SUT2, a putative sucrose sensor in sieve elements

Local endocytosis of sucrose transporter 2 in duckweed reveals the role of sucrose transporter 2 in guard cells

The local endocytosis of membrane proteins is critical for many physiological processes in plants, including the regulation of growth, development, nutrient absorption, and osmotic stress response. Much of our knowledge on the local endocytosis of plasma membrane (PM) protein only focuses on the polar growth of pollen tubes in plants and neuronal axon in animals. However, the role of local endocytosis of PM proteins in guard cells has not yet been researched. Here, we first cloned duckweed SUT2 (sucrose transporter 2) protein and then conducted subcellular and histological localization of the protein. Our results indicated that LpSUT2 (Landoltia punctata 0202 SUT2) is a PM protein highly expressed on guard cells. In vitro experiments on WT (wild type) lines treated with high sucrose concentration showed that the content of ROS (reactive oxygen species) in guard cells increased and stomatal conductance decreased. We observed the same results in the lines after overexpression of the LpSUT2 gene with newfound local endocytosis of LpSUT2. The local endocytosis mainly showed that LpSUT2 was uniformly distributed on the PM of guard cells in the early stage of development, and was only distributed in the endomembrane of guard cells in the mature stage. Therefore, we found the phenomenon of guard cell LpSUT2 local endocytosis through the changes of duckweed stomata and concluded that LpSUT2 local endocytosis might be dependent on ROS accumulation in the development of duckweed guard cells. This paper might provide future references for the genetic improvement and water-use efficiency in other crops.

Involvement of SUT1 and SUT2 Sugar Transporters in the Impairment of Sugar Transport and Changes in Phloem Exudate Contents in Phytoplasma-Infected Plants

Phytoplasmas inhabit phloem sieve elements and cause abnormal growth and altered sugar partitioning. However, how they interact with phloem functions is not clearly known. The phloem responses were investigated in tomatoes infected by “Candidatus Phytoplasma solani” at the beginning of the symptomatic stage, the first symptoms appearing in the newly emerged leaf at the stem apex. Antisense lines impaired in the phloem sucrose transporters SUT1 and SUT2 were included. In symptomatic sink leaves, leaf curling was associated with higher starch accumulation and the expression of defense genes. The analysis of leaf midribs of symptomatic leaves indicated that transcript levels for genes acting in the glycolysis and peroxisome metabolism differed from these in noninfected plants. The phytoplasma also multiplied in the three lower source leaves, even if it was not associated with the symptoms. In these leaves, the rate of phloem sucrose exudation was lower for infected plants. Metabolite profiling of phloem sap-enriched exudates revealed that glycolate and aspartate levels were affected by the infection. Their levels were also affected in the noninfected SUT1- and SUT2-antisense lines. The findings suggest the role of sugar transporters in the responses to infection and describe the consequences of impaired sugar transport on the primary metabolism.

Sucrose Transporter Gene AtSUC4 Regulates Sucrose Distribution and Metabolism in Response to Salt Stress in Arabidopsis thaliana

Sporadic reports indicated that salt stress induced the expression of sucrose transporter genes, and sucrose transporters (SUCs or SUTs) as the important carriers are responsible for the loading, unloading and distribution of sucrose, but the study that SUCs are involved in sucrose distribution and metabolism under salt stress at the whole-plant level has not been reported to date. AtSUC4, as the unique member of low affinity/high capacity SUT4-clade inArabidopsis thaliana, may play an important role in plant stress tolerance. Here, through analyzing two homozygous mutation lines ofAtSUC4(Atsuc4-1andAtsuc4-2), we found salt stress induced higher sucrose, fructose and glucose content in shoots and lower sucrose, fructose and glucose content in roots of these mutants compared with the wild-type (WT), resulting in an imbalance of sucrose distribution and fructose and glucose accumulation changes of sucrose metabolitesat the whole-plant level. Our results indicated thatAtSUC4is involved in salt stress tolerance by the regulation of sucrose distribution and metabolism.

Vacuoles release sucrose via tonoplast-localised SUC4-type transporters

Arabidopsis thaliana has seven genes for functionally active sucrose transporters. Together with sucrose transporters from other dicot and monocot plants, these proteins form four separate phylogenetic groups. Group-IV includes the Arabidopsis protein SUC4 (synonym SUT4) and related proteins from monocots and dicots. These Group-IV sucrose transporters were reported to be either tonoplast- or plasma membrane-localised, and in heterologous expression systems were shown to act as sucrose/H(+) symporters. Here, we present comparative analyses of the subcellular localisation of the Arabidopsis SUC4 protein and of several other Group-IV sucrose transporters, studies on tissue specificity of the Arabidopsis SUC4 promoter, phenotypic characterisations of Atsuc4.1 mutants and AtSUC4 overexpressing (AtSUC4-OX) plants, and functional comparisons of Atsuc4.1 and AtSUC4-OX vacuoles. Our data show that SUC4-type sucrose transporters from different plant families (Brassicaceae, Cucurbitaceae and Solanaceae) localise exclusively to the tonoplast, demonstrating that vacuolar sucrose transport is a common theme of all SUC4-type proteins. AtSUC4 expression is confined to the stele of Arabidopsis roots, developing anthers and meristematic tissues in all aerial parts. Analyses of the carbohydrate content of WT and mutant seedlings revealed reduced sucrose content in AtSUC4-OX seedlings. This is in line with patch-clamp analyses of AtSUC4-OX vacuoles that characterise AtSUC4 as a sucrose/H(+) symporter directly in the tonoplast membrane.

SUT Sucrose and MST Monosaccharide Transporter Inventory of the Selaginella Genome

Most metazoa use hexose transporters to acquire hexoses from their diet and as a transport form for distributing carbon and energy within their bodies; insects use trehalose, and plants use sucrose as their major form for translocation. Plant genomes contain at least three families of mono- and disaccharide transporters: monosaccharide/polyol transporters that are evolutionary closely related to the yeast and human glucose transporters, sucrose transporters of the SUT family, which similar to the hexose transporters belong to the major facilitator superfamily, but share only minimal amino acid sequence homology with the hexose transporters, and the family of SWEET sugar transporters conserved between animals and plants. Recently, the genome sequence of the spikemoss Selaginella has been determined. In order to study the evolution of sugar transport in plants, we carefully annotated of the complement of sugar transporters in Selaginella. We review the current knowledge regarding sugar transport in spikemoss and provide phylogenetic analyses of the complement of MST and SUT homologs in Selaginella (and Physcomitrella).

Evolution of plant sucrose uptake transporters

In angiosperms, sucrose uptake transporters (SUTs) have important functions especially in vascular tissue. Here we explore the evolutionary origins of SUTs by analysis of angiosperm SUTs and homologous transporters in a vascular early land plant, Selaginella moellendorffii, and a non-vascular plant, the bryophyte Physcomitrella patens, the charophyte algae Chlorokybus atmosphyticus, several red algae and fission yeast, Schizosaccharomyces pombe. Plant SUTs cluster into three types by phylogenetic analysis. Previous studies using angiosperms had shown that types I and II are localized to the plasma membrane while type III SUTs are associated with vacuolar membrane. SUT homologs were not found in the chlorophyte algae Chlamydomonas reinhardtii and Volvox carterii. However, the characean algae Chlorokybus atmosphyticus contains a SUT homolog (CaSUT1) and phylogenetic analysis indicated that it is basal to all other streptophyte SUTs analyzed. SUTs are present in both red algae and S. pombe but they are less related to plant SUTs than CaSUT1. Both Selaginella and Physcomitrella encode type II and III SUTs suggesting that both plasma membrane and vacuolar sucrose transporter activities were present in early land plants. It is likely that SUT transporters are important for scavenging sucrose from the environment and intracellular compartments in charophyte and non-vascular plants. Type I SUTs were only found in eudicots and we conclude that they evolved from type III SUTs, possibly through loss of a vacuolar targeting sequence. Eudicots utilize type I SUTs for phloem (vascular tissue) loading while monocots use type II SUTs for phloem loading. We show that HvSUT1 from barley, a type II SUT, reverted the growth defect of the Arabidopsis atsuc2 (type I) mutant. This indicates that type I and II SUTs evolved similar (and interchangeable) phloem loading transporter capabilities independently.

Regulation of RhSUC2, a sucrose transporter, is correlated with the light control of bud burst in Rosa sp

In roses, light is a central environmental factor controlling bud break and involves a stimulation of sugar metabolism. Very little is known about the role of sucrose transporters in the bud break process and its regulation by light. In this study, we show that sugar promotes rose bud break and that bud break is accompanied by an import of sucrose. Radio-labelled sucrose accumulation is higher in buds exposed to light than to darkness and involves an active component. Several sucrose transporter (RhSUC1, 2, 3 and 4) transcripts are expressed in rose tissues, but RhSUC2 transcript level is the only one induced in buds exposed to light after removing the apical dominance. RhSUC2 is preferentially expressed in bursting buds and stems. Functional analyses in baker's yeast demonstrate that RhSUC2 encodes a sucrose/proton co-transporter with a K(m) value of 2.99 mm at pH 4.5 and shows typical features of sucrose symporters. We therefore propose that bud break photocontrol partly depends upon the modulation of sucrose import into buds by RhSUC2.