Molecular cloning and expression analysis of turnip (Brassica rapa var. rapa) sucrose transporter gene family

In higher plants, sugars(mainly sucrose) are produced by photosynthetically assimilated carbon in mesophyll cells of leaves and translocated to heterotrophic organs to ensure plant growth and development. Sucrose transporters, or sucrose carriers(SUCs), play an important role in the long-distance transportation of sucrose from source organs to sink organs, thereby affecting crop yield and quality.The identification, characterization, and molecular function analysis of sucrose transporter genes have been reported for monocot and dicot plants. However, no relevant study has been reported on sucrose transporter genes in Brassica rapa var. rapa, a cruciferous root crop used mainly as vegetables and fodder.We identified and cloned 12 sucrose transporter genes from turnips, named BrrSUC1.1 to BrrSUC6.2according to the SUC gene sequences of B. rapa pekinensis. We constructed a phylogenetic tree and analyzed conserved motifs for all 12 sucrose transporter genes identified. Real-time quantitative polymerase chain reaction was conducted to understand the expression levels of SUC genes in different tissues and developmental phases of the turnip. These findings add to our understanding of the genetics and physiology of sugar transport during taproot formation in turnips.

Characterization and functional divergence of genes encoding sucrose transporters in oilseeds castor bean

Castor bean(Ricinus communis L.)is an economically important non-edible oilseed crop.Its seed oils are rich in hydroxy fatty acid,which are highly valuable with a wide range of industrial applications.Sucrose transportation is critical in regulating the growth,development and oilseed yield in castor bean.The transporters or carriers(SUTs or SUCs)play a central role in orchestrating sucrose allocation and aiding in plant adaptation to diverse stresses.In this study,based on castor bean genome,three RcSUCs(RcSUC2,RcSUC3 and RcSUC4)were identified and characterized.The expressional profiles of RcSUCs in different tissues such as leaf,stem,root,phloem and seed tissues exhibited a distinct divergence of gene expression,suggesting that the functions of RcSUC2,RcSUC3 and RcSUC4 are differentiated into long or short-distance transportation among tissues.Additionally,under abiotic stresses including hot temperature,low temperature,drought and salt stresses,the sugar allocation among leaf,stem and roots was tested.The expressional changes of Rc SUCs in leaf,stem and root tissues were associated with sugar transportation and allocation.Taken together,the differential expression of Rc SUCs among tissues responsing to abiotic stress suggested functional differences in sucrose transport and redistribution in different tissues.This study is helpful to understand the physiological and molecular mechanisms of sucrose transportation and allocation among tissues in heterotrophic oilseeds,and could provide clues for genetic improvement and optimization of cultivation practices.

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 iden- tification 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. MtSUTI-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 MtSUTI-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 bio- trophic interactions.

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. D^siree） 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 OsSUTSZ 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 OsSUTSZ 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.

Arabidopsis Sucrose Transporter SUT4 Interacts with Cytochrome b5-2 to Regulate Seed Germination in Response to Sucrose and Glucose

It remains unknown whether a sucrose transporter mediates sugar signaling. Here, we report that the Arabidopsis （Arabidopsis thaliana） sucrose transporter SUT4 interacts with five members of the Arabidopsis cytochrome b5 （Cyb5） family, and sucrose represses the interaction between SUT4 and a Cyb5 member Cyb5-2/A. We observed that down- regulation of SUT4 and three cytochrome b5 members （Cyb5-2, Cyb5-4, and Cyb5-6） confers the sucrose- and glucose- insensitive phenotypes in the sucrose/glucose-induced inhibition of seed germination. The sut4 cybS-2 double mutant displays slightly stronger sucrose/glucose-insensitive phenotypes than either the sut4 or cyb5-2 single mutant. We showed that the SUT4/Cyb5-2-mediated signaling in the sucrose/glucose-induced inhibition of seed germination does not require ABA or the currently known ABI2/ABI4/ABI5-mediated signaling pathway（s）. These data provide evidence that the sucrose transporter SUT4 interacts with Cyb5 to positively mediate sucrose and glucose signaling in the sucrose/ glucose-induced inhibition of seed germination.

Cloning and Expression Analysis of Rice Sucrose Transporter Genes OsSUT2M and OsSUT5Z

Two sucrose transporter （SUT） cDNAs, OsSUT2M and OsSUT5Z, were isolated from rice （Oryza sativa L.） by reverse transcription polymerase chain reaction （RT-PCR）. Sequencing results indicate they are 1 531 bp and 1 635 bp in length including complete open reading frame 1 506bp and 1 608 bp, which encode 502 amino acids and 536 amino acids, respectively. The TopPred program suggested that both sucrose transporter proteins, OsSUT2M and OsSUT5Z, consist of potentially 12 transmembrane domains. Semi-quantitative RT-PCR was carried out to investigate the gene expression patterns of OsSUT2M and OsSUT5Z. In vegetative organs, transcripts of OsSUT2M were higher in source leaf blades than in other organs at the same development stage, whereas transcripts of OsSUT5Z were less traceable in all organs investigated. In reproductive organs, both transcripts of these two genes were high in panicles from the booting stage to 7 days after flowering （DAF） and then sharply declined. The potential physiology functions of these two sucrose transporters are discussed.

Plant Sucrose Transporters from a Biophysical Point of View

The majority of higher plants use sucrose as their main mobile carbohydrate. Proton-driven sucrose trans- porters play a crucial role in cell-to-cell and long-distance distribution of sucrose throughout the plant. A very negative plant membrane potential and the ability of sucrose transporters to accumulate sucrose concentrations of more than 1 M indicate that plants evolved transporters with unique structural and functional features. The knowledge about the trans- port mechanism and structural/functional domains of these the current knowledge about the biophysical properties of nano-machines is, however, still fragmentary. In this review, plant sucrose transporters is summarized and discussed.

Polyphosphate Accelerates Transformation of Nonstructural Carbohydrates to Improve Growth of ppk-Expressing Transgenic Rice in Phosphorus Deficiency Culture

Crop yield and quality are often limited by the amount of phosphate fertilizer added to infertile soils,a key limiting factor for sustainable development in modern agriculture.The polyphosphate kinase(ppk)gene-expressing transgenic rice with a single-copy line(ETRS)is constructed to improve phosphate fertilizer utilization efficiency for phosphorus resource conservation.To investigate the potential mechanisms of the increased biomass in ETRS in low phosphate culture,ETRS was cultivated in a low inorganic phosphate(Pi)culture medium(15μmol/L Pi,LP)and a normal Pi culture medium(300μmol/L Pi,CP),respectively.After 89 d of cultivation in different concentrations of phosphate culture media,the total phosphorus,polyphosphate(polyP),biomass,photosynthetic rate,nonstructural carbohydrate(NSC)contents,related enzyme activities,and related gene expression levels were analyzed.The results showed that ETRS had a high polyP amount to promote the photosynthetic rate in LP,and its biomass was almost the same as the wild type(WT)in CP.The NSC content of ETRS in LP was higher than that of WT in LP,but slightly lower than that of WT in CP.PolyP notably promoted the sucrose phosphate synthase activities of ETRS and significantly down-regulated the expression levels of sucrose transporter genes(OsSUT3 and OsSUT4),resulting in inhibiting the transport of sucrose from shoot to root in ETRS.It was concluded that polyP can stimulate the synthesis of NSCs in LP,which improved the growth of ETRS and triggered the biological activities of ETRS to save phosphate fertilizer.Our study provides a new way to improve the utilization rate of phosphate fertilizer in rice production.

Dt1 inhibits SWEET-mediated sucrose transport to regulate photoperiod-dependent seed weight in soybean

Soybean is a photoperiod-sensitive short-day crop whose reproductive period and yield are markedly affected by day-length changes.Seed weight is one of the key traits determining the soybean yield;how-ever,the prominent genes that control the final seed weight of soybean and the mechanisms underlying the photoperiod's effect on this trait remain poorly understood.In this study,we identify SwW19 as a major locus controlling soybean seed weight by QTL mapping and determine Dt1,an orthologous gene of Arabidopsis TFL1 that is known to govern the soybean growth habit,as the causal gene of the SW19 locus.We showed that Dt1 is highly expressed in developing seeds and regulates photoperiod-dependent seed weight in soybean.Further analyses revealed that the Dt1 protein physically interacts with the sucrose transporter GmSWEET10a to negatively regulate the import of sucrose from seed coat to the embryo,thus modulating seed weight under long days.However,Dt1 does not function in seed development under short days due to its very low expression.Importantly,we discovered a novel natural allelic variant of Dt1(H4 haplotype)that decouples its pleiotropic effects on seed size and growth habit;i.e.,this variant remains functional in seed development but fails to regulate the stem growth habit of soybean.Collectively,our findings provide new insights into how soybean seed development responds to photoperiod at different latitudes,offering an ideal genetic component for improving soybean's yield by manipulating its seed weightandgrowth habit.

High nitrogen application rate and planting density reduce wheat grain yield by reducing filling rate of inferior grain in middle spikelets

Excessive use of nitrogen fertilizer and high planting density reduce grain weight in wheat.However,the effects of high nitrogen and planting density on the filling of grain located in different positions of the wheat spikelet are unknown.A two-year field experiment was conducted to investigate this question and the underlying mechanisms with respect to hormone and carbohydrate activity.Both high nitrogen application and planting density significantly increased spike density,while reducing kernel number per spike and 1000-kernel weight.However,the effects of high nitrogen and high plant density on kernel number per spike and 1000-kernel weight were different.The inhibitory effect of high nitrogen application and high planting density on kernel number per spike was achieved mainly by a reduction in kernel number per spikelet in the top and bottom spikelets.However,the decrease in 1000-kernel weight was contributed mainly by the reduced weight of grain in the middle spikelets.The grain-filling rate of inferior grain in the middle spikelets was significantly decreased under high nitrogen input and high planting density conditions,particularly during the early and middle grain-filling periods,leading to the suppression of grain filling and consequent decrease in grain weight.This effect resulted mainly from inhibited sucrose transport to and starch accumulation in inferior grain in the middle spikelets via reduction of the abscisic acid/ethylene ratio.This mechanism may explain how high nitrogen application and high planting density inhibit the grain filling of inferior wheat grain.