this review we evaluate the current literature on mesophyll-driven signals that may coordinate stomatal behaviour with mesophyll carbon assimilation. We explore a possible role of various metabolites including sucrose and malate (from several potential sources; including guard cell photosynthesis) and new evidence that improvements in WUE have been made by manipulating sucrose metabolism within the guard cells. Finally we discuss the new tools and techniques available for potentially manipulating cell-specific metabolism, including guard and mesophyll cells, in order to elucidate mesophyll-derived signals that coordinate mesophyll CO2 demands with stomatal behaviour, in order to provide a mechanistic understanding of these processes as this may identify potential targets for manipulations in order to improve plant WUE and crop yield.

Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation

Sucrose (Suc) is the predominant form of carbon transported through the phloem from source to sink organs and is also a prominent sugar for short-distance transport. In all streptophytes analyzed, Suc transporter genes (SUTs or SUCs) form small families, with different subgroups evolving distinct functions. To gain insight into their capacity for moving Suc in planta, representative members of each clade were first expressed specifically in companion cells of Arabidopsis (Arabidopsis thaliana) and tested for their ability to rescue the phloem-loading defect caused by the Suc transporter mutation, Atsuc2-4. Sequence similarity was a poor indicator of ability: Several genes with high homology to AtSUC2, some of which have phloem-loading functions in other eudicot species, did not rescue the Atsuc2-4 mutation, whereas a more distantly related gene, ZmSUT1 from the monocot Zea mays, did restore phloem loading. Transporter complementary DNAs were also expressed in the companion cells of wild-type Arabidopsis, with the aim of increasing productivity by enhancing Suc transport to growing sink organs and reducing Suc-mediated feedback inhibition on photosynthesis. Although enhanced Suc loading and long-distance transport was achieved, growth was diminished. This growth inhibition was accompanied by increased expression of phosphate (P) starvation-induced genes and was reversed by providing a higher supply of external P. These experiments suggest that efforts to increase productivity by enhancing sugar transport may disrupt the carbon-to-P homeostasis. A model for how the plant perceives and responds to changes in the carbon-to-P balance is presented.

Hexokinase mediates stomatal closure

Stomata, composed of two guard cells, are the gates whose controlled movement allows the plant to balance the demand for CO2 for photosynthesis with the loss of water through transpiration. Increased guard-cell osmolarity leads to the opening of the stomata and decreased osmolarity causes the stomata to close. The role of sugars in the regulation of stomata is not yet clear. In this study, we examined the role of hexokinase (HXK), a sugar-phosphorylating enzyme involved in sugar-sensing, in guard cells and its effect on stomatal aperture. We show here that increased expression of HXK in guard cells accelerates stomatal closure. We further show that this closure is induced by sugar and is mediated by abscisic acid. These findings support the existence of a feedback-inhibition mechanism that is mediated by a product of photosynthesis, namely sucrose. When the rate of sucrose production exceeds the rate at which sucrose is loaded into the phloem, the surplus sucrose is carried toward the stomata by the transpiration stream and stimulates stomatal closure via HXK, thereby preventing the loss of precious water.

The effects of a stimulating intron on the expression of heterologous genes in Arabidopsis thaliana

Introns are often added to transgenes to increase expression, although the mechanism through which introns stimulate gene expression in plants and other eukaryotes remains mysterious. While introns vary in their effect on expression, it is unknown whether different genes respond similarly to the same stimulatory intron. Furthermore, the degree to which gene regulation is preserved when expression is increased by an intron has not been thoroughly investigated. To test the effects of the same intron on the expression of a range of genes, GUS translational fusions were constructed using the promoters of eight Arabidopsis genes whose expression was reported to be constitutive (GAE1, CNGC2 and ROP10), tissue specific (ADL1A, YAB3 and AtAMT2) or regulated by light (ULI3 and MSBP1). For each gene, a fusion containing the first intron from the UBQ10 gene was compared to fusions containing the gene's endogenous first intron (if the gene has one) or no intron. In every case, the UBQ10 intron increased expression relative to the intronless control, although the magnitude of the change and the level of expression varied. The UBQ10 intron also changed the expression patterns of the CNGC2 and YAB3 fusions to include strong activity in roots, indicating that tissue specificity was disrupted by this intron. In contrast, the regulation of the ULI3 and MSBP1 genes by light was preserved when their expression was stimulated by the intron. These findings have important implications for biotechnology applications in which a high level of transgene expression in only certain tissues is desired.

Functional analysis of a rice late pollen-abundant UDP-glucose pyrophosphorylase (OsUgp2) promoter

OsUgp2, a rice UDP-glucose pyrophosphorylase gene, has previously been shown to preferentially express in maturing pollens and plays an important role in pollen starch accumulation. Here, a 1943 bp promoter fragment (P1943) of OsUgp2 was characterized by 5' deletion and gain-of-function experiments. P1943 and its 5' deletion derivatives (P1495, P1005, P665 and P159) were fused to GUS reporter gene and stably introduced into rice plants. Histochemical analyses of different tissues and pollens at different developmental stages of the transgenic plants showed that P1943 could only direct GUS expression in binucleate pollens. P1495 and P1005 could still drive GUS expression in binucleate pollens but at a lower level. On the other hand, neither P665 nor P159 transformant exhibited any GUS activity in pollens. Gain-of-function analyses showed that the region (-1005 to -665 relative to translation start site) combined with a minimal CaMV 35S promoter could direct GUS expression in pollens. Further analysis of 5' deletion truncated at -952, -847 and -740 delimited a 53 bp region (-1005 to -952) essential for pollen-specific expression. The 53 bp sequence contains two motifs of TTTCT and TTTC, which were known to be pollen-specific cis-elements. In addition, the same P1943-GUS fusion construct was introduced into tobacco to analyze its specificity in dicotyledon. Interestingly, the GUS expression pattern in transgenic tobacco was quite different from that in rice. High level of GUS expression was detected in mature pollens as well as leaves, roots, sepals and stigmas. These findings suggested a complicated transcriptional regulation of OsUgp2.

Characterization of the phosphate starvation-induced glycerol-3-phosphate permease gene family in Arabidopsis

Phosphate (Pi) deficiency is one of the leading causes of loss in crop productivity. Plants respond to Pi deficiency by increasing Pi acquisition and remobilization involving organic and inorganic Pi transporters. Here, we report the functional characterization of a putative organic Pi transporter, Glycerol-3-phosphate permease (G3Pp) family, comprising five members (AtG3Pp1 to -5) in Arabidopsis (Arabidopsis thaliana). AtG3Pp1 and AtG3Pp2 showed 24-and 3-fold induction, respectively, in the roots of Pi-deprived seedlings, whereas Pi deficiency-mediated induction of AtG3Pp3 and -4 was evident in both roots and shoots. Furthermore, promoter-β-glucuronidase (GUS) fusion transgenics were generated for AtG3Pp2 to -5 for elucidation of their in planta role in Pi homeostasis. During Pi starvation, there was a strong expression of the reporter gene driven by AtG3Pp4 promoter in the roots, shoots, anthers, and siliques, whereas GUS expression was specific either to the roots (AtG3Pp3) or to stamens and siliques (AtG3Pp5) in other promoter-GUS fusion transgenics. Quantification of reporter gene activities further substantiated differential responses of AtG3Pp family members to Pi deprivation. A distinct pattern of reporter gene expression exhibited by AtG3Pp3 and AtG3Pp5 during early stages of germination also substantiated their potential roles during seedling ontogeny. Furthermore, an AtG3Pp4 knockdown mutant exhibited accentuated total lateral root lengths under +phosphorus and -phosphorus conditions compared with the wild type. Several Pi starvation-induced genes involved in root development and/or Pi homeostasis were up-regulated in the mutant. A 9-fold induction of AtG3Pp3 in the mutant provided some evidence for a lack of functional redundancy in the gene family. These results thus reflect differential roles of members of the G3Pp family in the maintenance of Pi homeostasis.

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.

Membrane-transport systems for sucrose in relation to whole-plant carbon partitioning

Sucrose is the principal product of photosynthesis used for the distribution of assimilated carbon in plants. Transport mechanisms and efficiency influence photosynthetic productivity by relieving product inhibition and contribute to plant vigor by controlling source/sink relationships and biomass partitioning. Sucrose is synthesized in the cytoplasm and may move cell to cell through plasmodesmata or may cross membranes to be compartmentalized or exported to the apoplasm for uptake into adjacent cells. As a relatively large polar compound, sucrose requires proteins to facilitate efficient membrane transport. Transport across the tonoplast by facilitated diffusion, antiport with protons, and symport with protons have been proposed; for transport across plasma membranes, symport with protons and a mechanism resembling facilitated diffusion are evident. Despite decades of research, only symport with protons is well established at the molecular level. This review aims to integrate recent and older studies on sucrose flux across membranes with principles of whole-plant carbon partitioning.

Evidence for a DNA-Based Mechanism of Intron-Mediated Enhancement

Many introns significantly increase gene expression through a process termed intron-mediated enhancement (IME). Introns exist in the transcribed DNA and the nascent RNA, and could affect expression from either location. To determine which is more relevant to IME, hybrid introns were constructed that contain sequences from stimulating Arabidopsis thaliana introns either in their normal orientation or as the reverse complement. Both ends of each intron are from the non-stimulatory COR15a intron in their normal orientation to allow splicing. The inversions create major alterations to the sequence of the transcribed RNA with relatively minor changes to the DNA structure. Introns containing portions of either the UBQ10 or ATPK1 intron increased expression to a similar degree regardless of orientation. Also, computational predictions of IME improve when both intron strands are considered. These findings are more consistent with models of IME that act at the level of DNA rather than RNA.

Testing the IMEter on rice introns and other aspects of intron-mediated enhancement of gene expression

In many eukaryotes, spliceosomal introns are able to influence the level and site of gene expression. The mechanism of this Intron Mediated Enhancement (IME) has not yet been elucidated, but regulation of gene expression is likely to occur at several steps during and after transcription. Different introns have different intrinsic enhancing properties, but the determinants of these differences remain unknown. Recently, an algorithm called IMEter, which is able to predict the IME potential of introns without direct testing, has been proposed. A computer program was developed for Arabidopsis thaliana and rice (Oryza sativa L.), but was only tested experimentally in Arabidopsis by measuring the enhancement effect on GUS expression of different introns inserted within otherwise identical plasmids. To test the IMEter potential in rice, a vector bearing the upstream regulatory sequence of a rice β-tubulin gene (OsTub6) fused to the GUS reporter gene was used. The enhancing intron interrupting the OsTub6 5'-UTR was precisely replaced by seven other introns carrying different features. GUS expression level in transiently transformed rice calli does not significantly correlate with the calculated IMEter score. It was also found that enhanced GUS expression was mainly due to a strong increase in the mRNA steady-state level and that mutations at the splice recognition sites almost completely abolished the enhancing effect. Splicing also appeared to be required for IME in Arabidopsis cell cultures, where failure of the OsTub6 5' region to drive high level gene expression could be rescued by replacing the poorly spliced rice intron with one from Arabidopsis.

Lifespan profiles of Alzheimer's disease-associated genes and products in monkeys and mice

Alzheimer's disease (AD) is characterized by plaques of amyloid-beta (Abeta) peptide, cleaved from amyloid-beta protein precursor (AbetaPP). Our hypothesis is that lifespan profiles of AD-associated mRNA and protein levels in monkeys would differ from mice and that differential lifespan expression profiles would be useful to understand human AD pathogenesis. We compared profiles of AbetaPP mRNA, AbetaPP protein, and Abeta levels in rodents and primates. We also tracked a transcriptional regulator of the AbetaPP gene, specificity protein 1 (SP1), and the beta amyloid precursor cleaving enzyme (BACE1). In mice, AbetaPP and SP1 mRNA and their protein products were elevated late in life; Abeta levels declined in old age. In monkeys, SP1, AbetaPP, and BACE1 mRNA declined in old age, while protein products and Abeta levels rose. Proteolytic processing in both species did not match production of Abeta. In primates, AbetaPP and SP1 mRNA levels coordinate, but an inverse relationship exists with corresponding protein products as well as Abeta levels. Comparison of human DNA and mRNA sequences to monkey and mouse counterparts revealed structural features that may explain differences in transcriptional and translational processing. These findings are important for selecting appropriate models for AD and other age-related diseases.

The phloem pathway: new issues and old debates

The phloem is a central actor in plant development and nutrition, providing nutrients and energy to sink organs and integrating interorgan communication. A comprehensive picture of the molecules trafficking in phloem sap is being made available, with recent surveys of proteins, RNAs, sugars, and other metabolites, some of which are potentially acting as signals. In this review, we focus on recent breakthroughs on phloem transport and signalling. A case study was phloem loading of sucrose, acting both as a nutrient and as a signal, whose activity was shown to be tightly regulated. Recent advances also described actors of macromolecular trafficking in sieve elements, including chaperones and RNA binding proteins, involved potentially in the formation of ribonucleoprotein complexes. Likewise, long distance signalling appeared to integrate electrical potential waves, calcium bursts and potentially the generation of reactive oxygen species. The ubiquitin-proteasome system was also proposed to be on action in sieve elements for signalling and protein turnover. Surprisingly, several basic processes of phloem physiology are still under debate. Hence, the absence in phloem sap of reducing sugar species, such as hexoses, was recently challenged with observations based on an analysis of the sap from Ranunculaceae and Papaveraceae. The possibility that protein synthesis might occur in sieve elements was again questioned with the identification of components of the translational machinery in Pumpkin phloem sap. Altogether, these new findings strengthen the idea that phloem is playing a central role in interorgan nutrient exchanges and communication and demonstrate that the ways by which this is achieved can obey various patterns among species.

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.