Laurence Maurousset

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.

Cloning, Expression, and Characterization of Sorbitol Transporters from Developing Sour Cherry Fruit and Leaf Sink Tissues

The acyclic polyol sorbitol is a primary photosynthetic product and the principal photosynthetic transport substance in many economically important members of the family Rosaceace (e.g. almond [Prunus dulcis (P. Mill.) D.A. Webber], apple [Malus pumila P. Mill.], cherry [Prunus spp.], peach [Prunus persicaL. Batsch], and pear [Pyrus communis]). To understand key steps in long-distance transport and particularly partitioning and accumulation of sorbitol in sink tissues, we have cloned two sorbitol transporter genes (PcSOT1 andPcSOT2) from sour cherry (Prunus cerasus) fruit tissues that accumulate large quantities of sorbitol. Sorbitol uptake activities and other characteristics were measured by heterologous expression of PcSOT1 andPcSOT2 in yeast (Saccharomyces cerevisiae). Both genes encode proton-dependent, sorbitol-specific transporters with similar affinities (K m sorbitol of 0.81 mm for PcSOT1 and 0.64 mm for PcSOT2). Analyses of gene expression of these transporters, however, suggest different roles during leaf and fruit development. PcSOT1 is expressed throughout fruit development, but especially when growth and sorbitol accumulation rates are highest. In leaves, PcSOT1 expression is highest in young, expanding tissues, but substantially less in mature leaves. In contrast, PcSOT2 is mainly expressed only early in fruit development and not in leaves. Compositional analyses suggest that transport mediated by PcSOT1 and PcSOT2 plays a major role in sorbitol and dry matter accumulation in sour cherry fruits. Presence of these transporters and the high fruit sorbitol concentrations suggest that there is an apoplastic step during phloem unloading and accumulation in these sink tissues. Expression of PcSOT1 in young leaves before completion of the transition from sink to source is further evidence for a role in determining sink activity.

Regulation of sugar, amino acid and peptide plant membrane transporters

During the past few years, various cDNAs encoding the proton cotransporters which mediate the uptake of sucrose, hexoses, amino acids and peptides across the plant plasma membrane have been cloned. This has made possible some preliminary insight into the regulation of the activity of these transporters at various levels. The paper summarises the present status of knowledge and gaps relative to their transcriptional control (organ, tissue and cell specificity, response to the environment) and post-transcriptional control (targeting and turnover, kinetic and thermodynamic control, lipidic environment, phosphorylation). This outline and the description of a few cases (the sink/source transition of the leaf, the pollen grain, the legume seed) serve as a basis for suggesting some directions for future research.

Ethylene stimulation of latex yield depends on the expression of a sucrose transporter (HbSUT1B) in rubber tree (Hevea brasiliensis).

Hevea brasiliensis is an important industrial crop for natural rubber production. Latex biosynthesis occurs in the cytoplasm of highly specialized latex cells and requires sucrose as the unique precursor. Ethylene stimulation of latex production results in high sugar flow from the surrounding cells of inner bark towards the latex cells. The aim of this work was to understand the role of seven sucrose transporters (HbSUTs) and one hexose transporter (HbHXT1) in this process. Two Hevea clones were used: PB217 and PB260, respectively described as high and low yielding clones. The expression pattern of these sugar transporters (HbSUTs and HbHXT1) was monitored under different physiological conditions and found to be maximal in latex cells. HbSUT1, one of the most abundant isoforms, displayed the greatest response to ethylene treatment. In clone PB217, ethylene treatment led to a higher accumulation of HbSUT1B in latex cells than in the inner bark tissues. Conversely, stronger expression of HbSUT1B was observed in inner bark tissues than in latex cells of PB260. A positive correlation with HbSUT1B transcript accumulation and increased latex production was further supported by its lower expression in latex cells of the virgin clone PB217.

Water Deficit Enhances C Export to the Roots in Arabidopsis thaliana Plants with Contribution of Sucrose Transporters in Both Shoot and Roots

Mild water deficit enhances C export to the roots and modifies root architecture, with a subset of sucrose transporters involved in both shoot and roots. Root high plasticity is an adaptation to its changing environment. Water deficit impairs growth, leading to sugar accumulation in leaves, part of which could be available to roots via sucrose (Suc) phloem transport. Phloem loading is widely described in Arabidopsis (Arabidopsis thaliana), while unloading in roots is less understood. To gain information on leaf-to-root transport, a soil-based culture system was developed to monitor root system architecture in two dimensions. Under water deficit (50% of soil water-holding capacity), total root length was strongly reduced but the depth of root foraging and the shape of the root system were less affected, likely to improve water uptake. 14CO2 pulse-chase experiments confirmed that water deficit enhanced carbon (C) export to the roots, as suggested by the increased root-to-shoot ratio. The transcript levels of AtSWEET11 (for sugar will eventually be exported transporter), AtSWEET12, and AtSUC2 (for Suc carrier) genes, all three involved in Suc phloem loading, were significantly up-regulated in leaves of water deficit plants, in accordance with the increase in C export from the leaves to the roots. Interestingly, the transcript levels of AtSUC2 and AtSWEET11 to AtSWEET15 were also significantly higher in stressed roots, underlying the importance of Suc apoplastic unloading in Arabidopsis roots and a putative role for these Suc transporters in Suc unloading. These data demonstrate that, during water deficit, plants respond to growth limitation by allocating relatively more C to the roots to maintain an efficient root system and that a subset of Suc transporters is potentially involved in the flux of C to and in the roots.

Occurrence of plasmodesmata between differentiating vessels and other xylem cells inSorbus torminalis L. Crantz and their fate during xylem maturation

SummaryThe development of pit-pairs between differentiating xylem cells has been examined by transmission electron microscopy in young shoots ofSorbus torminalis. In some vessel-to-tracheid pits, as well as in previously studied intertracheid pits, a thickening of the pit membrane containing branched plasmodesmata was observed. A secondary wall-like cap was deposited over the thickening prior to cytoplasmic autolysis; some plasmodesmata, parallel to the plane of section, appeared to perforate the cap. At the end of the cell maturation stage, the central part of the primary wall thickening was hydrolysed, while the cap, including plasmodesmata remnants, appeared unaltered. In half-bordered pit-pairs between a parenchyma cell and a vessel or a tracheid, similar structures could be observed beside the conducting elements. When the vessel or tracheid matured, sealing of the pit membrane plasmodesmata resulted from the formation of a protective layer on the parenchyma-side rather than from the deposition of a cap on the conducting cell-side. These observations provide the first information on the presence of symplasmic connections in pits between differentiating vessels and neighbouring xylem cells. InS. torminalis, xylem differentiation is probably highly coordinated within a symplasmic domain; the persistence of such connections may account for the lack of specialization ofSorbus wood.

The promoters of 3 celery salt-induced phloem-specific genes as new tools for monitoring salt stress responses.

Genes induced by a progressive 3 week salt stress (final NaCl concentration 300 mM) were identified in the phloem of celery (Apium graveolens L., cv Vert dElne). A subtractive library was constructed and screened for salt-induced, phloem-specific genes. Work was focused on phloem due to its central role in inter-organ exchanges. Three genes were studied in more details, 2 coding for metallothioneins (AgMT2 and AgMT3) and one for a new mannitol transporter (AgMaT3). Expression of a reporter gene in transgenic Arabidopsis under control of promoter of each gene was located in the phloem. pAgMT2 has a typical phloem pattern with slight induction by salt stress. pAgMT3 and pAgMaT3 expression was induced by salt stress, except in minor veins. pAgMaT3 was highly active in stressed roots. The promoters described here could be regarded as new tools for engineering salt-resistant plants.

Carbon source–sink relationship in Arabidopsis thaliana: the role of sucrose transporters

AbstractMain conclusionThe regulation of source-to-sink sucrose transport is associated withAtSUC andAtSWEET sucrose transporters’ gene expression changes in plants grown hydroponically under different physiological conditions.n Source-to-sink transport of sucrose is one of the major determinants of plant growth. Whole-plant carbohydrates’ partitioning requires the specific activity of membrane sugar transporters. In Arabidopsis thaliana plants, two families of transporters are involved in sucrose transport: AtSUCs and AtSWEETs. This study is focused on the comparison of sucrose transporter gene expression, soluble sugar and starch levels and long distance sucrose transport, in leaves and sink organs (mainly roots) in different physiological conditions (along the plant life cycle, during a diel cycle, and during an osmotic stress) in plants grown hydroponically. In leaves, the AtSUC2, AtSWEET11, and 12 genes known to be involved in phloem loading were highly expressed when sucrose export was high and reduced during osmotic stress. In roots, AtSUC1 was highly expressed and its expression profile in the different conditions tested suggests that it may play a role in sucrose unloading in roots and in root growth. The SWEET transporter genes AtSWEET12, 13, and 15 were found expressed in all organs at all stages studied, while differential expression was noticed for AtSWEET14 in roots, stems, and siliques and AtSWEET9, 10 expressions were only detected in stems and siliques. A role for these transporters in carbohydrate partitioning in different source–sink status is proposed, with a specific attention on carbon demand in roots. During development, despite trophic competition with others sinks, roots remained a significant sink, but during osmotic stress, the amount of translocated [U-14C]-sucrose decreased for rosettes and roots. Altogether, these results suggest that source–sink relationship may be linked with the regulation of sucrose transporter gene expression.

Cloning and functional characterization of LjPLT4, a plasma membrane xylitol H+- symporter from Lotus japonicus

Abstract Polyols are compounds that play various physiological roles in plants. Here we present the identification of four cDNA clones of the model legume Lotus japonicus, encoding proteins of the monosaccharide transporter-like (MST) superfamily that share significant homology with previously characterized polyol transporters (PLTs). One of the transporters, named LjPLT4, was characterized functionally after expression in yeast. Transport assays revealed that LjPLT4 is a xylitol-specific H+-symporter (K m, 0.34 mM). In contrast to the previously characterized homologues, LjPLT4 was unable to transport other polyols, including mannitol, sorbitol, myo-inositol and galactitol, or any of the monosaccharides tested. Interestingly, some monosaccharides, including fructose and xylose, inhibited xylitol uptake, although no significant uptake of these compounds was detected in the LjPLT4 transformed yeast cells, suggesting interactions with the xylitol binding site. Subcellular localization of LjPLT4-eYFP fusions expressed in Arabidopsis leaf epidermal cells indicated that LjPLT4 is localized in the plasma membrane. Real-time RT-PCR revealed that LjPLT4 is expressed in all major plant organs, with maximum transcript accumulation in leaves correlating with maximum xylitol levels there, as determined by GC-MS. Thus, LjPLT4 is the first plasma membrane xylitol-specific H+-symporter to be characterized in plants.

Sugars en route to the roots. Transport, metabolism and storage within plant roots and towards microorganisms of the rhizosphere

In plants, the root is a typical sink organ that relies exclusively on the import of sugar from the aerial parts. Sucrose is delivered by the phloem to the most distant root tips and, en route to the tip, is used by the different root tissues for metabolism and storage. Besides, a certain portion of this carbon is exuded in the rhizosphere, supplied to beneficial microorganisms and diverted by parasitic microbes. The transport of sugars toward these numerous sinks either occurs symplastically through cell connections (plasmodesmata) or is apoplastically mediated through membrane transporters (MST, mononsaccharide tranporters, SUT/SUC, H+/sucrose transporters and SWEET, Sugar will eventually be exported transporters) that control monosaccharide and sucrose fluxes. Here, we review recent progresses on carbon partitioning within and outside roots, discussing membrane transporters involved in plant responses to biotic and abiotic factors.