Sugars not only fuel cellular carbon and energy metabolism but also play pivotal roles as signaling molecules. The experimental amenability of yeast as a unicellular model system has enabled the discovery of multiple sugar sensors and signaling pathways. In plants, different sugar signals are generated by photosynthesis and carbon metabolism in source and sink tissues to modulate growth, development, and stress responses. Genetic analyses have revealed extensive interactions between sugar and plant hormone signaling, and a central role for hexokinase (HXK) as a conserved glucose sensor. Diverse sugar signals activate multiple HXK-dependent and HXKindependent pathways and use different molecular mechanisms to control transcription, translation, protein stability and enzymatic activity. Important and complex roles for Snf1-related kinases (SnRKs), extracellular sugar sensors, and trehalose metabolism in plant sugar signaling are now also emerging.

https://molbio.mgh.harvard.edu/sheenweb/reprints/ARPB06sugar.pdf

Sugar Sensing and Signaling in Plants: Conserved and Novel Mechanisms

The structure

The natural environment for plants is composed of a complex set of abiotic stresses and biotic stresses. Plant responses to these stresses are equally complex. Systems biology approaches facilitate a multi-targeted approach by allowing one to identify regulatory hubs in complex networks. Systems biology takes the molecular parts (transcripts, proteins and metabolites) of an organism and attempts to fit them into functional networks or models designed to describe and predict the dynamic activities of that organism in different environments. In this review, research progress in plant responses to abiotic stresses is summarized from the physiological level to the molecular level. New insights obtained from the integration of omics datasets are highlighted. Gaps in our knowledge are identified, providing additional focus areas for crop improvement research in the future.

/pdf/effects-of-abiotic-stress-on-plants-a-systems-biology-19yoepwwvu.pdf

Effects of abiotic stress on plants: a systems biology perspective

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.

/pdf/source-to-sink-transport-of-sugar-and-regulation-by-3izapdndox.pdf

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

The environmental stress is a major area of scientific concern because it constraints plant as well as crop productivity. This situation has been further worsened by anthropogenic activities. Therefore, there is a much scientific saddle on researchers to enhance crop productivity under environmental stress in order to cope with the increasing food demands. The abiotic stresses such as salinity, drought, cold, and heat negatively influence the survival, biomass production and yield of staple food crops. According to an estimate of FAO, over 6 % of the world’s land is affected by salinity. Thus, salinity stress appears to be a major constraint to plant and crop productivity. Here, we review our understanding of salinity impact on various aspects of plant metabolism and its tolerance strategies in plants.

Effect of salinity stress on plants and its tolerance strategies: a review.

Wine is made up of more than one thousand compounds, the majority of which, such as vitamins and minerals, come from the grapes, while others, like ethanol and glycerol, are products of the winemaking process. Although sugars are either partially or completely transformed, sugar import and accumulation into the ripening berry is a major parameter of wine quality. Sugar status is directly related to the final alcoholic content of wine, and regulates several genes responsible for the development of its aromatic and organoleptic properties. Physiological ripeness is reached when the grapes achieve sufficiently high sugar levels without loosing too much acidity; however, aromatic and phenolic compound content must also be taken into account. Softening and water content are other essential characteristics of a ripe berry. From a winemaker point of view, optimal grape maturity is essential for wine quality, but is difficult to assess because it is under multifactorial control, involving grapevine cultivar variety and environmental parameters such as soil, temperature, exposure to sun, and hormonal regulation. Continued study of the key control points in grape ripening is crucial if we ultimately hope to improve grape and wine quality.

https://repositorium.sdum.uminho.pt/bitstream/1822/6820/1/Conde%20et%20al.2007%28review%29.pdf

Biochemical Changes throughout Grape Berry Development and Fruit and Wine Quality

Aquaporins are multifunctional water and solute transporters highly divergent in living organisms

The function of ASR (ABA [abscisic acid]-, stress-, and ripening-induced) proteins remains unknown. A grape ASR, VvMSA, was isolated by means of a yeast one-hybrid approach using as a target the proximal promoter of a grape putative monosaccharide transporter (VvHT1). This promoter contains two sugar boxes, and its activity is induced by sucrose and glucose. VvMSA and VvHT1 share similar patterns of expression during the ripening of grape. Both genes are inducible by sucrose in grape berry cell culture, and sugar induction of VvMSA is enhanced strongly by ABA. These data suggest that VvMSA is involved in a common transduction pathway of sugar and ABA signaling. Gel-shift assays demonstrate a specific binding of VvMSA to the 160-bp fragment of the VvHT1 promoter and more precisely to two sugar-responsive elements present in this target. The positive regulation of VvHT1 promoter activity by VvMSA also is shown in planta by coexpression experiments. The nuclear localization of the yellow fluorescent protein–VvMSA fusion protein and the functionality of the VvMSA nuclear localization signal are demonstrated. Thus, a biological function is ascribed to an ASR protein. VvMSA acts as part of a transcription-regulating complex involved in sugar and ABA signaling.

/pdf/a-grape-asr-protein-involved-in-sugar-and-abscisic-acid-3kiqacu964.pdf

A Grape ASR Protein Involved in Sugar and Abscisic Acid Signaling

Grape (Vitis vinifera) heterotrophic suspension-cultured cells were used as a model system to study glucose (Glc) transport and its regulation. Cells transported D-[14C]Glc according to simple Michaelis-Menten kinetics superimposed on first-order kinetics. The saturating component is a high-affinity, broad-specificity H+ -dependent transport system (Km = 0.05 mm). Glc concentration in the medium tightly regulated the transcription of VvHT1 (Vitis vinifera hexose transporter 1), a monosaccharide transporter previously characterized in grape berry, as well as VvHT1 protein amount and monosaccharide transport activity. All the remaining putative monosaccharide transporters identified so far in grape were poorly expressed and responded weakly to Glc. VvHT1 transcription was strongly repressed by Glc and 2-deoxy-D-Glc, but not by 3-O-methyl-D-Glc or Glc plus mannoheptulose, indicating the involvement of a hexokinase-dependent repression. 3-O-Methyl-D-Glc, which cannot be phosphorylated, and Glc plus mannoheptulose induced a decrease of transport activity caused by the reduction of VvHT1 protein in the plasma membrane without affecting VvHT1 transcript levels. This demonstrates hexokinase-independent posttranscriptional regulation. High Glc down-regulated VvHT1 transcription and Glc uptake, whereas low Glc increased those parameters. Present data provide an example showing control of plant sugar transporters by their own substrate both at transcriptional and posttranscriptional levels. VvHT1 protein has an important role in the massive import of monosaccharides into mesocarp cells of young grape berries because it was localized in plasma membranes of the early developing fruit. Protein amount decreased abruptly throughout fruit development as sugar content increases, consistent with the regulating role of Glc on VvHT1 expression found in suspension-cultured cells.

/pdf/pathways-of-glucose-regulation-of-monosaccharide-transport-2inj81z1v9.pdf

Pathways of Glucose Regulation of Monosaccharide Transport in Grape Cells

Different lengths of the promoter of grape ( Vitis vinifera ) VvHT1 ( Hexose Transporter 1 ) gene, which encodes a putative hexose transporter expressed during the ripening of grape, have been transcriptionally fused to the β-glucuronidase reporter gene. In transgenic tobacco ( Nicotiana tabacum ) transformed with these constructs, VvHT1 promoters were clearly responsible for the sink organ preferential expression. The potential sugar effectors of VvHT1 promoter were studied in tobacco cv Bright-Yellow 2 cells transformed with chimeric constructs. Glucose (56 mm), sucrose (Suc; 58 mm), and the non-transported Suc isomer palatinose doubled the β-glucuronidase activity conferred by the VvHT1 promoter, whereas fructose did not affect it. These effects were the strongest with the 2.4-kb promoter, which contains all putative sugar-responsive elements (activating and repressing), but they were also significant with the 0.3-kb promoter, which contains only activating sugar boxes. The induction of VvHT1 expression by both Suc and palatinose was confirmed in the homologous grape berry cell culture. The data provide the first example of a putative sugar transporter, which is induced by both glucose and Suc in higher plants. Although induction of VvHT1 expression by Suc does not require transport, the presence of glucosyl moiety is necessary for Suc sensing. These results provide new insights into sugar sensing and signaling in plants.

/pdf/sugar-regulated-expression-of-a-putative-hexose-transport-emyror8i0t.pdf

Alice Agasse

Papers

Biochemical Changes throughout Grape Berry Development and Fruit and Wine Quality

Aquaporins are multifunctional water and solute transporters highly divergent in living organisms

A Grape ASR Protein Involved in Sugar and Abscisic Acid Signaling

Pathways of Glucose Regulation of Monosaccharide Transport in Grape Cells

Sugar-Regulated Expression of a Putative Hexose Transport Gene in Grape