Identification and characterization of a sucrose transporter isolated from the developing cotyledons of soybean

Phaseolus vulgaris SUT1.1 is a high affinity sucrose-proton co-transporter

Plant sucrose transporters are required for phloem loading, and therefore are essential for plant growth and development. In common beans (Phaseolus vulgaris) there are only two sucrose transporters functionally characterized. Through a previous RNA-seq study, we identified a putative sucrose transporter in common bean, which we hypothesize to function in import of sucrose into plant cells. In silico analysis revealed that PvSUT1.1 is a putative sucrose-proton co-transporter distinct from other characterized sucrose transporters in common bean indicating that this is a previously undescribed transporter protein in beans. Further analysis revealed that PvSUT1.1 shares high protein sequence homology to the phloem loader Arabidopsis SUC2; both have 12 transmembrane domains, a typical characteristic of plant sucrose transporters. Heterologous expression in yeast further showed PvSUT1.1 to be functional and it imported sucrose into yeast cells with a Km of 0.7 mM sucrose. Import of sucrose through PvSUT1.1 is also pH-dependent with highest uptake at pH 4.0, and activity is lost in the presence of the uncoupler carbonyl cyanide 3-chlorophenylhydrazone. Consistent with identification of PvSUT1.1 as a Type I transporter, PvSUT1.1 also transports esculin. Finally, PvSUT1.1 showed expression in multiple tissues and the protein was localized to the plasma membrane. The results show that PvSUT1.1 is a sucrose transporter that is probably involved in the uptake of sucrose into source and sink cells. The potential role of PvSUT1.1 in leaf phloem loading of sucrose in common beans and its importance in heat tolerance of reproductive tissues are further discussed.

Defoliation, not gibberellin, induces tree peony autumn reflowering regulated by carbon allocation and metabolism in buds and leaves

Short and concentrated natural fluorescence hinders tree peony (Paeonia suffruticosa) annual production, and defoliation and gibberellin (GA) application is used to induce its reflowering in autumn. Here, the individual roles of defoliation and GA treatment were determined by monitoring morphological and soluble sugar changes in buds and leaves, and by investigating carbon allocation- and metabolism-related gene expression. Both defoliation and GA treatment induced early bud development, but induction was faster using the GA treatment. Only defoliation, not GA treatment, induced the final reflowering, although their combination accelerated it. Furthermore, defoliation decreased the sucrose content in buds much faster than the GA treatment. This sucrose reduction may play a key role in tree peony reflowering, and the higher carbon metabolism activity in young leaves after defoliation may further help the reflowering process. Defoliation enhanced the expression of sucrose transporters PsSUT4 and PsSWEET12 in buds, and their expression in young leaves was greater than after GA treatment. This indicated that PsSUT4 and PsSWEET12 may help transport carbon into buds after defoliation. In addition, the invertases, PsCIN2 and PsCWIN1 in young leaves were more highly expressed after defoliation, indicating that they may contribute to reflowering after defoliation by accelerating sucrose hydrolysis in young leaves. In addition, the expression levels of PsVIN1 and PsVIN2 in leaves, and PsVIN2 in buds were more highly induced by GA treatment than by defoliation, indicating that PsVINs may mainly respond to GA treatment. These results may help improve the tree peony forcing culture technology and related industrial production.

Sugar transporters in Fabaceae, featuring SUT MST and SWEET families of the model plant Medicago truncatula and the agricultural crop Pisum sativum

Sugar transporters play a crucial role for plant productivity, as they coordinate sugar fluxes from source leaf towards sink organs (seed, fruit, root) and regulate the supply of carbon resources towards the microorganisms of the rhizosphere (bacteria and fungi). Thus, sugar fluxes mediated by SUT (sucrose transporters), MST (monosaccharide transporters) and SWEET (sugar will eventually be exported transporters) families are key determinants of crop yield and shape the microbial communities living in the soil. In this work, we performed a systematic search for sugar transporters in Fabaceae genomes, focusing on model and agronomical plants. Here, we update the inventory of sugar transporter families mining the latest version of the Medicago truncatula genome and identify for the first time SUT MST and SWEET families of the agricultural crop Pisum sativum. The sugar transporter families of these Fabaceae species comprise respectively 7 MtSUT 7 PsSUT, 72 MtMST 59 PsMST and 26 MtSWEET 22 PsSWEET. Our comprehensive phylogenetic analysis sets a milestone for the scientific community, as we propose a new and simple nomenclature to correctly name SUT MST and SWEET families. Then, we searched for transcriptomic data available for our gene repertoire. We show that several clusters of homologous genes are co-expressed in different organs, suggesting that orthologous sugar transporters may have a conserved function. We focused our analysis on gene candidates that may be involved in remobilizing resources during flowering, grain filling and in allocating carbon towards roots colonized by arbuscular mycorrhizal fungi and Rhizobia. Our findings open new perspectives for agroecological applications in legume crops, as for instance improving the yield and quality of seed productions and promoting the use of symbiotic microorganisms.

In vivo transport of three radioactive [18F]-fluorinated deoxysucrose analogs by the maize sucrose transporter ZmSUT1

Sucrose transporter (SUT) proteins translocate sucrose across cell membranes; however, mechanistic aspects of sucrose binding by SUTs are not well resolved. Specific hydroxyl groups in sucrose participate in hydrogen bonding with SUT proteins. We previously reported that substituting a radioactive fluorine-18 [¹⁸F] at the C-6' position within the fructosyl moiety of sucrose did not affect sucrose transport by the maize (Zea mays) ZmSUT1 protein. To determine how ¹⁸F substitution of hydroxyl groups at two other positions within sucrose, the C-1' in the fructosyl moiety or the C-6 in the glucosyl moiety, impact sucrose transport, we synthesized 1'-[F¹⁸]fluoro-1'-deoxysucrose and 6-[F¹⁸]fluoro-6-deoxysucrose ([¹⁸F]FDS) analogs. Each [¹⁸F]FDS derivative was independently introduced into wild-type or sut1 mutant plants, which are defective in sucrose phloem loading. All three (1'-, 6'-, and 6-) [¹⁸F]FDS derivatives were efficiently and equally translocated, similarly to carbon-14 [¹⁴C]-labeled sucrose. Hence, individually replacing the hydroxyl groups at these positions within sucrose does not interfere with substrate recognition, binding, or membrane transport processes, and hydroxyl groups at these three positions are not essential for hydrogen bonding between sucrose and ZmSUT1. [¹⁸F]FDS imaging afforded several advantages compared to [¹⁴C]-sucrose detection. We calculated that 1'-[¹⁸F]FDS was transported at approximately a rate of 0.90 ± 0.15 m.h-1 in wild-type leaves, and at 0.68 ± 0.25 m.h-1 in sut1 mutant leaves. Collectively, our data indicated that [¹⁸F]FDS analogs are valuable tools to probe sucrose-SUT interactions and to monitor sucrose transport in plants.

Coordinated regulation of arbuscular mycorrhizal fungi and soybean MAPK pathway genes improved mycorrhizal soybean drought tolerance

Mitogen-activated protein kinase (MAPK) cascades play important roles in the stress response in both plants and microorganisms. The mycorrhizal symbiosis established between arbuscular mycorrhizal fungi (AMF) and plants can enhance plant drought tolerance, which might be closely related to the fungal MAPK response and the molecular dialogue between fungal and soybean MAPK cascades. To verify the above hypothesis, germinal Glomus intraradices (syn. Rhizophagus irregularis) spores and potted experiments were conducted. The results showed that AMF GiMAPKs with high homology with MAPKs from Saccharomyces cerevisiae had different gene expression patterns under different conditions (nitrogen starvation, abscisic acid treatment, and drought). Drought stress upregulated the levels of fungi and soybean MAPK transcripts in mycorrhizal soybean roots, indicating the possibility of a molecular dialogue between the two symbiotic sides of symbiosis and suggesting that they might cooperate to regulate the mycorrhizal soybean drought-stress response. Meanwhile, the changes in hydrogen peroxide, soluble sugar, and proline levels in mycorrhizal soybean as well as in the accelerated exchange of carbon and nitrogen in the symbionts were contributable to drought adaptation of the host plants. Thus, it can be preliminarily inferred that the interactions of MAPK signals on both sides, symbiotic fungus and plant, might regulate the response of symbiosis and, thus, improve the resistance of mycorrhizal soybean to drought stress.

The Medicago truncatula sucrose transporter family: characterization and implication of key members in carbon partitioning towards arbuscular mycorrhizal fungi

We identified de novo sucrose transporter (SUT) genes involved in long-distance transport of sucrose from photosynthetic source leaves towards sink organs in the model leguminous species Medicago truncatula. The identification and functional analysis of sugar transporters provide key information on mechanisms that underlie carbon partitioning in plant-microorganism interactions. In that way, full-length sequences of the M. truncatula SUT (MtSUT) family were retrieved and biochemical characterization of MtSUT members was performed by heterologous expression in yeast. The MtSUT family now comprises six genes which distribute among Dicotyledonous clades. MtSUT1-1 and MtSUT4-1 are key members in regard to their expression profiles in source leaves and sink roots and were characterized as functional H(+)/sucrose transporters. Physiological and molecular responses to phosphorus supply and inoculation by the arbuscular mycorrhizal fungus (AMF) Glomus intraradices was studied by gene expression and sugar quantification analyses. Sucrose represents the main sugar transport form in M. truncatula and the expression profiles of MtSUT1-1, MtSUT2, and MtSUT4-1 highlight a fine-tuning regulation for beneficial sugar fluxes towards the fungal symbiont. Taken together, these results suggest distinct functions for proteins from the SUT1, SUT2, and SUT4 clades in plant and in biotrophic interactions.

Early carbon mobilization and radicle protrusion in maize germination

Considerable amounts of information is available on the complex carbohydrates that are mobilized and utilized by the seed to support early seedling development. These events occur after radicle has protruded from the seed. However, scarce information is available on the role of the endogenous soluble carbohydrates from the embryo in the first hours of germination. The present work analysed how the soluble carbohydrate reserves in isolated maize embryos are mobilized during 6-24 h of water imbibition, an interval that exclusively embraces the first two phases of the germination process. It was found that sucrose constitutes a very significant reserve in the scutellum and that it is efficiently consumed during the time in which the adjacent embryo axis is engaged in an active metabolism. Sucrose transporter was immunolocalized in the scutellum and in vascular elements. In parallel, a cell-wall invertase activity, which hydrolyses sucrose, developed in the embryo axis, which favoured higher glucose uptake. Sucrose and hexose transporters were active in the embryo tissues, together with the plasma membrane H(+)-ATPase, which was localized in all embryo regions involved in both nutrient transport and active cell elongation to support radicle extension. It is proposed that, during the initial maize germination phases, a net flow of sucrose takes place from the scutellum towards the embryo axis and regions that undergo elongation. During radicle extension, sucrose and hexose transporters, as well as H(+)-ATPase, become the fundamental proteins that orchestrate the transport of nutrients required for successful germination.

The sucrose transporter HbSUT3 plays an active role in sucrose loading to laticifer and rubber productivity in exploited trees of Hevea brasiliensis (para rubber tree)

Efficient sucrose loading in rubber-producing cells (laticifer cells) is essential for retaining rubber productivity in Hevea brasiliensis, but the molecular mechanisms underlying the regulation of this process remain unknown. Here, we functionally characterized a putative Hevea SUT member, HbSUT3, mainly in samples from regularly exploited trees. When expressed in yeast, HbSUT3 encodes a functional sucrose transporter that exhibits high sucrose affinity with a K(m) value of 1.24 mm at pH 4.0, and possesses features typical of sucrose/H(+) symporters. In planta, when compared to the expression of other Hevea SUT genes, HbSUT3 was found to be the predominant member expressed in the rubber-containing cytoplasm (latex) of laticifers. The comparison of HbSUT3 expression among twelve Hevea tissues demonstrates a relatively tissue-specific pattern, i.e. expression primarily in the latex and in female flowers. HbSUT3 expression is induced by the latex stimulator Ethrel (an ethylene generator), and relates to its yield-stimulating effect. Tapping (the act of rubber harvesting) markedly increased the expression of HbSUT3, whereas wounding alone had little effect. Moreover, the expression of HbSUT3 was found to be positively correlated with latex yield. Taken together, our results provide evidence favouring the involvement of HbSUT3 in sucrose loading into laticifers and in rubber productivity.

Review: Nutrient loading of developing seeds

Interest in nutrient loading of seeds is fuelled by its central importance to plant reproductive success and human nutrition. Rates of nutrient loading, imported through the phloem, are regulated by transport and transfer processes located in sources (leaves, stems, reproductive structures), phloem pathway and seed sinks. During the early phases of seed development, most control is likely to be imposed by a low conductive pathway of differentiating phloem cells serving developing seeds. Following the onset of storage product accumulation by seeds, and, depending on nutrient species, dominance of path control gives way to regulation by processes located in sources (nitrogen, sulfur, minor minerals), phloem path (transition elements) or seed sinks (sugars and major mineral elements, such as potassium). Nutrients and accompanying water are imported into maternal seed tissues and unloaded from the conducting sieve elements into an extensive post-phloem symplasmic domain. Nutrients are released from this symplasmic domain into the seed apoplasm by poorly understood membrane transport mechanisms. As seed development progresses, increasing volumes of imported phloem water are recycled back to the parent plant by process(es) yet to be discovered. However, aquaporins concentrated in vascular and surrounding parenchyma cells of legume seed coats could provide a gated pathway of water movement in these tissues. Filial cells, abutting the maternal tissues, take up nutrients from the seed apoplasm by membrane proteins that include sucrose and amino acid/H⁺ symporters functioning in parallel with non-selective cation channels. Filial demand for nutrients, that comprise the major osmotic species, is integrated with their release and phloem import by a turgor-homeostat mechanism located in maternal seed tissues. It is speculated that turgors of maternal unloading cells are sensed by the cytoskeleton and transduced by calcium signalling cascades.

A suite of sucrose transporters expressed in coats of developing legume seeds includes novel pH-independent facilitators

A suite of newly discovered sucrose transporter genes, PsSUF1, PsSUF4, PvSUT1 and PvSUF1, were isolated from the coats of developing pea (Pisum sativum L.) and bean (Phaseolus vulgaris L.) seeds. Sequence analysis indicated that deduced proteins encoded by PsSUF1, PvSUT1 and PvSUF1 clustered in a separate sub-group under sucrose transporter Clade I, whereas the deduced protein encoded by PsSUF4 clustered in Clade II. When expressed in yeast, these genes were shown to encode sucrose transporters with apparent Michaelis Menten constant (Km) values ranging from 8.9 to 99.8 mm. PvSUT1 exhibited functional characteristics of a sucrose/H+ symporter. In contrast, PsSUF1, PvSUF1 and PsSUF4 supported the pH- and energy independent transport of sucrose that was shown to be bi-directional. These transport properties, together with that of counter transport, indicated that PsSUF1, PvSUF1 and PsSUF4 function as carriers that support the facilitated diffusion of sucrose. Carrier function was unaffected by diethylpyrocarbonate and by maltose competition, suggesting that the sucrose binding sites of these transporters differed from those of known sucrose/H+ symporters. All sucrose transporters were expressed throughout the plant and, of greatest interest, were co-expressed in cells considered responsible for sucrose efflux from seed coats. The possible roles played by the novel facilitators in sucrose efflux from seed coats are discussed.

Plant membrane proteomics

Plant membrane proteins are involved in many different functions according to their location in the cell. For instance, the chloroplast has two membrane systems, thylakoids and envelope, with specialized membrane proteins for photosynthesis and metabolite and ion transporters, respectively. Although recent advances in sample preparation and analytical techniques have been achieved for the study of membrane proteins, the characterization of these proteins, especially the hydrophobic ones, is still challenging. The present review highlights recent advances in methodologies for identification of plant membrane proteins from purified subcellular structures. The interest of combining several complementary extraction procedures to take into account specific features of membrane proteins is discussed in the light of recent proteomics data, notably for chloroplast envelope, mitochondrial membranes and plasma membrane from Arabidopsis. These examples also illustrate how, on one hand, proteomics can feed bioinformatics for a better definition of prediction tools and, on the other hand, although prediction tools are not 100% reliable, they can give valuable information for biological investigations. In particular, membrane proteomics brings new insights over plant membrane systems, on both the membrane compartment where proteins are working and their putative cellular function.

Altered expression of an ankyrin-repeat protein results in leaf abnormalities, necrotic lesions, and the elaboration of a systemic signal

The PR-like proteins, class I beta-1,3-glucanase (GLU I) and chitinase (CHN I), are induced as part of a stereotypic response that can provide protection against viral, bacterial, and fungal pathogens. We have identified two Nicotiana plumbaginifolia ankyrin-repeat proteins, designated Glucanohydrolase Binding Proteins (GBP) 1 and 2, that bind GLU I and CHN I both in vitro and when expressed in yeast cells. Sense as well as antisense transformants of tobacco carrying the GBP1 gene elaborated graft-transmissible acropetally moving signals that induced the downward curling of young leaves. This phenotype was associated with reduced starch, sucrose, and fructose accumulation; the formation of necrotic lesions; and, the induction of markers for the hypersensitive response. GBP1/2 are members of a conserved Plant- Specific Ankyrin- repeat (PANK) family that includes proteins implicated in carbohydrate allocation, reactive oxygen metabolism, hypersensitive cell death, rapid elicitor responses, virus pathogenesis, and auxin signaling. The similarity in phenotype of PANK transformants and transformants altered in carbohydrate metabolism leads us to propose that PANK family members are multifunctional proteins involved in linking plant defense responses and carbohydrate metabolism.

Reassessment of the pits and antipits in soybean seeds

A soybean (Glycine max (L.) Merr.) cotyledon has a concave region on its abaxial center called a pit, and the seed coat has a corresponding convex region on its inner surface called an antipit. While it is clear that the pit is lined with large epidermal cells, the anatomical identity of the antipit has been a subject of confusion. The inner surface of the seed coat consists of a compressed endosperm tissue that is subtended by an aleurone layer. In the antipit region of the seed coat, additional endosperm cells are situated between the aleurone layer and the compressed endosperm tissue. These endosperm cells, called cone cells, are similar to ordinary aleurone cells in basic structure. In the present study, it is demonstrated that the surface ornamentations of the antipit are a print of the pit on the compressed endosperm tissue. The functional aspects of the antipit–pit complex are not known. It was previously postulated that the antipit–pit region has an enhanced capability of nutrient translocation to the growing embryo, but there is little evidence to support this hypothesis. Alternatively, the antipit–pit association may provide a structural feature that anchors the embryo within the seed coat while contributing to an overall spherical shape of the seed.Key words: antipit, endosperm, Glycine max, pit, seed coat, soybean.