Cloning and expression analysis of rice sucrose transporter genes OsSUT2M and OsSUT5Z

MATE transporter GFD1 cooperates with sugar transporters, mediates carbohydrate partitioning and controls grain-filling duration, grain size and number in rice

More than half of the world's food is provided by cereals, as humans obtain >60% of daily calories from grains. Producing more carbohydrates is always the final target of crop cultivation. The carbohydrate partitioning pathway directly affects grain yield, but the molecular mechanisms and biological functions are poorly understood, including rice (Oryza sativa L.), one of the most important food sources. Here, we reported a prolonged grain filling duration mutant 1 (gfd1), exhibiting a long grain-filling duration, less grain number per panicle and bigger grain size without changing grain weight. Map-based cloning and molecular biological analyses revealed that GFD1 encoded a MATE transporter and expressed high in vascular tissues of the stem, spikelet hulls and rachilla, but low in the leaf, controlling carbohydrate partitioning in the stem and grain but not in the leaf. GFD1 protein was partially localized on the plasma membrane and in the Golgi apparatus, and was finally verified to interact with two sugar transporters, OsSWEET4 and OsSUT2. Genetic analyses showed that GFD1 might control grain-filling duration through OsSWEET4, adjust grain size with OsSUT2 and synergistically modulate grain number per panicle with both OsSUT2 and OsSWEET4. Together, our work proved that the three transporters, which are all initially classified in the major facilitator superfamily family, could control starch storage in both the primary sink (grain) and temporary sink (stem), and affect carbohydrate partitioning in the whole plant through physical interaction, giving a new vision of sugar transporter interactome and providing a tool for rice yield improvement.

An overview of sucrose transporter (SUT) genes family in rice

INTRODUCTION

Photosynthesis provides the energy basis for the life activities of plants by producing organic compounds, mainly sugar. As the main energy form of photosynthesis, sugar affects the growth and development of plants. During long-distance transportation, sucrose is the main form of transportation. The rate of sugar transport and the allocation of carbohydrates affect the biomass of crops and are closely related to the reproductive growth of crops.

MAIN TEXT

The transportation of sugar is divided into active transportation and passive transportation. So how does the sucrose transporters (SUT) genes, which are the main carriers of sucrose in active transportation, affect the performance of rice agronomic traits is still to be explored. In this article, we describe the structure of inflorescence and review the transport forms and metabolic processes of sucrose in rice, such as how CO₂ is fixed, carbohydrate assimilation, and transport of organic matter. Sucrose transporters exhibited remarkable effects on the development of reproductive organs in rice.

CONCLUSIONS

Here, the effects of different factors, such as the effects of anthers morphology on starch enrichment of pollen, effects of biotic and abiotic factors on sucrose transporters, effects of changes in trace elements on sucrose transporters, were discussed. Moreover, the regulation of transcription or translation level provides ideas for future research on sucrose transporters.

Down-regulation of the sucrose transporters HvSUT1 and HvSUT2 affects sucrose homeostasis along its delivery path in barley grains

Sucrose transport and partitioning are crucial for seed filling. While many plasma-membrane-localised sucrose transporters (SUT1 family members) have been analysed in seeds, the functions of vacuolar SUT2 members are still obscure. In barley grains, expression of HvSUT1 and HvSUT2 overlap temporally and spatially, suggesting concerted functions to regulate sucrose homeostasis. Using HvSUT2-RNAi plants, we found that grains were also deficient in HvSUT1 expression and seemingly sucrose-limited during mid-to-late grain filling. Transgenic endosperms accumulated less starch and dry weight, although overall sucrose and hexose contents were higher. Comprehensive transcript and metabolite profiling revealed that genes related to glycolysis, the tricarboxylic acid cycle, starch and amino acid synthesis, grain maturation, and abscisic acid signalling were down-regulated together with most glycolytic intermediates and amino acids. Sucrose was increased along the sucrose delivery route in the nucellar projection, the endosperm transfer cells, and the starchy endosperm, indicating that suppressed transporter activity diminished sucrose efflux from vacuoles, which generated sugar deficiency in the cytoplasm. Thus, endosperm vacuoles may buffer sucrose concentrations to regulate homeostasis at grain filling. Transcriptional changes revealed that limited endosperm sucrose initiated sugar starvation responses, such as sugar recycling from starch, hemicelluloses and celluloses together with vacuolar protein degradation, thereby supporting formation of nucleotide sugars. Barley endosperm cells can thus suppress certain pathways to retrieve resources to maintain essential cell functions.

Sucrose Transporter AtSUC9 Mediated by a Low Sucrose Level is Involved in Arabidopsis Abiotic Stress Resistance by Regulating Sucrose Distribution and ABA Accumulation

Sucrose (Suc) transporters (SUCs or SUTs) are important regulators in plant growth and stress tolerance. However, the mechanism of SUCs in plant abiotic stress resistance remains to be dietermined. Here, we found that AtSUC9 expression was induced by abiotic stress, including salt, osmotic and cold stress conditions. Disruption of AtSUC9 led to sensitive responses to abiotic stress during seed germination and seedling growth. Further analyses indicated that the sensitivity phenotype of Atsuc9 mutants resulted from higher Suc content in shoots and lower Suc content in roots, as compared with that in wild-type (WT) plants. In addition, we found that the expression of AtSUC9 is induced in particular by low levels of exogenous and endogenous Suc, and deletion of AtSUC9 affected the expression of the low Suc level-responsive genes. AtSUC9 also showed an obvious response to treatments with low concentrations of exogenous Suc during seed germination, seedling growth and Suc distribution, and Atsuc9 mutants hardly grew in abiotic stress treatments without exogenous Suc. Moreover, our results illustrated not only that deletion of AtSUC9 blocks abiotic stress-inducible ABA accumulation but also that Atsuc9 mutants had a lower content of endogenous ABA in stress conditions than in normal conditions. Deletion of AtSUC9 also inhibited the expression of many ABA-inducible genes (SnRk2.2/3/6, ABF2/3/4, ABI1/3/4, RD29A, KIN1 and KIN2). These results indicate that AtSUC9 is induced in particular by low Suc levels then mediates the balance of Suc distribution and promotes ABA accumulation to enhance Arabidopsis abiotic stress resistance.

Identification and characterization of the three homeologues of a new sucrose transporter in hexaploid wheat (Triticum aestivum L.)

BACKGROUND

Sucrose transporters (SUTs) play important roles in regulating the translocation of assimilates from source to sink tissues. Identification and characterization of new SUTs in economically important crops such as wheat provide insights into their role in determining seed yield. To date, however, only one SUT of wheat has been reported and functionally characterized. The present study reports the isolation and characterization of a new SUT, designated as TaSUT2, and its homeologues (TaSUT2A, TaSUT2B and TaSUT2D) in hexaploid wheat (Triticum aestivum L.).

RESULTS

TaSUT2A and TaSUT2B genes each encode a protein with 506 amino acids, whereas TaSUT2D encodes a protein of 508 amino acids. The molecular mass of these proteins is predicted to be ~ 54 kDA. Topological analysis of the amino acid sequences of the three homeologues revealed that they contain 12 transmembrane spanning helices, which are described as distinct characteristic features of glycoside-pentoside-hexuronide cation symporter family that includes all known plant SUTs, and a histidine residue that appears to be localized at and associated conformationally with the sucrose binding site. Yeast SUSY7/ura3 strain cells transformed with TaSUT2A, TaSUT2B and TaSUT2D were able to uptake sucrose and grow on a medium containing sucrose as a sole source of carbon; however, our subcellular localization study with plant cells revealed that TaSUT2 is localized to the tonoplast. The expression of TaSUT2 was detected in the source, including flag leaf blade, flag leaf sheath, peduncle, glumes, palea and lemma, and sink (seed) tissues. The relative contributions of the three genomes of wheat to the total expression of TaSUT2 appear to differ with tissues and developmental stages. At the cellular level, TaSUT2 is expressed mainly in the vein of developing seeds and subepidermal mesophyll cells of the leaf blade.

CONCLUSION

This study demonstrated that TaSUT2 is a new wheat SUT protein. Given that TaSUT2 is localized to the tonoplast and sucrose is temporarily stored in the vacuoles of both source and sink tissues, our data imply that TaSUT2 is involved in the intracellular partitioning of sucrose, particularly between the vacuole and cytoplasm.

Sucrose Transporter Gene AtSUC4 Responds to Drought Stress by Regulating the Sucrose Distribution and Metabolism in Arabidopsis thaliana

Sucrose transporters (SUCs or SUTs) are considered as the important carriers and responsible for the loading, unloading and distribution of sucrose, but at present there is no report that SUCs are involved in sucrose distribution and metabolism under drought stress at the whole-plant level. AtSUC4, as the unique member of SUT4-clade inArabidopsis thaliana, may be important for plant stress tolerance. Here, by analyzing two homozygous mutation lines ofAtSUC4(Atsuc4-1andAtsuc4-2), we found drought stress induced higher sucrose, lower fructose and glucose contents in shoots, and lower sucrose, higher fructose and glucose contents in roots of these mutants compared with the wild-type (WT), leading to an imbalance of sucrose distribution, fructose and glucose (sucrose metabolites) accumulation changes at the whole-plant level. Thus we believe thatAtSUC4regulates sucrose distribution and metabolism in response to drought stress.

A novel fluorescent assay for sucrose transporters

BACKGROUND

We have developed a novel assay based on the ability of type I sucrose uptake transporters (SUTs) to transport the fluorescent coumarin β-glucoside, esculin. Budding yeast (Saccharomyces cerevisiae) is routinely used for the heterologous expression of SUTs and does not take up esculin.

RESULTS

When type I sucrose transporters StSUT1 from potato or AtSUC2 from Arabidopsis were expressed in yeast, the cells were able to take up esculin and became brightly fluorescent. We tested a variety of incubation times, esculin concentrations, and buffer pH values and found that for these transporters, a 1 hr incubation at 0.1 to 1 mM esculin at pH 4.0 produced fluorescent cells that were easily distinguished from vector controls. Esculin uptake was assayed by several methods including fluorescence microscopy, spectrofluorometry and fluorescence-activiated cell sorting (FACS). Expression of the type II sucrose transporter OsSUT1 from rice did not result in increased esculin uptake under any conditions tested. Results were reproduced successfully in two distinct yeast strains, SEY6210 (an invertase mutant) and BY4742.

CONCLUSIONS

The esculin uptake assay is rapid and sensitive and should be generally useful for preliminary tests of sucrose transporter function by heterologous expression in yeast. This assay is also suitable for selection of yeast showing esculin uptake activity using FACS.

Impaired function of the tonoplast-localized sucrose transporter in rice, OsSUT2, limits the transport of vacuolar reserve sucrose and affects plant growth

Physiological functions of sucrose (Suc) transporters (SUTs) localized to the tonoplast in higher plants are poorly understood. We here report the isolation and characterization of a mutation in the rice (Oryza sativa) OsSUT2 gene. Expression of OsSUT2-green fluorescent protein in rice revealed that OsSUT2 localizes to the tonoplast. Analysis of the OsSUT2 promoter::β-glucuronidase transgenic rice indicated that this gene is highly expressed in leaf mesophyll cells, emerging lateral roots, pedicels of fertilized spikelets, and cross cell layers of seed coats. Results of Suc transport assays in yeast were consistent with a H(+)-Suc symport mechanism, suggesting that OsSUT2 functions in Suc uptake from the vacuole. The ossut2 mutant exhibited a growth retardation phenotype with a significant reduction in tiller number, plant height, 1,000-grain weight, and root dry weight compared with the controls, the wild type, and complemented transgenic lines. Analysis of primary carbon metabolites revealed that ossut2 accumulated more Suc, glucose, and fructose in the leaves than the controls. Further sugar export analysis of detached leaves indicated that ossut2 had a significantly decreased sugar export ability compared with the controls. These results suggest that OsSUT2 is involved in Suc transport across the tonoplast from the vacuole lumen to the cytosol in rice, playing an essential role in sugar export from the source leaves to sink organs.

A transgenic study on affecting potato tuber yield by expressing the rice sucrose transporter genes OsSUT5Z and OsSUT2M

In many plants, sucrose transporters are essential for both sucrose exports from sources and imports into sinks, indicating a function in assimilate partitioning. To investigate whether sucrose transporters can improve the yield of starch plant, potato plants (Solanum tuberosum L. cv. Désirée) were transformed with cDNAs of the rice sucrose transporter genes OsSUT5Z and OsSUT2M under the control of a tuber-specific, class-I patatin promoter. Compared to the controls, the average fructose content of OsSUT5Z transgenic tubers significantly increased. However, the content of the sugars and starch in the OsSUT2M transgenic potato tubers showed no obvious difference. Correspondingly, the average tuber yield, average number of tubers per plant and average weight of single tuber showed no significant difference in OsSUT2M transgenic tubers with controls. In the OsSUT5Z transgenic lines, the average tuber yield per plant was 1.9-fold higher than the controls, and the average number of tubers per plant increased by more than 10 tubers on average, whereas the average weight of a single tuber did not increase significantly. These results suggested that the average number of tubers per plant showed more contribution than the average weight of a single tuber to the tuber yield per plant.

Sucrose importation into laticifers of Hevea brasiliensis, in relation to ethylene stimulation of latex production

BACKGROUND AND AIMS

The major economic product of Hevea brasiliensis is a rubber-containing cytoplasm (latex), which flows out of laticifers (latex cells) when the bark is tapped. The latex yield is stimulated by ethylene. Sucrose, the unique precursor of rubber synthesis, must cross the plasma membrane through specific sucrose transporters before being metabolized in the laticifers. The relative importance of sucrose transporters in determining latex yield is unknown. Here, the effects of ethylene (by application of Ethrel on sucrose transporter gene expression in the inner bark tissues and latex cells of H. brasiliensis are described.

METHODS

Experiments, including cloning sucrose transporters, real time RT-PCR and in situ hybridization, were carried out on virgin (untapped) trees, treated or untreated with the latex yield stimulant Ethrel.

KEY RESULTS

Seven putative full-length cDNAs of sucrose transporters were cloned from a latex-specific cDNA library. These transporters belong to all SUT (sucrose transporter) groups and differ by their basal gene expression in latex and inner soft bark, with a predominance of HbSUT1A and HbSUT1B. Of these sucrose transporters, only HbSUT1A and HbSUT2A were distinctly increased by ethylene. Moreover, this increase was shown to be specific to laticifers and to ethylene application.

CONCLUSION

The data and all previous information on sucrose transport show that HbSUT1A and HbSUT2A are related to the increase in sucrose import into laticifers, required for the stimulation of latex yield by ethylene in virgin trees.